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View Full Version : Ducane furnace not lighting. Draft inducer or pressure switch?



Dean M
10-04-2010, 12:50 PM
OK, I have narrowed down my problem to the draft inducer motor or the pressure switch. Here is what I have tried:

When I apply suction to the pressure switch the ignitor will glow and the furnace will light! How much suction is needed to operate the pressure switch correctly? Is it very little pressure?

The draft inducer motor starts up and seems to be operating properly but there is very little suction from the pressure switch tube. What is the proper way to check the correct operation (volts?) of the inducer motor?

The mornings are getting cold here in N.M. Thanks

Dean

Dean

Thatguy
10-04-2010, 01:59 PM
How much suction is needed to operate the pressure switch correctly? Is it very little pressure?

0.7" WC, about 0.03 PSI, depending on what switch you have. With

a ruler
a clear flexible hose from a h/w store
some water
and with a drop of hand dishwashing liquid

you can test the switch yourself.

The detergent eliminates the meniscus by reducing surface tension.
http://www.google.com/imgres?imgurl=http://chemsrvr2.fullerton.edu/HES/volume/volume_files/meniscus.gif&imgrefurl=http://chemsrvr2.fullerton.edu/HES/volume/volume.htm&usg=__-R881CRwMokSI1rhELQ6E1UWKjw=&h=173&w=219&sz=2&hl=en&start=0&zoom=1&tbnid=4G4ynV3qfY_zlM:&tbnh=138&tbnw=175&prev=/images%3Fq%3Dmeniscus%2Bwater%26um%3D1%26hl%3Den%2 6safe%3Doff%26client%3Dsafari%26rls%3Den%26biw%3D1 530%26bih%3D738%26tbs%3Disch:1&um=1&itbs=1&iact=hc&vpx=353&vpy=152&dur=139&hovh=138&hovw=175&tx=113&ty=66&ei=rU2qTIHPK4S8lQew_bmxDA&oei=rU2qTIHPK4S8lQew_bmxDA&esq=1&page=1&ndsp=30&ved=1t:429,r:1,s:0

Dean M
10-04-2010, 03:19 PM
Thanks for the info Thatguy.

I will purchase some hose and try the test. If that is succesful and I need a pressure switch can you recomend a web site to purcahse one? The furnace is a Ducane MPGA07583A SN # 6891790229. The switch has no brand on it. The part number on the switch is: 20011140 IS20152-3387. I have had zero luck finding a replacement so far.

Thanks!

Dean

jadnashua
10-04-2010, 04:05 PM
The pressure switch appears to be working if you can force it to work. You should only have to suck on it slightly to close the switch. There isn't much volume in the line to the bladder, so you should not get a continuous stream of air - the switch has a rubber bladder that flexes, that is attached to the switch. If it is split, you might get it to move enough with higher vacuum, but it would continually leak air. I'd look for a split hose, fitting, or possibly a crack in the exhaust line. If the draft inducer blower motor is not turning, or the fan blades have broken, that could cause it as well.

Dean M
10-04-2010, 07:19 PM
The pressure switch appears to be working if you can force it to work. You should only have to suck on it slightly to close the switch. There isn't much volume in the line to the bladder, so you should not get a continuous stream of air - the switch has a rubber bladder that flexes, that is attached to the switch. If it is split, you might get it to move enough with higher vacuum, but it would continually leak air. I'd look for a split hose, fitting, or possibly a crack in the exhaust line. If the draft inducer blower motor is not turning, or the fan blades have broken, that could cause it as well.

Thanks Jim, I took apart the draft inducer, the exhaust pipe and cap and the hose. I then cleaned everything thoroughly and checked the hose for any cracks or splits. There were no obstructions in the vent pipe or the cap. I did have to put heavy suction on the pressure switch to get it to work. The inducer motor is turning freely and seems to be operating properly. Now my problem is finding the pressure switch. I have not been able to locate it locally. I found one on the Internet but it seems high priced at 78.00 plus shipping. I am still searching. Any ideas on where to find one? Thanks again for the info.
I will keep you all posted.

Dean

Dean M
10-06-2010, 05:08 PM
After replacing the pressure switch the furnace ignitor will glow and the burners will kick on. After about a minute the main blower motor fails to start and the unit shuts down. The next thing I checked was the main blower motor which was working fine when I by passed the control board and powered the motor directly. I then tested the Heat terminal on the control board and after the burners lit I waited for approximately 40 seconds plus to see if I had 110 volts...no reading. I then pulled the circuit board and there was a burnt spot where one of the relays was soldered in. Next step is replacing the control board at a very reasonable cost of $59.00 The pressure switch I finally found in town for $29.00 which did need replacing as well.

Hopefully this will take care of the problems. I will post and let everyone know if this works.

Dean

Dean M
10-09-2010, 07:05 PM
Finally a working furnace!!! I replaced the control board which had a bad motor relay. It was burnt right behind the relay where it is soldered in. I ended up paying $49 for a new control board. A very reasonable price. Once replaced the furnace fired right up!

Thanks Estrogen Hostage (first post: Ducane furnace will not light then shuts down ) Thank you Thatguy and Jadnashua for the help and advice. Much appreciated. Hopefully this will help others with similar Ducane furnace problems. Stay warm this winter and thanks again guys.

Dean