I noticed my heat stops working during the night when the temp drops below 32 degrees. I went top the attic to check it out and noticd the refrigerant was unplugged. Last summer I had a pvc pipe crack and maybe the plumber unplugged it. I'm not sure. Anyway, do you think the refrigerant is the culprit? If not, what could be the problem? It's been in use for only 2 1/2 yrs.

It's unlikely that a disconnected refrigerent line is going to prevent a gas furnace from providing heat.

Take the cover off the burner compartment of the furnace. Obtain the make and model of the furnace off the rating plate in the burner compartment and post it here.

Observe the sequence of events that occurs at the furnace when you turn up the thermostat and post that sequence of events in order and in detail.

Not for sure what your talking about when you say refrigerant unplugged. But if you have a 90% furnace(Vents with PVC pipe) It creates condensation, If it can not drain and move all the water out of the furnace it will lockout. Check for restricted or frozen drain line or faulty condensate pump if you use one.

Good point, T Top.

Kcvab, are you sure the refrigerant line you refer to is a condensate line for an air conditioning coil? If so, it would be upstream from the furnace in the warm air plenum.

Or is it a drain line coming from the furnace, in which case a frozen drain line might well be the issue as T-Top suggests. Condensing furnaces in attics are prone to freezing up unless special steps are taken to protect them.

What kind of outdoor overnight outdoor temperatures are you experiencing, and what kind of temperatures is this furnace in the attic experiencing?

The model number is 90RT07EES01. It reads ICECO on the plate but has a RUUD label on the cover. When the temp outside gets below 30 degrees, the furnace will not fire. Once temp outside gets above 30 degrees, it will work. However, the first 2 years the furnace worked fine in sub 30 degree temps. This is the first time I'm having trouble with heating. The refrigerant I was referring to is actually BBU (back boiling unit). I understand that shouldn't be a factor.

I have two pipes; the one pipe that is insulated with commercial material doesn't connect to furnace. The pvc pipe that is connected to furnace and carries the water from furnace, the pipe that cracked last summer and flooded attic floor, is not insulated. Do you think I should insulate that pvc pipe with blankets or heating tape?

Well, the pressure switch was changed and the drain pipe was flushed, and the furnace is still not working in the morning. The temp outside is 39

The blink code reads pressure switch stuck open. But the switch was just changed. It seemd to only get stuck when the temp is in the 30s.

Actually the blink code reads PS Circuit open

Okay, so the drain on the outside of the furnace was flushed, but was the drain system in the furnace it self cleaned, including the drain trap. Clean all drain hoses and port in the furnace. If that doesn't do it, take a close look at the venting of the furnace. Make sure it is sized correctly, and sloped correctly (should slope to the furnace at 1/4 inch per foot to drain condesate back to furnace, no bellys or sags in the vent, okay? Something is causing the PS to no close correctly. Usually its either a plugged drain system, incorrect or restricted venting, bad draft inducer, faulty pressure switch (not in your case).

Now, here is the hard truth, and don't get me wrong, Im not trying to be insulting in any way, just stating the way I see it. The fact that you thought a refrigerant line had something to do with your system not working correctly, leads me to believe that you really need to call in an hvac tech on this. They really should be able to locate and fix the problem without too much agony. Make sure to have them show up when the furnace is not working, so they can test for themselves what is happening (hard to troubleshoot a working furnace, yes?) We try our best to help out here, but some things we just can't do, we would need to see the system and be able to put our testing equiptment on it first hand. Truly, Im wishing you the best, and please do let us know how things work out.

The tech replaced the idm. Everything seems to be working fine

Thanks for the update, and yep, a weak draft inducer would cause your problem. Glad its working, and hope it stays that way for years to come. Take care.