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Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 5648
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Wall thermostat display is gone! How can I make display on?

Customer Question

Wall thermostat display is gone!
How can I make display on ?
Submitted: 3 months ago.
Category: HVAC
Expert:  airheatman replied 3 months ago.

Do you know model on thermostat and has anything happened such as work being performed or electrical power been off?

Customer: replied 3 months ago.
No model # on thermostat, but I found "Model 58STA/STX" on manual booklet.
Customer: replied 3 months ago.
No work performed and electrical power been off.
Expert:  airheatman replied 3 months ago.

OK usually there are two reasons for losing display. If battery operated low batteries, otherwise loss of power from furnace.

Can you do some hands on troubleshooting and do you have a multimeter for checking voltage?

Customer: replied 3 months ago.
It is not battery operated.
How can I retrieve power from furnace?
Expert:  airheatman replied 3 months ago.

The furnace sends 24v to the thermostat.Your furnace has a small led light inside on the control board. Can you look in furnace and see if the red light is on solid?If you have to remove panel to see light it will be necessary to hold door switch in to have power to board.

Customer: replied 3 months ago.
I have no multimeter.
Expert:  airheatman replied 3 months ago.

You will not need a multi meter to look to see if the light is on control board.

The furnace sends 24v to the thermostat.Your furnace has a small led light inside on the control board. Can you look in furnace and see if the red light is on solid?If you have to remove panel to see light it will be necessary to hold door switch in to have power to board.

Customer: replied 3 months ago.
I can see red light is on, but different control board!
Customer: replied 3 months ago.
I see model # as " 58STA070-12".
Thermostat display was on and off from yesterday afternoon.
Expert:  airheatman replied 3 months ago.

Look and see if there is a small auto type fuse on control board. Remove the fuse and see if it is blown

http://i.imgur.com/UxPpnn4.png

http://i.imgur.com/kYe4Epz.png

Customer: replied 3 months ago.
I can not see an auto type fuse!
Expert:  airheatman replied 3 months ago.

You need to remove the panel to get to the board

Customer: replied 3 months ago.
I think the fuse is not blown.
Expert:  airheatman replied 3 months ago.

Fuse looks good. Next lets follow the control wires coming from the furnace board going to the thermostat.See if perhaps the red wire is routed through a drain float switch that may be breaking the circuit.If so lift the float and see if we get display on thermostat.

Customer: replied 3 months ago.
Where is a drain float switch?
Customer: replied 3 months ago.
Do you mean SAFE-T-SWITICH as the photo I sent?

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