HVAC Guys........Furnace problems

[srxsrule]

I figure there has to be some hvac guys on this great site.

I have an Heil DC 90 (92% high eff) model NTGM075EGA1 I would guess its around 15 years old and I just bought it last year for my shop. It worked fine up untill about a week ago. I noticed the fan didnt stop running. After taking a closer look at it I noticed both the main blower fan and the draft inducer fan were both running but the Stat was not calling for heat the the gas valve was off.

Anytime there is power to the furnace both these fans run regardless of what the t-stat says, I can even unhook it and no change. Also it will not go through a cycle at all, just the fans run and it never trys to light the burner.

I replaced the fan timer board with the new updated version and it still does the same thing. Any help would be great! Thanks Eric

 

[skidoo4fun]

Furnace

Could be the high limit switch, if the high limit switch opens, the blower fan will run continous and the burner will shut off. You will need to consult the wiring diagram to locate the high limit switch. Once you find the high limit switch you can check it for continuity with a Voltage ohm meter whith the power off, or with the power on check for voltage, a closed switch will have no reading, a open switch will have a reading, most likely 24 volts. If the switch is open you will need to replace it. This is one possibility, if you do not have a VOM and cannot read a wiring diagram, your best bet is to call a qualified service tech. You could end up replacing a bunch of unneccesary parts. Most electrical parts cannot be returned. Good Luck!

 

[srxsrule]

Thanks for the reply.

I do have a DVOM and know how to read a wiring diagram 14 years as a ASE master auto tech.

I have been looking at the different switches and there is a wiring diagram on the inside of the unit. Would it be possible to by-pass these switches with a jumper lead to test and make sure thats the problem. Eric

 

[skidoo4fun]

Yes you can test the switch with a jumper to test. The high limit is usually located above the gas valve center of furnace. With your VOM and power off disconnect one wire and check for continuity. If it is open, you will have no reading. Connect your jumper to test and turn on power, if our theory is correct the jumper will close the open high limit switch and the burner will cycle on if the thermostat is calling for heat. If the furnace does not cycle on locate your model number and google it, you may find an online trouble shooting guide for further assistance.

 

[srxsrule]

Is the high limit switch wired in series with the pressure switch that has the vac lines going to it? I have tried jumping that wiring with no luck.

What is the switch that is located down by the main blower fan? That wiring comes from the control board, goes to the switch by the blower and then goes to the gas valve. IF I put 24v to that wiring the furnace starts up and starts working..But then it never cycles off even after the t-stat reaches the desired temp.

If this furnace was for my house I wouldnt even mess with it, But since its just for my shop I figured I would try and figure it out. Not much to loose I only paid 200.00 for it last year. Thanks Eric

 

[02edgehead]

check the rollout switch which is on the right side of the burner box. 2 wires going to it with a little button in between. push on the little button to see if it clicks. these furnaces have had problems with the primary heat exchangers but seeing how its just for your garage it shouldnt pose any issues. what size did you use to vent the exhaust and intake?

 

[srxsrule]

I found a manual thats similar to mine. It does say if the roll out /manual rest switch trips it will cause both the blower fan and draft inducer fan to run continuosly.

I have been messing with the rollout switch and it doesnt feel right, doesnt seem to do anything when trying to push the button.

If I jump the roll out switch, the fans DO stay off. Then when I call for heat with the t-stat the main blower turns on but nothing more happens, the gas valve never opens.

Will a jumper wire work to by-pass the roll out or does it need to see that signal open and close at different times? Thanks Eric


Oh yeah, the exhaust has 2" or 2 1/4" and the intake side is just open, its not run outside. The exhaust pipe is only about 3 feet long and its free and clear.

 

[02edgehead]

yes you can jump the rollout. i would leave it this way permently. maybe with the intake not vented out was bringing in too warm of air which caused the rollout to trip. have had this prob before.

 

[02edgehead]

i mean wouldnt leave it that way permanatly

 

[srxsrule]

Ok I figured it out. Wish I would have started here first!

Its a bad roll out switch. With that switch jumped everything works fine. I also watched and made sure the flame was not getting back to the switch to make sure the switch was actualy bad and not just doing what its ment to do.

So thanks for the help from both of you!

Eric