Furnace fan won't switch on--unpredictable intermittent problem
1. We have a Rheem central furnace (with AC) circa 1995. It worked w/o a hitch until about 2008. On a cold, wet morning the furnace was not working when we got up.
2. Since then this same problem has recurred repeatedly--maybe a dozen times.
3. We had our HVAC licensed contractor come out. (He'd been our go-to guy for about 35 years. Honest guy who really knew his stuff; he's passed on now, so I can't pick his brain anymore.) First time out he replaced the pilot module, Honeywell S8610U 1003. Afterwards, furnace worked fine for about a year, then same thing. He came out again, did a bunch of tests, and replaced the same part again PLUS replaced the gas control valve VR8304 M 3509. The retail on these parts is a little spendy. I think the combined cost of the two service calls was about $600, almost all parts.
4. Since then the SAME THING has happened several more times. The 2nd time he replaced the pilot module (circuit board with chips, diodes, etc.) I asked him about getting a new furnace. He said this one should have lots of life left in it. Still, it's become undependable, and I hate to just keep throwing money at it. (He did A BUNCH of tests, specific electrical tests on the pilot module. He even did some kind of pressure test with a long glass tube with water hooked up to the incoming gas line somewhere. He said it was a little low but okay.) I specifically asked him about the fan limit switch (part #L4064A 2030) since that's about the only part, at least in the vicinity, that he hadn't replaced; he said that part was fine.
5. This just happened again yesterday morning. As always I switched the house thermostat to off, turned the power to the unit off, and took the panel off to get to where these parts are housed in the unit. It was DRY as always, no water in there, but RH outdoors including in there was probably 100% or more. It's been raining hard. Yesterday for the first time, I took the fan limit control switch out (with an 8" thermostat "finger") and looked at it. When I reinstalled the fan limit switch, before I closed back up the panel, I adjusted the on setting down a hair from 150 F to about 140 F.) (The lower off limit is set at 100F and the higher off limit 200F.) The furnace AND FAN fired up and have been working okay; this happened also around Thanksgiving, but before that it had been about a year or more. 'Course, in this country, we only use the furnace about 7 months of the year. This may work fine for another week, another month, or until next fall. It's UNPREDICTABLE.
5. A couple other points. The problem has NEVER been with spark or the burners firing. Burners fire with a nice blue flame and continue doing so at short intervals, but the burners shut down (I'm guessing because it reaches 200 F in there quickly because the fan hasn't turned on in time to blow the heat out of there into the house).
6. CONCLUSION: I'd be grateful if anyone has any ideas what to try. And I have a couple specific questions.
A. The fan limit switch appears to be an all-manual, non-electronic device, BUT it is hooked into the pilot module. Am I correct in thinking that the fan limit switch is only a relay switch that triggers an electronic switch in the pilot module to actually turn the fan on? Is it likely that the pilot module could have gone out a THIRD time in only a few years? The original one had lasted about 12 years, IF that was ever even the problem to start with.
B. What's the best brand or what are the best BRANDS if we end up just replacing this furnace?
C. If I go ahead and replace the pilot module a third time, would anyone recommend a different brand of pilot module? I noticed there's an ICM brand made in USA. The Honeywell is made in Mexico. Also, I see both the ICM and the new Honeywell (S8610U 3009 replaces 1003) have LED lights that blink to give trouble codes (sort of like newer garage-door openers?) Would THIS maybe help diagnose my problem?
*** I'd be grateful for any input and ideas! ***
Ok i think i got it this time.......the fan is not coming on when it should? Can you post a picture of your fan/limit control?
If the fan is really not coming on soon enough then the fan/limit would cut the power to the gas valve circuit. If the electrical connections within the Fan/limit are bad enough then furnace will also not operate. It's an unusual type of intermitant problem with that type of fan /limit control but not impossible. It's also just a $70.00 part.
But it may also be responding to an overheating problem from any number of other causes. When the furnace has been running steadily for 15 minutes, what does the temp dial on the fan/limit control read. How close does it get to the 200F limit?
Some of the more common reasons for overheating are dirty air filter, dirty a/c coil/ dirty fan cage/too many heat registers turned down, something in front of the return air grill, too slow a fan speed set up, capaciter problem, overheating blower motor, overgassed gas valve ( the water filled tube was testing for that),a cracked exchanger blowing on the fan/limit probe,
I would do a temp rise test first on the furnace just to see what's what.
Look for a 1/8th hole in the furnace plenum just beyond the direct sight line of the heatexchanger. It may be covered with tape. If you can find it, make a hole of your own and stick in a thermometer when the furnace has been running for a while. Subtract that measurement from the house air temp and compare it the rating plate inside your furnace that lists the recommended temp rise for your furnace.
Let us know what you find.
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