In Which I Try Not To Set A Reader's House On Fire

A reader writes:

I've been searching the internet (including your articles) for information on putting together a simple back-up power supply for my central heating system. One that when the power fails (which it will here in Greece when we get a good old storm), I just go down to the boiler room, disconnect the boiler system from the mains and hook it up to a back-up supply (for the two or three hours that it takes to get the utility repair man out of the taverna, up the pole and get my 230v back on line). Automatic systems are all very well, but isn't it nice to know what is actually going on, and also be in charge!

Most of the back-up power systems that I have found on the internet seem to be designed for computer systems (oh, and maybe a fridge). Well when it's wet and cold here and the power goes down, I am more interested in keeping warm than keeping my beer cold (although I do understand the importance of the later) and if my computer doesn't work, well, I still have my Ipod.

The reason I need back-up power is because my oil fired central heating system has a wood burning stove linked to it and must keep the system (in particular both the pump and the system controls) running. The boiler is rated at 140w, the pump 160w, 5 motorised valves 30w and the control box and lamp 70w - 400w in total.

Is there such a back-up supply suitable for my heating system or do I put up with shivering, writing unnecessary e-mails in the dark over a can of cold beer?


Anything involving monkeying with heating systems raises red flags with me, but I'm pretty sure I'm not about to give you advice that will lead to your death. I have, however, been repeatedly demonstrated to have very poor judgement in this regard. (Some friends of ours have officially notified their small children that not everything Daniel says should be accorded the same respect as things said by other adults. They were fine with this, though.)

There may also be some local law that makes this illegal, or requires a licensed electrician to install it, or something; I know nothing of Greek law.

OK, disclaimers over. If you're happy to have a setup that you have to go into the boiler-room to connect, then I think the best option would be an appropriately robust petrol-powered generator. You have to duct the exhaust outside, of course, or set the generator itself up outside. (It might be possible to plumb the generator exhaust into the boiler flue or something, but this could also be another piece of extremely dangerous advice.) Apart from the exhaust issue, though, it'd probably work nicely. Modern generators from the major manufacturers are reliable, quiet and not even all that expensive.

(Generators that serve the purpose of a UPS, cutting in automatically when power fails, are fancier and more expensive. Way more expensive, if you want one that won't give you even half a second of blackout.)

You could probably also use a suitably large off-the-shelf UPS, though, if the tromping into the basement and switching the cables and pulling the starting rope starts to pall. The wattage figures on your heating system's specification stickers are, like most such figures, likely to be over-estimates, so it's possible a quality UPS with as small a rating as 700 volt-amps (which are not quite the same as watts, as I discuss here) could do the job.

The power-rating issue is the same for generators as for UPSes, but I think generators are better at handling the initial "inrush" current when a motor starts. That can be high enough to cause a UPS to beep and shut down, even if the UPS is perfectly able to power the motor if it's already running. This is particularly the case for refrigerators, whose run power is quite low but whose compressors suck a lot of watts for a brief moment when they click on. A UPS trying to power such a motor will therefore work OK if you lose power when the motor's already running, but not if the motor needs to start from UPS power. In the same situation, a similarly marginal generator should just bog down and deliver lower voltage than it's meant to, which is in this case perfectly fine and should let the motor start up with no trouble.

The solution to this whole problem is, of course, to just get a UPS or generator with a higher volt-amp rating, or with a specific high surge capability that it may only be able to deliver for one second, but that'll do. A "home"-model UPS or small generator rated for a genuine 1500 VA (as opposed to the suspiciously high numbers on suspiciously cheap off-brand UPSes) might be adequate; if you turn out to need more than that, and decided to go with a UPS, then you'd have to pay extra for a commercial-market one.

Easy enough to find out what works, of course, if you can get a local dealer to let you borrow likely-looking generators and/or UPSes and try them out. Overload won't actually damage any half-decent UPS or generator; at worst, they'll just complain and shut down.

The real killer for a UPS solution would be run time. Generators can run for as long as you have fuel, of course, but three hours is a long time for a home-or-small-office UPS to be delivering a few hundred watts. Smaller UPSes may even overheat and die in such a situation.

If we presume the constant draw is, say, 400 watts, then that for three hours is 1200 watt-hours, and the battery-to-UPS-to-appliance chain is not 100% efficient, so you'd need more than 1300 watt-hours of batteries to get it done. The capacity of the standard little sealed-lead-acid brick batteries in small UPSes is maybe 90 watt-hours. Less, actually, if you don't want them to die young; standard lead-acid batteries don't like being run flat.

Commercial UPSes can usually be had with extended batteries, but regular readers will know that I recommend just hooking up some car batteries instead. The very cheapest car batteries are still good for 240 watt-hours or more, so it'd be inelegant but feasible to build an array out of them that could meet your needs.

Drop some extra dough on quality "deep-cycle" batteries that are actually meant to do this kind of job and you can easily get well over a thousand watt-hours from one 12V battery that two normal humans can probably move. Graduate to proper industrial batteries that only strongman contestants can move by themselves and you probably won't actually get a whole lot more capacity per kilogram, but probably will get a setup that'll work for many, many years with no more maintenance than an occasional distilled-water top-up. Industrial batteries and a commercial UPS should actually easily outlive a generator.

The smallest batteries in the current Trojan Industrial Line, for instance (PDF here), are six-volt with a 355-amp-hour rating even if you're running them flat over only five hours; two of those in series will give you 4000 watt-hours at 12 volts even into quite a large load.

But, again, none of this is necessary if a generator's acceptable to you. Since you specifically asked for a system that requires you to switch it over manually, a standard, quite inexpensive pull-start generator looks like just what you want.

(I invite commenters to point out the many ways in which I have, in the above, unknowingly endangered Bill's life.)

8 Responses to “In Which I Try Not To Set A Reader's House On Fire”

  1. Stoneshop Says:

    In our hackerspace we have a wood-burning heat-exchanger stove, which MUST have its pump running, lest it reduces to a glowing heap of molten metal and lotsa steam, a rather non-enticing prospect.

    The power group for the 100VA pump, the valve and its controller is routed through a 1000VA APC SmartUps, and I have not found any problem in keeping the pump running, or even starting it. The circuit is equipped with a bypass relay, so that the system normally runs directly off the mains. Cutting the power causes the relay to drop and cut over to the UPS output (something which an online UPS wouldn't actually need), but this doesn't cause any hiccup in the controller and the pump.

  2. Mohonri Says:

    As long as he's moving a power cord from the regular outlet to the generator and back, I see no issues with the "just buy a generator!" advice. Our local Harbor Freight was offering 1000-watt generators for under a hundred bucks US on Black Friday. I'm almost tempted to pick one up just to see how loud it is.

    • Fallingwater Says:

      I'm not sure how black friday prices work, but at <$100 I'd be worried it's a two-stroke model. They're noisy, smelly and the engines last a lot less.

      If it actually is a four-stroke then holy crap, I've never seen one sold so cheaply.

  3. Kernelpanic Says:

    I would recommend a generator.

    If an outage was a yearly event I would get a cheap generator and an extension cord or two. (1kw for $100? sold.) If the outage was monthly or more often I would get a good 1 or 2 kw generator and wire it in. I prefer the Honda Inverter series generators which are quiet and have long run times on a tank of gas. If you are going to the time and expensive of adding the wire go with a larger generator and add some lights and maybe even a microwave oven or lower power hot plate to the generator backed circuit.

    When I lived in the middle of nowhere I had the furnace, well pump, sump pump, microwave, fridge and some lights connected to a sub panel. The subpanel was connected to a 5.5kw generator and the main power through a transfer switch that insured my generator and the power company's generators were never ever connected. The switch was expensive for what should be a mechanically simple device.(likely due to all the safety standards it claimed to pass.) Another way would be to connect the sub panel to the main panel with a large high current plug like the ones used (in North America) for electric clothes dryers and ovens/cooktooktops/stoves/cookers. Wire in a similar outlet from the generator. In my area it is debatable if it would pass code. But it should be perfectly safe as long as the wire, outlet, and plug were all rated for at least as much current as the generator could produce.

    BTW: running a generator indoors is a classic way to die. Even if you are sure you know what you are doing, don't.

    • Red_October Says:

      You're talking about "back-feeding" the system through a high-capacity socket. It's quite easy to make this legal. All you do is make a little metal plate that is bolted to the face of the service panel. Form the plate so that it slides on its moorings, and has two positions: one where the main breaker is blocked, and one where the socket fed by the generator is blocked. Presto, legal requirement of not being able to feed generator power down the line satisfed. Sucks if you planned to use that outlet for something else like a dryer or a mill or something, but if that's the case you should probably spring for a panel-mount male plug (avoiding the neccessity of a man-killer cheater cord in whatever the highest capacity residential circuit you've got is rated for) and an extra high-amp breaker to wire it to. If you do that, be absolutely sure to make the block-out plate to avoid energizing the male plug when nothing is connected to it.

      Of what Dan has written, my only piece of advice is to take care plumbing the exhaust, as exhausts get furiously hot in operation, so don't try to use a length of vacuum cleaner hose for this purpose, and be sure that there's no open window or other place for the gasses to sneak back in near the exit point, since the love to do that. I'd go so far as to say that the exhaust plumbin should be accomplished with proper automotive exhaust tubing, it's not expensive. The other concern with that job is mating the tube to the generator, which may be easy or difficult depending on the generator itself. Don't leave leaks!

      • matguy Says:

        Wouldn't fire place smoke stack tubing work ok for this? Or does it need to be more leak-proof?

        • Stark Says:

          If you are running an engine inside the house, you want the exhaust line as airtight as it possibly can be. The danger here is carbon monoxide, as it is odorless and tasteless you might just assume you were feeling drowsy due to the nice meal you just had when in fact you were soon to be dead due to asphyxiation. Any leak at all in your exhaust could result in a slow buildup of CO with disastrous results. Even if you do put in a nice and airtight exhaust I would strongly recommend putting a CO detector in the house as well - they aren't terribly expensive and will save your life.


          • Firethorn Says:

            Given that the OP is talking about powering a boiler, and not at insane wattage levels, he's most likely burning some sort of hydrocarbon, thus should have the monoxide detector anyways.

            My concern is that non-professional level gensets aren't made to have tight exhausts, so should be outside anyways.

Leave a Reply