Shouldn't burner come on under these conditions?

This is not a diagnosis question (but if you happen to know the answer to the problem, I'll take it) - this is a question about how my system should operate under normal conditions.

I have a Nordyne MH furnace on LP with a heat pump.

Outside temp was 35F and the indoor temp was 68F. I ran the setpoint up to five degrees above the indoor temp as displayed on the thermostat.

The heat pump came on, not the burner. I was under the impression that a difference of two or three degrees between the setpoint and the inside temp would cause the burner to come on.

I moved the setpoint to 85F. The burner still did not come on.

Switching to Emergency heat caused the burner to come on.

I turned the ODT down to below the outdoor ambient temp and the burner came on as it should.

This system has had a lot of mysterious issues that neither I nor the HVAC guy can definitively diagnose.

My question is simply:

Under normal conditions, should the burner have come on when I moved the setpoint 17 degrees above the indoor temperature?

 

If you have an indoor fossil fuel relay board, I think I'd look there first. If wired properly the burners should have come on above what the set point programed at the stat was.

 

Thanks, tinmantu - I received the replacement fossil fuel kit yesterday and I'll install it in a couple of days - so you may have confirmed what I suspected (plus everything else except the compressor and reversing valve has been replaced).