Furnace ignites but cool air comes out of ducts

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Furnace ignites but cool air comes out of ducts

October 23, 2014, 04:57:02 pm

I have a 1986 Grand Villa. When I turn the front furnace on the fan blows cold air through the ducts at a reasonable rate but when the furnace ignites, the air-flow diminishes and only cool air comes out of the ducts. I am 99% certain that there are no blockages in the 3 separate ducts. The fan speed does not appear to change after the furnace ignites.

Any ideas?

Re: Furnace ignites but cool air comes out of ducts

Reply #1 – October 23, 2014, 05:59:25 pm

Marty, my guess is that the furnace ignites but then the flame goes out within a few seconds. It will keep blowing. If the sail switch or any of the other functions do not send the right signal to the control board, the board kills the gas flow. You also may need to get the gas pressure checked. If the gas pressure is incorrect, bad regulator, the gas will shut off after initial ignition.

Re: Furnace ignites but cool air comes out of ducts

Reply #2 – October 23, 2014, 06:46:13 pm

Well diagnosed Kent. Thermocoupler.

Re: Furnace ignites but cool air comes out of ducts

Reply #3 – October 23, 2014, 06:46:34 pm

"Air flow diminishes" is the key here. Less air flow means the sail switch opens, so the PC board does NOT open/or keep open the gas valve.

I would start by checking voltage at the furnace PC board. If 12.5 or more, then look for restrictions in air return or closed vents.

Re: Furnace ignites but cool air comes out of ducts

Reply #4 – October 23, 2014, 08:22:35 pm

Marty ,
Just to be sure it's not a bad connection, unplug and replug the connector on the control board. Do that three or four times to wipe the connections.
Do the same on any other connections you can get to on that furnace.
JD.

Re: Furnace ignites but cool air comes out of ducts

Reply #5 – October 25, 2014, 08:39:27 pm

I had a similar problem with my furnace 4-6 years ago and about drove me crazy and everything I did as a cure, was a failure.
I finally, after a LOT of further investigation found that Mud Dauber wasps had built TWO of their famous nests inside the burner tube. I had found the first one most outboard) but missed the fact that they built another nest AROUND THE CORNER IN THE TUBE, where I was unable to see it without a long handled mirror and a high powered flashlight. Used my battery drill and a piece of cable to tear up the next and then blew it out.
Furnace fired right up after that.
You just never know what you are gonna find inside tubes.
Same thing happened to my air horns.
Regards,

Re: Furnace ignites but cool air comes out of ducts

Reply #6 – October 25, 2014, 08:59:28 pm

One other little trick after checking all of the electrical connections is that if you coach has two furnaces the chances are they both use the same control board. I have taken the control board out of the one that works and put it in the one that isn't working to see if it is the board or not. If the board fixes the problem you know you need a new board. If not you can move on to other problems like the mud dobbers in the burner or a bad or corroded sail switch.

Re: Furnace ignites but cool air comes out of ducts

Reply #7 – October 26, 2014, 10:44:13 am

@Marty since you have an older coach you might want to check for oil in the LP line. See this post for more info.

LP Maintenance

see ya
ken

Re: Furnace ignites but cool air comes out of ducts

Reply #8 – November 01, 2014, 03:03:12 pm

Thank you all for your suggestions. Kent, you are correct in that the furnace ignites for between 3 and 4 seconds and then goes out (the rear furnace and the stove work perfectly). Now I have to try to reach the electrics which are hidden behind 2 tightly-packed duct hoses. I am beginning to wonder if the coach was built around the furnace, lol.

Re: Furnace ignites but cool air comes out of ducts

Reply #9 – November 01, 2014, 03:27:36 pm

Marty, the electronics on my coach's furnaces are on the left hand side of the furnace enclosure. Once you open the outer door they should be in a black or gray plastic box on the left wall. Open the box and take the electronic board out of the back furnace and install it in the front furnace and see if works. If that doesn't fix it then you need to start checking the gas pressure at the furnace. In my 93. U225 there were two gas regulators, one for the front furnace and one for the back. My symptoms were just like yours but it was the back furnace that wouldn't stay lit. It was a bad regulator and nothing to do with the furnace. I found that out by spending over $200 at Camping World for nothing. Then I found a certified Atwood tech at a local RV dealer. He used a water column to check the gas pressure and then fixed it in 20 minutes and a cost of $45 for a new regulator.

Hope this helps.

Re: Furnace ignites but cool air comes out of ducts

Reply #10 – November 02, 2014, 02:32:06 pm

I have checked a few things. With the furnace switched off at the wall thermostat there are 13.2 volts at the circuit-breaker on the furnace. When the blower starts up, the voltage at the circuit-breaker drops to 12.2 volts and the voltage from the sail switch at the relay is 10.4 volts. When the burner ignites, the voltage from the sail switch drops to 9.4 volts. When the burner goes out after ~ 4 seconds, the voltage from the sail switch reverts to 10.4 volts. The burner ignites 3 times whereafter a red light flashes on the circuit board. I cannot actually see the sail switch but I assume that the burner would not ignite at all if it were malfunctioning.

I have not tried switching the circuit boards (from the rear furnace). I gave the rear board a cursory inspection and it looks different to the front one. I shall continue scratching my head.

Re: Furnace ignites but cool air comes out of ducts

Reply #11 – November 02, 2014, 02:42:28 pm

Marty,

That is way too much voltage drop-- assuming voltage at the battery stays steady at 13.2. At most, repeat most, you should loose 1.0 VDC. If more, check connections. Worse case, blow $15 and power the furnace using a relay and fuse with heavier gauge wire from better source of power.

Re: Furnace ignites but cool air comes out of ducts

Reply #12 – November 02, 2014, 03:03:30 pm

Thank you, Brett. I was pretty sure that there should not be such a drop in voltage. I shall clean all the connections that I can get to and see if there is any improvement.

Re: Furnace ignites but cool air comes out of ducts

Reply #13 – November 02, 2014, 03:48:59 pm

Hi Marty,

From the dropping voltages with more furnace parts turning on...it might be a poor ground.Â Some circuit boards donot function well at less than about 11 volts.

Re: Furnace ignites but cool air comes out of ducts

Reply #14 – November 02, 2014, 04:04:12 pm

Absolutely check the ground side as well as positive side. No problem running an additional ground wire to clean bare metal on the frame.

Re: Furnace ignites but cool air comes out of ducts

Reply #15 – November 02, 2014, 05:17:24 pm

SUCCESS! Well it's working now. I removed the plug on the circuit board and as I was fiddling with it, it came apart into 2 separate and distinct pieces. I cleaned it up (although it looked clean) put it back together, plugged it back in, switched the furnace on and - Hey Presto! - it ran and ran.

Thank you all for your kind help. So often it is something simple.

It is darn hot in here!

Re: Furnace ignites but cool air comes out of ducts

Reply #16 – November 02, 2014, 11:20:10 pm

Oh dear. I spoke too soon. The furnace worked fine for several hours then reverted to the same fault. The outside temperature is below 40 so I shall try to check for any oil in the line as Ken suggested.

Re: Furnace ignites but cool air comes out of ducts

Reply #17 – November 03, 2014, 12:28:49 pm

Marty,
If moving the connector on the control board made it work for awhile I would concentrate on that plug and the wires that feed that plug.
Also look carefully at the board where the plug connects to the board.
It's not likely a propane supply problem would fix it's self then fail again.

Re: Furnace ignites but cool air comes out of ducts

Reply #18 – November 03, 2014, 12:53:44 pm

Check voltage again when symptoms reappear.

Re: Furnace ignites but cool air comes out of ducts

Reply #19 – November 22, 2014, 12:54:14 am

I gave some logical thought to my problem. The furnace ignites thus the blower, sail switch, relay and those parts of the CB must be working - for a short time at least. So, I thought, what is required to keep the furnace going. Answer: the heat sensor. So I took it out, cleaned it (although there were no heavy deposits), squeezed the connector on the wire a bit, refitted it and so far the furnace has worked. Fingers and everything else crossed.

Re: Furnace ignites but cool air comes out of ducts

Reply #20 – November 22, 2014, 01:55:44 am

Occam's razor!

Brilliantly applied.

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