No Heat: Pressure switch & Hoses forming traps

Yooperman906

Aire Flo Model# AF90-MPB075U3A

• It has been converted to LP (propane)
• Inducer is installed vertically
• Furnace is direct-vented horizontally through the basement wall
• I seem to have good air flow through the exhaust

On to the issue at hand: we have no heat

Details
When the thermostat calls for heat, nothing happens. I have tried performing the shut-down and start-up sequences as written on the furnace label. When I restart the furnace, it initially displays the "heartbeat" LED code indicating normal operation. Then when I call for heat, the LED code indicates "Pressure switch CLOSED, should be open".

After receiving that code, I followed the shut-down procedure. First I turn down the thermostat, then turn off the gas control switch. When I turn this off, the inducer starts running almost immediately. It runs for 20-30 seconds, then stops. At this point, the LED fault code changes to "Pressure switch OPEN, should be closed".

I am going to try some of the methods found on other posts regarding the pressure switch (e.g. blowing on the hoses, etc.).

Hose Traps
I have been unable to locate a manual for this furnace (if you've got one to share, I'd be tickled pink), but I found one for a similar model that warned about installing the hoses in a manner that would form a trap in the hose. It recommended trimming the hoses to avoid this situation.

From what I can tell, both of the hoses connected to the pressure switch have a loop with a low point that could form a trap. In fact, when I disconnected them the one from the combustion area did have liquid in it. Did the installer make a mistake when putting these in?

I'm tempted to trim them to try and eliminate the potential for traps, but I'm not sure this is possible given the configuration of the pressure switch. It is mounted on the inducer, and the hose connectors point downward. The drainage hose from the combustion chamber (at the top of the furnace) must come down to where the pressure switch is, and then go underneath it and turn upwards to make the connection on the bottom of the pressure switch. Is this installed in the correct fashion?

I'll try to put up a photo later if I can. In lieu of that I made a quick drawing of the furnace.

THANKS IN ADVANCE FOR ANY IDEAS OR ADVICE!

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Yooperman906

I'm trying to figure out how to test the functionality of the pressure switch by sucking on the hose...gonna break out the multimeter next...try to see what's happening electrically...

hvactech126

if you have water in the p/s hoses, then your drain trap is probably plugged or the hoses or other components are plugged. find drain trap and clean. follow tubes connected to drain traps and ensure their connecting components and hoses are clear.

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Yooperman906

Here's a picture of my actual furnace.

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Yooperman906

A little closer view showing what I think is the pressure switch, but I am very confused as to how this has anything to do with air pressure...so maybe I'm wrong...?

Also, you can see the "hose traps". What do you think about these...issue?

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hvactech126

no. do not remount p/s the "traps" actually saved your p/s from being ruined by water. drain trap is the white thing on the left side of furnace toward bottom. remove other end of hoses on trap and blow into them. try to restart furnace. make sure there is no water in p/s hoses

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yuri

CAREFULLY remove the white condensate trap and flush it with bleach and hot water. May be blocking and forcing water into the press switch hose. Had to do that procedure on a couple of them.

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serviceermanman

NOTHNG WRONG WITH THE TRAPS LEAVE THE DOORS OFF TURN HEAT ON GO BACK TO THE UNIT TAKE ORANGE WIRES OFF THE PRESSURE SWITCH ONLY IF YOU CONFIDENT COMBUSTION MOTOR HAS GOOD AIRFLOW PRESS DOORSWITCH COMBUSTION MOTOR WILL START THEN PUT LOW VOLT WIRES ORANGE WIRES TOGETHER SEE IF IT STARTS UP

serviceermanman

IF THE DRAIN IS STOPED UP THER SHOULD BE WATER NOTICIBLY WERE THERE SHOULDENT BE WATER SOUNDS TO ME LIKE U NEED A PAPER CLIP AND CLEAN ALL THE LITTLE TITS THE PRESSUR HOSES MOUNT ON RUN THE TEST I SUGGESTED IF IT WORKS GET A PRESSURE SWITH COULD BE THE CONTROL BOARD TOO.

Yooperman906

I tried bypassing the switch as you suggest, but did not get a different result. I've read elsewhere that the furnace might be somehow sensing the bypass and not allowing it - is this truth or fiction?

Yooperman906

Thanks to all for the ideas, I will try and work on the condensate trap tonight and report back my findings here. In order to remove the condensate trap, I'm think I'm going to have to cut the pvc coming out the bottom, but I'll look a bit more carefully before I get out the pipe cutters.

Would the removal of this trap usually involve the removal of the entire side panel of the furnace? At this time, that is the only alternative to cutting the pvc that I can find, as all the joints are glued rather than threaded.

Thanks again for the ideas, and I'll share my findings...

Yooperman906

Am I blowing back into the pressure switch, or blowing into the condensate trap? Thanks for clarifying!

hvactech126

blow into the trap

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Yooperman906

Update: I was able to blow some liquid out of the condensate trap. However, the furnace still would not light, despite the LED showing "heartbeat" for normal operation.

BIG NEWS: Though something is still wrong, I was able to get the furnace to fire up - here's how:

After I blew out the condensate trap, I switched the gas control to "Off". As noted previously, when I do this the inducer fan kicks on (this is with the thermostat calling for heat). Instead of letting it run and then quit after 30 seconds or so, I turned the gas control switch back to "On" after the inducer started up. At this point, the furnace cycles on normally, burner starts up, blower goes, etc. There seems to be good air flow out through the exhaust, and condensation was coming out the drain as in normal operation.

It seemed like all was well, but...when the thermostat stopped calling for heat, the furnace shuts down as expected. But the LED code for "Pressure switch closed, should be open" comes back and the furnace will not cycle on again when the thermostat calls for heat.

If I follow the same steps above (i.e. turn off gas switch, then turn on when the inducer starts) the furnace will fire up and seem to operate normally until the house comes up to temp and the thermostat stops calling for heat. Then the pressure switch code comes back and here we are again...

Any ideas on these symptoms?

FYI: I meant to say this before, but have forgotten. Just-in-case it's relevant, this furnace worked flawlessly for the entire 2009-10 heating season. We haven't used it all summer long (June - Sept) and the troubles outlined in this post began when I turned the furnace back on in October.

THANKS FOR READING!

Jackofall1

Just a guess but, remove the hose (ambient pressure side) that runs from the PS to the trap and try and cycle the furnace.

My experience is on large industrial burner systems so I can't be certain that this would apply to your switch but, some switches do have a built in resistance that the control board must see to provide protection against a jumper being installed.

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