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Thread: Battery Backup for Gas furnace (boiler)

  1. #1
    JDeT Guest

    Default Battery Backup for Gas furnace (boiler)

    After the recent ice storm that disrupted the electrical grid in central NE, I would like to add an a battery Backup and inverter to my gas fired hot water heating system. Are any systems commercially available (why reinvent the wheel)? The power demand when running is less than 50 watts . A heating cycle runs about twenty minutes at a pop. The house is fairly energy efficient and the boiler only cycles about every two to three hours during the night. There is sufficient solar gain during a sunny winter day that the house gains heat during the day (3-4 degrees). Has anyone built of contemplated such an insurance policy? Naturally I would like to accomplish this project in as low a cost as possible. Any input would be appreciated.

  2. #2
    Join Date
    Jun 2003
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    pittsburgh, pa
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    Default Re: Battery Backup for Gas furnace (boiler)

    yes people do it all of the time. i do find that your statement that your boiler only draws 50w to be somewhat low i would think because the pump should draw much more than that. in any case you need to know how much the total draw will be and for how long it will be needed to provide this backup power. once you find out that info you should add another 25% or so for inefficiencies in the inverter and charging inefficiencies. this power is in watts which is volts x amps. if the system isn't large you can go with 12v, but do be sure to size your wires correctly as too small of wires will introduce too much resistance and of course lessen the overall efficiency more. any way, when you decide on the battery voltage suitable for the system divide the watts by the battery voltage to come up with the amps. this figure must be doubled because you shouldn't drain the battery or batteries below 50% depth of discharge.
    on the inverter's requirements, be sure to get a sinewave inverter and that it can take not only the constant power needed by the boiler, but also the surge power it will require upon startup. the surge could easily exceed several times the constant power the boiler requires even though it's to be for a few seconds. if you place other items on the inverter then it all must be figured into the requirements additively.
    one more requirement would be a way to introduce or switch the inverter into the household circuits as most do not get connected to the grid. disconnects are fused and protect you from having multiple power sources active to the same wire. without this this could be as dangerous as connecting a generator up to the grid when it's not designed to do so. there are more costly inverters out there with grid tie ability and builtin chargers.
    the charger is another consideration as without it the batteries would die. the charger should be a quality design and not a run of the mill automotive battery charger. it should have at least 2 stages of charging being bulk and float and not be less than 5% of the battery capacity. be sure of the maximum charge current the batteries can take as per manufacturer's specs as some cannot take more than 10%, but some do go higher. you don't want to exceed the manufacturer's max charge rate recommendations. i think this covers most of it and if you decide later on to want to add solar you could by buying and installing the pvs and an appropriate controller all wired to run parallel to the backup system just outlined.
    NIEL

  3. #3
    JDeT Guest

    Default Re: Battery Backup for Gas furnace (boiler)

    Thanks for the prompt reply. The boiler for the house is only two years old and apparently has a fairly efficient pump. I have a "kill-a-watt" wattmeter that I plan to use to monitor the power demand during both standby and operation for several days to determin the actual consumption. Am I correct in my assumption that you definitely advise a "true Sine" inverter rather that a "modified sine" (el-cheapo) inverter.

    Since ice storms aren't all that common in this part of NE, I don't envision the system to be used very often. I am wondering about the quality of battery to use. My fishing boat has two deep cell batteries that I use for trolling. Since this is an emergency backup system would you consider this an acceptable power source since the actual demand for emergency power might only be once every ten years. I have a multi-stage charger for these batteries currently. Strictly from a cost standpoint I think this would be an effective approach. I keep the batteries in the basement during the winter anyway. My reason for leaning this way is that I wonder about the reliability of a battery that is maintained on a float for extended periods without any use.

    I have a 1 kw Yamaha generator that we use for camping which I would use to recharge the batteries if a power outage lasted an extended time. Am I missing anything in my current line of thought . at this point my main concern at this point is the source of battery power- dedicated or the trolling motor batteries.

  4. #4
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    Default Re: Battery Backup for Gas furnace (boiler)

    use a sinewave inverter as the el cheapos could cause overheating, use more power for the same load, and possibly cause damage to the pump and that's if the el cheapos would work at all. i'm not sure of the rest of the boiler's system as to how modsine would effect it.
    your choice of battery is up to you, but even i know that you may need it for more than just outages due to ice. keep in mind that you may want to have enough backup to power your fridge too for you don't want to heat the house to find your food has gone bad.
    you also have my curiousity on the power requirement of my new boiler. i will have to readup on it. mine is a crown 160kbtu input.
    NIEL

  5. #5
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    Default Re: Battery Backup for Gas furnace (boiler)

    I would say since you already have the batteries go ahead and use those. If in the future you decide to upgrade you can. Once we get the numbers from the kill-a-watt we can figure out how long those batteries might last with your system.

    I would guess your boiler has a glow plug on startup? Or does it have a standing pilot? If it uses a glow plug you will see something like 300-600w for 20 or so seconds during start up. This will likely be the biggest load. I would also suggest a sine wave inverter if the unit has any kind of electronics on it, they sometimes work on mod sine, but there is a chance you could fry them as well, that would be bad. I am assuming its hot water since you didnít mention a forced air blower.

    For a connection I would suggest making the boiler a male Edison plug-in (standard plug like you se on anything that plugs in). Then in a pinch you can just unplug it from the grid or wall outlet and plug it in to the inverter.

    I would also suggest upping the size of the inverter to potentially allow a fridge or freezer to be run. If you have an outage in summer it would be quite handy although you could use your genset for that as well.

    What the heck is the small standalone sine wave inverter everyone seems to like and last forever? The Prosine?
    XW6048, 4 KC 120's, 4 KC 130's and 4 Evergreen 200's totaling 1800w of PV, MX-60 charge controller, Trimetric meter and eight AGM 8A8D's or 490 amps at 48v. 4 tons of geothermal and 3 tons of air source.

  6. #6
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    Default Re: Battery Backup for Gas furnace (boiler)

    JDeT,
    i would like to hear what make and model of boiler you have there just for the record. on mine, even though the pump would be less than 100w as it's marked, the side of my boiler has 120vac 60hz less than 12amps stamped on it. this may be way overstated i am not sure as they may be accounting for the initial surge power. i could wire up my killawatt meter to measure mine, but i feel it's a hastle not worth my time right now to wire it up due to the wiring having an armored jacket. knowing more about yours would give me a better idea about my own and give you a basis for designing a backup system. see if your boiler also has a power requirement placed onto it. mine also listed min and max gas pressures the boiler can handle so if you see that you may be on the right track.
    as to a sinewave inverter, i was generalizing. those with builtin chargers are from xantrex and outback, but i think that other sinewave inverters could be used(like samlex or exeltech) if one comes up with a charger for the batteries(like iota).
    NIEL

  7. #7
    JDeT Guest

    Default Re: Battery Backup for Gas furnace (boiler)

    The boiler is a Lennox GWB8-070S (70 K BTU input). I had misread the draw on the circulating pump, it is .71 amp so the pump draws about 80 watts. I opted for a standing pilot when I installed the boiler so beside for the power vent (80MW at 24 volts) and the digital thermostat I doubt that the total use will be over 100 watts. Startup of the pump might push a 150 watt inverter. I am planning to use a twistlock connector for the grid power connection and make a twistlock pigtail connector for the Inverter. Do you think a 150 watt inverter would be marginal, should i opt for a 300 watt inverter to be sure of adequate startup margin? Does it seem as if I have everything covered?

  8. #8

    Default Re: Battery Backup for Gas furnace (boiler)

    The peak efficiency of many inverters is at ~1/3 of their rated power. If your fan's nominal continuous power requirement is ~80 W, then a 250 W inverter should work nicely. A quality 250 W inverter would have a surge capability of ~ 2X it's full-power rating, so that should be plenty for your fan. One unit definitely worth considering would be the Exeltech XP 250 true sine wave inverter.

    See: http://www.exeltech.com/ex_root/ex_p..._xp_series.htm
    and: http://store.solar-electric.com/exsiwain.html

    HTH,
    Jim / crewzer
    120618: System off-line for a while...

  9. #9
    Join Date
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    Default Re: Battery Backup for Gas furnace (boiler)

    be sure of the draw of the whole system as my pump also states .71amps and i too have a standing pilot. you could buy a larger inverter and be safe, but if you buy too small there's no way around it except to buy another. be sure of your need.
    NIEL

  10. #10
    Join Date
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    Green Bay, WI
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    Default Re: Battery Backup for Gas furnace (boiler)

    I would second the suggestion to go with at least a 250w inverter, again if you ever want/need to power a fridge or freezer I would go even larger. If you can get an amp meter in line that can measure the peak the load and start the unit. Honestly I have had issues with undersized inverters. Your better off getting larger then you might need, then having a smaller one not start the load or fail on startup.

    But it sounds like with a standing pilot and just the exhaust blower a 250w should do the trick.
    XW6048, 4 KC 120's, 4 KC 130's and 4 Evergreen 200's totaling 1800w of PV, MX-60 charge controller, Trimetric meter and eight AGM 8A8D's or 490 amps at 48v. 4 tons of geothermal and 3 tons of air source.

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