Furnace will not get up to temp

[Douglas Brown]

I have a fairly new Guardian furnace that will not get up to temp. If the thermostat is set for 65 degrees at night and then tries to get up to 70 degrees in the morning, it can only achieve about a 3-4 degree temp rise. The unit shuts off and on with an error code of 4 red flashes indicating a open fuse OR air movement issue and then goes into a soft lockout around the 3rd attempt. I have tested the high limit switch and it is within voltage drop specs of less than .01 and is in a closed loop once the soft lockout happens. I can reduce the temp on the thermostat below room temp and then above room temp and the unit will kick on again and continue the heat cycle. I have completely removed the filter assure proper airflow while testing to no avail. Any thoughts would be great. I'm leaning towards a control board, but just not sure. PS I also removed and cleaned the heat probe.

 

[Plumber69]

I have a fairly new Guardian furnace that will not get up to temp. If the thermostat is set for 65 degrees at night and then tries to get up to 70 degrees in the morning, it can only achieve about a 3-4 degree temp rise. The unit shuts off and on with an error code of 4 red flashes indicating a open fuse OR air movement issue and then goes into a soft lockout around the 3rd attempt. I have tested the high limit switch and it is within voltage drop specs of less than .01 and is in a closed loop once the soft lockout happens. I can reduce the temp on the thermostat below room temp and then above room temp and the unit will kick on again and continue the heat cycle. I have completely removed the filter assure proper airflow while testing to no avail. Any thoughts would be great. I'm leaning towards a control board, but just not sure. PS I also removed and cleaned the heat probe.

The return air grill isnt covered? or filtered

 

[Sponsor]

 

[Plumber69]

And all your supply vents are open

 

[Plumber69]

Check the fuse to

 

[Dana]

Given the wide array of Guardian furnaces out there, is it TMI to share the model number? (It's a bit like asking to debug what's wrong with your Ford car without any hint other than there's a flashing light on the dashboard.)

If the ducts are way too small for the cfm of the furnace's air handler or there's a bear hibernating on top a flex duct trunk squishing it, a family of raccoons nesting in the return plenum or some other major restriction in the air flow any number of problems like this might occur. Measuring & mapping the static pressure drops on the system might point to something if all of the dead-obvious stuff checks out. A cheapie $100 dual port manometer is probably more than enough instrument for figuring that out.

If yours like 9 out of 10 residential furnaces in service it probably ~3x oversized for the design heat load, with duct sized less about half of what's called for by the air handler. It's somewhat rare that duct impedances alone would create a fault condition (pulling the filter was a good for eliminating that static drop), but not unheard of.