exhaust blower on-off every 5 seconds


Old 12-22-00, 11:28 AM
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A few weeks ago my exhaust blower motor started turning on and off every 5 seconds (no exaggeration). The thermostat (digital Honeywell CT3400) and high efficiency gas furnace (Tempstar DC90) had been working fine through a heating and air conditioning season.

I thought it was the blower motor, so had it replaced but same thing happened. Bypassing the thermostat makes the furnace run fine, so replaced it with a mercury thermostat. Everything ran fine for a few weeks, then I attempted to replace the mercury thermostat with a newer model of my original thermostat (Honeywell CT3500). Same exhaust blower problem.

Here's some clues: only the exhaust blower is cycling on and off, not the circulation blower. This only happens when the system is not calling for heat. When the system calls for heat, everything runs fine without turning on and off until it reaches the set point, at which time the circulation motor cuts off and the on-off behavior of the exhaust blower starts up again.

Not sure if I should include this last clue, but will because it's so strange. If I take the thermostat off and connect red to white wires together, heat is called for and the furnace runs OK. But if I try to measure the voltage across red to white, the exhaust blower starts its on-off thing! Hesitant to include this because I think the multimeter is bad (it doesn't measure correct voltages for other things), but perhaps that's part of the clue.

The answer is not on honeywell.com. Honeywell tech hotline says perhaps install an isolation relay?

Help anyone? Thanks.
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Old 12-23-00, 07:12 AM
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Hi: grb

I'll take a few pot shots at the possible causes for the problem you accurately discribed. But first, let me state the Honeywell tech/rep. you spoke to on the phone, in my opinion, suggested a remedy for the problem but not a solution. May not be the thermostat causing this problem.

Most likely the electronic switch on the main board is defective. Another possibility could be an electrical short somewhere in the low voltage circuit. Other possibilities could be a defective "SAIL" switch, fan on relay, etc.

The fan should not have started up when you tested it for current. Unless the red and white wires were connected by the tester <using a defective tester> as you mentioned or the wires where shorted locally. Trace the red and white wires to the furnace, then try testing for current at the gas valve.

Be advised that you should not jump the wires at the furnace while the thermostat is connected. Doing so will distroy a heat anticipator should the thermostat have one.

You may need to contact the local service agency for the furnace brand and have this problem professionaly corrected. There could be electrical problems within the main controls only an experienced service person can find.
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