UKC

Morning all,

The boiler was given its annual health check and service yesterday and I was having a chin wag with the engineer over a cuppa and we got on to the subject of efficiencies. He was quite persuasive and said that because I have a large tank that it would be better to keep the hot water on permanently as this would prevent the 'kettle' effect of constant heat-cool-heat-cool cycle. We have a three girls in the house and the hot water can be used quite quickly.

He also said that, as with underfloor heating and using the same principle as above, it's better to have the heating on at a constant at this time of year for pretty much the same reasons. Set to 18-21 and forget and this will ensure that not only is the boiler not overworking getting the house up to temperature but when it does get there, it just stays ticking over.

I have to say that it seems logical in both cases but a quick Google search brought up the below which rather debunks the idea. Are there any engineers in UKC who have knowledge of this?

Ta

Richard

https://www.cse.org.uk/advice/energy-saving-tips/energy-mythbuster

In reply to TheDrunkenBakers:

It all depends how efficient your house insulation is, the efficiency and power of you heating system, and how long you are in the house

In reply to TheDrunkenBakers:
There are several factors at work... gas/electricity cost (not as simple as sometimes projected as the constant set value can be a lower temp when always on to give the same level of comfort) reduced brick thermal conductivity from preventing bricks from getting damp when cool and finally the costs of and risks to your heating system. Its easy enough to evaluate what works for you... sock it and see... plenty of people have reported lower heating bills from keeping heating on in the winter.

Sadly its all rather got muddled up with climate change politics; but the daily use you measure is what matters and the reduced strains on the heating system will increase its life irrespective. You certainly can't beat the laws of thermodynamics but the complexity of how they apply to heat loss from a house is clearly oversimplified if the energy usage evidence sometimes contradicts the simple theory and brick dampness seems to be a logical explanation (though I'm not aware of any proper scientific analysis).

In reply to TheDrunkenBakers:
I've never tried our underfloor heating on continuously, a few hours per day is enough in frosty weather.

I think 60 minutes was sufficient this morning but it takes another hour or two for living room to heat up.
Based on the thermostat the heating would be on for 2 or 3 hours and the room would get far too hot.

I'd like a heating controller that did just increased the on time based on external temperature

Our hot water tank is sufficient for a few days so it gets a daily top up (30 mins?) so again 24 hours seems crazy.

In reply to TheDrunkenBakers:
I'm not sure who wrote that page, but it's seems a bit unscientific to me. There will be a break even point for any of those methods, and as people already said it depends.

It is inefficient to have a gas boiler on and off and better to use it at its most efficient point. Is your hot water heated by the gas boiler?

Mr Marsbar is an expert on this stuff if you want any specific questions, but generally I suspect your guy is right from what I've learnt from Mr Marsbar.

In reply to elsewhere:

You can get weather based heating controls.

In reply to TheDrunkenBakers:

Interesting.

My home is reasonably large, 2150 square feet I seem to recall from the drawings, with rooms that are individually quite large. Built in 2011 and fully cavity insulated. The boiler is the same age as the house and is both heating and water.

In reply to TheDrunkenBakers:

It's probably quite efficient already then

In reply to TheDrunkenBakers:

Simple physics tells me that ideally water and your house should be heated as quickly as possible to the temperature required just before you need it to be at that temperature - the wasted energy you are paying for is the heat that escapes to the environment and the rate at which it is lost increases with the difference in temperature between your water/house and the environment, so less heat is lost if the temperature of your water/house is as close as possible to that of the environment when you don't need it to be warmer.

In reply to TheDrunkenBakers:

The thing to understand is that the bigger the difference in temperature between the hot thing (that you are heating) and the cold thing (that heat is lost to), the greater the rate of heat loss from the hot thing.

Given that, the only way it can be more efficient to heat something at times when you don't need it to be hot is if there is a big energy cost associated with starting and/or stopping the heating system.

Which there isn't for any modern boiler. There's perhaps half a second of high current draw as an induction motor in the blower or the oil burner fires up. Utterly insignificant.

Systems with condensing units to recover exhaust heat are also more efficient with colder return water, so that shifts the efficiency more in favour of periodic operation.

It's more complex if you consider boiler wear and tear but I'd struggle to believe a modern boiler prefers low intensity constant running.

Its got one of those too. Me thinks he was talking babble then.

UKC never ceases to amaze me. Top responses

In reply to Robert Durran:
Your simple physics is not good enough. If you measure usage for when the heating is on all day its usually much better than your simple physics predicts. No physics laws are being broken so your simple model is simply wrong. Its a simple experiment for you to try as an intelligent man.... just find two weeks with similar temp and weather patterns and measure it both ways. Keeping dampness out of the bricks makes sense as one major explanation of the unexpected difference.

In reply to wintertree:

I did some calculations for savings with a new combi boiler and decided to stick with my old one until it breaks. It would need to last well over a decade (many dont) ... a decade is a point where parts can become sparce for repairs so you might need another new one.

In reply to Offwidth:

There is nothing wrong with my model as a starting point, but it is an idealised model using an unignorable and perfectly correct principle, though it may well need refining for a realistic situation involving, as you say, extra factors such as dampness in walls. My model would, for instance, be perfectly realistic for, say, a kettle - you boil it rapidly when you want hot water rather than keep it hot all the time.

In reply to the dislikers

Experimental physics did for the aether. As a muliple graduate in physics I find it depressing to see other people well educated in physics writing off actual measured experience with over simplified models. The whole basis of Physics is the model has to explain the experimental data.

In reply to Offwidth:

I haven't disliked your post, but I'm far from convinced by the damp bricks argument. Unless being warmer physically seals the bricks to stop damp entering, broadly speaking the same arguments apply as with heat loss - doing more work than needed is never more efficient.

In reply to TheDrunkenBakers:

In terms of boiler wear and tear, if you set your thermostat at say 19C and leave the heating turned on the boiler will boiler will kick in and out repeatedly during the day as the temperature rises just above and just below 19C. Won't this cause as much wear and tear as it coming on and staying on for a long time and then going off for a long time?

Just a thought, I don't know the answer but I put our heating on at certain times and off at others, never on permanent (as in 24hrs a day with a low thermostsat setting). I've never had a boiler breakdown and I've never had a boiler serviced. Perhaps I've been very lucky?

In reply to Offwidth:


For what it's worth, it wisnae me!


Of course, so, if your experimental data shows that it is in fact more efficient to keep your water/house warm all the time, the onus is on you to come up a model which involves factors which outweigh my correct heat loss argument.

In reply to Robert Durran:
So, you are telling me the onus is on the householder (who has discovered in their house that consumption is little changed by constant heating) to produce a better physics model? As opposed to the energy agencies responsibility to stop peddling over simplistic models dressed up as public information.

In reply to pec:


Same here.
It's what I do as well, it gets turned on when the children have frost on their hair.
Apart from the crappy first combi I had, my current Vaillant is fine after 20 years service.
Having said that though, I would say that regular exercising of its components is better than leaving it idle for long periods of time.

In reply to TheDrunkenBakers:

I was just reading your CSE link...

It takes more electricity to turn lights off and on again than to leave them on. Myth.

Until recently this was true.

Now, given the low, low cost of running LED lighting it's not always true. Consider a light that's always on and doesn't have a switch. Now price up the extra mains cable, switch box, switch and additional trades time (running cables, fitting box, chasing and repairing cable runs in walls, drilling holes, plastering and painting around box etc.). When I did this for our house rebuild I estimated we'd have to turn the lights off 16 hours a day for several years to recoup the cost savings of fitting the kit for turning them off. Given the environmental impact of making all the cabling and stuff it's probably more energy efficient to leave them on as well.

Although as I'm not an insane weirdo I had light switches fitted.

In reply to Offwidth:

Surely you can't accept such anecdotal evidence as a proper experiment.
Vaguely similar weather! One degree difference or a couple of mph different wind conditions and you've a massive difference in cooling of the building.
Also, you haven't got a proper measuring system. Are the rooms away from the thermostat heated more or less if you leave the heating on?

I'm not convinced by the constant heating idea. Assuming the outside conditions are identical and constant an object warmer than the surroundings will lose heat at a constant rate so if you increase the time that the object is kept hot you lose more heat. If you go for the damp brick hypothesis then you are assuming the house is already damp and if it is damp then you are constantly losing energy to evaporate the moisture which will continue to ingress.
The only I hypothesis I could suggest to support such a constant heating plan would be that the boiler is most efficient with a specific return water temperature and a specific temperature increase but, saying this, you can determine these factors by setting up the boiler correctly.

Aside from all these technicalities, who wants the heating on all night? It's much more pleasant to sleep in a cooler room.

In reply to Offwidth:


No. I was talking about the physics discussion between you and me.

In reply to Offwidth:

... brick dampness seems to be a logical explanation

go on then - explain.

In reply to Offwidth:

The only way I can think of your "model" making sense:

"who has discovered in their house that consumption is little changed by constant heating"

Is that your boiler is incorrectly sized (or is broken in some way) and ran incredibly inefficiently whilst re-heating.



Obviously any heat made available when you do not need it is wasted energy.

In reply to TheDrunkenBakers:

Why not just do the experiment? Just note down the meter reading every day and swap between methods every day. Compare the sum of energy used on over all the days with the first approach with the sum of energy used over all the days with the other approach. After a week or two it will be pretty obvious if one is significantly more energy efficient.

In reply to TheDrunkenBakers:

We have a wood pellet boiler and it's definitely more efficient to have it on a slow burn all day rather than turning on and off several times a day.

In reply to TheDrunkenBakers:
Damp Brick has twice to three times the thermal conductivity of Dry Brick. With regards to cyclic heating vs constant heating - I've not yet found any studies that indicate that cyclic heating is more likely to cause damp bricks. I would have thought that there will be a dew point somewhere in the brick layer, and when the internal temp is higher, the dewpoint would be closer to the external wall, therefore a smaller proportion of the brick could be damp. If you cycle the internal temp, more of the brick could become damp when the heating is off, which would then require drying out (lots of energy) plus there would be more heat transfer (i.e. loss) to the surroundings....but those are just my thoughts...

I know for sure that poorly installed cavity wall insulation can cause thermal bridging, damp, and subsequent poor insulation/cold room. (often worse than no cavity wall insulation!)

I've also read somewhere that underfloor heating powered by heat pumps are more efficient if left on low, rather than used to rapidly heat rooms, but I can't see any reason why this would be true for non-heat pump systems (electric/gas). Off to bed so no more time to look that up...

In reply to TheDrunkenBakers:

I performed an experiment on two sucessive years. On one I allowed the house to get cold overnight, on the second year (similar outside temperatures) I left the heating running all the time, controlled by a thermostat.

Result - no material difference in heating bill.

Your mileage may vary.

In reply to Offwidth:

I actually did the empirical test in my own house over 2 winters which were reasonably comparable. Easy enough to do as we had an oil fired system at the time which only fed the heating, so no confusion with other usage.
Why? Well Mrs Paul in Sheffield had a theory that given the high C values of our very thick stone walls, and well insulated roof, we were better off keeping the house at a reasonable temp during the day/night and a small increase during the usual 'habitable' periods.
Result? Around 25% reduction in our oil bill, like for like.
I had some people doing some Matlab a while back on whole house modelling, and it turns out to be non linear, and highly sensitive to combinations of U and C values, external thermal loading profiles and layout.
Pretty difficult to linearise or put on the back of an envelope.

In reply to TheDrunkenBakers:

Just saw John Hat's email which kind of supports the thesis that it's very specific and sensitive to a lot of parameters. However, it is interesting in that case that the house was kept warm for longer periods without using extra fuel.

In reply to paul_in_cumbria:


I'm not clear to me from your post whether that is by maintaining the temperature or by letting it drop when not required!

I presume that it will tend towards not being worth maintaining the temperature if you only require it for a short period each day; I generally only require about 20min in the morning and about an hour at night and I would be very surprised if it was worth maintaining that temperature the rest of the time.

In reply to Robert Durran:

I read it as the result of trying Mrs Paul's suggestion was a 25% reduction. This makes sense as heating up a thick stone wall from cold will use a lot of energy, but keeping it warm once heated won't and it will act as a heat storage.

In reply to marsbar:


As "common sense" as that sounds, it's wrong. It just doesn't work like that - the hotter it is, the worse the heat loss.

There has to be more at play if this is true. My best guess is that in either way of operating the thermal mass of the wall is sufficient that the variation in interior temperature is averaged out by the outer layer of the wall, meaning that as long as the same energy is spent per day heating the inside, it doesn't matter - in terms of energy loss - when it is spent.

This gives insensitivity to how/when you run the boiler, but still not increased efficiency. Indeed the link between heat into the inside and outside temperature never fully disappears, so there's still some very small inefficiency from heating when not needed.

You get increased efficiency only if the way you run the boiler is more efficient in one way than in another. Regular "top up" heating is more efficient with a condensing boiler than occasional long-run heating because you'll have on average colder return water going into the condensing heat recovery unit in a continuous "top up" system.

This only applies to building above some level of thermal mass and insulation however, and to a boiler that has a significant efficiency range linked to the temperature of the return water.

In reply to TheDrunkenBakers:

Always find the efficiency word interesting, the above argument changes depending on how you define your goal...

If you're trying to keep your house warm while you're in it, having the heating on while you're in it will do that, so if you're in and out all the time, then on all the time makes sense. Expect government advice is based on a typical families requirements, with their typical family clearly indicating the kind of people who work for the organisation...

Our house is an old concrete prefab with an external insulated shell, the main goal of heating for us is to keep dampness out of the internal structure, so ours is a constant 16degrees - we've then got a log burner to give us a cosy room for humans in the evenings. You could argue that it would be better to do x or y, but if that resulted in a drop below temperature at any point that would be less efficient for us.

For the record, I'm a physics drop out - got too annoyed by the whole 'assume x is negligible' thing, in this case x would be the opinion of the boiler owner...

In reply to wintertree:

I assumed that some of the heat "loss" from the wall is back into the house, making the house feel warmer and so the heating can be turned down.

In reply to marsbar:


That doesn't make it correct. What you describe shifts - in time - when some of the boilers heat is put into the people and the air in the room. That in itself worsens efficiency as the more heat that goes into the wall to be time-shifted "storage heater" style, the more heat is lost from the wall to the outside world.

I can only see it improving efficiency if it shifts the boiler into a more efficient opperating reigeme.

In reply to TheDrunkenBakers:

I would assume that a lot depends on the insulating properties of the house.
If they are quite good and you have thermostats that work then keeping the heating on at a constant would be more efficient. If insulation is poor then you are constantly losing heat to the outside

In reply to Alasdair Fulton:

I get the dampness; brick is a poor insulation material all round.
water gets in by capillary action - and if you're heating damp brick (or you've got thermal bridging going on) your heating isn't going to be efficient either way.

I don't buy the idea that yer average house-brick is somehow better off at a consistent warm temperature; you want to stay well well away from putting heat into the (external) brick of a building.
(internal is less of an issue - but it's not some fantastic thermal mass heat store)

......more conjecture for yer hypothesis......

In reply to Offwidth:

No, the model has to predict reality which is a very different statement .One fundamental problem with science is deciding whether to throw away experimental results that contradict well-known theory, or to throw away the theory. Which is wrong? I think writing off large chunks of heat transfer theory on the basis of anecdotal heating costs is asking a bit much.

In reply to paul_in_cumbria:

Did this model predict in any case it using less energy to have heating on full-time? If so, how? Was there coupling between temperature and conductivity somewhere, as is being suggested for moisture in walls (I find this a very improbably explanation)?

In reply to daWalt:
Get away with ye and your brick built houses, we dream of brick built houses, livin int cardboard box at end of road.

In reply to wintertree:



That's part of the idea.

The other part is that by heating the stone and keeping it warm, the temperature difference between the stone inside and the room disappears, so the heat will then not pass into the stone from the room. There will obviously be some heat from the outside of the wall to the outside world but thick stone walls are not the most efficient conductor of heat. They are good at holding heat.

In reply to MG:

Not necessarily less energy, but more efficient at the boiler.

In reply to marsbar:


I'm sorry but that's just wrong. That just isn't how it works. Further; If the walls are that insulating then why is heating needed? Losses through air leaks, floors, ceilings, windows etc. Again, against this heating only when needed is more efficient.

So far possible explanations I think are viable for increased efficiency with always on thermostatic heating are eliminating conduction altering damp from outside walls and using high thermal mass walls to shift the boiler into a more efficient operating zone.

In reply to paul_in_cumbria:


You'd be wasting a ton of energy trying to heat stone walls.

Most people either dry line or use something like hemcrete to insulate the stone.

In reply to wintertree:

>

The most likely is that the whole idea is cobblers. British Gas, the Energy Saving Trust and many others point this out. But as is the way with internet memes it won't go away, and arguing the point probably just embeds the idea more.

In reply to MG:


I tend to think it's all cobblers, but I'm curious about the posters above who've say otherwise based on (poorly controlled) experiments.

In reply to Wayne S:

cardboard box at end of road?
ours was int' middle ov motorway - and we used to have to lick the road clean every morning....
etc.

In reply to TheDrunkenBakers:

We moved into our present home just under 2 years ago. The house has cavity wall and loft insulation. Wherever we could we added under floor insulation. We also replaced the double glazing as we didn't want the faff of maintaining wooden window frames.The heating system was 25 years old and very inefficient. We replaced the boiler with a condensing boiler and after a lot of research we bought the Honeywell Evohome control system. This consists of thermostatic radiator valves which wirelessly connect to the boiler. Each valve is fully programmable in terms of temperature and time. For example the radiator in the main bathroom goes on twice a day, once for a couple of hours in the morning and the same in the late evening at a temperature of our choosing. This means the bathroom is warm when we are likely to shower. Every room in the house is set in the same way. For this system to work to maximum efficiency you need to keep the room doors closed. Another advantage of this system is that you can control the system remotely. For example we went to the climbing wall last week and just before we left we were able to switch on the bathroom radiator.

This system was not cheap and was a faff to set up but as we are going to be in this house until we either die or need nursing home care we felt it was worth while.

http://getconnected.honeywell.com/en/evohome?gclid=CjwKEAiAu6DBBRDDr6-e_669...
http://www.trustedreviews.com/honeywell-evohome-review?category_id=44&c...

In reply to TheDrunkenBakers:

In all liklihood he was a technician, not an engineer.

In reply to TheDrunkenBakers:

In a rather pointless exercise of procrastination I have avoided doing my uni maths revision by doing some maths on the problem.

It would seem that a condenser boiler is approximately 10% more efficient when running at a low load as seen here http://m.hpac.com/heating/small_boiler_efficiency

I believe you stated your house was approximately 2100 square feet and built in 2011 I have therefore assumed it's built with cavity walls using a mid weight block and 50mm of polyurethane insulation with a ceiling height around 2.2m which is fairly typical of a new build.
Given this and assuming a temperature difference of 20 degrees I estimate that it will cool by about 75% in 10 hours at this point the heat loss is approximately half that when it is 20 degrees.

If you were to keep the temperature difference constant at 20 degrees then over 10 hours the heat lost would be slightly larger than the initial amount which would suggest that it is about 25% more efficient to allow the house to cool.

A similar house with 0.5 meter thick limestone walls would take approximately 5 days to lose 75% of the heat so over a 10 hour period the rate of heat loss doesn't change much so it is probably better to heat constantly taking advantage of the small increase in boiler efficiency.

Given these examples it seems the key factor is how close the inside temperature will approach the outside temperature before you wish to re warm the house therefore it is probably more efficient to constantly heat an extremely well insulated house also.

Right now I'm going to climb something.

In reply to TheDrunkenBakers:

I think the whole point of this discussion is how badly most brick built houses are insulated, a couple of cm between bricks or some cavity fill if the building company actually even bothered to put it in, as many newish home owners discover they haven't. We have an average of 100mm external insulation and only 10-30mm internally, perhaps 150mm of solid wood in middle. Heat loss minimal, generally limited to hot spots of thermal bridging where wood of internally / external framing is at exact same spot. Can tell in winter when snow and haw frost on the outside does not melt, apart from the odd dot where big nails go in. We have no night time heating on and it's been constant below zero for a few weeks now. Wish I knew years ago everything I do now when renovating houses in the UK years ago.

In reply to Offwidth:


So, so tempting...

In reply to TheDrunkenBakers:

My gas boiler has a 'c' mode which keeps the hot water for taps at temperature as opposed to heating it up when needed - it drinks gas when this mode is on.
Best thing we did was secondary glazing (we have single-glazed sash windows which we cant' change due to being a listed building).

In reply to Toerag:


This in itself is a bit of a myth, you can replace the windows with modern energy efficient ones but you have to do a proper job of it (not just throw in some uPVC) and this will cost you a small fortune, probably a lot more than the secondary glazing and the energy saved.

In reply to gethin_allen:

Its not even *that* much more. We have double-glazed sash windows that work well and cost, from memory, about 30% more that the equivalent PVCs ones. They also suit the house, rather than looking completely out of place and destroying the aethetics. Oh yes, you can open them more than 2" too.

In reply to GrahamD:

Personally I'd draw the line by saying if he makes calculations to design and optimise a system then he could be considered an engineer. If he just fixes existing systems then I'd say technician. So he could be either.

In reply to marsbar:

I'd draw the line at having professional engineering qualifications. You don't need those to optimise a central heating system.

In reply to gethin_allen:


The answer to that is that it depends, often it will be possible to install double glazing, but not always.

In reply to gethin_allen:


Nah, we've already asked as we were happy to put in wooden double-glazed ones to match what's there. Ours are original from 1895ish and have 'wobbly' glass in them as opposed to modern plate glass. The planning authority's view was that it's impossible to get that sort of glass anymore so we're not allowed to replace it. I do live in Guernsey though, and our laws are different to yours.

In reply to Toerag:

But yet they allowed you to fit secondary glazing, this is silly.

In reply to gethin_allen:


The external view of the building is the meat of the listing, gutters, doors, windows, materials all important to maintain the exterior look, the interior, meh.

In reply to Toerag:

had a similar discussion nearly a decade ago in a Yorkshire conservation area. We did change the front door which was very modern, installed in the 70s before the rules were created or enforced, we were told to pull some of the trim off it, because it was too ornate and more Victorian, when it should have been Georgian. Made my blood boil when there plenty other properties breaking the rules in pretty horrendous ways, the mistake I made was asking and applying for person. I should have just done it, as I now know the council were too scared to take people to court as they couldn't afford the legal costs.

In reply to winhill:


It depends on the listing, 9 times out of 10 you will be right.

In reply to gethin_allen:


Secondary glazing doesn't remove what's there, and is removable in the future (ours is essentially a whole lightweight wooden sash window whose frame gets screwed onto the inside of the existing frame). From the road you can't tell its there unless you look hard.