UKC

I am considering how to protect my oil boiler from power outages as it is the sole heat source for the house. Max power draw of the boiler is 0.168kW.

Would a petrol genny be ok for the job, I have a vague feeling these do not provide a stable current flow and could fry the PCB.

Second option would be a leisure battery and inverter combo, which I could trickle charge when not in use, I suspect this wouldn't last long.

Applying some pessimisms

1amp drawn from boiler full time

100aH battery will give 50ah

30% inverter efficiency

Gives me less than an hour.

Any experts out there who can challenge/support my thinking?

Any other options to consider?

Should I give in and install a multi fuel stove as a back up?

With newer heating and inverter generators there is a problem when the generator has a floating neutral (a lot do) as the heating computer needs a stable neutral, it will see this as a fault and not start. You need a special plug or if you are brave bridge the neutral to earth on the generator. A normal generator should be fine. Try and see (and make sure you have completely removed your heating from the house electrical circuit including the earth bonding).

Would a jackery box be a suitable alternative, keep it charged in the house and wire a simple manual changeover switch into the boiler supply with a flying lead to plug into the jackery box?

It sounds doable to me.

Heating draws 168W max. I've got a cheap little 300W inverter and it's over 80% efficient. That's at least 3 hours run time at max without running your battery down too far. Assuming you can run your heating for a bit then leave it off for a while that should cover you for best part of a day. I'd get a second battery to be safe though.

If the neutral is floating then I can't see any reason why you shouldn't just bond it to earth. You would need to be sure that your generator earth point was actually earthed though. I assume the reason they leave it floating is that a lot of people won't bother earthing a generator.

All the above applies to an inverter too.

Does that include the circulator pump?

Inverter ought to give way better than 30%.

We had a domestic Turco oil boiler with conventional circ pump on our boat. 

All electrics fridge lights shower etcetc ran from the 900ah battery bank 240ac via 2.5kw inverter. Would last a week between charges

Caveat, I'm not a sparky.

I think the earth bonding would be an issue. The oil boiler is probably earth bonded to the pipes and also to the house earth, which is probably connected to the neutral phase at the consumer unit (Protective Multiple Earthing). This is where I knew I was way out of my depth and thought it best to rely on the log burner and LPG hob during power cuts. It's no doubt doable, but not as easy as just sticking a plug on the boiler feed and sticking it into an inverter/genny when the power goes off.

I also seem to recall the flame sensor in the boiler won't work without adequate earthing.

This is where it's difficult, combining or seperating the earths is a real hassle and knowing when and why a job for experts. That neutral return (or not) is a nightmare. In Germany houses have 3-phase supply and in my house there are just 3 cables coming in and earth is a spike in the ground, across the river they get 4 wires and earth return (a legacy of the hydro plant feeding the system). 

On the superyachts we built you have 5 seperated (and identified) earth circuits, the colour coding was a nightmare and the whole concept difficult to get over to the German trained electricians in our area as they knew nothing about ships. Seeing someone using a normal 220v jigsaw in a supposedly 24v circuit is worrying to put it mildly!

We were without power for 3.5 days after Storm Corrie. Consider this when you work out how long your battery backup needs to last. Our neighbour used a small petrol generator to keep his oil boiler powered up and running and kindly loaned it to us for an hour a day to run our boiler long enough to keep the frost at bay. We've a modern (10yr old) condensing oil fired boiler and it worked fine.

A multifuel stove is an option if you don't have any neighbours who'll be upset by the smell, and you don't mind filling your house with carcinogenic particles.

Whether you decide on a generator or use the enormous battery inside an electric car, use a qualified electrician to set up the hook-up.

you can assume 80% efficiency for a sinewave inverter. Even at 50% efficiency and your pessimistic 50% duty cycle on the battery, you should get a days worth of 1 amp draw.

Before I moved from the Peak, the house was in a valley with above ground cables which used to go down regularly. I initially had a generator, but it was so much faff and niggly little repairs, I got a couple of 120ah leisure batteries  off the web, in parallel feeding one inverter. 

That setup would run the central heating, some led lights, internet router, wifi and ipad chargers. Cant remember the exact figure but that current draw on average was less than 1A, and the system ran over 2 days on one occasion without problem. There wasn’t anything on overnight, but there was still charge available when the power eventually came back on.

Are you trying to fix an unlikely problem? How many power outages have you had, how many were mid winter and what would be the worst case scenario if you had no heating for a day or two?

Or are you a prepper?

Storm Arwen left our village without power for about 48 hours, and nearby houses for 11 days.  After one day, the houses with neither backup power nor a fire/stove were down to 2C *inside* and people were having a miserable time eating cold food and struggling to stay warm.

For them it was somewhere between miserable and dangerous in the cases of households with newborns, very elderly etc.  going to relatives didn’t work for many due to the scale of the outages.

Many places in our general rural area had no power for 36 hours after storm Isha.  We missed the nearest cluster of outages by < 2 miles.

Spending £100 on an inverter to repurpose a car/car battery is hardly going all Burt Gummer now is it?

(We calmly rode out Arwen with a battery/inverter system that also ties in our rooftop solar PV in a microgrid mode.  We’ve had a couple of other day long outages where it’s helpful to have a house on UPS.  Northern rural area and all that. Cost less than £1.9k self installed with a professional electrician for the AC parts.  Been in for close to a decade now so costs about 50p/day, a fifth of what I spend on fizzy pop and snacks…)

3 or 4 a year here. Most sorted in 4 to 12 hours, one was 36 hours before ENW set up a generator. The joys of semi rural life.

Edit: Having back up power for 7 days loss of grid is the current standard we're expected to have in place at work. It was 72 hours not so many years ago. The grids not getting any more reliable.

Thanks some interesting reading. I am due a boiler service soon so will discuss with my heating engineer.

Those that have powered a boiler using either a Genny or an inverter, did you link the earth's? I don't (yet) understand floating neutrals. 

Is there a chance of doing serious damage to my boiler taking either approach? If so, how big a chance?

I have 2 spare recond leisure batteries which could provide a base for this project. Swapping the fused spur for a plug and socket is a simple job.

Prepping to stay warm. Rural location, much like ridge 3 to 4 a year. Rural circuits are often switched off first to prioritise urban areas, fewer customers to compensate.

Of course, my tin foil hat might keep me warm.

Fair enough...

Probably worth spending a few pounds extra on a decent inventer for the most stable output. 

 In some ways it's a shame we run on AC, as it's pretty likely the PCB with recifty back to DC anyway... 

Shame they don't provide a DC input for backup power! 

(I guess most of the pumps will be AC though - I have absolutely no idea what an oil boiler system circuit would look like!) 

Local village currently in their third day without power...

Agree with the inverter, lots of fragile electonics in an oil boiler.

As I understand it, linking neutral and earth in domestic wiring permits quicker tripping of RCDs if theres an appliance fault. In my house (overhead supply) the sheath of the SWA mains cable (earthed at the substation) is linked to the neutral supply at the consumer unit in the house, and to the house earth (the incoming water pipe).

Most gennys are used for powering tools and lights. They have a separate earth spike linked to the conductive parts, but earth and neutral may not be linked. This (IIRC) is a floating neutral. You'd have to link earth and neutral yourself and provide some fom of RCD protection. The fact I've reached the limits of my understanding of earthing led me to conclude that DIYing my oil boiler to a genny/powerbank and inverter wasn't a good idea...

Possibly. The big thing to remember it isn't just your boiler you need to worry about. There will be someone working on restoring power to your house. Messing up on the disconnection from mains supply (not saying you'd do this) could end up with the supply unexpectedly becoming live, which wouldn't be nice for the bloke up a pole in the pouring rain.

I'm reasonably happy with doing work on the house wiring, and thought about using a plug and socket rather than the hardwired fused spur to power the boiler, but just didn't feel confident with the earthing aspects. YMMV.

Err, you buy an inverter generator, most of the smaller ones are anyway, a couple of hundred quid and you are sorted.

In reply to 

Lots of mis-information on this thread with regards to how and when DNOs turn off the power, or react to outages. Most DNOs do their utmost to keep the power on. I should know, as I work for one of the Hot glove teams that do all our work live so as not to impact the supply. DNOs don't turn off some customers just to keep others on, it doesn't work like that.

In the case of a big storm with lots of customers off, a small job that get lots of customers back on, will get prioritised over a larger job getting one or two customers back on. That should be obvious. The use of Generators is now the preferred way to get customers back on if they can't be permanently reconnected to the mains within a timeframe.

With regard the Neutral. Try and get an Electrician in and ask if you can have a separate Earth installed (PMB) and get the PME removed. Not always possible, but worth an ask. There are other ways to earth the electrical system in your house. 

I’d add that some cheap/generic/unbranded transformerless inverters become lethal under a neutral/earth link with good grounding, as their AC neutral can be a large offset from the 0V and 12/24/48 V DC lines.  As both sides of the battery are ungrounded, there can be a large potential difference between ground and the battery system, which people aren’t reasonably going to expect or be protected against.

The moral is not to buy cheap, high power inverters from online marketplaces!

I stick with Victron gear and I know some others around use them.  Decent kit may be 2x to 3x the price, but it’s still much cheaper than the kit it’s powering.  A fried boiler from dodgy gear would rather defeat the purpose.