This thermostat I have has a mechanical relay. But.... When closed is a closed circuit or dead short, fine that works for me. BUT when it's open it still has 20 ohms and will not shut off a circulator. Why in hell do they make it have 20 ohms when it's open?????? It should be infinity.
Not if you want the anticipator to work.
I suspect that might be the resistance of the "anticipator"... a small built-in heater, which is normally energized when the furnace is running. The heat it generates, helps cancel out the effect of thermal lag (i.e. the room air heats up faster than the bulky thermostat does) and improves the ability of the thermostat to maintain good temperature control.
All of the heat anticipators I've seen, have been in series with the heat contact, not in parallel. The anticipators for cooling are generally across the cool contact. That's the way I've seen it for mechanical thermostats. But, most of the good electronic thermostats have the anticipator function built into the software. So if the thermostat is electronic with a relay driving the output, I'd suspect something is wrong. Some electronic thermostats have triacs in the output, which can have leakages. I've had one that actually slightly hold open the gas valve. Honeywell confirmed that they had a problem with some gas valves playing nice with some of the triac models and sent me a new one, with a relay output.
I put voltage across the relay contacts that were supposed to be open and fried the electronics that was holding it partially closed. Everything else still works and the relay still clicks. Now I can use the contacts like a switch like I wanted in the first place.
Well...that's one way to fix it.
Jeff
The Bill Turner way. :(
There was probably a contact protection network across the relay contact that got partially destroyed. There are lots of possible protection networks including varistors, diodes, RC or RLC, etc. You say it measured 20 ohms. Was that done with a standard ohmmeter? And if so, if you reverse the meter's polarity, what does it read. Well, I know you can't do that because it was smoked. Of course, you are not supposed to put raw power across the contact, even if it is open.
That's ok. The contacts will probably weld themselves together before long, and he'll have to replace it.
It would not shut off the circulator so current still flowed. It could have been 200 ohms instead of 20. It was useless that way.
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