TL;DR
A freeze protection lockout almost always means cold outdoor air reached the coil before the burner raised the air temperature. Check the outdoor air damper end-switch and burner ignition sequence before resetting the freeze stat.
What you might see
- freeze protection thermostat or sensor tripped on the unit controller
- unit locked out and not starting on cold mornings
- water or ice visible at the unit drain or base pan
- coil leaving water temperature below 35 degrees F
Likely causes
Outdoor air damper stuck open on unit shutdown, allowing cold air to soak the coil overnight
Burner ignition delay too long relative to the outdoor temperature, allowing cold air to chill the coil before heat is established
Low glycol concentration in the hydronic coil circuit allowing freeze-up during cold ambient temperatures
Freeze protection sensor calibrated incorrectly, giving a false trip at a higher temperature than actual
Required tools
- Freeze protection sensor readout from unit controller
- Refractometer for glycol concentration check (if hydronic coil)
- Flashlight for coil inspection
Safety first
- Never use an open flame to thaw a frozen coil. Hydronic coil freeze-up under pressure can rupture the coil.
- Rooftop access requires fall protection. Follow site fall protection procedures.
- Confirm the unit is in a safe, non-operating state before opening any access panel with the fan potentially freewheeling.
Procedure
- 1
Confirm the unit is in lockout and read the freeze protection sensor temperature from the unit controller. A genuine freeze-up shows a temperature below 32 degrees F.[1]
- 2
Do not reset the unit until you have checked the coil for ice. Open the access panel carefully and inspect the coil face for ice accumulation.
Warning: Do not attempt to force ice off the coil or use an open flame to thaw it. Allow it to melt naturally or use warm water and ensure drain pan is clear. Check that the drain is not frozen. - 3
Once the coil is confirmed to be ice-free, check the outdoor air damper for proper closure when the unit is off. A damper stuck open is the most common cause of freeze-up.
- 4
Test the damper end-switch: with the damper closed, the end-switch should confirm a closed position to the unit controller before the fan is allowed to start.
- 5
If the unit uses a hydronic preheat coil, check the glycol concentration with a refractometer. The glycol percentage must be sufficient for the lowest expected outdoor temperature at the site.
- 6
Reset the freeze protection lockout from the unit controller and run the unit through one complete start sequence. Verify the burner fires and heats the air before the fan ramps to full speed.
Sources
Greenheck Greenheck RV / RVE / IGX Make-Up Air Unit (MAU) general technical documentation, Greenheck
Make-up air unit freeze protection systems, damper and preheat coil troubleshooting, general HVAC references (general)
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