Furnace Repair Report August 18, 2014, 11:22:18 pm Yahoo Message Number: 148012On a recent trip to MT, when using the furnace for warm-ups on cold mornings, there began a serious time lag between turning the thermostat to heat and the blower coming on (heater ignition follows). Finally, the furnace would not come on even though I could hear the "click" of the thermostat sending the ignite message to the furnace. After troubleshooting fuse, propane pressure, thermostat batteries/connections, I called a recommended mobile RV tech in the Denver area. Today, after checking nearly every connection and moving part and connecting a new circuit board (not the problem), he checked the sail switch. "micro switch with an arm that extends into the blower air stream which closes when the blower reaches approximately 75% of its rated speed. Also called an air prover switch." New sail switch and it's working fine now. Chris2002 30' IB
Re: Furnace Repair Report Reply #1 – August 19, 2014, 01:10:32 am Yahoo Message Number: 148014I'm glad you were able to get it fixed, Chris, but the explanation you were given is certainly an odd one. The way I understand it (and somebody please correct me if I'm wrong about this), the startup sequence works like this:1. Thermostat contacts close 2. Blower immediately starts running 3. Airflow deflects sail switch 4. Controller board senses sail switch deflection and turns on burnerNow, you said that there was "a serious time lag between turning the thermostat to heat and the blower coming on." But the sail switch is not involved in turning on the blower; it only acts after the blower is running. So it's hard to see how replacing the sail switch would have any effect on the blower starting up. I could be wrong, but I'm going to hazard a guess that the real cause was a bad contact, and the furnace was actually fixed by the various cleanings and re-seatings of connectors before the tech replaced the sail switch. I know how easy it is, when troubleshooting, to forget to retest after each step. That can lead you to think that the last thing you did was the magic fix, when it was actually the next to last thing you did. In fact, the problem may well have been the thermostat, not the furnace. You wrote, "the furnace would not come on even though I could hear the 'click' of the thermostat sending the ignite message to the furnace." With a mechanical thermostat, that click means the two contacts have physically come together. However, I can tell you from experience that it does not necessarily mean a signal is being sent to the furnace. If the contacts in the thermostat are dirty or oxidized, it's possible that not enough current will get through to turn on the blower. It's happened to me several times over the years. The solution is to pull off the thermostat's cover and clean those contacts; the easiest way is to slide a thin strip of emery paper between them. I usually dose them with DeoxiT after that, to stave off future corrosion. I've experienced exactly the symptoms you described--delayed blower startup, and eventually no startup--on several occasions, and each time the problem was those pesky thermostat contacts. Next time this happens, it's worthwhile to clean those first before resorting to more drastic measures.Andy BairdTravels with Andy
Re: Furnace Repair Report Reply #2 – August 19, 2014, 11:41:12 am Yahoo Message Number: 148015Thanks for your comments and analysis, Andy. Regarding the thermostat, I am nearly certain that it was not defective nor was it a contact problem. The first thing I did was check it out by cleaning the contact points, checking for wire tightness and changing the batteries. When that did not cure the ill, I replaced the thermostat with a new one. It did not work either (I returned it). As for the sail switch, the tech commented that the reason he did not check it first was because the failure symptoms did not seem to be attributable to the sail switch. This reinforces your comment about the start-up sequence. However, when he did check it - and I was watching, there was no electrical continuity (I think this is what he said) in the switch which would indicate that it was defective. In any event, when it was replaced, the system worked. This was an education for me because the outside access panel to the furnace had never been taken off and I had no idea what was inside. It's actually pretty easy to access most of the contact points and it didn't appear to be that difficult to pull the circuit board out for checking contact points, cleaning, etc., which he did. I have learned from experts such as you and others on this forum that the first thing to do with any seemingly electrical failure is to clean the contact points in combination with re-seating the wires to them. Often this is a cure and prevents an expensive repair. I posted this somewhat boring experience in hopes that others may learn from it.Chris 1 Likes
Re: Furnace Repair Report Reply #3 – August 19, 2014, 12:07:17 pm Yahoo Message Number: 148016"I posted this somewhat boring experience in hopes that others may learn from it." --- Not boring at all; this type of detailed "symptom/resolution" report adds to the knowledge base! Thanks for posting it.As ever, YMMV.Joan
Re: Furnace Repair Report Reply #4 – August 19, 2014, 03:17:00 pm Yahoo Message Number: 148017"this type of detailed 'symptom/resolution' report adds to the knowledge base!"Absolutely, and I'm grateful to Chris for explaining what happened."The first thing I did was check it out by cleaning the contact points, checking for wire tightness and changing the batteries. When that did not cure the ill, I replaced the thermostat with a new one. It did not work either"OK, you did what I would have done. (I'm curious about your mention of batteries, though. I thought you had a 2003 rig, which would have come with a low-tech mechanical, battery-less thermostat. Sounds as if you must have replaced it with something fancier, yes?)I should have mentioned one other tip for troubleshooting these older mechanical thermostats: short the terminals together.If you remove the case front, you'll see a screw terminal at upper left with red wires, and one at lower left with white wires. Short those two screw terminals together with a paperclip, or something similar, and the furnace should start immediately. (It'll continue to runafter you remove the short, and won't stop for a couple of minutes, so don't panic.) That's an easy way to test whether the thermostat is the problem.The newer high-tech thermostats use cat-5 (Ethernet-type) cable to connect to the furnace, and I don't know how to troubleshoot that type. If anybody has any tips on that, I'd love to hear them!Andy BairdTravels with Andy
Re: Furnace Repair Report Reply #5 – August 19, 2014, 04:20:06 pm Yahoo Message Number: 148019Yes, I did replace the thermostat in '08 on my '02 30'. It has a battery error function/readout. The original batteries functioned until I replaced them preemptively while checking the thermostat out. Chris
Re: Furnace Repair Report Reply #6 – August 19, 2014, 09:42:12 pm Yahoo Message Number: 148020My previous 1998 SOB had a Suburban furnace. The blower started at least 10 seconds after the thermostat contacts closed. I assume that was normal, since it did it for 16 years with two different thermostats (one mechanical, one digital). I don't remember why it had that delay, but I sort of think it was safety related. Our present SOB has an Atwood furnace, and works as you describe. I first thought it was malfunctioning when the fan started immediately! Maybe other brands, other years of furnaces also have start-up delays for the fan?Eric Greenwell [lifewithalazydazerv] wrote, On 8/19/2014 12:17 PM:
Topic: Furnace Repair Report (Read 27 times)
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Topic: Furnace Repair Report (Read 27 times)
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