Original message
| sigmawave
 |  "Problem with HONEYWELL RTH7400 thermostat."
, posted Sun 2 Nov 06:40  
I bought a RTH7400 thermostat to replace the old manul one.
My heating system is a very old oil furnace.
After I set up the temperature on the thermostat, For example 20 degrees in the evening.
The furnace starts to work and mantain this temperature. In maitaining period, thermostat can start the furnace to work for a short time. I understand the thermostat finds the temperature is dropping down. But the problem is:
Because the fan system can not be started after the furnace starts. There is a pre-warmming time (5-6 minutes). The thermostat stops the furnace before the fan system works. So every time the thermostat let the furnace start to work and stop it before the hot air comes out.
How can fix this problem. It wasting the oil.
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Replies:
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| Houston204
| "Re(1):Problem with HONEYWELL RTH7400 thermost" , posted Sun 2 Nov 09:06
You can configure your stat for electric heat (function 180 = 1) untill you can have your unrelated furnace problem repaired.
The problem sounds like a faulty fan limit or camstat.
This is a fan limit pic...
"Tip of the Day" Remember to remove power first.
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| | sigmawave
| "Re(2):Problem with HONEYWELL RTH7400 thermost" , posted Mon 3 Nov 19:05
The technician from ULTRAMAR investigated the oil furnace. I was told the furnace is in good condition. No problem at all.
He believes the problem comes from THERMOSTAT.
I am wondering how can I change the sensitivity of the thermostat. For example:
If I set the temperature is 18 degrees. I just want the thermostat let the furnace work when the temperature drops down to 17 degree.
Certainly, I haven't found the solution for this issue.
Any suggestion will be appreciated.
Thanks!
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| | sigmawave
| "Re(2):Problem with HONEYWELL RTH7400 thermost" , posted Sun 2 Nov 15:55
thank you
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| | Houston204
| "Re(3):Problem with HONEYWELL RTH7400 thermost" , posted Mon 3 Nov 19:56
The furnace initiates fan operation in heat mode, usually in a lower speed. Since the stat does not normally control fan operation for heating, it seems unlikley that it is failing to bring in the fan before you hit your high limit setpoint.
You can address this by configuring the stat to energize G (fan) on demand for W (heat). This will blow cold air upon startup and will probably run in a higher speed than intended for heat mode. Whatever control normally energizing your lower speed is probably failing.
If your old stat energized the fan immediately with heat demand, set this one to do the same (180 = 1). Though this would cause me to suspect that something has been bypassed, I'm not there and have not seen your furnace.
"Tip of the Day" Remember to remove power first.
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| | sigmawave
| "Re(4):Problem with HONEYWELL RTH7400 thermost" , posted Tue 4 Nov 20:03
Thank you a lot!
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| | sigmawave
| "Re(4):Problem with HONEYWELL RTH7400 thermost" , posted Tue 4 Nov 20:03
Thank you a lot!
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