Trane AUD080 & AUD100

I have two Trane Furnaces. One for 1st floor. One for 2nd floor. The 1st floor furnace, the Trane AUD100 will not go into second stage and stay there. The house is currently 63 degrees. The few times it does go into 2nd stage it goes on for 2 seconds and then drops back to 1st stage. I have checked the flame sensor and the compression valves. Both work. The code light gives a troubleshoot number of 3, which is the compression valves. But the valves work fine. They were tested by an HVAC shop, and I also switched them out with the AUD080, proving they worked fine. What do you think the problem is and what should be my next step? Thanks. RemodelerJay

 

I'm not familiar with the furnace, but you must have a two speed vent blower on the AUD100. It may be that it is not tripping the 2nd stage pressure switch on the higher speed. That could possibly be an obstruction in the vent, or possibly the blower is not switching the the higher speed.

You could get a pressure gage attached at the switch point and watch what happens when the furnace attempts to go into the 2nd stage. If the pressure doesn't change at all, then it may be a blower problem or control board problem. If the pressure changes, but not enough to trip the 2nd stage switch, then it may be an obstruction problem.

If the pressure does not change when it goes to 2nd stage, then you can check your vent blower operation (if you have the same vent blower on the AUD080 and AUD100). Take your vent blower from the AUD080 (which you know is operational) and move it to the AUD100. That should tell you if it is the control board or the blower.

 

Thanks Bentlerf. Both furnace vents were and are clear. I removed both and cleaned them. The 2nd floor furnace worked and works fine. The 1st floor furnace still refuses to stay in 2nd stage.

I have so far removed, cleaned and changed out (one at a time) the following parts from the 1st floor furnace and placed them in the second floor furnace: flame sensor, low compression valve, hi compression valve, control board, vent blower/inducer. All of them worked in the 2nd floor furnace, proving that all the parts are fine.

After changing out blowers the 2nd floor furnace works fine as before. The 1st floor furnace stays in 2nd stage a few seconds longer than with the original blower that is currently properly working in the 2nd floor furnace. I am running out of parts to trade out.

What parts are left for me to trade out and what else should I try?
Thanks, Jay

 

So it sounds like you not only put the 1st floor components in the 2nd floor furnace, you also put the 2nd floor components into the 1st floor furnace. You have been busy.

The only other thing left that I can think of is some kind of air leak into the AUD100 vent from the heat exchanger or vent ducting between the heat exchanger and the vent blower. That is only a remote possibility though, because if it were a sizable leak, it would keep it from operating in stage 1 also.

The AUD100 will have the additional vent channel through the heat exchanger (versus the AUD080) so the vent blower needs to pull 20% more air through the vent in order to reach the negative pressure that trips the stage 2 pressure switch.

It sounds like your vent blower has enough capacity to reach the trip point for the AUD080, but is just not quite doing the job on the AUD100.

A pressure gauge to read the actual pressures at stage 1 and stage 2 on both furnaces would provide additional clues, but I am afraid it will just confirm that your vent pressure on stage 2 on the AUD100 just doesn't reliably trip the 2nd stage pressure switch.

Anybody else have any ideas?

 

Hello bentlerf: As you wrote I switched out from the 1st floor furnace in to the 2nd floor furnace and from the 2nd floor furnace into the first floor furnace the following: flame sensor, low compression valve, hi compression valve, control board, vent blower/inducer.

This proved that all 1st floor furnace parts worked fine as 2nd floor furnace has worked well with every part change out. But still the 1st floor furnace is intermittent on 2nd stage. On last item 1st floor furnace worked for four days then died out, all with 2nd floor furnace parts that worked perfectly. 2nd floor furnace chugging along as always, even with 1st floor parts.

I am now thinking that the problem might be deterioration of the very thin fiberglass gasket between the furnace and the blower/inducer. Do you or anyone know the name of this part and where I can get one? I think the deterioration of this gasket over 20 years might be the problem. It might be allowing just that tiny bit of air leakage that is preventing the 1st floor furnace from staying in 2nd stage. Is there a part number or a name? Where can I get another one of these very thin fiberglass gaskets? (If that is the proper word.)
Thanks!