Any furnace experts here?

[Dave Sherman]

My gas furnace is giving me some fits right now. I called some folks I've used before to come check it out, and their technician pronounced it "dying" and quoted me a new furnace. He did give it a cleaning and found that the gas pressure was high, so he readjusted the gas pressure. He noted there was a small crack in the sheet metal near where the pilot jet goes (not in the heat exchanger), and he reasoned that the higher gas pressure was making the pilot flame a little too big, which torched the sheet metal and cracked it. FYI, the furnace is about 14 years old.

Here's what it's doing. It seems to be overheating. It will start normally, but the burners cycle on and off frequently while the thermostat is still commanding heat. It never did that before. Sometimes the thermostat will command heat, but the burners don't start at all until several minutes after it commands heat. Other times, it will shut down normally, but a few seconds after it shuts down the blower motor comes on again. When I did some troubleshooting, one of the rollout switches seemed to be opened, but the technician said they are not self-resetting. He was thinking the big pilot flame was getting too close to the rollout switch and tripped it, but I never had to reset the switch.

The filters are clean, but it sure seems to keep a lot of heat inside it. Maybe getting a second opinion would help, but I'm trying to figure out whether the heat exchanger is worn out, dirty, or cracked and that's why he's recommending a new furnace, or if it's just "old" and he wants to sell me a new one.

 

[Quest]

what is the make/model?

Also: are you using mercury switch based thermostat or digital?

 

[SrDriver]

Don't know about the "Expert Part" but there is a Fan Limit Switch is located on the front of the furnace that both controls the fan and also is a safety device which will shut off the burners in case of a problem.

They can get out of adjustment - need replacement - indicate a problem.

Sounds like you could use the help of a professional 2nd opinion.

Make sure that your have some working carbon Monoxide detectors in your home.

The professional can check for a cracked heat exchanger.

If you must replace the unit and plan on living in the home for an extended period you may consider replacing the heater and the air conditioner while you are @ it with a energy star high efficiency unit so the A/C coil in the furnace matches the new condensing unit.

I worked in the HVAC field for many years before retirement and the first place I would look because of the short cycling you are discribing is the fan/limit control.

 

[bepperb]

It sounds to me like the limit switch which turns the gas off at a certain temp is getting tripped because the temp is too high. This may be a bad limit switch, and that's only a five~ten dollar part, but it's more likely the heat exchanger is on it's way out.

I had this same thing two years ago. I was able to point a box fan at the combustion area and keep it going longer but every night I'd have to go hit the reset button on the limit switch. I then replaced that limit switch, but that wasn't the problem. The exchanger was full of corrosion/crud and restricting flow. 400 for a new one, or 800 for a new furnace (installation of either was free) and really, the new furnace has paid for itself in energy savings in just one year. Obviously your mileage will vary on that if you don't have a free labor source, but both furnaces were over 90% but the new one has significantly lowered our gas bill (maybe the old was crudded up for some time).

 

[Bob Woods]

My daughter and SIL had that problem, the guy that finally fixed it cleaned the flame sensors.

http://arnoldservice.com/furnace_flame_sensors.htm

 

[Kestas]

A dirty flame sensor will cause shut down of the gas shortly after flameup. The computer allows some seconds (8?, 15?) for the flame sensor to get up to temperature before it shuts down the gas. I'm not so sure if Dave's problem matches this.

 

[wander2039]

My grandma had a similar problem with her Lennox couple winters ago. She had too many vents closed and the furnace would overheat and shut down.

 

[Dave Sherman]

Well, I'm very glad I got the second opinion. The new guy took many more measurements than the first one, and couldn't believe the first guy determined it was on its last legs without making these measurements. Bottom line, the heat exchanger is not cracked or leaking, it's not worn out, the burners are burning cleanly and efficiently, and the only trouble was the blower was set for the lowest speed, which was not moving air quickly enough through the new brand of filter. He kicked up the blower speed a notch, temperatures are more reasonable, and he suggested a different brand of filter. Great. I think the first company might be getting a letter of disapproval, and the second company will be getting a letter of commendation!

 

[doitmyself]

Dave,

Thank you for sharing your outcome, with several lessons for all of us.

I had to come up with a new filter source for my workplace furnaces last fall so I educated myself about MERV, restriction, etc..

I could imagine an older furnace might not be engineered to handle a more restrictive, new style filter. Please tell us more about the "new" filter you installed and how or if it contributed to the problem. Was it a HEPA filter or otherwise more restrictive filter?

As you may have learned, sometimes the "best" filter might not work, especially in an older furnace.

Thanks.

 

[Cutehumor]

interesting. I was going to put a more restrictive air filter because I'm down a half a lung. I may be keeping with the cheap basic ones.

 

[Dave Sherman]

The filters I was using were the fiberglass ones, but those apparently are becoming scarce. The replacements I got were made by Flanders, and they had a paper filter element. The good furnace tech suggested Filtrete instead, and said he has seen problems using the HEPA variety. Clean air, but the blower motor has to work harder. As I found out, slowing things down may overheat.