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Matthew
Matthew, HVAC Technician
Category: HVAC
Satisfied Customers: 1994
Experience:  14 years experience in the HVAC field of service, installation, Manual J & D calculations
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The furnace wont stay on. The themostat goes blank. not blowing

Resolved Question:

The furnace wont stay on. The themostat goes blank. not blowing air.
Submitted: 3 years ago.
Category: HVAC
Expert:  Matthew replied 3 years ago.

Matthew :

Hello, My name is XXXXX XXXXX I will do my best to help you today.

Matthew :

If the thermostat display goes blank then your furnace is losing voltage. Is the thermostat blank all the time or can you get the display to come back on?

Customer :

It comes on when the furnace starts then turns off after it runs about 3 minutes

Customer :

it does not give an error message or anything - just goes blank

Matthew :

How do you get it to reset?

Matthew :

Does it trip the breaker to the furnace every time?

Customer :

no the breaker was only tripped once.

Customer :

I f I turn the emergency switch from off to on the furnace will come back on.

Customer :

run for a few minues, then turn back off. The thermostat turns off when the furnace stops working.

Customer :

The thermostat will turn off when the furnace turns off. It comes back on by itself when the furnace comes back on.

Matthew :

You are losing voltage for some reason on your furnace. Is there a condensate pump next to the furnace that pushes water out of the house?

Matthew :

That model number doesn't pull up anything it looks more like a serial number.

Matthew :

Full Size Image

Customer :

No condensation pump.

Matthew :

Is the furnace in a basement, attic, crawl space? Is it horizontal with a pan under it?

Customer :

mdl # XXXXX think is correct other wise C16-31-FF. Furnace in basement of split level in utility room

Matthew :

c16 is the evaporator coil on top of the furnace.

Matthew :

That's ok. I need to know how the thermostat is wired in. For one reason or another your control board is losing 24v. Find where the wires from the thermostat tie into the control board and see what they have to each terminal and make sure you trace the wires back out and see if they are split by any type of float device or humidifier or anything like that.

Customer :

There is a whole house humidification system tied into the furnance - but havnt used in two years - kept in the off position. will trace wires and let you know.

Matthew :

I think I found your furnace it is a G20Q3 I think

Matthew :

There is a damper prove switch on these furnaces that cut 24v power when they are tripped. Try bypassing this switch by hooking the two wires going to it together. It is located at the end of the burner box hooked to a damper looking baffle that is over the burners.

Customer :

The thermostat is on the main level of split level. Think it is powered by splitter from a plug located below it.

Matthew :

On one end is a little round cylinder that is the damper motor gear and on the other end under a cover plate is the prove switch with two wires hooked to it.

Customer :

if there is a problem with the plug socket would that cause the thermostate to turn off and if the thermostat turns off or lose power would that \make the furnace shut down?

Matthew :

I think you have to take the burner box top off to get to all this.

Matthew :

If this damper isn't opening properly anymore it will set this limit off.

Customer :

k. I bypassed the switch and connected the two wires together. Seems to be working. Will bypassing the switch cause a problem long term? or is it ok to leave as is indefinately

Matthew :

This damper is suppose to open and close based on the temperature build up in the heat exchanger. If it is not operating you need to get a new one or it will cause the high limit to trip because of too much heat build up. Personally i would just open the damper and rig it to stay open then you never have to worry about it. It was a poor design and they don't do this anymore with the new inducer motor direct draft furnaces. It was an early experiment with the natural draft models before the induction motor came about.

Customer :

Thank you so much for your help. sorry for the delay accepting - had a family emergency. Anyway, ended up taking the fan off auto on the thermostat and putting in on position. Seems to be working - keeping the heat on and circulating it throughout house. I think I'm going to replace the control unit for a more permanent fix.

Matthew, HVAC Technician
Category: HVAC
Satisfied Customers: 1994
Experience: 14 years experience in the HVAC field of service, installation, Manual J & D calculations
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