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Backup power for an old steam boiler with a standing pilot

109A_5
109A_5 Member Posts: 1,945

Since Wcurtis requested it, here it is.

Decades ago I woke up early one day since I was getting cold. The power was off, so no heat. Since this boiler was old and very simple and had a standing pilot all I needed to do was get the Gas Valve to open and there would be heat. Isn't steam heat great !!!

I had an idea, I happen to have an old car battery in the garage that would not start a car any more but still had about 12 volts across the terminals. So I went out and got the battery. I disconnected the 24 VAC transformer and connected the battery to the wires and the Gas Valve opened. We have heat.

Since the Gas valve is an AC solenoid and AC solenoids have different electrical properties than DC solenoids (often the resistance is lower) I did not want to overheat or damage the Gas Valve's AC solenoid, even though I was already at half the AC voltage with the DC voltage. So I inserted an old Snap-On Tools test light in series with the battery, I believe the old test lights had a #53 Incandescent Lamp .

The #53 lamp did a few good things, some of the battery voltage was dropped across it so the Gas Valve did not see the full battery voltage. It acted as a current limiter (it has resistance) and since at its normal operating voltage it can only draw about 0.12 Amps.

A couple things were in my favor. With an incandescent lamp the cold resistance is lower than the hot resistance. A solenoid typically has a higher 'Pull In' current than the 'Hold Current' to keep the Gas Valve open. So with the cold filament of the #53 lamp the Gas Valve reliably opened and then the lamp warmed up a bit the circuit current is reduced, so longer battery life and less current through the gas valve to minimize the heating of the solenoid. This worked well, and the boiler was still operating well (with a junk car battery) when I returned home from work. After work the utility power was back on so everything was restored to normal.

Years later due to a storm (the power does not go off much here) I did the same basic thing with just two 6 volt lantern batteries connected in series and it ran for days.

Here is that basic circuit drawn without the Limits, LWCO and Pressuretroll.

With the car battery in mind for backup boiler power, I now always keep the last battery and give the auto parts store the previous old battery for the core.

Since I am playing with batteries that are old and have diminished capacity I wanted to improve the circuit above. So I measured the actual DC electrical properties of the Gas Valve. I wanted it to work reliably with a minimum voltage and current. Sadly I have lost that scrap of paper with the data, I can always redo the tests. However with that data I came up with a better circuit and I remember the basic needed component values.

I was even thinking of automating the circuit with a battery charger and a relay to automatically transfer to battery when the power goes out. Since the power here does not go out much and I can cobble the updated circuit together in minutes I have not had the motivation to refine and build the automatic version.

So with the updated circuit, I liked the advantages that the incandescent lamp brought to the original method. So with that in mind I came up with this simple circuit that is fine tuned for my Gas Valve.

Starting with the thermostat off, the Capacitor is discharged, at the the instant the thermostat switch closes there is current through the Resistor and the Capacitor which satisfies the 'Pull In' current of the Gas Valve. The Gas Valve opens. Within a second or two the Capacitor fully charged and the only remaining current is through the Resistor, the 'Hold Current'. This current is a lot less than if the Resistor was not in the circuit, so the battery will last a lot longer. When the thermostat opens the Resistor in parallel with the Capacitor discharges the Capacitor so it is ready for the next call.

If memory serves me correctly I used a 250 Ohm resistor and a 3000 micro-farads, 25 VDC aluminum Electrolytic Capacitor. Basically any larger Capacitor that will pull in the Gas valve will work and the resistor is picked to supply the minimum 'Hold Current' plus about 10 % for reliability as the battery discharges. Other Gas Valves may need other component values for optimum performance.

National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System
bburdMad Dog_2delcrossv

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Comments

  • Wcurtis
    Wcurtis Member Posts: 125

    Very good post, thanks for reply

  • TKPK
    TKPK Member Posts: 71

    I still say a 12v battery with an inverter is 20x simpler and all you have to do is swap the power going to the disconnect to the inverter to isolate it from the house.

    if I was a geek and didn’t want to spend 2 minutes taking the box apart and putting a plug on it I would put a transfer switch on it …

    Instead got a whole house generator since we were losing power 3-4 times a year and I travel a lot so could not guarantee I would be home. Of course after that money was sunk the utility gets their act together and replaces much of the 100+ year old circuit in the neighborhood and went on a tree trimming campaign.

    Simpler is not always simpler

  • Long Beach Ed
    Long Beach Ed Member Posts: 1,460
    edited December 2024

    The car battery system that 109A_5 uses is great idea, and for many years served me well in rental properties.

    Lately I've purchased rechargeable lithium battery packs with built-in inverters that will power a solenoid for days, and even a circulator for a few hours. Jackery makes an affordable bunch, marketed as 120V "solar generators" in all different capacities. I use 240 watt-hour batteries that give perhaps 8 hours of solenoid heating at about $200.

    This part of Long Island, with an ancient distribution system loses power about monthly. We've wired boxes with dpdt isolation switches and a recessed male connector on every boiler to accept the plug-in battery/inverter. This saves us from freeze-ups should we lose power in the cold, and eliminates the need for a generator.

    In a long term situation batteries can be recharged with a generator or solar panel every 15 hours or so,figuring on a 50% boiler cycle. Generally, you can figure on this replacing one 100 AHr car battery.

    The drawback with this vs. 109A_5's car battery is a higher cost and working with 110 VAC wiring, which requires code compliance and a licensed installer. The advantage is that you have a small, portable easily rechargeable battery pack and inverter that puts out 120VAC and can be used for a bunch of other stuff.

  • Lance
    Lance Member Posts: 318

    My backup heat source is my girl sitting real close.😉

  • Dan_NJ
    Dan_NJ Member Posts: 262
    edited December 2024

    >With the car battery in mind for backup boiler power, I now always keep the last battery >and give the auto parts store the previous old battery for the core.

    I took the approach of keeping a spare group 35 battery - same size that the car uses - that from brand new day one lived on a trickle charger. This way I have a spare for the car at all times that doubles as power supply with 50-60 amp hours available in case of power outage. From experience I can tell you it works for both scenarios. Edited that to say Amp hours, say 600+ watt hours total in a pinch but I'd not want to drain more than 50% in one go.

  • 109A_5
    109A_5 Member Posts: 1,945

    Modern technology has added a lot of options depending on your boiler system. Back in the day when I first did this, inverter technology was so poor it would kill a good car battery in a matter of hours, you had to leave the engine running to use an inverter and the output waveform and frequency was terrible. Also at the time I first did this the longest part of the effort was to carry the battery from the garage to the basement, and it was done with stuff I had already had so basically free to implement.

    Even with steam heat, with modern Boiler trim you are forced to use an inverter and a battery in good shape if you expect to have any significant run time. luckily inverter technology has improved a lot over the years. If you have an older boiler you may have more options.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
    Long Beach Ed
  • Wcurtis
    Wcurtis Member Posts: 125

    How feasible would a portable inverter work?

    I have one of these that was plugged into my truck “power cord” for phone charging etc.

    Brown wire goes to pressuretrol and LWCO which is in series with gas valve and T-stat

  • Wcurtis
    Wcurtis Member Posts: 125

    Disregard, I’m not seeing how this could work

  • 109A_5
    109A_5 Member Posts: 1,945

    Hello Wcurtis,

    With that equipment (pictured) you would unplug the primary side of the transformer and plug it into the inverter.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
    TKPK
  • Wcurtis
    Wcurtis Member Posts: 125

    That’s what I thought as well but the wiring diagram from the manual appears different that what I see but I’m not an electrician so can’t be sure

  • 109A_5
    109A_5 Member Posts: 1,945

    Can you take a photograph of the wiring diagram in the manual and post it ?

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • Wcurtis
    Wcurtis Member Posts: 125
  • 109A_5
    109A_5 Member Posts: 1,945

    Hello Wcurtis,

    That overall wiring diagram provided here is for inverter and step down transformer use. The wires on the Right side of the transformer (the primary) would go to the 120 VAC output of the inverter, and that in general would probably work fine. This method has the extra heat loss of the inverter and the transformer that are constantly energized.

    In my case I disconnected the boiler wires from the 24 VAC side of the transformer and ran it from a 12 VDC source with current limiting as described earlier in the thread. I believe my method of directly using the 12 VDC with the current limiting strategy would greatly extend the battery runtime compared to an inverter powering a step down transformer method. Additionally with my method when the thermostat is is not calling there is no draw on the battery.

    My boiler wiring is very similar to what is shown in this wiring diagram.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • Wcurtis
    Wcurtis Member Posts: 125

    You are awesome, thanks for all the details, extremely helpful

  • Long Beach Ed
    Long Beach Ed Member Posts: 1,460
    edited December 2024

    How we do it, with male socket and isolating switch. (IN5 steam boiler with a basement hot water loop)

  • 109A_5
    109A_5 Member Posts: 1,945

    Hello Long Beach Ed,

    Is that just a 3-way switch with the common terminal going to the boiler ? Or something else ?

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • Jamie Hall
    Jamie Hall Member Posts: 25,351

    @109A_5 — that should be an SPDT switch, but not just any SPDT: it must be a break before make design. Not all of them are. The standard 3 way switch from the big box or what have you isn't.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • 109A_5
    109A_5 Member Posts: 1,945

    A common 3-Way switch is actually a SPDT switch. I agree in this case it should be break before make type, the ones I have played with are, however I agree it should be verified. And what the actual switch is, is a question for Long Beach Ed since it is his equipment I was questioning about.

    Most actual Transfer Switches I have seen and worked with have a lot of physical isolation between the Utility and the back up power source.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • Long Beach Ed
    Long Beach Ed Member Posts: 1,460
    edited January 2

    I used a DPDT switch. The double pole switch completely isolates the boiler from the line power when the battery is connected. My battery supply has a bonded neutral; it is grounded at the inverter.


    Without getting overly technical, with a floating neutral generator (one that is not grounded) you would use a SPDT switch and keep the neutral wire contiguous and not switched, exactly as you drew. Your boiler's neutral would remain grounded at your service panel.

    Any switch should be a break-before-make as Jamie mentions. Most switches are, but you would want that specified on a UL listed switch.

    Some jurisdictions require an electrician or engineer sign off on this sort of thing. He would know your local inspector and know what he wants. Naturally, that's another story.

  • 109A_5
    109A_5 Member Posts: 1,945

    Yes Long Beach Ed, you don't want the neutral bonded to the ground in two separate, different places, so you don't have neutral current in the ground conductors. So a DPDT switch would provide the neutral isolation.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
    Long Beach Ed

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