Furnace fan sequence

[boo coo tracks]

HEIL dc90 ultra high efficent furnace.
1. Thermoasat call for heat.
2.Small fan kicks on to vent burnt gas out plastic pipe.
3. Gas burner kicks on.
4. Furnace blower kicks on.

Problem-Steps 1&2 occur then shuts off, 20 seconds repeats several times but burner never kicks on.

Repairman replaces theromsat, 2 days later same problem!
Repairman finds lose wire. 2 weeks later same problem.

This seniro never happens when repairman is here.

This problem has occured 20 times over last six months.

Any suggestions?????????????

 

[HoosierBuddy]

I'll take "Pressure switch not making all the time for $100."

Does that unit have a diagnostic light on it? An LED that blinks a code when it goes into lockout?

Phil

 

[scofo]

Hey Tracks, that sounds like what the heater in my house did, but only if there was a large temp drop in a short time. My heater is an Intertherm, if you remove the panel you may find that your heater has a self diagnostic capability. There was a problem with the pressure differential switch circuit. This switch informs the processor that the"small fan" has done its job. Anyway condensation was dripping onto switch causing a short in the circuit. Dried out switch and relocated it away from condensation source. So maybe your heater will help you determine whats wrong with it. As HoosierBuddy said.

 

[tdkkart]

Ours did this once a few years ago.

Your sequence in more detail.

1. Thermostat
2. Small Fan on
3. Electric Ignitor glows
4. Gas on
5. Flame lights
6. Flame detector(on the opposite side of burner from igmitor) checks for flame
7. Good flame--OK, fan comes on.
8. No flame--Bad, sequence stops and retries

Ours was going bad between numbers 5&6, the first section of the burner lit, but didn't spread to the other side. Flame detector didn't sense the flame.

Pulled the burner assembly and found a tiny cobweb inside one of the flame spreader tubes(small tube that connects the individual burners), blew the tubes out and all was good...............

 

[Kevin54]

I'll take "Pressure switch not making all the time for $100."

We have a Lennox that has done this for the past couple of years. After it would try to fire five times it would shut down for an hour until it would reset then start again. I have had countless service calls on it and finally decided to get a new furnace. The last guy that we had work on it had been on the phone to Lennox numerous times, So one day he stops and tells me they came out with a retrofit kit to cure the problem. Finally after thousands of complaints. It was four little plastic tubes the size of a ball point pen ink cartridge and about an inch or inch and a half long. They went thru the little orifice in the pressure switches (I have two) Presto. Another $100.00 service call but took care of the problem and has been working great for the last 6 months. The next time that yours does it, locate the pressure switch and you should see a red and black, maybe, square rubber tube. One tube red, one tube black. These are hooked to the pressure switch. Blow thru this to see if it is open, or leave the tube off to see if the furnace fires. Also run a long straight pin thru the orifice in the switch to see if it is plugged. You can leave the hose off to see if the furnace works but do not leave it off as it is a safety device. If that lets the furnace fire, let me know and I can find out the size of the plastic tube that my repairman used on mine. I guess what the pressure swith was doing was not allowing enough in so it was detecting abnormal pressure then when it does it will not fire. And relying on electronics now, it was built in that if there is a problem to only try to fire five times before shutting down. Also when yours does it, kill the power to the furnace for 5 minutes and apply power and see if it fires back up. I feel your pain though. I paid out enough in service calls the last few years to buy another furnace. Luckily I kept the one we took out and put it in the garage. On a few occasions the wife and dogs had to go down there because the house was so cold. Nothing worse than going to bed at 70 and waking up to 50 or less. But I'm putting my money on your pressure switch(es).

 

[Vicegrip]

If you blow into a pressure switch to test blow gently as they can be damaged by too much pressure. Sometimes I find dirt and or rust in the fitting that is in the side of the combustion air blower on Lennox units.

 

[6768rogues]

TDKKART, between steps 2 and 3 there is a sensor that detects that air is moving. A bad sensor, wiring, connection, etc will keep it from proceeding through its cycle. I had to replace one on my Carrier unit.

 

[kbs2244]

Have you checked for a blocked chimmeny?
Sometime a squirlw or bird will set up there to get warm, OP on the CO2 and fall down the pipe.
That blocks it, so the pressure switch trips.

 

[6768rogues]

Good point, kbs. That is why the sensor is there. It is easy to overlook the obvious. A cursory visual inspection of everything is taken for granted, I guess.

 

[Kevin54]

If you blow into a pressure switch to test blow gently as they can be damaged by too much pressure.

Vicegrip...after reading what you wrote I do see I failed to mention that. Good catch. Don't use an airhose either. Just a little blow with the mouth.

But ys there are many other things that can cause it also. Controllers are one, chimneys plugged, etc. Do an online search for "Heil DC90 problems" and see what crops up. I know with our Lennox...I did a search and ended up on a board that was specific to problems. I quit reading after about 300 responses that people had posted about similiar problems. So a search engine can be a huge benefit.