Ask Me Help Desk - 2-Wire Thermostat Issues

Hello,

I have a hot water gas boiler with hydronic baseboards. There is a junction box near the boiler which powers the pump and transformer that activates the gas valve. Recently my thermostat stopped working, and I'm trying to figure out why. I have two wires, one black, and one white. The black one seems to maintain about 3.5V AC and the white wire fluctuates from 15V to 25V depending on when I test it. Combined together, they reach a full 120V. I realized this *after* I blew up some thermostats trying to fix it. Is there another transformer or something somewhere that Im not seeing? If I jumped the wires together, the system activates properly. Should I consider just getting a line voltage thermostat and using that instead? What voltage should be going to my gas valve?

Thanks in advance!

From what you stated it must be a line voltage system. Most systems are 24 volt but older systems can be 110 line volt.

Quote:

Originally Posted by wumarkus

Hello,

I have a hot water gas boiler with hydronic baseboards. There is a junction box near the boiler which powers the pump and transformer that activates the gas valve. Recently my thermostat stopped working, and I'm trying to figure out why. I have two wires, one black, and one white. The black one seems to maintain about 3.5V AC and the white wire fluctuates from 15V to 25V depending on when I test it. Combined together, they reach a full 120V. I realized this *after* I blew up some thermostats trying to fix it. Is there another transformer or something somewhere that Im not seeing? If I jumped the wires together, the system activates properly. Should I consider just getting a line voltage thermostat and using that instead? What voltage should be going to my gas valve?

Thanks in advance!

If you fllow the wires you should be able to determine voltage - yes there are 120V zone valves -also you do not mention if you have end switches, It would be better to get a tec in which may be cheaper - depending on how many zones you have there could be another transformer and there could be a short - ( your first thermostat should have a rating of the voltage on it - look at it - also the zone valve should have a rating for either 120 volt or 24 volt - but you may have a system that someone dreamed up with 120 V thermostat going to a relay which could control a 24V zone valve - wiring would also tell you what the voltage is but again it depends if the installer knew what they were doing ( I have seen thermostat wire on 120V and this is a fire hazard.it depends on the amp draw. It would be a lot safer to havea tec look at this one. Good luck and let us know

Quote:

Originally Posted by wumarkus

Hello,

I have a hot water gas boiler with hydronic baseboards. There is a junction box near the boiler which powers the pump and transformer that activates the gas valve. Recently my thermostat stopped working, and I'm trying to figure out why. I have two wires, one black, and one white. The black one seems to maintain about 3.5V AC and the white wire fluctuates from 15V to 25V depending on when I test it. Combined together, they reach a full 120V. I realized this *after* I blew up some thermostats trying to fix it. Is there another transformer or something somewhere that Im not seeing? If I jumped the wires together, the system activates properly. Should I consider just getting a line voltage thermostat and using that instead? What voltage should be going to my gas valve?

Thanks in advance!

I tested the transformer and verified that it was outputting 24V, so I think that was OK.

Then, I replaced the thermostat with a line voltage model (5-2 Day Programmable Baseboard/Line Voltage Thermostat RLV430A (120 - 240V) by Honeywell - Medwing.com, Inc.) and it worked immediately. I had to change the "temperature setting", though, or it would try to adjust the voltage, which doesn't work in my case.

So I think I'm fine now. For the record I had a single zone configuration and valve was rated for 24V. The system is original from 1953 (including the boiler and pump, but the valve is newer), so that would explain the high voltage.

The one thing I don't get is how the last thermostat worked at all. It was a 24V programmable model (LUX TX1500, BlueRidge Co.: Radiant and Hydronic Heating) and it ran fine for a few years.

Gave technical feedback and information on things to check

The old boilers are not efficient but they last forever. If you can afford the gas bill I would never replace it. Years ago they used line voltage 110/120 volts as the main controller for most boilers manufactures. You never had to worry about the transformer going south just when you need it.

Back then they also considered a transformer a parasitic load that used current just to stay ready in case the transformer was called upon to do its job. How funny that today we waste over 10% of our national electric supply because of those same parasitic loads from all our appliance.

Glad you got it fixed.

I feel you still have a problem- if you installed 24V thermostat on a 120V line it will not work and will fry the thermostat. If you install a 120V thermostat on 24V it will not do anything but allow power through when it wants - it is safer but will not function properly. Again I would recommend getting someone who can determine what it is you have. Just to be on the safe side - as you have 120V thermostat on the system now there is no danger except if the boiler runs too long - and honestly if you need to know how your boiler works if you are to work on it so - if you only have one zone valve why is it in there? It could be that the thermostat is switching the pump or power to the boiler. But without seeing the install I can not tell you what is done or wired. There are many ways that you can control a system Let Us know