View Full Version : Thermostat question...

Dec 12, 2006, 03:27 PM
I have a gas fired boiler for a radiant (water radiators) system. I know the following:

1. It has a V15-5 gas valve
2. It has a very dysfunctional TM850A Honeywell Thermostat (it's very old, but I have the installation manual)
3. It has a L170A Aquastat (the wires pass through)
4. I have a wiring diagram and blueprints for the entire system. It was installed by the original owner, a plumber. He's dead and his company doesn't know the system, except to say it's more efficient than a newer system.

The heat works well, but the thermostat makes the system run at odd times. I wanted to replace the thermostat with a programmable thermostat. Never having dealt with an old system like this, I just bought a Lux TX500b.

It has connections for G,Y,W,RH,RC. As I understand it, the RH is power from the transformer. The W would act for the gas valve. But my old thermostat also has a B wire and I don't know where to put it.

Any help you can provide would be appreciated.

Dec 12, 2006, 08:46 PM
Blue wire is prob' common. See where it is connected to in furnace

Dec 12, 2006, 09:29 PM
blue wire is prob' common. see where it is connected to in furnace

OK, learned a couple things. Big things.

1. The original thermostat had five wires coming in, two that were for running a real box of a clock. Those two wires turn out not to be connected at the other end.
2. Going on the fact that there's juice sparking if you touch B to W, I disconnected RH and connected B. The thermostat now functions, turning the furnace on/off based on programmed temperature.

According to the wiring diagram, and my visual inspection confirms it, The wires run as follows:

1. 110v/60cycle power to disconnect switch (two wires - 70 year old house that I'm rewiring with grounding wires)
2. switch to valve transformer (two wires)
3. transformer to V15-5 gas valve (two wires, connect at T & R)
4. V15-5 AV to pilot safety (X and W wires)
V15-5 AV to L170A Aquastat (Three wires, R B & W)
5. L170A AS to TM850A Thermostat (Three wires, R B & W)

I should add that the TM850A install paper says it can be installed two or three wired. The two wire config is with B and W. Since W was already labeled, I kept it and swapped B for RH.

But here's the new question. There's a circulating pump (that the Aquastat is for, judging from the position it's in). I'm wondering if the RH should be connected to the same place as the B.

Thanks for the help!