No Heat AGAIN!!!

[Broken Cold]

Okay I've had many service calls on this furnace, which was installed about 7 years ago.  Noone seems to be able to fix it.  I have a Comfortmaker furnace, the model number is C9MPV075.  When the service man gets here he said the furnace has a flash code, which means the pressure switch is stuck closed.  He is always able to get things going again, but it usually konks out again on the coldest night!  Any ideas how to solve this?

[Hgye]

With that little bit of information it could be anything.  Do you mean the pressure switch is stuck open?  Check to see that your venting outside is free and clear of ice and snow.

[Admin]

Sounds like a 2 flash lockout - Low pressure switch closed when it should be open.

First thing to check is if your furnace has a white ptrap.  It will have two 5/8" black drain tubes connected to it.  See if there is a third 1/4" black hose that connects to the middle of the ptrap, between the two 5/8" tubes.  Tell the technician to order the three port ptrap kit, if there is no 1/4" hose connected to the ptrap.

Next see if there is one or two pressure switches in your furnace.  If there is two, remove the lower pressure switch, and disconnect the black tubing that connects to it.  Look behind the nipple on the pressure switch, where the black tubing connected.  There is a tiny pin hole located there, that is most likely full of corrosion.  Take a pin and clean that hole out.  If you only have one pressure switch, do the same to it.  The tiny hole is sometimes hard to see if it is plugged.  It is there though.

You're having a drainage problem that is causing a vacuum and holding that pressure switch in the closed position.  Once you clean that hole, pressure will force the system to drain and the switch will open.  Also installing the three port ptrap will help the system drain.  The older models didn't have it, but the new ones do.

I would also have him verify the exhaust venting arrangement is correct.  Alot of installers seem to have problems with this model.  A black 2" rubber coupling with a yellow plug should be on the ventor motor, with the arrow facing a certain way.  Refer to the installation manual, pages 14 - 18.  Download the manual - Here

[Hgye]

Admin,

Is the pinhole common to that furnace only, or is it on all pressure switches?

[Admin]

I have seen this air hole on Goodman furnaces as well, but only find the problem on ICP furnaces.

[Hgye]

Thanks Admin, you taught me something today!  I went to work and checked out the pressure switches for a Carrier and the low fire switch had the pin hole.