GLAS
Heat Call Not Reaching Furnace
Thermostat calls for heat but the furnace does not start; no error code is displayed on GLAS.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Loose W wire, Furnace safety lockout, Blown furnace fuse, Failed furnace control board relay
How to Fix / Troubleshooting
Safety warning: Turn off power to the furnace at the breaker before opening panels.
- Step 1 – Verify GLAS is calling for heat: Confirm the thermostat shows a heating demand (heat icon or status).
- Step 2 – Inspect W wiring: Check W at the GLAS wall plate and furnace control board for secure connections.
- Step 3 – Check furnace status: Look for LED error codes on the furnace control board and follow the furnace manual for diagnostics.
- Step 4 – Bypass test: With power on and caution, jumper R to W at the furnace board. If the furnace starts, the issue is between GLAS and the board. If it does not, the furnace itself needs service.
- Step 5 – Call HVAC technician: If the furnace does not respond to an R‑W jumper, contact a professional to diagnose the furnace control board, pressure switches, or ignition system.
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Repair Difficulty
Professional Required
5/5
Required Part
Furnace Control Board or GLAS Main Control Board
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