Lennox 80UHG Pressure Switch Problem?

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Old 09-21-09, 08:09 AM
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Lennox 80UHG Pressure Switch Problem?

Hi,

I think I know what my problem is but I just wanted to get a second opinion.

My furnace is about 10 years old and has mostly worked the last few years without problem. I did have to clean the flame sensor a couple years ago.

Anyway, I'm now getting a pressure switch problem (led 1 off, led 2 slowly flashing).

I have removed the tubing and cleaned everything. There's no water or anything.

When I lightly suck on the pressure switch I can hear it open (or is that close?) but once I let the air escape I hear a light click. I think the switch is closing. Is that expected?

Anyway, here's what happens when I fire up the furnace.

As far as I can tell everything is working as normal. The air blower starts up, the pilot starts, then all the burner start up, the blower kicks in and I get heat for about 2.5 minutes. I think that's significant because the troubleshooting mentions 2.5 minutes as being the limit for a pressure switch not working.

The burners go out, I hear a click on the switch I think, and then the fan and blower go out after the heat dissapates. Finally the leds changes from 2 quick flashes to the off, slow flash.

I've open the blower and it looks good. I've cleaned the fire sensor and I've checked the venting. All wiring looks good with no rust or loose ones.

Is this a pressure switch problem? Are they easy to install?

PS I have a roaming multi meter should I check the switch? How would I do this? Does it has to be running?

Any help would be appreciated.

Thanks

Jason
 
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  #2  
Old 09-21-09, 04:21 PM
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I removed the clips on the pressure switch and used the multimeter while the blower was engaged. I didn't get an infinity reading so I guess that means the pressure switch is working???

I'm stumped.

The furnace has worked for almost 8 minutes sometimes and only 3 minutes other times so I'm not sure what the problem is.

I guess I'll have to get a pro to diagnose it.
 
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Old 09-21-09, 04:49 PM
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You can't ohms test while 24 volts ac is passing through the switch.

If you want to quickly find out if the pressure switch or related problem with the way the furnace takes in combustion air and then vents it out - you can remove the wires on the pressure switch and connect them together, so that 24 volts continues on it's way.

!!This is for testing purposes only. Your life can depend on that pressure switch because the reason they have it is to help protect you agaisnt possible carbon monoxide poisoning in case the furance is blocked up or something.!!

Report back with your results and we can help hone in on the actual problem.

..............................................................................

You can do a voltage test however on those pressure switch terminals. One terminal will be activated by starting the funace. The other will remain dead until the blower runs and applies vacuum to the pressure switch. Then both terminals are live with 24 volts ac. To test, find the incoming wire/terminal. That one will not be the issue, I am sure. It will be the other one that the pressure switch activates. So set your meter to same volts you do for 120 volts, and put red probe to the outgoing pressure switch terminal and the black probe to any good metal on the furnace. Then see when the flame goes out that the 24 volts goes out also, while the ventor blower is still running(which it should be, as long as the thermostat keeps calling for heat).

If you have a 3-wire pressure switch, then you would test between NO and metal, once ventor blower comes up to speed, and see if that goes out.
 

Last edited by ecman51; 09-21-09 at 05:05 PM. Reason: added more below dots
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Old 09-26-09, 05:55 AM
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I had a pro come in for a service call and here's what he found:

The combustion air inducer(blower) appears to be the culprit. During operation at the location where the two wires connect to it the heat appears to be causing the connection to expand and it cuts in and out. After 5 times the pressure switch shuts everything down and I get a pressure switch code.

You can wiggle the connection loose just by lightly pulling down or up or the appropriate wire to move it away from the other. To be clear, it's not the wires, it's at the plug in part.

Two questions. Is this a security feature? Is it possibly expanding because it's getting too hot? The blower gets too hot to touch but I suspect this is normal?

If I can tie the wires in firmly what material should I use? Is there fire proof string or tape?

I'm thinking of using rotisserie elastic string. I'm not sure if it will work of course.

Lastly, anyone know where I can buy this part?

J238-087-8171
120 V 60Hz 1.3A
T.P.L. B#88K8401
T.P. 3000 R.P.M.

This is on the power part of the blower. I should be able to install this myself right?
 
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Old 09-26-09, 04:29 PM
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At this connector that you wiggle, does it have male pins inside the male jack portion, that plug into the female jack portion? If so, these pins are usually split, and can be spread some, with a sharp knife tip, to make better contact. I have had to do that to regain lost power. Sometimes you can actually see the pin that looks smaller diameter than the others next to it.

Often you can't buy the blower motor separate from the fan assembly, even though it is apparent one bolts to the other, believe it or not. I think the reason is they do not want people tampering with the gaskets and fan blade alignment/centering/ locking clip, and who knows what else, as perhaps they figure that liabilty could fall back on the mfg.'er.
 

Last edited by ecman51; 09-26-09 at 04:40 PM. Reason: added more
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Old 09-27-09, 05:05 AM
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Inside the connector is a male pin. It's not split. The wires have female pins.

The wires and connectors are solid. It's the connector base, it actually moves apart (up for the top plug and down for the bottom plug) very easily. This causes the inducer to flicker.

I'm wondering if this is a safety feature. Or if I should try to clamp down on the plug so that hopefully the connection will stay.
 

Last edited by lonehanded; 09-27-09 at 05:29 AM.
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Old 09-27-09, 05:39 AM
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I've attached a clamp to the draft inducer blower at the connector plug. It's working for the short term.

Is this dangerous? Should I just buy anothe draft inducer blower?

Thanks
 
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Old 09-27-09, 11:25 AM
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Originally Posted by lonehanded View Post
It's the connector base, it actually moves apart (up for the top plug and down for the bottom plug) very easily. This causes the inducer to flicker.

I'm wondering if this is a safety feature. Or if I should try to clamp down on the plug so that hopefully the connection will stay.
It moves apart from what? The case of the motor itself?

I have never heard of any electrical connector that has to come loose at times for some safety reason. I can assure you that should not be the case. I have seen plug in jacks right at the entry to a motor, that you can jiggle some, and yet there are no repurcussions from this. I have that on condensor fan motors that I have to deal with on large commercial air handlers. If you can, you might send us a close up pic of this.
 
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Old 09-27-09, 11:37 AM
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Originally Posted by lonehanded View Post
I've attached a clamp to the draft inducer blower at the connector plug. It's working for the short term.

Is this dangerous? Should I just buy anothe draft inducer blower?

Thanks
It's hard on the system since the inducer is supposed to stay on to keep the flame going, and not have to have the control board keep retrying to ignite the furnace redundantly. There will be some minor arcing perhaps at that connector also, that you might not even be able to see.

I cannot really say much more than this as I am not sure about your description of the design and location of that plug/connector.
 
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Old 09-27-09, 01:20 PM
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Here's a pic of the inducer draft blower normally.



And here's a pic of me pulling down on the wire to break the connection. NOTE: I usually pull down on the plug itself but then you wouldn't see the gap in the picture because my hand would be blocking it in the shot.



It's very subtle. Using a clamp seems to keep the connection working.
 
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Old 10-03-09, 05:41 AM
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I installed a new inducer blower. The furnace is working perfectly (touch wood).

Thanks for everyone's help.
 
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Old 01-09-12, 12:40 PM
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Removal of Combustion Air Blower from Heater

Lonehanded,
I was looking at your threads and I'm having the same problem with my heater. How did you remove the combustion air blower from the heater? Did you have to remove the rectangle flue box that the blower is connected to? I found 3 screws with nuts attached on the blower and removed the nuts. I can wiggle the blower but it will not come off and I did not want to force it if I'm not supposed to.

Thanks,
Brian
 
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