TL;DR
A MAU that cannot meet discharge temperature on cold days usually has a gas supply issue, a failed igniter, or a fouled heat exchanger. Check gas pressure at the unit first before opening the burner section.
What you might see
- supply air temperature below setpoint on cold days
- indoor temperature not reaching setpoint despite unit running
- gas valve position at full open but discharge temperature not meeting design
- draft inducer running but no ignition
Likely causes
Gas supply pressure below the unit design minimum, especially during peak demand with multiple heating appliances running
Failed igniter or flame sensor preventing the burner from lighting reliably
Fouled or cracked heat exchanger reducing heat transfer efficiency
Dirty air filters increasing static pressure and reducing airflow across the heat exchanger
Required tools
- Manometer for gas pressure check
- Replacement air filter
- Replacement igniter and flame sensor (unit-specific part numbers)
- BAS or unit controller access
Safety first
- Gas work must be performed by a licensed technician where required by local code. Confirm gas supply is isolated before opening any gas connections.
- A cracked heat exchanger allows combustion gases into the supply air stream. Shut down the unit immediately if a crack is found and do not return to service until the heat exchanger is replaced.
- MAUs are often installed on rooftops. Follow fall protection procedures when accessing the unit.
Procedure
- 1
Check the supply air temperature trend on the unit controller or BAS. Confirm the unit is calling for heat at full capacity.[1]
- 2
Measure the gas supply pressure at the unit inlet with a manometer. Compare the reading to the minimum gas pressure specification on the unit nameplate.
Warning: Gas pressure measurement requires a qualified technician. All gas work must comply with local codes and be performed by a licensed gas fitter where required. Never open gas connections without first shutting off the upstream gas valve. - 3
Inspect the burner sight glass or use the HMI diagnostic screen to confirm ignition and sustained flame on a heating call.
- 4
Check the filter differential pressure. Dirty filters above the maximum pressure-drop specification reduce airflow and cause over-temperature limit trips.
- 5
If the unit is tripping on high temperature limit, inspect the heat exchanger for scaling or physical damage. A cracked heat exchanger is a combustion gas in-leakage safety concern and requires immediate unit shutdown.
- 6
Replace the igniter and flame sensor if the unit is failing to light on a cold start and the gas pressure is confirmed good.
- 7
Return the unit to service after repairs and verify discharge temperature reaches setpoint at the design outdoor temperature.
Sources
Greenheck Greenheck RV / RVE / IGX Make-Up Air Unit (MAU) general technical documentation, Greenheck
Make-up air unit heating system troubleshooting, gas burner and heat exchanger maintenance, general HVAC references (general)
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