UKC

10 years ago we bought a wreck of a house with no heating system to speak of, single glazed windows, literal holes in the roof etc etc.

Our village isn't on mains gas and at the time I didn't like the idea of an oil fired system from an environmental standpoint. Given I generate quite a bit of wood waste as a furniture maker we installed an entirely wood fired central heating system with a log burning stove that heats a thermal store to provide c/h and hot water.   We burn all my waste wood plus a fair few logs a year that we buy in and gather / split ourselves.  Always burnt well below 20% mc. I think about 10 cubic meters a year.

It's not a thermally efficient house by a long stretch,  victorian, stone walls with no cavity to insulate, it does now have double glazed wooden windows everywhere.  We dismissed an air pump, or rather the manufacturers did as our house is just too leaky, something I might address more going forwards.

Get to the point George...

How much better/worse has burning all that wood been for the environment than if we'd saved ourselves a huge amount of work and just stuck an oil boiler in?

Hard to quantify perhaps but go on have a stab at it. Per KG of wood/oil how do the two compare in CO2 and other stuff?  I'd like to think it's been better than oil but maybe I'm wrong about that.

Not sure about the answer to your question but planting trees is always good karma

It's better than fossil fuels, assuming you (or someone) are planting  at least what you  burn. The net emissions are zero.in this case, pretty much.  There may be even better uses for the wood, and there are questions about about polution in some areas,  but better than any fossil.fuel for heating.

To a first order, it’s not about what you burn but about what you plant.  Timber and logs are more likely to be matched by tree planting than oil.

Wood burners are extremely bad for some types of pollution. 8% of population use them yet they cause more small particle pollution than traffic:

https://amp.theguardian.com/environment/2021/feb/16/home-wood-burning-bigge...

Don’t know how they compare with an oil boiler. 

Ok, I've had a quick stab at this using some googled figures. 

Firewood is about 600kg per cubic metre. You're using 10 cube per year so about 6,000kg. Co2 released by burning wood in a stove is about 1.9 times the weight of wood, so you're producing around 11,400kg of Co2 per year. BUT that is effectively net zero as the trees absorbed that during their life. (Assuming you're not traveling large distances to collect it) 

On average firewood produces 5.1 Kwh per Kg so you use about 30,600 kwh per year.

Oil contains around 10.35 Kwh per litre so your wood is the same as 2956 litres of oil. Oil gives off 2.52 Kg Co2 per litre when burnt which would be 7,449Kg per year. BUT that's 7,449Kg of fresh Co2 added to the cycle. 

As I say, average figures quickly grabbed off Google and not accounting for boiler efficiency, wood type or moisture content etc. Also the trade off to lower Co2 levels is the increase in air pollution and smoke particulates.  

(Happy to be corrected if I've cocked up the maths!)

On another note, your system is likely to be all steel and copper so 100% recyclable once it's life is finished. 

This linked article is worth a read. Wood burners are mainly a problem for particulate output but you can significantly reduce that impact by using the better efficiency burners and only burning dry wood. Improving the energy efficiency of the home (insulation, double glazing etc) and supplementing with renewables and battery storage of electricity can also help (Paul from UKC has posted on this a few times).

Also the various industry bodies have pointed out (correctly) that the cumulative national problem includes burning of coal in wood stoves and all sorts of other burners outside the home. I've no idea how they deal with the uncontrolled output of bonfires and other fires (eg Nottingham's waste recycling center went up in flames this week).

https://www.which.co.uk/reviews/wood-burning-stoves/article/wood-burning-st...

well... the wood is going to give up its carbon, burning it just speeds up the process

the oil, coal and gas should be left in the ground

simple yeti logic : )

Most waste wood gets pelleted and burned somewhere like Drax power station.

If you burn it for heat yourself you cut out the processing and transport etc. Since it's seasoned the particulate concern is greatly reduced.

I can't quantify it, but I'm certain you are reducing your carbon footprint

How does this work? Not disputing, just interested in the argument here.

I have a bunch of stuff with carbon locked away in liquid or solid hydrocarbons - wood, oil, coal, whatever.  That carbon is mostly inert and is not contributing to global warming.

I burn it.  I get energy out and I convert the carbon in to CO2, which goes in to the atmosphere and contributes to global warming.

To a crude approximation, I get the same amount of useful energy out per kg of CO2 released.  So it doesn't matter what I've burnt, I just made global warming worse.

If I burnt oil, it's come from a hole in the ground. That hole in the ground does not go on to remove any CO2 from the atmosphere in response to the oil being removed.

If I burn logs that come from a managed forest, the trees that were cut down for my timber are replaced with more trees.  These remove CO2 from the atmosphere making global warming less bad.

Hopefully a well managed forest represents a net removal of CO2 from the atmosphere as lots is more permanently sequestered in the root systems and the leaf mulch, although there are plenty of ways of making things worse with the wrong practices I think.

If oil burning was matched by carbon sequestration efforts elsewhere, e.g. through additional tree planting, it would not be such a problem in terms of greenhouse gasses.  

I'm sure I've got a grossly over-simplified view of things here...  The elephant in the room with my argument is that I very much doubt we could support the scale of managed forestry needed to replace all fossil fuel use...

I think he means that in order to keep the cycle going we need to grow as much wood as we consume. Recapture the carbon released during burning (or rotting), in whatever way, by growing more trees to mop it up again. On a global scale I very much doubt that's happening at the moment. Oil on the other hand is gone for a few million years once it's been used. 

I have to declare an interest because we had a burner(a multi-fuel one)

I read the Guardian article but I haven't read all the government papers but that 8% of the population cause 38% of the particulate emissions doesn't sound likely.

For about six months of the year I think wood burning stoves(other than those which power central heating and provide hot water, which are probably in the minority) will not burn any wood. As we have had some mild winters our stove hasn't been lit for over two years.

Motor vehicles have become more efficient but they are used all year and it seems odd that the approximately 31million petrol/diesel vehicles in the UK produce only 12% of the particulate emission while wood burning stoves/fires produce 38%.

Dave   

I’d like a wood burner (and could get cheap-ish logs from my brother who sells logs as a business - kiln dried etc).

I hear people with log burners making their case, but I wonder what things would be like if every house that could have one, had one. Would log production be sustainable and manageable? How bad would particulate pollution be?

As always UKC comes up with some great reasoned discussion and further reading.

I’d like to add that for 6 months of the year we don’t light our wood powered heating (it’s warm enough not to need heat) we switch over to an electric shower on a green tarrif for washing,  we don’t bother with hot water for the rest of the house, use a dishwasher for washing up so don’t really miss hot water in that regard.  I feel that heating the small amount of water electrically on demand is better than having a tank full constantly losing heat and being topped up again.  Our system does have a 3kw immersion coil in the thermal store but I dread to think what the bill would look like if we let that try and heat the house.   We could add some solar thermal panels to the store and let that provide summer hot water and contribute in the winter but the £5-6k
Cost of adding those to a not very sunny roof seems like a long payback when I have cheap wood to burn.  

Ill ask our neighbours (semi detached house) who burns oil, how much they get through for comparison.  I don’t know how hot they keep their house though. They have a 1000l tank in the garden and get 2 or 3 deliveries a year. 

The CO2 in the atmosphere is effectively made up of two things - CO2 in the short-term carbon cycle, and that in the long-term carbon cycle. Short term cycle is CO2 in air->carbon in plants/trees/life-> burning/decay-> CO2 in air.  Long term is CO2 in air->carbon in plants/trees/life-> fossilisation -> carbon underground-> extraction and burning -> CO2 in air.

The climate change problem is due to the long term cycle being heavily exploited and thus releasing carbon quickly into the air that has been locked away underground for millions of years.  It will take millions of years to lock it away again unless carbon capture is used to accelerate the process.

So, you burning wood means you're running purely in the short-term cycle and not really adding any extra carbon to the atmosphere - the wood you burn was CO2 in air within the last 100 years whilst the trees were growing.  You're effectively more or less CO2 neutral - you're not adding to atmospheric CO2 (because you're not burning fossil fuels), but you're also not removing CO2 from the atmosphere by allowing the trees to rot in a bog and eventually become fossilised and locking it away underground. In essence, you're not making things worse, but you're not making things better either.

It's a massive elephant. Mankind is not managing forestration at all effectively. And we don't talk enough about all the other natural cycles (ecosystems, water, oxygen...) In which trees play a vital role.

See the Which article I linked above (and if required, search for the industry bodies named for more information). Yet, even if the particulate pollution numbers are much smaller it's still significant and not everyone with a stove (but struggling to afford an alternative) is doing the best they can to cut particulate emission.

Well the article is actually only talking about one size of particulate (PM2.5) so the figure is easy to believe since most of the other particulates from other sources are much bigger. The government report is easy to read and looks correct from equivelent figures from Germany.

Well, just about every house outside of smokeless zones could have one, apart from flats. So if everyone who could, did have one then, no, I can't see how that would be sustainable.

Wood stoves don't give off as much smoke as the old coal fires if managed properly so I doubt we'd return to the city smogs of the past even allowing for the population increase but I'm sure there would be a noticeably poorer air quality, especially on still days. 

Particulates (ands specifically PM2.5s) have been discussed here quite recently.  My feeling is that there is a world of difference between a poorly managed stove (or - worse - an open fire) burning wet or treated timber, and a decent stove burning dry wood as hot as hell.  Even so, I wouldn't choose to install a stove in an urban environment now, and I think twice about lighting mine on very still days.

In terms of CO2 the point has been well made between fast and slow CO2 cycles.  Burning oil (or indeed gas or coal) is releasing carbon that would otherwise not have re-entered the cycle at all, or would have taken a very long time to do so.  Burning wood is releasing carbon that was relatively recently taken out of the atmosphere.  

It's certainly true that some woodburning is nothing but greenwashing, especially if it involves felling trees that would otherwise have stood for decade to come.  But if you're burning logs that needed to have been felled anyway, then that's approaching carbon neutral.  Equally, you're going to have the offcuts from your business regardless.  What would have happened to them if you didn't have a woodburner?  One way or another they would presumably have returned their carbon to the atmosphere, so you may as well heat your house in the process.

Thanks - that makes sense.

Also I am sure I read somewhere that it is actually better that felled wood is burnt (well) rather than being allowed to rot down.

Something to do with co2 being less harmful than methane ?

Edit, beetles etc  might disagree

I was speaking to an arborist (on one of my frequent “can I have those logs please” roadside stops) he said the same thing and seemed quite well read/nerdy about it.  He was delighted I heated my house with wood and gave me some lovely cherry and oak to take away. 

That's true: CH4 has a much higher warming potential than CO2, approximately 30 times worse per kg (depends on the timescale you measure it over).  So it's arguable that if the fate of some wood was to rot and become methane, then you're having a positive effect by burning it into CO2 instead.

It doesn't appear to be clear cut that burning is better from a co2 POV than leaving to rot, 5 minutes of googling didn't come up with anything definitive but then there are probably far too many variables to ever have a definitive answer. The below website claims that the EPA assumes that leaving to rot releases approx. 1/6th of the CO2equivalent compared to burning it. And of course, from all other environmental POVs, leaving to rot is much better than burning. If the EPA figure is correct then it does somewhat destroy the environmental arguments for wood burners

https://thehikingauthority.com/is-it-better-to-burn-wood-or-let-it-rot/

That website is quite scientifically illiterate.  He claims that there are several types of carbon monoxides, and that CO and NOx are only harmful to women, children and those with breathing problems.  I'm still trying to parse my way through "It’s hard to say before, but a pound of compost emits enough methane to methane to make a quarter pound of carbon dioxide."  Without a link to the source of the factor of six claim, I'm struggling to give it much credence.

You're right, though - the factors are probably too complicated to make any definitive comparison.  If some wood were allowed to rot completely anaerobically then the methane would be far, far worse than CO2 emitted by combustion.  What's unknown, and probably unmeasurable, is what fraction of the wood's carbon might end up as methane rather than CO2.

The methane vs CO2 question is a bit of a sideshow for the OP, because they're trying to compare the wood to the displaced oil.

The Findhorn Foundation  have done a lot of building research in conjunction with Cambridge University. They recognise that in many places throughout the world including the UK, we need to live in high density settlements. So although they are situated in open sites in Scotland, they've tried to build their community housing as if it is part of a town.

Originally all their houses were heated by wood burners as it was the 'obvious' hippy solution, but they soon realised that especially during cold, still, high pressure periods in winter, the air quality in the village was appalling. They had to accept that wood burners could not be part of any model for sustainable living in the real world and about 15 (20?) years ago removed them all from their properties.

If proof were needed it's now to be found here in our little corner of suburbia. From a base of none, I should now think that 30% of the properties have installed wood burners over the last 5 years. Between 6 and 6.30 in the evening, it's the "Honey I home - lets light the wood burner" death zone when walking outside beings tears to your eyes.

Wood burners are only 'acceptable' when you either don't have any neighbours or your neighbours don't have woodburners. As soon as a sizeable number of people have them, you soon realise why the 'smokeless zones' were instigated years ago.

But the EPA also have graded wood burning from managed forests as CO2 neutral. Methinks the writer of the article is scientifically illiterate.

I live in Northumberland and have had 6 days and further 2 days without power (Storm Arrwen and Malik).  I was so thankful we had a wood fired stove.  The whole house was heated to about 15-18 degrees just through leaving doors open.  The oil fired boiler wouldn't work (needed electric), the three neighbors have air pumps (which they complain about as being pretty useless) and had no heat. 

One guy locally died of hypothermia!

We need alternative fuel sources (as the current Russia/Ukrain thing is showing). 

You use waste wood and mostly locally collected would (fallen branches etc) by the sounds so CO2 aint an issue.

We have a higher tree coverage now than in the Medieval period when wood was the only fuel source (as well as the main building material) and the population was lower, so I guess it certainly wouldn't work if everyone went for a wood burner now.

If you're in the sticks then particulates may not be a big issue as they'll blow and settle in the empty surrounds.

How green is your wood?

That's pretty awful.  I'd guess that quite a few of these are being run by people who have no idea of how to use them properly, and are pushing them hard to try to get unseasoned wood to burn.  

If you don't have the capacity to season wood properly - I aim for two years minimum and three for less ideal wood like pine or willow - then you just shouldn't be burning it.

Dave - 'cos the greener your wood, the less green it is?

After seeing the fiasco with the power supplies this winter If I were living in a situation supplied by overhead cables I would be investing in a small generator, just enough to power the boiler. 

But if I were living in such a situation I'd probably also have a wood burner.

burning wood is carbon neutral, if you offset extra for transportation and processing, then wait 80 years for the sapling to mature. 

If folk burn wood from a forest floor, apart from loss of habitat, they've accelerated carbon release. 

It varies,  my waste wood is all kiln dried so 8-10%.  Stuff I buy has sat for a year and I leave it for another year if possible but rather than go strictly by ‘age’ I test it with a moisture meter and burn it at about 18% or lower.  I can tell straight away if a batch is not quite there and usually identify it by where it is in one of the stacks and move to a different stack.   It’s quite an operation my wood stacking process. 
 

It’s one of life’s little joys swinging a good splitting axe.  I do feel bad about a 2-stroke chainsaw but I’m not switching to battery yet.

buying kiln dry is a waste and destroys any carbon neutral claim ( what powered the kiln?). plus if you store it, it won't be kiln dry by the time you use it! 

Why are you telling me this?

If you are mainly burning your waste then I would (ha) say that is the best option.

The waste would (not again) go to landfill where it would (groan) decompose anaerobic ally releasing methane. Or be incinerated, commercial incineration would (give up man) likely be cleaner but transporting it there would (kill me now) likely take up the differential.

Sorry, not sorry

Edward Woodward. 

There is little better in life.

Crossed wires there, the waste wood I produce at work is kiln dried, ie it is bought as furniture grade hardwood for making erm.. furniture.  This goes from my workshop waste bin into the burner, it’s great for starting the fire but burns very quickly with a lot of heat.  I don’t buy kiln dried logs, agreed that’s a stupid idea. 

I'm not sure how sound my logic is, but for me electric heaters make the most sense. As our energy progressively becomes more sustainable, my heating becomes more sustainable. I know UK energy comes from many non-renewables too, but there's almost no way you could grow the amount of wood you burn in a year so it's hardly sustainable either, whereas a lot of the UK's energy is now solar, wind, hydro etc. 

Ironically my workshop that produces all the wood waste is heated electrically.  The major difference being I built it 3 years ago with a very high insulation value so the heat I put into it stays in there for days.   If I heated the house the same way it would cost a fortune.   
 

Basically I need to start insulating my house walls internally and with great attention to air tightness but the disruption to life would be pretty big.  Also our downstairs floors are stone, laid onto earth so the moisture control would become another massive issue if I built an airtight envelope on I’ve them.  So I dig up the stone and put in insulation and a dpm… and so it goes. 

If you're thinking purely resistive heating, then ( as the OP says) you will need to be very well insulated for it to make financial sense.  The problem is that electricity is three to four times the price of gas, so unless your heating load is very low then it'll be ruinously expensive.

Of course, this is where heat pumps with efficiencies of 300% or more can make things viable; it doesn't matter if the 'fuel' is three times the price if you're getting 3kWh of heat for every 1kWh of electricity.

In terms of carbon: it is fair to say that you can assume that the UK grid is always meeting the last bit of demand with gas.  At the time of writing the grid is getting 18% of its power from CCGT, and that number very rarely gets to zero.  So if you have the choice of gas or electric heating, then it's approximately carbon neutral - you can either burn the gas in your house to heat water directly, or you can burn it in a CCGT to add electrical energy to the grid.  Thee are broadly comparable in terms of overall efficiency.

So if, at a given moment, you want to add more heat to your home and can do it by resistive electrical heating or a gas boiler, you might as well use the gas boiler.  There's little CO2 difference and it'll cost you much less.  I look forward to a time when we don't have any gas burned for the grid but it's still a bloody long way off.

Wouldn't the amount of renewable energy in the grid make that untrue? Supposedly about 43% in 2020, hopefully increasing every year. 

I appreciate that the OP's home is a bit of a construction outlier, I suppose a terribly insulated house in a cold environment is just never going to be sustainably heated. 

I thought that the kilns used to dry firewood quickly were fuelled by the many tiny scraps of wood left over from splitting and felling that are hard to get rid of cleanly and easily any other way. Am I wrong?

You’re right they are.  

A few years ago there was a incentive from the government to fire such a kiln as it was classed as a biomass boiler.  A friend with a tree surgery business ran his regardless of whether there was wood drying in there as every hour it was lit it made him money.

Kilns for drying wood for construction and joinery are gas or electric I think as the process is much more controlled.  They also have dehumidifiers in there. Drying wood too fast isn’t good for it in terms of stability.  

So why is kiln dried firewood considered bad? I know that naturally seasoned logs would be better ( basically I have a wood burner and love it but am beginning to feel a bit guilty about it.....)

I don’t know why, I suppose you are burning wood to speed up a process that occurs naturally given time.  That wood could go off to a biomass power station to make electricity.  Lots of it does.   Like lots of thinks it’s convenience instead of patience.

The amount of renewable energy is getting better all the time, that's true.  But any additional demand is currently met with gas.  We don't yet have extra renewable generation (or storage) that can be switched in or out; we use all the renewable energy that's generated, all the time, and meet the additional demand with gas.

Imagine you're sitting at home, cold, with a gas boiler and a bar heater.  If you switch the gas boiler on you burn gas and turn it directly into heat.  If you switch the bar heater on the electrical demand goes up and, somewhere, a CCGT fires up a bit more to supply that demand.  Either way, gas gets burned (and it's approximately the same amount either way).

We can look forward to a time when we have so much renewable energy - and good enough storage to get over the intermittency - that additional demand on the grid is still met with green energy.  But at current rates of progress it'll still be some time (decades) before gas stops being necessary. 

At least we are approaching the point when we no longer need to use coal to top off the demand because we have deployed all the gas capacity.  Drax is the last plant left.  Mind you, right now it's a bright but windless Saturday morning and I see that Drax is putting out about 500MW.

The exception to all this is using a heat pump, because then you get the CoP multiplier.  If burning 1kWh of gas gets you 3 or 4kWh of heat then that's a significant CO2 reduction. 

Retrofitting our old housing stock is a massive problem.  Even a fairly typical house is pretty bloody bad.  If you put me in charge I would make the standards for new build much, much higher - it's simple to build a well insulated house but really hard to upgrade an already built one - once you've dealt with windows, cavities and loft insulation it's very expensive to go much further.  With older properties such as the OPs it's a real difficulty.

I manage the RMC for the estate I live on and we have a block of flats here.  The U-values are poor, but we've found that when they were built they stuck 20mm of crappy sheet insulation to the inside leaf in the cavities.  Most of these have now fallen down and are providing no real insulation at all, and instead are thermally bridging across the cavities.  We can't inject fresh insulation because the sheets will cause voids.  The cost to clean the cavities out (by systematically removing bricks and hoicking it all out) is in the order of £5000 per flat, with projected energy savings of about £500pa.  No absent landlord will go for that.  So the tenants go on paying insane heating bills.

Kiln drying is kind of OK, if it's genuinely powered by waste arisings that would genuinely have no other viable use.  But a heck of a lot of it is fossil fuel powered.

And there's very little point in taking wood to drier than ambient, anyway, because it just reabsorbs from the atmosphere!  I guess if you only have room to store one winter's wood but not to season the next two or three years, or if there were several people with burners (each of whom had very little storage area) sharing a pallet, then kiln dried could make sense.  

The farm where my workshop is makes and dries firewood as do others in my area using waste heat from their bio-gas plants, economically it's better than air drying (the amount of space and labour to stack and dry for years is unthinkable) but also there is the forest hygeine to consider, roughly 1/3 of the trees have to be disinfected before transport to prevent disease spread anyway and stopping infection in air dried is difficult. Previously diseased trees had to be burnt on site.

Their excess heat would otherwise just be dumped in the winter, the rest of the year they are drying agricultural products (seed, hay and grain mostly) which would otherwise be done using conventional heat sources and produces a better product.

I believe the main issue with kiln dried wood is the transportation. It seems to be mainly large operations that do this and then sell online, with the wood nicely crated up and shipped to you by a pallet courier. So if you're buying online and going purely on price you may well be buying timber that has been shipped half way across the country rather than sourced locally. 

As someone else pointed out up thread, all timber will settle at the ambient moisture levels in the air during storage anyway and as the uk has a moist, maritime climate it's never going to be particularly low.  

I'm currently buying some kiln dried firewood from a very local tree surgeon who powers his kiln with the waste scraps from the business. I'm still getting about 50% through my work, where I have dangerous trees felled reasonably frequently and air dry the chopped wood at home in shelters. So my conscience is still reasonably clear given that I have a DEFRA approved stove for smokeless zones and only burn well dried logs. What a complicated (and fascinating) subject though!

I still don't understand how you say we can be using 43% renewables, but when I turn on an electric heater still be using the same amount of gas compared to if I had a gas boiler. The huge hydroelectric reservoir in Snowdonia supplies massive amounts of electricity to the local area during surges, for example. 

So lets say that the demand on the grid is a million kW.  Of that, 100,000kW is being provided by gas.  All the renewables are already running flat out.

If you now want 1kW of heating, then you could burn 1kW of gas at home.  The total gas being burned is now 100,001kW.

Or you could turn on an electric heater.  The grid demand goes up and it now has to supply another kW.  It can only do this by increasing the gas supply, so it's now burning 100,001kW of gas.  

In both scenarios the total gas being burned goes up, and the only difference is where it got burned.  The only scenarios in which using the electric heater won't increase the gas consumption are ones where there was untapped renewable power that could be switched on, or where we had so much storage of green energy that we could call upon that.

Neither of those are the case.   If we doubled or tripled our solar, hydro and wind capacities, then we'd start to see periods when there was spare low carbon power on the grid.  Or if we massively increased our storage (battery, or pumped storage like Dinorwic) then we could charge it up overnight and deploy it in the daytime peaks.

In fact, at periods when we have maxed out the gas CCGTs and are having to draw on coal, then adding demand to the grid is substantially worse than burning it at home.  At this moment, burning 1kWh of gas at home adds around 490g CO2 to the atmosphere.  But drawing an additional 1kWh from the grid will mean running Drax harder at 820g CO2/kWh.

You can argue that the grid is 43% renewable, but whilst additional demand is met with gas or coal, then adding demand drags that percentage downwards.  You don't get more energy out, still at 43%, you reduce the average to 42.99%

(there are a lot of complicating factors in this, such as our use of the various interconnectors to France, Belgium, the Netherlands and Ireland, and our balancing of charging versus discharging things like Dinorwic.  But ultimately the extra demand has to come from somewhere)

edit: Dinorwic stores about 9GWh.  Right now the UK is using about 35GW, so fag packet maths suggests we get through something of the order of 500GWh every day (35 times 24, minus a bit for the overnight lull).  So Dinorwic could drive the UK for 26 minutes, then it needs to be recharged.  It's a fantastic fast-response system (apparently about 12 seconds to spin up to full output!) but we could do with about 20 more of it...

Wouldn't you actually need to burn a lot more than 1kw of gas in a power station to generate 1kw of electric heating at home because of the inefficiency of a generating electricity with gas and the power lost during transmission and voltage changes? 

I can't afford one yet, but from reading New Scientist, in terms of climate change the 'ungreeness' of wood stoves appears to come from the transportation and processing of the wood burned (but it will depend on the wood), and the particulates which can darken the atmosphere (to whatever degree), and add to any heat absorption of the sun's rays. I seem to manage to scavenge my wood, and once I can afford one, am getting a particulate filter to address what goes out of my wood stove flue. Unless the climate has warmed up enough by then that I don't feel the need, but rising energy bills may mean that I do.

There may be other more economically sensible approaches to heating at home, but as far as I know, that'd be how to make having a stove as green as possible, to scavenge any wood and to have a particulate filter. 

Yes.  You'll get something around 55% efficiency at the power station itself and then lose another 10% or so in transmission, so you probably need to burn around 2kWh of gas to deliver 1kWh of electricity.  Resistive heating is more or less perfectly efficient so you'll get that entire 1kWh delivered as heat.

But the gas boiler won't be 100% efficient either.  Old non-condensing boilers will be in the 50-60% range, so comparable to the electric option.  A perfectly set up condensing boiler at optimum conditions might get towards 90%, but in most real world conditions you'll probably see 70-80%.  

Of course, the fact that a gas boiler is likely to be more efficient than electricity generated by CCGT only adds weight to my point, which is that choosing resistive electric heating over a gas boiler would be an expensive way to achieve no overall CO2 savings, given our current grid.

I'm surprised electricity generating gas boilers never appeared.

Very inefficient electricity generation.  But still cheaper, and all the losses would heat your water.  Either switch between that and mains, or save on your standing charge.

Hmm, combined gas heat and power plants for households are kinda a commercial thing, there's a firm down the road from me makes them. Ecomically centralised generation is better.

Why do you think that ?

A mill will sometimes heat their own, if they already have a combined hot water / electricity power plant on site, but it's often more financially viable to sell all scrap wood chipped to those who use it to power plants. Kiln drying on industrial scale is quite a precise process, not just a hot oven, they still need electricity to run pumps, filter water etc.. 

There isn't really such a thing as scrap wood, even chipped it's worth £6/m3 to burn, or for chipboard. Bark can be chipped for power generation too. Some don’t like oak though because of the tannins, which i presume cause emission problems.

Why do I think what?

Ecomically centralised generation is better.

Is there any chance of using J, MJ, or GJ as the units of energy in these discussions, instead of multiples of kWh? It is so tedious having to do endless conversions into consistent units.

Currently BHKW (what they are called here) are expensive, depending on the type need more space and maintenance and still require a reliable back-up. They are more efficient than centralised power generation as long as you have a relatively constant heat and power demand, otherwise a lot of storage of both power and heat is nescessary which is why generally you need to be hooked up to some other sources as well. 

Good for things like heated indoor swimming pools but problematic for domestic housing especially small units.

Companies like Viessmann make smaller ones, both gas i.c. powered and fuel cell variants.

They are probably expensive mostly because the mass market was never addressed.  Although I expect it would be difficult to achieve the burning efficiency of a normal boiler, the specification that gets priority.

purely residential systems have distinct peaks and trough, whole community (shops, factory, schools, housing etc) help spread demand to something more consistent through the day, when that early morning residential spike eases factories get going etc.. 

I've never considered the concept before - you mean a tiny gas power station that sits in your house, and uses waste heat from electricity generation to heat water for space heating?  And at times of the year when you don't want the waste heat you just switch back to grid import?

My first thought is that efficiency on such a small generator is going to be pretty low.  From a CO2 point of view that's not going to be an issue, as long as the reclamation of waste heat into the thermal store is high efficiency, but that's hard because you're going to be trying to pull heat out of the flue gases.  Embedded CO2 will probably be quite significant over the lifetime of the unit too.  And you'll need a big thermal store, which not many houses can take.

Could be an interesting idea if you built a new house around the concept, but tbh if I was building a new house now I'd be looking to insulate it so well that it barely needed heating at all.  I understand that when you get near passivhaus levels then you don't bother with any heating beyond a simple resistive heater, because the demand is so low that it really doesn't matter that electricity is expensive.

John: kWh/MWh/GWh have rather become the standard for this sort of discussion.  I know off the top of my head that my house uses about 6kWh a day, and I'd have to think to convert that into Joules...  equally, when I read that the UK is using 35GW then it's easier to do 35GW x 24h = 840GWh than to go via 3600 sec x 24 hours.  

Yes, and yes.  Since gas boilers are on the way out.  The time for it has now passed.

They run around 90% efficiency BUT only if you can balance use of the two outputs perfectly, in practice for private dwellings this is impossible by a long way. Integrated into a large PV system and solar water heating and storage you should get it to work but the cost and complication is getting out of hand and you are still missing the back-up being hooked to a large system gives you. Our bio-gas plant is effectively 95% efficient in the generation/heat use side but it's a struggle to use the waste heat in the summer and get much money from it. The average house isn't going to be drying hay!

Then there's maintenance! The fuel cells should be pretty good, the Stirling motor ones okay but the i.c. engines need oil changes and stuff like that which means an engineer travelling to the plant to do it. We do the oil and filter changes and service for 500kW generators in two hours. Big is cool!

maybe just go electric only, perhaps it's time to just phase out piping gas into every house? Insulate properly, yes it's a pain and costs but it's lasts, instead of just changing heating source every few decades, houses last hundreds of years, insulating is a no brainer.

Actually, on reflection, I wonder if it might not have been a decent solution for old and very poorly insulated properties?  A low temp ASHP is always going to struggle with this sort of property.

I was about to say 'such as the OP's house' but then I remembered that they're off the gas network in the first place! 

Yeah, sounds like implementing this on a single house scale is going to have more than a few issues!  And as David says, getting off all fossil fuels does need to be the end goal.

Better insulation is always the key.  But it does have limits.  We have a three story 140m^2 semi detached house.  I've brought the loft insulation up to 450mm, filled the cavities with the best material I can, put 50mm celotex onto the single skin walls that connect to next door (part of the connection is onto a garage so it's an unheated space), and I'm slowly replacing the crappier windows with triple glazed.  Judging by how things are going, by the end of this our annual heating demand will be around 5000kWh, less than half what our EPC claims.  But to get any lower than this we'd need to put in much more internal or external wall insulation throughout, and that's expensive and a lot of work.

I do exactly the same calculations with my electrical usage, which (like yours) is recorded in kWh, so kWh are OK at that superficial level. But using units of kWh for energy is about as logical as using kmph.sec for length (e.g. my height is 6.281 kmph.s, and the distance from my house to the centre of town is about 3600 mph.s)

I think it's just horses for courses, isn't it?  If I'm given a power in kW then working out a day's energy seems simpler in kWh than having to multiply by 3600.

But I've been thinking a lot about energy on this scale whilst planning my house renovation over the last year, so maybe I'm just more used to using them.

Doesn't need to be fossil fuel (ours aren't), the Stirling motor ones work with wood just fine. The ÖkoFen ones which have been around for a decade or more are added into a relatively normal pellet heating system to take over from PV at night. I'd think about it since I've a pellet system but the €25000 in the bank is also nice to think about!

Of course, a Stirling engine will work with any heat source.  Silly of me - I missed that.

What kind of split between electrical and heat output do they achieve?

That's the rub, while Stirling motors are somehow cool they are gutless as well! About 15:1 heat to electricity, the first ÖkoFen one puts out 600W so enough to run the heating system and a couple of lights. They were developing a 5kW plant but I've not looked if it came on the market.

The hassle with the idea is modern heating plants have already taken most of the energy out of the system leaving barely enough to actually get the smoke up the chimney let alone produce worthwhile power.