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new thermostat, now no heat.

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Posted: Wed Nov 4th, 2009 01:20 am

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Wip
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Installed a setback thermostat (robertshaw) on a Nordyne high efficiency gas furnace. set it to heat and the induced draft fan starts, runs for about 30 sec then kicks off for a couple of seconds and starts again. This could go on all night apparently.

To make a long story short if I jumper from R to W the furnace works properly. Seems the thermostat has some resistance to it. I didn't have my meter to check this but I can't imagine anything else causing this to happen.

I plan to add a small relay, have the thermostat pick it up and use contacts from the relay to start the furnace. Strange, though. Anybody run into this kind of thiing?

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Posted: Wed Nov 4th, 2009 03:20 am

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RegUS_PatOff
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Thermostat model number ?

Furnace model number ?

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Posted: Wed Nov 4th, 2009 10:03 pm

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ACtechGUY
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Many furnace control boards have problems with electronic thermostats that switch the control signal electronicly vs. an onboard relay. I have not seen that problem for years , most thermostats these days do have relays due to the above problems.
Also problems occur with "power robbing" thermostats and furnace control boards . If this stat does not use batteries and does not have a 24v common wire It needs resistors installed at furnace to allow it to work properly .

Your relay idea WILL solve the problem though.

Last edited on Wed Nov 4th, 2009 10:06 pm by ACtechGUY

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Posted: Wed Nov 4th, 2009 10:37 pm

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Wip
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I put the relay in and it works now. It was a Robertshaw Digital Model 300-252 that I got on ebay. It has a battery in it. I knew the relay would make it work but just wondered if anyone else had run into this problem.

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Posted: Thu Dec 10th, 2009 06:34 pm

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jb8103
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I did not understand the relay solution so I asked our instructor about it. Here is the schematic he drew for the HVAC class:

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Posted: Thu Dec 10th, 2009 06:36 pm

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jb8103
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ACtechGUY wrote:
Also problems occur with "power robbing" thermostats and furnace control boards . If this stat does not use batteries and does not have a 24v common wire It needs resistors installed at furnace to allow it to work properly .

How would these resistors work?

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Posted: Fri Dec 11th, 2009 03:56 am

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ACtechGUY
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If you have a thermostat that is powered by the system you would have the 24 volts hot and 24 volts common side of the transformer feeding power to the stat.

If You have a power robbing type of thermostat, you only have 24 volts hot feeding power to the thermostat. What the resistor does is allow A tiny amount of the 24 volt common side of the transformer to trickle back to the thermostat powering the thermostat on either the W or Y (depending on mode).
The resistors go between C and W and C and Y at the furnace terminal strip.

Attachment: isorelay123005.jpg (Downloaded 12 times)

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