I'm a retired electrical engineer and believe me, I've come to learn more about furnaces in the past week than I ever wanted to know before. I could write a book about all I've learned in the last week. I now know more than at least some furnace techs about how to correctly test a differential pressure switch thanks to simply watching some good YouTube videos on the subject. First there was a low spot in the flue exhaust tube and water building up in that low spot that caused the first issue. After we had run the new furnace a total of maybe 10 cycles it stopped working and started putting up Error Code 3, Pressure Error. A tech came out, fixed that issue and the furnace worked (though I didn't watch to see if it ever struggled to start) for a month. But in that same visit that first time, while fixing the low spot in the flue pipe, the tech wiggled the pipe and it broke free (quite easily) from it's attachment point at the furnace flue output. This company glues that connection though I know Trane "recommends" using High Temp RTV for that connection "so you can take it apart later". For a time I wanted them to RTV it but I then relented after they convinced me they've used glue on hundreds if not thousands of installs (this is a big company who does all the furnaces in this large retirement community and lots more as well). But I told them I didn't like the idea of "regluing" a previoiusly glued part. So they agreed to purchase a new flue output pipe for the furnace and it took a month to get that part. When the tech put that on is when all hell broke loose. He also chose to change the flue pipe to get rid of most of any horizontal run by putting an upward 90 almost right at the furnace where before the first 90 was about 14 inches away from the furnace. He did that, the furnace worked and he left. The next morning, however, it wouldn't start. Error Code 3 again. And it wouldn't run but maybe once in 10 tries (Error Code 3, not enough differential pressure) for the next two days. By this time I and one of their techs are up in the attic making manometer readings on the two pressure inputs to the pressure switch. It was also in this time that this tech discovered that Trane does not allow 2 inch flue pipe on this 100,000 BTU furnace (I've since seen that clearly called out in the install manual). And this install had precisely that, two inch pipe. So last Friday, a major undertaking, cause this now involved roofers, was undertaken by the builder (these are new homes still under warranty) to come out of the furnace with a short piece of two inch because the Trane output port is 2 inch and then as soon as possible switched to three inch pipe (2 1/2 is allowed but the furance company decided to go to 3) and ran a new 3 inch pipe up to and out of the roof through a new 3 inch roof jack the roofers installed. All worked great the rest of that day. No more Error Code 3 and we were now getting differential pressure readings of 1.8 inches of water against a pressure switch that closes at 1.4 inches of water. Before we were getting only 1.5 inches of water (which if everything was calibrated should have been good enough) but these manometers looked to be ancient (I'm sure never calibrated) and who knows if a 1.4 pressure switch migh have an error bar of say +/- 0.1 inches of water??? Anyway, we had much better differential and the furnace worked. No more cycling or Error Code 3. But then came Friday night. The furnace ran a few cycles. Then just before going to bed, I chose to want to force it to run one more cycle and then I was going to shut it off for the night (my policy since all this trouble started). I went to the furnace. It was not running but was at the set point. I wanted to hear it run once more so I upped the set point one degree. I waited the 68 +/- 2 seconds I've determined it takes to start and IT DIDN'T START. I raced to the garage, got my ladder, popped the attic cover, watched and listened. It was NOT showing Error Code 3. Instead the LED was flashing rapidly which means "calling for heat". But the inducer fan was doing somethin I had never heard before. It would run up for maybe 5 seconds, you'd hear a click and it would shut down only to immeidately start up again, five more, down, up and so on. Once in a while you'd hear it run for maybe 10 seconds, then cycle again. I even had a neighbor listen with me as I wanted a witness (should have recorded the audio on my iPhone). After letting it do this "new error" for a short time, it finally did start and run one cycle. But this was new and bad. That prompted the furnace company to send out one of their experts the following morning who spent two hours with the furnace. He then finally reported that there is an issue with the seondary heat exchanger that can't get rid of water fast enough. He told me I should be able to run the furnace as long as I don't let it run two cycles close together. Since the furnace was at the setpoint when I did my before bed test, perhaps it had just run minutes before I tried to get it to run again??? But would a good furnace do that??? Sit up in the attic and try and try and try because it had just run some minutes before and needed more time to get rid of its water?? He said water was building up high enough that it was getting into the line going to the pressure switch and causing the symptom I observed. So he sent this long report (that I wasn't shown) back to this company and that info is what is prompting a replacement. I was told that Monday morning this tech "put in a call and was waiting for a call back from Trane tech support for further guidance". Not long after I was told they were going to replace the entire furnace.
So is this unit really bad??? One part of me says no but could a good furnace do what it did Friday night??? Run once, then ask for an almost immediate second run, only to sit and constantly (for a good 20 minutes)(I began to fear burning out the inducer by starting it so often) cycle every 5-10 seconds until it finally starts and runs a successful heat cycle?? If I went up in the attic of other houses, would I hear a "good furnace" occasionally get corrupted by condensate such that it has to cycle a bunch of times before running??? I tend to think not but I will admit, I have many times asked the same question. Are we about to replace a good unit with a new, unknown unit??? And am I stressed about doing this??? VERY... thanks.. bob
