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airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 8327
Experience:  Furnace, A/C and heat pump specialist.
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Honeywell thermostat has a blank display. it does not have

Customer Question

Honeywell thermostat has a blank display. it does not have batteries and is connected directly. THe AC is not turned on.
Submitted: 1 year ago.
Category: HVAC
Expert:  airheatman replied 1 year ago.

If you have a volt meter check for 24v ac between R and C terminals

If no voltage look in furnace or handler for a small fuse that may be blown

Customer: replied 1 year ago.
I don't have a volt meter. What are R and C terminals? Where is the fuse located (Handler - what and where is it?)
Expert:  airheatman replied 1 year ago.

Giv me model on your furnace and I will tell you where to look for fuse.

Customer: replied 1 year ago.
Expert:  airheatman replied 1 year ago.

Can you give me complete model from inside furnace in burner compartment so I can provide guidance with this specific unit.Model will be on a label just above burners on right or left inside panel

Customer: replied 1 year ago.
GMS8, GHS8, GDS8, Gas fired warm air furnace single stage
Expert:  airheatman replied 1 year ago.

Excellent.Remove blower (bottom) panel and look inside for a control board. Check to see if fuse may be blown

Customer: replied 1 year ago.
what does the bottom blower panel look like?
Customer: replied 1 year ago.
do I need to shut any power to prevent injury or can I go ahead and open the panel (which I still need to locate(
Expert:  airheatman replied 1 year ago.

See below

Customer: replied 1 year ago.
got the fuse and it is blown. Where do I get the fuse and what type (It says E on it)
Expert:  airheatman replied 1 year ago.

That s a 3, not an E. It is a 3 amp fuse. You can get one at an auto parts store or hvac supply house for les than a dollar.

Customer: replied 1 year ago.
I put in a new 3 amp fuse, but there was a spark and blew the fuse. I tried total with 3 new fuses. All were blown away. Each time there was a spark when I turned on the switch. What next?? What is wrong?
Expert:  airheatman replied 1 year ago.

WE have to have a dead short somewhere. It may take us several fuses o locate the short.

Below are some of the most likely causes for a short and blown fuse.

1.Short in thermostat

2.Chewed or knicked control wire between the units

3.Relay or component in air handler

4.Wire touching hot gas line in outdoor unit

5.Short in switch or contactor in outdoor unit.

Below is a step by step procedure I have typed up to help in finding the short.

Finding a short can be difficult. Easiest way to find is by process of elimination.If you can get several fuses, we can try different things until we no longer blow fuse to locate the short.

1.Disconnect all thermostat and outdoor unit control wires from the terminals on indoor unit control board.Turn thermostat to call cool and see if fuse blows.If it blows, the control board in unit is shorted and will need to be replaced.If not move to step 2.

2.Reconnect all wires previously disconnect at control board. Go to outdoor unit. Disconnect the 2 control wires going to Y and C where they first enter the unit. Turn thermostat to call for ac. If fuse still blows we have a short in those wires between indoor and outdoor units.Could be chewed or skinned, possibly nicked by weed eater, etc. allowing wires to touch and create short.If it does not blow,we know short is inside of the outdoor unit.Move to next step.

3. Reconnect those 2 wires .Then locate the contactor relay in outdoor unit. Remove the small control wires on the contactor coil.Turn on AC.If fuse does not blow, the contactor coil is shorted and contactor will need to be replaced.If fuse still blows we know short is between where wires enter unit and the contactor.Move to next step.

4.Reconnect the contactor wires.Follow the control wires from where they enter unit and see what component may be in that circuit .(Pressure switches, time delay, defrost control board, etc).Follow same procedure of disconnecting wires to each of those controls and see if we find one that seems to cause fuse to blow when in circuit.Also look for any place the small control wires may be touch a hot gas copper line in unit that may have melted sheathing on wire and be allowing copper to ground to pipe.

Let me know what you find and we wlll go from here.