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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22627
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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Furnace blower keeps running with thermostat off

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Carrier gas furnace. The blower runs continuously even with the thermostat turned off or disconnected from the unit. I have even unplugged the blower from the board,and the relay still clicks to turn it on.

Does the heat function still work just the blower wont shut off?

The Carrier line had a board that a relay stuck on..but one of the safety features makes the same symptom. So does the unit heat?

Customer: replied 7 years ago.
No the burner wont come on because the fan is running.No voltage to the gas valve at all.I've tried turning the thermostat OFF,and disconnecting it from the system and the fan still runs as soon as you push the safety button on the cabinet in.

Ok, so it is on safety...was/is the filter plugged?

Customer: replied 7 years ago.
No filter is new.Unit was working fine a week ago,then I noticed the fan running constantly,the thermostat is a Honeywell programable digital and it shows that it wants to light the burner but I know it can't unless the blower is not running.And even with the blower unplugged and off line,the relay for it clicks in soon as power is restored.

The 2 main things are fusible link bad or furnace on limit. I know you said the furnace is new. But was it when this problem started.

There are 2 different directions to take and try not to waste your time. Was the filter new when the problem started?

Customer: replied 7 years ago.

The furnace is not new.It is 22 years old.

The filter was not brand new,about 2 weeks old,but clean.

Thank you,

see page 11 here -

The fusible link is a soft solder fuse in the wire that is above the burner. If you get flame rollout from a bad exchanger it melts the inside and cuts power. That is one choice....the other is an open limit, but if filter was kept clean it is unlikely. The limit can be tested by removing the 2 wires from the flat rectangle plate behind the gas valve and touching the wires together thus bypassing the switch.

The final choice if the previous 2 items are ok is a bad circuit board.

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