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How did I burn out my 24v transformer?

So, I’m replacing the two zone valves on my hot water boiler (switching from White-Rodgers 1311 to Taco Sentry) and I needed to drain the system to put in isolation valves and unions so that I can change valves more easily in the future.

While trying to figure out how to rewire the new valves—a challenge that may need help in a separate post—I thought I’d get a jump on refilling the system (and get some much needed heat in the house) by manually opening the valves and firing up the boiler by bypassing the thermostat controls and wiring the transformer directly to the boiler controls. Of course, the plan was carefully monitor the situation and turn it off after a few minutes.

It worked fine for a few minutes, but then the transformer conked out. Testing confirmed that the primary winding lost continuity.

What did I do wrong? Perhaps the transformer was faulty?

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 17,679
    Pity there wasn't a fuse in there...

    Which burner control terminals? Some of them aren't looking for a voltage, only a closed switch... and I can envision ways in which you were drawing way too much current from the secondary.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Steamhead
    Steamhead Member Posts: 15,022
    I think I know what you did. The boiler is looking for a switch, which is what a thermostat is. If you put voltage on the T-T (thermostat) terminals there may not have been enough resistance and it would look like a short-circuit to the transformer. Don't forget, the boiler's transformer is also supplying voltage to the T-T circuit, so if a second transformer is added the current can run away.

    But- to be sure, post the make and model of the boiler, and the type of control where the T-T terminals are.

    When the zone valves are hooked up, their end switches should be wired in parallel to T-T.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
    ZmanHomerJSmithSuperTechEdTheHeaterMan
  • hot_rod
    hot_rod Member Posts: 15,363
    The zone sentry should have a good wiring diagram with it, or use the wiring guide at the Taco site.
    It is always easier to wire zone valves via a zone control box to keep the various 24V circuits separated, and fused :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    ZmanEdTheHeaterMan
  • SuperTech
    SuperTech Member Posts: 1,712
    I would definitely recommend a zone control board.  Its not 100 percent necessary but it makes it easier to have cleaner looking wiring, fuse protection for the 24 volt circuits and generally makes troubleshooting easier.  Also if you want to add thermostats with wifi capability it makes adding a common wire simple.  
    EdTheHeaterMan
  • REKBDR
    REKBDR Member Posts: 39
    I think I might have really screwed up.

    The boiler is an old Utica Series M-AGB. The controller is a Honeywell L8148E.

    I think I misread the wiring diagram for the White-Rodgers valve, thinking that the system was set up to send transformer power to the controller when calling for heat. It apparently just closes a switch to make a circuit powered by the transformer in the Honeywell controller.

    The question now is whether I also screwed up the Honeywell transformer. How might I easily test that?
  • HVACNUT
    HVACNUT Member Posts: 4,320
    Does the L8148E have just TT, or T, TV, and W?
    With the wires removed from TT, there should be 24v there.
    If its cooked, there are better choices than the L8148E. Probably cheaper too.
  • Zman
    Zman Member Posts: 7,103
    REKBDR said:

    I think I might have really screwed up.

    The boiler is an old Utica Series M-AGB. The controller is a Honeywell L8148E.

    I think I misread the wiring diagram for the White-Rodgers valve, thinking that the system was set up to send transformer power to the controller when calling for heat. It apparently just closes a switch to make a circuit powered by the transformer in the Honeywell controller.

    The question now is whether I also screwed up the Honeywell transformer. How might I easily test that?

    Do you have a multimeter?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 15,363
    Those WR zone valves are power open and power close, they typically have 5 or 7 wires, so when you replaced them with a 2 or 4 wire zone valve you need to make sure you wired correctly. Probably find some YouTube videos on how to wire the Taco valves.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • REKBDR
    REKBDR Member Posts: 39
    Thanks to everyone! I do have a multimeter and found the schematic for the L8148E. Figured out how to test the primary coil but still puzzled by how to test the secondary without taking the controller apart.

    Not optimistic about what I might find, and definitely interested in a quick and cheap alternative to the L8148E without having to change the well. Recommendations? Whole boiler is due for replacement in a few years, and so I just need something to tide me over.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 2,524
    edited March 2021
    So the L8148E is designed with a larger transformer compared to the L8124A, L8148A, and others. That transformer is there so someone can power 2 zone valves without the need for a separate transformer. If you have more than 2 zone valves, the valve manufacturer and Honeywell suggest that you wire the zone valves with a separate transformer. That said, Your existing wiring may be really confusing, so you will need to start from scratch with the low voltage control wiring.

    The first step is to determine if your L8148E is still operable. The secondary of the transformer is connected to Z and TR Z=common and TR=R. If you power the control and you get 24v. to Z and TR then your control is most likely still good. If not then you need a new aquastat relay and I would replace it with a Honeywell L7224U. There are others on the market that may have different features, but I find this one to have everything you need without going crazy. It is easy to understand and has some energy-saving features that can be enabled (Or Not).



    Let me know how you make out with your test on your L8148

    Mr.Ed
    Edward Young
    Retired HVAC Contractor from So. Jersey Shore.
    Cleaned & services first oil heating system at age 16
  • REKBDR
    REKBDR Member Posts: 39
    Well, no luck. Tried testing Z to TV and Z to B2, and even jumpered T and TV hoping for a miracle.

    No visible damage to the transformer, but the coil on the relay looks really fried.

    So, what to get to replace the aquastat? The L7224U appears to be for an oil boiler whereas I need a controller for gas. Any other suggestions to save me some of the $200+ for a new L8148E?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 2,524
    OOps you are right. That is a line voltage burner control.
    Edward Young
    Retired HVAC Contractor from So. Jersey Shore.
    Cleaned & services first oil heating system at age 16
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 2,524
    Edward Young
    Retired HVAC Contractor from So. Jersey Shore.
    Cleaned & services first oil heating system at age 16
    SuperTech
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 2,524
    edited March 2021
    here is a diagram. This will get you where you need to be



    Also, get yourself a zone control panel like this one.
    https://www.supplyhouse.com/Taco-ZVC405-4-5-Zone-Valve-Control-Module-with-Priority

    I like to get one more Zone than necessary. If you have 3 zones get a 4 zone panel. If you have 6 zones then get a 6 zone because no one makes a 7 Zone control. LOL
    Edward Young
    Retired HVAC Contractor from So. Jersey Shore.
    Cleaned & services first oil heating system at age 16
    SuperTech
  • Kickstand55
    Kickstand55 Member Posts: 32
    If the L7224U is all you can find, attach a 24volt 40va to the electrical knockout and wire the primary leads to B1(hot) & B2 (neutral). Then attach your gas valve wires to the 24volt terminals. That will only power the gas burner. The zone valves will be wired separately as mentioned before.
    A Hydrostat 3200 for gas is a good low expense choice. There are others, research necessary.
    Please be sure you feel safe around 120 volt electricity. If not, call a tradesperson. We would like to hear back from you!!! Really!
  • SuperTech
    SuperTech Member Posts: 1,712
    I've done the L7224U with a transformer for gas boilers several times.  Its a better,  more modern aquastat than the old mechanical L81848E. 
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 2,524
    edited March 2021
    Don't need a 40VA to operate a gas valve. You can use a doorbell transformer. They are less expensive and come in a style that fits in a knockout. 20 VA is plenty of power for a gas valve.
    Edward Young
    Retired HVAC Contractor from So. Jersey Shore.
    Cleaned & services first oil heating system at age 16
    SuperTech
  • JUGHNE
    JUGHNE Member Posts: 9,225
    Ed, the doorbell transformers I have seen are 12-16 volts.
    Of course gas valves used that 24 vac.
    There are 20 VA 24 VAC transformers that come with some humidifiers.

    Although I did replace a 16 volt transformer on a boiler that used 24 volts.
    It did work somewhat on the 16 volt.
  • REKBDR
    REKBDR Member Posts: 39
    Well, in the end I simply replaced the aquastat with another L8148E, and things are working fine. “The Boss” put her foot down on the idea of rigging up a cheaper alternative, convinced that I might screw up again— and the expense is coming out of my beer fund, anyhow.

    But, thanks to all of you, I learned a lot and I appreciate your contributing your expertise to help me out.