blower motor turns on and off

[jammer111]

Hi was wondering if any can help.I have a keeprite High-Efficiency Furnace and i have on cool well came home today and its was 21 in the house and the thermostat was set to 22 so the ac shouldnt be on.The furnace motor would turn on run for 10 seconds and shut off then about a minute latter it would do the same thing.So i turned the thermostat to off and the furnace motor turned on ran for 10 seconds and shut off.Anyone have and idea whats wrong?Thanks

[txv]

You might have a power stealing or triac switched thermostat...

Take the t-stat off the wall - if the blower stops cycling, disconnect the G terminal and put the t-stat back on.

If it still cycles with the t-stat off the wall, either there's a very minor short in the control wiring (R-G), or the circuit board is bad.

If disconnecting the G terminal solves the problem, replace the t-stat.

If the problem occurs with the t-stat on the wall and the G wire disconnected, an isolating relay or blead resistor should be installed. (Hopefully TKG26 will see this thread and give more info)

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Power stealing: The t-stat takes it's power from the heating or cooling control circuit, batteries are only for backup.

Battery powered, triac switched: Electronic components are used to switch the system on/off instead of mechanical relays; triacs bleed a little bit of current when off. Does the t-stat click?

Battery powered/transformer common powered, relay switched: T-stat is powered by the furnace's control transformer (minimum one extra wire) or batteries which need to be replaced once a year.

[wgauci]

If it's a Magic Stat, just save yourself some time and change it out.

[jammer111]

as far as i know when when the t stat is off and the blower cycles i dont here a click on the t stat.I though maybe the is a fan switch that might be not working properly but dont know much about this.the t stat is honeywell and it has 2 batteries which i changes also when i remove the front panel of the t stat which just snaps on the fan will still cycle on and off for 10 seconds so i dont think its a t stat problem?

[timmaywpg]

I used to have similar problems in my house. It turned out that on days with a strong North wind, the wind would find it's way into my central exhaust venting.

The central exhaust line has a differential pressure switch in it, which turns on the furnace fan when it detects wind getting into the central exhaust lines.

Picture of differential pressure switch:



If you have a similar setup to mine in your house, that may be the cause of your issues.

I fixed my problem by getting a better exhaust louver.

[jammer111]

Thx i will have a look

[txv]

Originally Posted by timmaywpg

I used to have similar problems in my house. ...

I have never heard of or seen that setup - strange.

In the OP's case, the furnace vents through dedicated PVC or ABS pipe (all high efficiency units do) - no B vent or chimney involved.

[jammer111]

timmaywpg i think it could be what you said differential pressure switch.I have the same set as you central exsuast fan going to the both bathrooms and the furnace motor comes on when running.it also has differential pressure switch going into the duct.Today wasnt windy and it wasnt happening so i think you are correct.Thanks for the help.probably going to leave the louver alone as the stucco all over the screw holes and i think i will make a mess trying to change it

[TKG26]

the power robbing stats where never known for brining on the Main fan motor. The only time we had seen these stats being an issue was on ICP mid efficiency furnaces and it would cause the ventor motor to cycle on and off. The fix was a new stat or a bleed resistor...

Sounds like the OP has figured it out, and its something totally new to me. I have not seen such a setup in the Ottawa area. At least the problem has been solved.