Running central heating off a 12v battery!

[redtag]

I am investigating whether I can run the central heating for a short while off a 12v leisure battery (110Ahr) with a suitable inverter.

The problem will be the load (and the time that the inverter will run for before melting). Has any one tried this? We get a lot of power cuts and the lights and cooking are easy, but the heating is not! The water pump only draws 90W or so, but I can not work out the power for the oil-fired boiler. The ignition unit has 3AV as a rating but my electrical knowledge doesn't do AV :-)

If this might work, then what is the best way of keeping the battery charged and ready to go?

If this can not work, then I am off to by a small generator!! Thanks

[usenet]

It should be no problem in terms of the capacity of the inverter. There *might* be a problem if the electronics of the boiler or the pump motor don't like the waveform from the inverter, but it's certainly worth trying in my opinion.

The main load will be the pump. Have you misread the '3AV' on the ignition unit, if it was 3VA that would mean 3 VoltAmps which is, effectively (though not exactly) 3 watts. (It can be anything up to 3 watts, depending on the power factor).

Or it could just be that 3AV is a code of some sort and not a power rating.

[The Natural Philosopher]

This should work, and if it does I'd be interested.

The way to keep the battery charged is to use a simple trickle charger off the mains. One could be constructed with a LV lighting transformer (torooidal) a bridge rectifier and a resistor.

As far as teh boler goes - well it has an air blower motor and an oil pump (assuming oil) and an igniter. That plus pumps and motoruised valves has to be less than 500W. I am pretty sure my whole shebang runs of a 3A fused spur. A cheap clamp on ammeter would probably tell you. Lots on ebay at the moment.

So you probably need a big tractor battery and a 500W inverter. Big tractor battery might be 120A/h, ao assuming draw9ng one amp from 250v, thats 250w. The battery will be 12*120 watt hours = 1440watt hours, so maybe 4 hours of safe running.

YMMV

Another trick would be to have an old MOT failed vehicle parked outside the boiler room, and some jump leads coming in...so you can simply pour petrol in that and run it to keep the C/H going. Most alternators will do 20A or so.

[usenet]

A vehicle battery isn't ideal for this application, though it may be cheaper for a given capacity.

A car/tractor/lorry battery is designed to cope with high charge and discharge rates so that it can manage starting etc. well but is not designed to be discharged to a large proportion of its total capacity.

Batteries which cope with deep discharge well are those designed as traction batteries or as backup batteries in alarm systems, UPS' etc.

If the battery never gets fully discharged and is trickle charged all the time that the mains is on then a vehicle battery may be OK but a much smaller capacity backup/traction battery would also do the job.

[Andrew Gabriel]

Alarm batteries are not normally deep discharge types. Alarms either contain deep discharge protection circuitry, or expect the SLA battery to be replaced if it was completely flattened. Similarly for UPS's I've used -- they contain deep discharge protection circuitry so the UPS doesn't need more expensive batteries to cope with being completely flattened. Many inverters also include deep discharge protection circuitry.

[Dave Plowman]

Good grief. How the other half live...

[Bob Eager]

I remember an article in Practical Mechanics (yes, a LONG time ago) where someone did this with a large battery and a dynamotor. His first stab at it ran the dynamotor the whole time; the second was more sophisticated and provided power only when needed. If it was doable then (40 years?), it's doable now...

I always remember what he called it - BEBOP.

Battery Energised Boiler Operation Plant!

[IMM]

A new genny will cost approx £2000. This will do all essential services in the house: lights, CH, energy efficient fridge, etc. I would tend to go that way. Tractor batteries are expensive as are inverters. A genny is also very useful for other tasks.

Although too early, the Microgen Combined Heat & power boiler is due out in Spring, so worth investigating in advamnce, with an LPG version to follow. The LPG version may be cheaer to run "overall" than oil, and no cut in CH or DHW.

Whisper gen is out around now, but not sure if an LPG version is available.

[usenet]

Yes, I was just realising that too, I should have put only traction batteries in my response.

[Andy Dingley]

You don't even need that - just a constant (or temperature dependent) output voltage, but it's lower than that used for trickling. Like I said, it's a two transistor circuit. I once made an 8-headed version of this, for permanent use on a battery storage shelf.

-- Die Gotterspammerung - Junkmail of the Gods

[Owain]

"Bob Eager" wrote | I remember an article in Practical Mechanics (yes, a LONG time ago) | where someone did this with a large battery and a dynamotor. His first | stab at it ran the dynamotor the whole time; the second was more | sophisticated and provided power only when needed. If it was doable then | (40 years?), it's doable now... | I always remember what he called it - BEBOP. | Battery Energised Boiler Operation Plant!

Ah, how times change. We're up to High Integrity Power Heating Operation Plant these days :-)

Owain

[Andrew Gabriel]

Don't assume it will run with no mains power though. When I looked at a couple of datasheets a while back, it was quite clear that the units would not work during a power cut. One of them was thinking about an add-on box (probably a battery and charger to provide power for the electronics) to enable this at some unspecified time in the future.

[Gordon Henderson]

The easiest way (possibly not the best or cheapest) would be to simply buy a UPS. (uninterruptable power supply) Battery, charger and inverter in one box. I ran my CH unit of one of mine (A small APC unit) for a short while to test it. Worked OK. (But it's only a pump and a few mW for the gas boiler ignition)

Go to

and enter UPS into their search box. Their top of the range one is £129 which isn't a bad price for a UPS of that capacity. Watch out for run-time quotes for UPSs though - they are usually quoted at less than half load. The typical run-of-the-mill figures I've seen in the past are about 5 minutes on full load and 15-20 on half load (it's very non-linear above half load due to the discharge curve of the sealed lead acid batterys they typically use)

90W is approx 1/6th of the rated capacity of that Maplin one (give or take power VA -> W conversions), so expect it to last a fair old time.

Gordon

[Mike Harrison]

In most cases this should work, but there may be some very cheap & nasty UPSs out there that won't like the somewhat inductive load presented by the pump.

[IMM]

Good point. The literature I looked at (initial release) for the Microgen did say that it would work in power cut. I assumed it would have to be running when the cut came. Starting it in a cut would be a problem.

[David W.E. Roberts]

Talked to someone years ago who lived in the wilds of Derbyshire and had oil fired central heating. He had a 12v pump for the CH so he was covered for power failures. Had a quick Google but only found marine applications - however a CH pump for a large barge might do several radiators. This would avoid all the messing about with inverters - although if your boiler requires 240V then you still have a problem :-)

Cheers Dave R

[IMM]

All the major pump makers do low voltage versions.

I know some makers did have low voltage LPG boilers for boats etc, then no problems. I still say a genny is the best option, in price, less hassle, function given and versatility.

[Dave Plowman]

If he's got a 12 volt DC pump, then presumably that means messing about with a power supply for that when running off mains?

The inverter isn't the problem - it's the battery, as lead acid types don't like deep discharge. And this would happen with a 12 volt pump equally.

[Jim Hatfield]

Why not just use a UPS? That's what they are for, after all.

[Grimly Curmudgeon]

Basic Universal Low Leakage Standby Heating Integrated Thermal System.