How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Tom Your Own Question
Tom, HVAC Technician
Category: HVAC
Satisfied Customers: 770
Experience:  16+yrs HVAC/R service. Residential Commerical Industrial
Type Your HVAC Question Here...
Tom is online now
A new question is answered every 9 seconds

Heil DC90 gas furnace not working. How to troubleshoot?

This answer was rated:

My Heil DC90 gas furnace stopped heating yesterday. The blower continues to blow cold air through the vents. I've checked everything I know of (which is limited), including turning the thermostat off and on, "resetting" the furnace by taking the bottom cover off and on, and replacing the (very dirty) air intake filter. It's 12 years old and I had a similar problem 4-1/2 years ago. The gas company came and replaced the "mother board" but I don't know what type of repair this represents. He left the box with me that the part came in and it says "Electronic Fan Timer". Do you suppose this is the same thing again? I know my particular model and serial number of furnace were found to be subject to to premature heat exchanger which resulted in a service bulletin being issued but no real recall, to my knowledge. Is an electronic fan timer and a heat exchanger the same thing? I'm wondering if there's anything else I can try before calling the Gas & Electric company?

The fan timer and heat exchanger are two different things. The fan timer controls when the blower runs. The heat exchanger is the 'guts' of the furnace that the flame shoots into to make heat. The blower then blows air over the heat exchanger.


If you pull the front cover off of the furnace see if the small black exhaust motor is blowing. On the DC 90 if the hi temperature limit switch is open then the exhaust motor and the main blower will run. This could have been triggered by the dirty air filter, however it should have reset after the unit cooled down and you cycled power on and off.


The only other thing that would make the blower run continuously is if there is a stuck relay on the electric fan timer board. This doesn't occur all that often. I would suspect the high limit. You may even have a faulty high limit that needs replace.


So again, check to see if the main blower and the small black exhaust blower is running. If so then you probably have a defective high temperature limit that would have to be replaced. If only the main blower is running could be the electric fan timer.


If you or someone you trust could disconnect the fan wire from the thermostat you could diagnose a little more, but it sounds like you may need a new part no matter what.


Besides resetting your thermostat you might also try switching off the main power to the furnace and then back on. It should have a switch on the side of the furnace.


Let me know if you have further questions. Thank you.


Customer: replied 7 years ago.

I'm at work right now and won't get home until after midnight. I will check those things you mentioned once I get home. Oh, I forgot to mention that I did try turning the power off as well. I think that's what I did, anyway, when I removed and replaced the fuse (yes, old house wiring)! I've been searching the internet, thinking I'd find a manual for the furnace but to no avail. I thought they'd have some "troubleshooting tips", along with pictures, for me to try before calling in a service person. There is a regular light-switch on the side of the furnace but that turns the blower off and on. I have that turned off so at least I got it to stop blowing cold air into the house! So I'm hoping that I can check in again with you tomorrow after I've tried those things? Thanks for the help so far--I only wish you were my neighbor!

With the front panel removed off of the furnace, look behind the gas valve and you should see a brown insulator board with 2 red wires going to it. (You won't see the round switch device shown in the photo as it is inserted into the heat exchanger). This is the high limit cutoff device. If it senses too high of temperature in the furnace it will automatically shut off the gas and turn on the blower to cool the furnace off. What sometimes happens in the case of a really dirty air filter is that the furnace has been cycling on this limit switch instead of the thermostat. In milder weather this sometimes goes unnoticed if it keeps the house warm.


There is a simple test you can try or someone you trust to check it can try.


Shut the power off to the furnace. Remove the 2 red wires and connect them together with a paper clip or a short piece of wire. Be very careful not to let any bare wire touch anything or it will blow the fuse on your mother board down in the blower compartment.


This is only a 24 volt circuit so it's not like you are working with household current. Now turn the furnace back on and if the blower does not come on then you can assume it is a faulty limit switch. Even better if you or someone else has access to a multi meter check the switch contacts to see if they have continuity (with power off!).


If you shut the power off again and reconnect the wires and the blower starts, then you have a bad hi limit control. These can sometimes fail due to repeated cycling due to a plugged filter (poor airflow over the heat exchanger) or it could be a faulty heat exchanger causing the problem also. Since you said the filter was very dirty I would assume that first.


You must not leave the jumper in place even if it makes your furnace work. It is a safety device and the jumper wire is to be used for diagnosing the problem only.


You will have to locate the exact replacement part through a Heil dealer, a local HVAC supply house or you can likely find one online at


Joy, you can check in tomorrow and let me know how it turns out. Don't start a new question, this one will still be open. When you log in, just go to your questions and reply like you did today. Good luck!



Customer: replied 7 years ago.

Thanks. That's incredibly helpful. About how big is this piece? And is it behind the upper panel or the lower one? (I told you I was technologically-challenged!) I've printed out our entire dialogue and will have it in hand when I'm checking this stuff! I will check in with you tomorrow. Thanks again!

Upper panel, behind the gas valve. Aprox 2" X 3" square. It's easy to find. It will be the only thing back there with 2 red wires on it. If you use a bare wire or paper clip to jumper out the control it would be a good idea to wrap a little tape around it to avoid making contact with any furnace parts when you turn power back on. Good luck. Be careful.



Customer: replied 7 years ago.
Got it!

As in got it working? Or just found the limit?

Let me know how it turns out.


Thank you.



Customer: replied 7 years ago.

Hi Tom


Sorry, I didn't have any time to write today until I just finished work--I had cases all day long...we were busier than a one-armed paper hanger!!! You probably thought I'd blown up the block when you didn't hear from me!


So I went home last noc and futzed with the furnace until well after 2AM but didn't accomplish anything. I found the high limit control switch (you were right, it was very obvious)! Now, the left wire had a rubber cover over the end and it came off very easily. However, the right side had no rubber cover on it and I met a great deal of resistance when I tried to pull it off so I was reluctant to try and force it. I even tried some gentle, steady pressure with some pliers and also tried rocking it back and forth just a little but it didn't want to give at all and in fact, it was pulling on the insulator board. I don't think that connection will come off unless you bend the crimps out a little and I didn't want to do that. I figured if it was that hard to remove, it wasn't supposed to come off (at least not for some hack like me)! So I decided to just leave that alone.


I did find the owner's manual (it was in the pocket behind the service/maintenance sheet--I thought it would be thicker so didn't even think about looking for it there). One thing I didn't try was starting the furnace up "from scratch" by turning off the gas valve on the smart valve and clearing the line and then firing it up. Chances are that wouldn't help anyway, would it?


Also, I noticed that the "peep hole" has a thick, rusty-brown covering on the inside--I'm not sure you'd even be able to see if the pilot light did come on! Is that normal?


The other thing I noted is that I don't have a "condensate trap" on my unit like it shows on the schematic in the manual. I know that I have a humidifier on my unit, however. It's an ancient "Chippewa 224". I do have three drain hoses that come from the furnace and converge together at the drain in the floor, though. I'm assuming that's what these are for? I did read one thread where a guy talked about first and foremost, making sure that nothing is clogging the system and went on to talk about taking one drain hose off and realizing that sludge had caused a buildup and once he cleared that out (by blowing on it!) he immediately had heat again! I'm guessing my problem isn't that simple.


So I'm beginning to think that I just may have to bite the bullet and call the repair guy. Would that be your take on the situation as well? I really don't want to mess around and make things any worse; I just thought that maybe there was some simple solution that I could try before calling in the "big guns"!


I'll wait for your suggestions. Thanks so much!


I just realized after I sent that last response that the the condensate trap is shown in "cutaway"--so it's really there, it's just under the metal plate/shelf where you can't see it unless you're laying on the floor, looking up at it, eh?



Well, you certainly gave it the old 'college try'. The wire terminal should come off. Some of them are very hard and yes, sometimes they break.


If you turn the power on and your blower is still coming on immediately then I still believe you have a high limit problem. If the condensate trap is plugged it will allow the vent motor to start but won't let the furnace go into ignition.


Did you check to see if the small exhaust motor to see if it comes on with the main blower? If it does, that is a sure sign of the high limit fault.


I figured you were busy! Sorry I cannot be of much more help. If you are uncomfortable with any part it is always best to get a pro on site.


To answer your other question, no the sight glass shouldn't be discolored. Possibility of heat exchanger problems, but no way to really tell without being there and doing a few tests.


Good luck with your problem, hopefully will turn out minor.

Let me know if I can be of further assistance.



Customer: replied 7 years ago.

Thanks so much--I'll call the repair guys today. I wish you lived somewhere near Madison, WI--since you know my furnace and all... It is starting to get a little cold--the house was 40 when I got home last noc! It's supposed to start warming up from now through the weekend so I shouldn't have any problems with pipes or anything. Fortunately, I have two big dogs so I can make myself a "sandwich"--that works really well! On the up side, my hot flashes now feel like mere "power surges"! :)


When you mentioned that the discolored glass could be heat exchanger problems, it reminded me of what I read about this particular "batch" (of which mine is one) having problems with premature failure.



Hi Joy,


I'm curious. I have the same furnace in my own home, and while I like it, I have put a heat exchanger in it a few years back. That is why, without a 'hands on' it may be a little difficult to diagnose 100%. While I'm inclined to go with the dirty filter causing the problem initially, there is still a chance it is the heat exchanger.


Good luck, keep me posted.



Tom and other HVAC Specialists are ready to help you
Customer: replied 7 years ago.



Well, the MG&E guy just left. Come to find out it was the igniter/flame rod assembly that needed replacing. He showed me the little ceramic post that was all white at the tip. He said there's probably a little crack in it somewhere. So I'm feeling a little better about it--it was nothing I'd done, like stupidly waiting too long to replace the air filter. (Unless he just said that to make me feel better.)


So I'm back in business--I'm going to go stand in front of the register and soak up the heat for a while!


Thanks again for all your help!