Dual Furnace Question

[a-b3]

I have two Carrier 58pav070-12 furnaces that trip the limiter whenever they run together. Resetting the switches will give about 15 mins of runtime before tripping again and throwing error code 31 on both units. If I reset the switches and run one furnace at a time, both run perfectly normal with no issue. I’ve disconnected all the vent pipes to the first 90 and cleaned out any dust/debris and checked for blockage. Two story home with vertical vent pipe out through the roof still has cap intact and I can see it blowing smoke when one furnace is running. When it stops raining, I’m going to take the roof cap off and see if there is any visible obstruction in the vent pipe.

Any one have any other ideas? Just seems odd to me that it is only tripping while both are running. Gas water heater is also connected to the vent pipe and has no issues.

 

[SGKent]

do they share common ducting, electrical etc., or is the vent pipe and gas line the only things they share in common?

 

[a-b3]

Each unit have their own ducting and dedicated power switch on the sides.

 

[SGKent]

reading articles I found thru Google I would be checking the vent system from beginning to end to be sure there are no obstacles such as rust, nests, leaves, etc.. that are partially blocking it. If that fails Google the code and there are several causes that are common. In your case I would start with the vent based on other articles online. I would also make sure there is ample combustion air in the area the furnaces are if you have recently tightened up the house, the units do monitor inlet and outlet pressures if I understand the other articles.

 

[Jim greengo]

Have you checked the filters and A coils to make sure they're clean?

 

[yeldogt]

what is code 31 ? they both go off on 31

Limit ... heat ?

Each furnace has It's own intake and exhaust to the exterior .

no common ducting

 

[a-b3]

what is code 31 ? they both go off on 31

Limit ... heat ?

Each furnace has It's own intake and exhaust to the exterior .

no common ducting

Code is for "Pressure, Draft Safety Switch, Blocked Vent Switch"

Both furnaces share one vertical exhaust vent out through the roof.

 

[Caddybill]

Either not enough combustion make up air with both units running. Or exhaust not large enough for both units to run at same time.

 

[BD1]

Check manual for venting. We always run independent intake and exhaust for EACH furance AND water heater.


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[a-b3]

I should also state that these furnaces have been in place for roughly 15 years with no issues and started tripping the draft safety limiters on Friday evening.

 

[Jim greengo]

Do you have a picture of the install?

 

[Caddybill]

Any changes to the building , like new kitchen or bathroom exhaust?

 

[yeldogt]

There can be confusion with venting.

Years ago when everything was atmospheric the temp of the exhaust going out the flue was great enough to provide a "stack effect" .. pushing the gasses up and out. Like a fireplace. Nothing mechanical.

It was common to see both a gas water heater and furnace tied together. Or two furnace

Enter fuel saving models. Without getting to technical ... they are simple 80% furnaces that use a fan ..and inducer fan to provide negative pressure to the heat exchanger. The fan helps provide proper flow for combustion. They do not pressurize the flue. These can be tied together if property done.

Now -- as you move up the fuel savings tier. The next step becomes units that pressurize the flue .... you can not have these tied together.

My guess is since the unit has the ability to monitor -- it's a pressure system. But -- you have to find out. The fact that it's been working for 15 years .. who knows.




EDIT: That is an 80 inducer model .. so it may be fine to tie together. If it was just one unit it's often the switch or the small tube you see from the chamber being clogged (spiders). With both I would be checking the flue. it's obviously not something you want to play with or bypass. Find and down load the manual -- often you can find the service ones as well.

It can be a failed heat exchanger -- again odd for both at the same time

 

[a-b3]

I believe I am good. There were 2 45 degree bends in the attic before the vent pipe exited through the roof. The one before the exit had a nest of some sort in there. I was able to remove a bunch of debris and the furnaces have been running solid for about 20 mins so far with no issues. Fingers crossed! Also I witnessed a bird fly out of there when I went onto the roof to check the cap. The vent stack will be replaced this year when we do our roof, but in the meantime, I am going to wrap some type of chicken wire or mesh screen around the small openings.

 

[BD1]

Do you get freezing temperatures where you are located ?? Small screen openings will form moisture on it and freeze in winter.
This will shut down furances.


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[a-b3]

Yes, I live in a suburb of Chicago. But, the vent will be replaced before next winter.

 

[acmikee]

I once found a bird in a combustion fan blower wheel. I removed the bird and the returned the new motor and wheel

 

[Jim greengo]

I once found a bird in a combustion fan blower wheel. I removed the bird and the returned the new motor and wheel

I think my record for birds in an inducer motor was 5-6
in 1 motor, years ago.
The time I pulled a piece of flue pipe out of a chimney and came face to face with a barbecued ducks face I'll never forget though!