# Heil DC90 Limit Trip



## JohnnyV (Jan 15, 2012)

_ have a 13 year old Heil DC 90
Last week it was blowing cold air. Called my friend the HVAC Guy and we determined - guessed - it was the control board, $150.00 later it was not.
I suggested testing limit switches & we discovered the limit on the side of the fire box - heat exchanger was tripped, looks like a small transducer, with a reset between the 2 terminals. pushed the reset and all was good. 
He suggested testing for CO with his special tester, all is good - 1ppm or less. the limit switch popped again a few day later, I installed a new switch and a week later that popped as well. Any Ideas - Suggestions?
It is winter the furnace is running fine 98% of the time. 
_


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## harleyrider (Feb 20, 2007)

*when THIS TYPE OF LIMIT TRIPS, ITS ALMOST ALWAYS A SURE SIGN OF PROBLEMS WITH THE HEAT EXCHANGER. THE HEAT EXCHANGER IS WERE THOUSANDS OF BTU'S OF GAS ARE BURNT, A POTENTIALLY DEADLY SITUATION IF LEFT UN-CHECKED.......THE REASON I AM TYPING ALL THIS IN CAPS AND UNDERLINED IS TO DRIVE HOME THE IMPORTANCE OF THE ADVICE I AM ABOUT TO GIVE YOU.......HAVE IT LOOKED AT BY A PROFESSIONAL HVAC SERVICE TECHNICIAN A.S.A.P.*


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## yuri (Nov 29, 2008)

the heat exchanger is probably cracked and that unit has had problems with them cracking and some were because the burner was overfired (too much gas pressure) from Day 1 and the installer did not check/set it like he should have. VERY unsafe to use. Cut a hole in the top of the plenum and look at the top of the exchanger or right where the flame enters and they usually crack there. should have at least a 10 yr warranty and probably a 20 yr one.


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## JohnnyV (Jan 15, 2012)

I thought cracked heat exchanger also.
I used a professional grade CO detector & reads less than 1ppm
I put the probe in the plenum as well as rooms & heat vents throughout the house.
It's obvious there's an problem, but it doesn't appear to be leaking CO


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## yuri (Nov 29, 2008)

it won't necessarily produce CO as the burner is getting enough air for proper combustion. lack of air causes poor combustion and CO. it however is VERY dangerous and a fire hazard that situation you have.


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## JohnnyV (Jan 15, 2012)

Yuri, Thank you, is the heat exchanger replaceable or do I need a new furnace?
What brand do you recommend?


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## yuri (Nov 29, 2008)

At 13 yrs old it is replaceable but the labor to install it may be $600 or more and it has 13 yr old parts etc. Like a 13 yr car do you decide to keep investing $$ in it or buy a new one. Not my favorite unit so personally I might go with a new one. Should probably have a 20 yr warranty on the exchanger but not on the labor. If it has the infamous SV9500 smart valve gas valve then I would ditch the unit as that valve can be a big problem. Depends on how you like to spend your funds $$ and what your value system is. You get what you pay for quality wise when buying furnaces and we don't like recommending brands here as feelings get hurt but my favorite is the one Dave recommends. You need to confirm that the exchanger is cracked but most likely it is. If the gas valve is overfiring you may have the same problem.


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## JohnnyV (Jan 15, 2012)

I believe in buying quality, I thought Heil as a reputable brand, although never had a problem till now.
Who's Dave, Are you sure you don't want to suggest a replacement brand?

Thanks again


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## yuri (Nov 29, 2008)

......


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## JohnnyV (Jan 15, 2012)

I just realized that one of the return was blocked - due to a recent remodel.
Could this restriction also be the cause of the exchanger over heating & the 
limit to trip?


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## hvac5646 (May 1, 2011)

heil is a good brand if you don't buy the entry level model.


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## hvac5646 (May 1, 2011)

JohnnyV said:


> I just realized that one of the return was blocked - due to a recent remodel.
> Could this restriction also be the cause of the exchanger over heating & the
> limit to trip?


 clean up the debris. they sure don't help your furnace.


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## how (Feb 26, 2011)

Your furnace is overheating the limit switch. This could be from a cracked exchanger, overfired gas valve, dirty filters, dirty fan cage, dirty a/c coil restrictive heating ducts and definitely a restricted return air.

Do a temp rise test on your furnace and compare it to the rating plate on the furnace. Look for a small 1/8" hole (might be covered with duct tape) on the plenum *just beyond the sight range of the heat exchanger*. If not, just make your own hole and put a thermometer in it when the furnace is running. Subtract the current house temp from this measurement and see how it compares to the rating plate. 
You could do it with the return air restricted like you found it and also with it cleared. 
Let us know the results.


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## JohnnyV (Jan 15, 2012)

The limit has only tripped 2 X in a month.
I'm not a HVAC guy, but some what mechanically inclined.
I have two digital probe thermometers. The plenum is 24" high, I put the thermometer 18" up from the top of the furnace. Ran the furnace for 15 min
Max temp achieved 140 F - I opened a small seal around the filter giving it another 1/4" X 6" inlet opening and the output temp dropped 2 - 3 degrees.
I have to believe, the return restriction caused by my WIFE"S curtains being sucked into the return is causing the issue.
It seems my return ratio is too small / restricted? 
Does any of this make sense?
I also put a thermometer in the return 18" from the furnace that was reading 69 F does this seem like an OK difference, or maybe means nothing??
Thanks


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## how (Feb 26, 2011)

The rating plate in your furnace will list the recommended temp rise. Compare that against your measured 70 degree rise to know if air flow/gas output is within the designed spec's. Let us know what it is.
The drapes may be the reason the limit tripped or they might only be one of a number of contributions to the limiting.
Do something to prevent the drapes from resticting the return air supply again so that the diagnostic's can continue if need be.


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## JohnnyV (Jan 15, 2012)

Temp Rise 40 - 70 F


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## Doc Holliday (Mar 12, 2011)

140 minus 69 would equal 1 degree over max temperature rise ALL due to restricted return air. The limit switch is working.

Clear the returns.


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## how (Feb 26, 2011)

I'm guessing that you probably did your temp rise test with the drapes away from the return air grill & the remodel problems sorted out so you still have a furnace temp problem.
All you've determined is that your limit *is* working on a furnace that is running too hot. 
You can clock your gas meter to see if the furnace is overfired.
See if there is a higher speed tap to use on your blower.
Check the other possible causes of an air flow restriction that have already been posted here.

or call a pro.


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## JohnnyV (Jan 15, 2012)

How - Excellent suggestions
Thank you again!


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