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Propane furnace exhaust blower cycles on/off - thermostat NOT calling for heat

Propane furnace exhaust blower cycles on/off - thermostat NOT calling for heat

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Old 02-24-13, 10:12 AM
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Question Propane furnace exhaust blower cycles on/off - thermostat NOT calling for heat

Hi,

The exhaust blower on our furnace has started cycling on/off continuously when the thermostat is NOT calling for heat. If I turn off thermostat, it stops. If I set thermostat to heat, it seems to function normally - i.e., exhaust blower will immediately start running constantly, gas solenoid will trip, furnace will ignite, and main blower will start. But then when heat has been satisfied and room temperature is below thermostat set-point, exhaust blower will once again start cycling on/off every 3 seconds or so.

Just to clarify, this is the small exhaust blower that vents to the outside, not the main furnace blower.

Any ideas? Thanks!
 
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Old 02-24-13, 10:44 AM
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That is called a draft inducer blower.

It should only run when the thermostat calls for heat. You said when you turn the thermostat off it doesn't cycle. How do you turn a thermostat off ? The OFF between heat and cool ?


This problem has appeared here recently and I don't think we were able to determine what was causing it.
 
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Old 02-24-13, 10:54 AM
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Yes, if I physically switch the thermostat to off (between heat and cool) the draft inducer blower stops cycling. But of course, this leaves us without a furnace that functions independently.

The thermostat is digital, made by Hunter. I replaced the batteries yesterday. It is at least 10 years old. The furnace is ~19 years old.

Thanks!
 
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Old 02-24-13, 12:49 PM
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Do you think this could be an issue with the thermostat? I have a multimeter - is there anything I can trouble-shoot?
 
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Old 02-24-13, 12:55 PM
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Sorry--- your comments are too vague to be of any use in identifying the problem.


Describe in detail and in order the sequence of events that occurs when you turn up the thermostat to call for heat.

Describe in detail what happens when you turn the thermostat doesn so that it isn't calling for heat.
 
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Old 02-24-13, 01:02 PM
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S.P. This problem crops up every once in a while. It almost seems like the thermostat is creating just enough of a load to activate the draft inducer blower but not enough to cause the burners to light. I know...sounds crazy.
I was trying to dig up some of the other threads. In the other thread the OP claimed he replaced the thermostat and yet it still had the problem......and on more than one unit too.
 
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Old 02-24-13, 01:14 PM
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Yes, and just to clarify:

When the thermostat is calling for heat, there is no problem whatsoever (as far as I can tell). The furnace, including both blowers, functions normally.

The problem is ONLY when the thermostat is NOT calling for heat. That is when the draft inducing (exhaust) blower cycles on/off every 3 seconds or so. It does this continuously. The only way to quiet it is to turn the thermostat off, and by "off" I mean, slide the heat/cool switch to the middle (off) position.

So, even taking into account the experience of the other poster, do you think replacing the thermostat would be the logical first thing to try before calling a service technician?
 
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Old 02-24-13, 02:02 PM
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Yes......replacing the thermostat would be the logical first step.
 
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Old 02-24-13, 04:01 PM
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Thumbs up

I have replaced the thermostat and I am happy to report that the furnace is now functioning normally. The draft/exhaust blower is no longer cycling every 3 seconds and heat comes on, runs, and shuts off as it should.

Thanks very much for your time today. Greatly appreciated!

-Joe
Middletown, Delaware

(FYI, new thermostat: basic Honeywell programmable, mod. RTHL2310B1008, $25 from Lowes.)
 
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Old 02-24-13, 04:08 PM
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You are welcome

Those are lower end thermostats but they are easy to program and very reliable.

I like them because I can install them and it doesn't take hours to teach the homeowner to program them.
 
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