GLAS Intermittent Power Drop or Internal Overcurrent

Thermostat powers on but frequently reboots or restarts by itself, especially when heating or cooling starts.

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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

Weak 24 VAC transformer unable to handle GLAS load, Loose R or C connection at wall plate, Short or intermittent short in thermostat cable, Internal power regulator fault on GLAS main board

How to Fix / Troubleshooting

Safety warning: Turn off power to HVAC equipment at the breaker before tightening or moving wires.

  • Step 1 – Tighten connections: Remove GLAS and firmly tighten R and C terminals at the wall plate. At the furnace board, tighten R and C screws as well.
  • Step 2 – Wiggle test: With GLAS powered, gently wiggle the thermostat on the wall plate and the cable behind the wall. If reboots occur, suspect a loose connector or damaged cable.
  • Step 3 – Measure voltage under load: Use a multimeter to measure R‑C voltage at the wall plate while the system is actively heating or cooling. If voltage dips significantly, the transformer may be undersized or failing.
  • Step 4 – Inspect cable: Look for signs of damage where the cable enters the wall or furnace. Replace if crushed or chewed.
  • Step 5 – Replace transformer or GLAS: If voltage is unstable at the furnace board, have a technician replace the transformer. If voltage is stable but GLAS still reboots, replace the thermostat.
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Repair Difficulty

Hard 4/5

Required Part

HVAC 24 VAC Transformer or GLAS Main Control Board
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