'open high limit' problems with Goodman furnace

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  #1  
Old 12-17-09, 09:30 AM
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'open high limit' problems with Goodman furnace

I have a 2002 5 ton goodman furnace. The problem is that I get very short (2 minute) burner cycles. When the burners go out there are 4 blinking red lights on the control board which indicates 'open high limit' per the troubleshoot references.

When the furnace is working the light is solid red.

So far I have tried the following in the chronological order:

1-tested the RITETEMP thermostat by connecting "R" and "W". The short cycles continued

2-Made sure the flue was not clogged by letting it exhaust directly into the utility room (for a short time) and the short cycle continued

3-Jumpered the high limit switch and the burners stayed on and provided ample heat for the house

4-bought a new (not OEM) high limit switch but the short cycles continued.

5-A HVAC contractor (seemingly not very experienced based on poor diagnostic)suggested the exhaust fan was noisy and not running properly. He changed that but the short cycles continued. now he think he needs to rebuild the return/supply ducting as he feels they are undersized.

Any suggestions? i.e. check control board, add combustion air into the furnace...

thanks
 
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  #2  
Old 12-17-09, 09:39 AM
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Well, 1,2,3 and 5 were all wrong. #4, a bad limit switch was VERY unlikely to be the problem, but at least you were barking at the wrong tree.


The furnace is overheating, which causes the limit switch to shut off the burners. The most likely causes of this are plugged or dirty furnace filters, a return air grill that is covered over or too many warm air vents closed off.

Check those possibilities first.
 
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Old 12-17-09, 10:28 AM
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thank you for your suggestions. The filter is new. The supply air grills are not blocked. All the vents in the house are open.

any other thoughts?
 
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Old 12-17-09, 10:38 AM
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If you have air conditioning, the evaporator coil in the furnace can get plugged. Checking this involves being able to get access to the furnace side of the coil, which may take some work and ingenuity.

Is this a condensing gas furnace, that uses white PVC pipe for venting? If so, the secondary heat exchanger can get plugged up much like the AC evaporator coil.

The furnace fan can get plugged up with dust and dirt and it's efficiency impaired, so that should be inspected as well. Also, check to see that the fan is actually starting and running and coming up to speed.
 
  #5  
Old 12-17-09, 11:00 AM
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I do not have A/C.

The exhaust piping is metal.

How do I test the fan motor? Check for resistance?
 
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Old 12-17-09, 11:12 AM
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The fan should be on and running within a minute or so after the burners turn on. If that's not happening, very likely it's the problem.

Turn the power off and try turning the fan by hand. It might be seized up from lack of oiling, a bad starting capacitor or a failed motor.

If the motor is very hot, it's probably failing to start and shutting off on it's thermal limit switch.
 
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Old 12-17-09, 11:23 AM
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fan starts and runs. I am not sure how to determine if it is "coming up to speed'.

There is no combustion air intake. Only two intakes, both ambient inside air.

There is a metal knock out in the combustion area of the furnace. Would running outside air through some PVC directly into the combustion box help out?
 
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Old 12-17-09, 07:03 PM
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Originally Posted by dakar View Post
...... now he think he needs to rebuild the return/supply ducting as he feels they are undersized.
What is odd about that theory though is why has it worked from 2002 to now?

Is this only an 80% non-condensing furnace?, since you say you have a metal chimney. Then there would be no secondary exchanger to plug.

Any kids that may have put things in ducts?

Does the ductwork itself have damper controls on them? Could someone have fooled with them?

Does a lot of air seem to blow out of registers or not?

I can think of ways to possibly make it not high limit, but that will not answer the question as to what has caused this high limiting to occur. Such as: If you had a multi-speed blower you could put blower on high speed. Or you could run the furnace with the blower door off, to see what it does then. But you still will not know the cause, because you could be both allowing more air and colder air into the furnace.

Have you religiously changed filters through the years when needed without having them get plugged and/or not in there the whole way? -or otherwise can you see in the blower good enough to make sure the cupped squirrel cage fins are not filled with dust?

I wonder how common it is, for the gas pressure by whatever cause to have increased. ??? If you have natural gas, you could time the gas meter and we could calculate the approximate btus, to see if figures just don't jive, if we know your furnaces input btu rating......, since both more air, and colder air, would be entering the furnace.

!!Just so you know, with an open combustion furnace, especially, it is dangerous to rig up the furnace to run with the blower door open, as combustion gases can get drawn into the house blower air!!
 
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Old 12-17-09, 07:10 PM
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Thanks for all the help

THanks folks. The issue has been resolved.

I took apart the a coil on top of the furnace. I do not have a complete AC system as we use the roof mounted swamp cooler. But there is an a coil that was never hooked up.

On the bottom side of the a coil I found 1/4" thick sheet of dust that just peed off in one piece.

The unfortunate thing is the HVAC contractor told me that he checked the inside of the a coil and it was clean. This was right before he gave me an estimate for $1000 to redesign the supply air ducting.

THhank you so much for directing me in the right direction.
 
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Old 12-17-09, 07:39 PM
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Originally Posted by dakar View Post
On the bottom side of the a coil I found 1/4" thick sheet of dust that just peed off in one piece.

The unfortunate thing is the HVAC contractor told me that he checked the inside of the a coil and it was clean. This was right before he gave me an estimate for $1000 to redesign the supply air ducting.

Another victim of a lazy/incompetent/dishonest "repairman."

That's disgusting.


Furnaces shutting off on a high limit is a very common problem, and a "repairman" who can't figure that out ought to be fired ----or at least trained to do an elementary job properly.

Telling you that the coil had been checked and was cleaned suggests he was too lazy to check, and didn't mind being dishonest as well by telling you an important check had been done when it wasn't. Usually that's kind of a pain in the neck to check ---- was that true?


Very sad. I'd ask for my money back on that one, and expect to get it. The outfit that employs the guy ought to know, and they ought to be glad to refund your money. Save the dust mat as evidence.
 
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Old 12-18-09, 07:25 AM
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It is a shame to have to live in a world where one must be skeptical at all times. Even with cars at garages, there may be certain things you request the mechanic to do, and you wonder if they might even bother. Such as taking bearings apart, greasing, and such stuff. Certain things that might be a PITB, yet mean so much in making your car last longer. But what if they did not do it? You going to get the car up on the lift and check yourself? Same with your furnace. Like SP said...........

When I started to take care of these commercial big airhandlers(that used to be under the watch of previous buildings operations directors), I found the large(surface area) condensor fins so clogged, one would never have known that that is what was there (fins, that is). Amazing how people must think mechanical things will just run themselves forever without checking out anything.
 
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Old 12-18-09, 07:30 AM
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A BIG gold star for everybody who helped in this thread. You just "saved" the OP $1000! Beer 4U2

Very sad. I'd ask for my money back on that one, and expect to get it. The outfit that employs the guy ought to know, and they ought to be glad to refund your money. Save the dust mat as evidence.
I agree with this as well.
 
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Old 12-18-09, 10:34 AM
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It is people like this that call themselves service techs but are, at best, a hack that give this industry a bad name. The licensing procedures in this country are no more than money makers for the state. I guess all you need is a pick-up and a tool box to be qualified as a tech. This kind of incompetence pisses me off.
 
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