using 24vdc to open a 24vac gas valve on a low pressure steam system

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by DaveMe, Oct 28, 2011.

  1. DaveMe

    DaveMe New Member

    Messages:
    4
    Location:
    Denver
    My Peerless low pressure steam boiler has a 24vac gas valve. During a recent snow storm (seems too early to be saying this), we lost electricity for over 24 hours.

    A friend of mine told me that I could hook 2 12 volt dc batteries together in series to get 24 volts, and that the 24volts of dc power would open the gas valve. I did a goggle search and found confirmation, and the confirmation basically said you can open a 24vac valve with 24v of dc power, but not vice-versa. My friend said basically all you are doing is operating a magnetic valve, and that is why 24vdc will open a 24vac valve.

    Being somewhat skeptic, and not wanting to replace a gas valve, I thought I would ask the forum.

    Since there are no pumps on a low pressure steam system, it sure would be a good backup to have it really works. At least you could get the house warm, although be it manually. I guess if you had a 24 volt dc thermostat, you could potentially operate the furnace automatically if it is true about using 24dvc on a 24vac vavle.
  2. Tom Sawyer

    Tom Sawyer In the Trades

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    Location:
    Maine
    Potentially yes. Practically no. So you get the burner to fire but unless you run through the thermostat, low water cutoff and the pressuretrol you have created a manual bomb there.
  3. jimbo

    jimbo Plumber

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    Location:
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    24 DC might damage a 24 volt ac coil. IF the system is rated for ac or dc, the rating would likely say something like 24 vac/16 vdc. Knowing nothing else, I would be concerned that there are other system components...limit switches? relays? that need the ac. But if the gas valve is truly the only energized component, then I would be more comfortable first gettting a recommendation from the manufcaturer.

    I will say that when I was growing up, my father had our oil furnace changed out for a gas boiler with a millivolt gas valve, for just that reason. After some storms, we were the only house with heat!! It was a simple single pipe steam system, so all you need was to get the burner on!
  4. DonL

    DonL Jack of all trades Master of one

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    Location:
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    I think that you would be better off using a small cheap DC-AC Inverter, and run the 24VAC transformer off of it.

    Then You only need 1 12volt battery, and have the ability to run other Household appliances.

    That is the way that I would do it.


    Enjoy your project.


    DonL
  5. hj

    hj Moderator & Master Plumber Staff Member

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    Thermostats are just "switches" so they do not care whether it is AC or DC.
  6. DonL

    DonL Jack of all trades Master of one

    Messages:
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    Location:
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    Switches are normally rated at a much lower DC voltage than AC voltage.

    It depends on the Thermostat.

    I have seen contacts stick together when a 120 VAC relay was run at 120 VDC.

    At 28 Volts it may work fine, You would just have to try it. The difference between the AC/DC Electromagnetic field that affects the switch would have to be put into the formula.

    I would use Limiting Resistors while doing any testing.

    Or maybe a Computer UPS would be safer.
    Last edited: Oct 29, 2011
  7. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,185
    Location:
    NW Ontario, Canada
    or maybe a 12V DC to 120V AC solar or car inverter which is essentially what is in many basic UPSs.
  8. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
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    Location:
    Maine
    Gentlemen.......Just say no before someone gets hurt
  9. DaveMe

    DaveMe New Member

    Messages:
    4
    Location:
    Denver
    Gentlemen - The transformer that powers my boiler is a Honeywell AT 20D1047 1, O.C. 25V. N.E.C. CLASS 2, 120V. 60CY. 20V.A. I metered it - 25 volts.

    What I would do if I lost power is take the + and - leads off the AC transformer and put them on a 24v transformer that is operated off a car battery. 24 volts vs 24 volts.

    My question really should have been - is 24 volts coming from a AC source the same as 24 volts coming from a DC system. If the answer is yes (which I am not sure it is), then shouldn't I be able to power the boiler with 24 volt dc transformer coming from a car battery ? If the juice comes back on while the battery is being used, there should be no problem - the AC transformer would not be connected, and once the juice is back, I just change the wires coming from the boiler back to the 24v AC transformer.

    I did find this converter that looks like it could do the job - http://www.outsidesupply.com/24vdc-...ve-dc-ac-inverter-for-irrigation-40-watt.aspx - says Converts 12 volt dc to 24 volts ac.
    Last edited: Nov 1, 2011
  10. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
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    Location:
    Maine
    Sure, since you have no intention of listening to reason and don't give a crap about safety and all, go ahead and hook up any way you feel like doing it. I suggest you warn the neighbors and the fire department first though. Fair is fair
  11. BobL43

    BobL43 DIY Senior Member

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    Location:
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    You obviously don't want to listen to anybody that cautions you on danger and safety, but AC is not the same as DC and AC has no + or - . Aside from that, you are dealing with a double acting bomb: gas and steam, and getting that boiler to fire without the controls working 100% as designed is not too bright in my humble opinion. you're not rigging up a toilet here
  12. DaveMe

    DaveMe New Member

    Messages:
    4
    Location:
    Denver
    Thanks for all the concerns.

    Instead of messing with 24vdc vs 24vac, I think I will get a 12vdc to 120v ac inverter and if the power goes off, just unplug the transformer from the AC house circuit and plug it into the inverter. The transformer will be getting AC current, no worrying about effect of 24v ac vs. dc, and when the power comes back on, just plug it back into the house circuit.
  13. DonL

    DonL Jack of all trades Master of one

    Messages:
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    Location:
    Houston, TX
    You can buy a Computer UPS for about $50 with the battery and charger build in.
    It turns on automatically, And would be a lot safer.
  14. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    In answer to one of the questions, the power delivered to a resistive load by 24 volts AC ( rms ) is in fact equal to the power delivered to the same load by 24 volts DC. One issue in this equation is that a coil or transformer is not a pure resistive load. That is why I mentioned that in similar situations I have seen a solenoid ( coil) type device listed as 24 volts AC / 16 volts DC.

    You mentioned something about a DC transformer, and of course transformers don't work with DC.
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