Attic TempStar over temp issue

[casadaurora]

My 7-year-old TempStar forced air gas furnace, installed in the attic, is confused.

This original as-built installation was probably flawed from the start, but it is now causing heating problems.

The furnace is on its side in the attic, and is supposed to heat the upper (2nd) floor living space. All of the heating registers are located in the ceiling of that living space. There are three cold-air returns: one in the MBR, and two in the open hallway overlooking the first floor (the hallway is really a bridge that spans a great-room that is 20-feet high...the great room ceiling is the 2nd floor ceiling), outside the other three BRs; all three returns are also in the ceiling of that living space.

The house floor plan is such that the basement and first floor are heated by another TempStar in the basement.

I suspect that the heat from the basement furnace is rising up in the great-room and infiltrating the cold-air returns for the attic furnace, causing the over-temp limit switches in the blower compartment to cut out. The result is as follows: with the basement furnace on, when the attic furnace thermostat calls for heat, the induction blower comes on, but there is no pilot ignition (or subsequent burner ignition). There is no 24V from the control board to the pressure switch or anywhere else. The temperature inside the blower compartment is about 90F. If I manually turn on the blower (Fan ON), that lowers the blower compartment temperature enough (usually) to allow the limit switches to close and the burner to light and heat to be produced. I can set the Fan back to AUTO at that point and the furnace continues to produce heat. Once the thermostat heat demand is satisfied, the furnace shuts down as it should.

Aside from having the furnace ripped out and installed correctly, is there anything I can do to solve the problem? I'm considering relocating the cold-air returns from in the hallway to inside the individual bedrooms. That should eliminate the heat infiltration and help with the false over-temp situation, shouldn't it?

Any suggestions/solutions would really be appreciated.

Thanks, in advance!

[LazyPup]

I would consider changing the fan control circuit on the attic unit. Set it up for the fan to come on instantly when the thermostat calls for heat and put a 60 second delay sequencer on the igniter circuit. That would cause the fan to come on and purge the hot ambient air before it starts the ignition sequence.

[Hube]

The heat from the lower level will not cause a high limit to kick in if has a proper temperature cut-off point which is normally around 180. Even a 125 degree blast from below would not activate the high limit, unless of course the high limit is faulty. Have it checked out.the 90 temp you say is in the unit (I doubt it)will not in any way effect your burner from firing up on a call from the T stat. All this heat from below is doing to your attic unit is making the t stat get satisfied without the attic unit kicking in. The use one or two ceiling fans (whirly birds) in the upper ceilings will certainly be a worthwhile quick cure. (worth a try)
Other than that, this is what needs to be done;
First, isolate the lower level from the upper level ( the lower level heat is escaping to the upper area and influencing the operation of the upper level T stat.
Secondly, why not get those r/a inlets at a low level instead of the ceiling. In the heat mode all your doing is drawing the heat back into the unit. The cold air remains on the floor.
Thirdly; Another bad installation is the location of the heat grilles. They work best in a low opposite, wall to the return.
IMO, someone sure goofed when it came time to install heat to this home. It would have been best to install a split system /zoned type of layout. But thats too late now as it would be too expensive to rectify any of the duct layout.
As I suggested, do these 3 items that I mentioned and that should make a huge difference.
Keep us posted, and good luck....Hube