Furnace Won't Respond to Old or New Thermostat


Old 04-05-09, 11:39 AM
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Furnace Won't Respond to Old or New Thermostat

Hi. I have forced hot water baseboard heating. There are 3 levels, each with it's own pump and thermostat. The first and 3rd floors are heating fine. The middle floor has stopped heating. I assumed it was the 10 year old Honeywell programmable thermostat for that floor, and installed a new model Honeywell programmable thermostat. The boiler still doesn't respond to it. When I turn up the first floor thermostat, the boiler comes on within a couple of minutes. Ditto for the 3rd floor thermostat. However, there is no response to the middle floor thermostat at all. I have checked and double checked the connections. it's a 2 wire system, and the red wire is correctly hooked to the RH socket, and the white wire to the W socket. The instructions said to remove a jumper for 2 wire systems, which i did. There is no response from the system. I doubt very strongly that I've had 2 thermostats go bad (old and new), so it must be in the wiring between the thermostat and the boiler. Before I call a service man, I thought I'd check for any advice. Thanks.
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Old 04-05-09, 03:04 PM
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If you have a voltmeter, you can troubleshoot by seeing where you're losing the 24-V control voltage to the pump, etc. When there is not a call for heat from the middle floor, you should measure 24V across the thermostat terminals.
Old 04-05-09, 06:06 PM
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You may have a bad zone relay also...

Depending on how your system is set up, there could be two or three additional relay boxes running the circulator and firing the boiler. The t'stats should be wired to these. You should hear a distinct 'click' when the t'stat for each one is turned up.

As Doug says, a voltmeter and knowing how to use it is pretty much the next step in troubleshooting this type of problem.
Old 04-05-09, 06:43 PM
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Thanks to both of you for your responses. This problem crosses into unfamiliar territory for me, so I'll be calling the serviceman. But thanks again for taking the time to post.

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