[jimdwest]

I have two Heil furnaces, 90%. One 75,000 and other 125,000 BTU. Both have problems with the Roll Out switch.

The basement one continued to open the switch. A good dealer inspected and saw no reason for it. But did find a cracked heat exchanger. Replaced under warranty. Still roll out switch problem. Changed it and seemed okay last winter. Now it is starting over again with the switch.

Upstairs furnace roll out switch has had to be rest twice this heating season.

I thought I was buying a good brand of furnace, but apparently not.

Any ideas ? Everything has been meticulously checked and nothing found to be wrong. If I jumper the switch, all is fine. Furnace guy says no indication the flame is causing the problem.

 

[yuri]

May be that the gas pressure/manifold pressure is too high. With that brand I have seen some come from the factory set as high as 4"WC instead of 3.5". In their defence the installer is supposed to check that and the gas valves are made by HWell, WRodgers or Robertshaw. Get the tech to check the manifold pressure and make sure there is no obstructions in the venting also.

 

[jimdwest]

That was all checked when he replaced the upper and lower heat exchangers. It is odd that both furnaces, bought and installed same time have the same problem. And that it took a few years for the problem to begin on both. Thanks for the reply.

 

[yuri]

Apparently and Beenthere can chime in as he may have said it, if the venting is partially plugged the burner does not get enough oxygen and in rare cases will burner hotter than normal and trip a rollout or the flames/flue gases don't travel fast enough thru the burner box/heat exchanger and trip the rollout. Correct me if I am wrong Been. Even though it has a pressure switch partially plugged intake with leaves/debris/spiders web/snow/ice in rare cases can cause that. I had a Carrier get the first chamber/burner where the air enters get overheated/warped and damaged from a partially plugged intake pipe. Rare but it happens.

Especially if it has a concentric vent kit where the intake/exhaust go thru a donut shaped vent outside. The intake goes in behind the donut hood and is notorious for spiders webs/debris getting in there/

 

[beenthere]

Restricted intake or exhaust can both cause a somewhat hotter flame.

Strange that the dealer didn't think a cracked heat exchanger would cause the roll out to trip. Since that is the most common reason for it to trip.

undersized duct work can also cause premature heat exchanger failure.

 

[yuri]

Restricted intake or exhaust can both cause a somewhat hotter flame. WHY? I am too tired to look up or study gas burning to figure out the science.

Strange that the dealer didn't think a cracked heat exchanger would cause the roll out to trip. Since that is the most common reason for it to trip.

undersized duct work can also cause premature heat exchanger failure.

.....

 

[beenthere]

It gets to a point that the gas isn't burning where it should. The inducer doesn't pull the gas and air as far into the heat exchanger as it should. So the roll out can trip. Also burns a little dirty cause not enough air/oxygen. Need excess air to ensure complete combustion. And the excess does cool the flame some.

 

[jimdwest]

He figured it was being caused by the cracked heat exchanger and that's why it was discovered. But still happening. Nothing has changed as far as intake air or exhaust. Intake (125,000 BTU) is just open to good-sized basement air and exhaust is 4" plastic running for maybe 15' to outside foundation and pointed down. He removed a screen in the exhaust in case that was blocking too much. Upstairs (75,000 BTU) is open intake to large hallway and attic air, and exhaust 2" plastic thru roof maybe 6'. The roll out switch on the bigger furnace has been replaced once by me and once by repairman. He said he could not see any flame close to the switch, with the panel on or off.

 

[yuri]

Could be the gas valve(s) are dying/malfunctioning and occasionally producing more than 3.5" WC pressure. They can act up and after 20 yrs may do so. They DO NOT make them to last more than 20 yrs nowadays. 13-15 is not uncommon with some WRers I know of.

 

[beenthere]

A combustion analyzer should be used on both of them. It would tell more about what is going on. Strange that a new heat exchanger would be having a problem in one years time.

Since both are acting up, I would expect either their gas valves as Yuri suggest or the gas company has increased their gas pressure.

 

[yuri]

Or the regulator B4 your gas meter is failing/ overpressuring the line. Get the gas co. or a tech to check the inlet pressure B4 one of the valves. If you get ice or snow blocking the vent on the regulator that can also cause it to overpressurize the line. Or that vent may be plugged with spider webs and has a screen on it. Get the pressure checked ASAP.

 

[hvac benny]

Clock the furnaces individually at the meter to determine actual input. Checking the manifold pressure is good, but it's only one part of the equation, and gives you an incomplete picture. Ive seen plenty of furnaces set to 3.5" wc that were over-fired.

 

[Marty S.]

Dragging draft inducers can also cause roll outs to trip. As beenthere said get somebody with a combustion analyzer to see what's going on. They're 23 years old,which is the life of a furnace, so expect some large repair bills in the near future. If you have not done it already start saving for new furnaces