Ask Me Help Desk - Ducane Gas Furnace intermittent fire goes out, fan still on

Manufacturer: Ducane model # FPBB100A5, manufacter # 9874569843
Age: year is 1995 printed on it
80% efficiency: There is metal flue pipe and a fan I think is draft inducer blower so System: Gas Fired Forced Air Furnace
Fuel: Natural Gas

Sometimes before it reaches the desired temp the furnace fire goes out but the fan continues to blow.
The thermostat is communicating and the fire comes on and then the fan comes on, hot air comes out and if requested temp is reached it all shuts off. But sometimes the fire shuts off before and it just blows.

I had a service check up and they could not find anything wrong. It is intermittent.
Thank you

Could be a limit switch killing the power to the gas valve, caused maybe by over heating,some have a reset between the 2 connection wires that you can push to reset, at the time it happens,, possible the flame senser needs cleaning,, does it have a control board that is coding with a flashing light?

I don't know if it has a control board -- There is a metal box that has the label about the Ducane but it doesn't have any lights on it. The thermostat wires go into it.

I took a photo but don't see how to post it here.

Limit switch killing the power to the gas valve sounds exactly like what is happening. But would the fan continue to run? I think I read that happens

Quote:

Originally Posted by jtuchin

I don't know if it has a control board -- There is a metal box that has the label about the Ducane but it doesn't have any lights on it. The thermostat wires go into it.

i took a photo but don't see how to post it here.

Limit switch killing the power to the gas valve sounds exactly like what is happening. But would the fan continue to run? I think I read that happens

Yes, depending on some models they do run the fan in event of an open limit. Check your filter! If you have a dirty filter, that will cut down air flow and the heat exchanger will overheat causing the limit to open. If this has already occurred in the past, the limit may now be weakened and overly sensitive.

Other things to check-make sure all vents in the home are clear, blower wheel isn't packed with crud (does it make a whawhawhawha building noise when it starts indicating out of balance?).

The metal box you speak of probably contains the transformer and fan relay, maybe a terminal board-or it could have furnace control. Usually the control board is in the bottom in the blower compartment. Some models have a cover on them. Usually a 1/4" nut driver is all you need to open it up-but I don't think you have to worry about this control-it seems to be doing its job. If the control board is in the bottom, is there a window to see the flashing LED through the bottom door?

Also keep in mind that this WILL happen on some models if you shut the furnace down while the main burner is firing, as the unit will not cool down properly before re-firing, causing the limit to trip.

You can test the limit's performance by placing a type k thermocouple type thermometer inside the heat exchanger (remove limit and attach hot point to limit with a piece of wire or some such, re-install limit). Cycle the furnace with this set up, and watch and see what temperature the furnace drops out at. If it's below the rating on the limit (170-220 degrees F-rating is printed on switch itself) when it trips, the limit is bad. If your temps are high, and the limit is doing its job, something needs to be looked at. I realize you may not have a type K (Fluke is most common) but that's the ideal tool for this job and nothing else I've tried works as well.

You could also simply change the limit (less than $50) and see if it keeps happening. If it does, call your contractor back and request the senior tech to solve it for you.

Thank you for your help! The filters are clean and it's not making funny noises. I am going to go back up there with some lights and see if I can identify the furnace parts and look for the LED.
And I will find the correct limit switch to order. Since I'm not confident, I will pay the service man to install it. Thank you and I will let you know what happens.

Quote:

Originally Posted by jtuchin

Thank you for your help! The filters are clean and it's not making funny noises. I am going to go back up there with some lights and see if I can identify the furnace parts and look for the LED.
And I will find the correct limit switch to order. Since I'm not confident, I will pay the service man to install it. Thank you and I will let you know what happens.

If you're going to have a service tech out to install it, have them check the unit over for you. You can provide the model and serial number to them and request they bring the switch out as well-saving you a goose chase. It should still only cost around $50 or so, even with a contractors markup (unless the company uses a "flat rate" program, in which case you may have to fork over a limb or two-depends on the greed factor of the contractor). A good mechanic should be able to solve the problem very quickly (unless it's one of those "twilight zone" calls that never fails while the mechanic is present. It NEVER hurts to develop a good relationship with a good contractor. Our shop strives to make our customers happy to the extent that we have people calling us asking us which plumber or electrician to use. Word of mouth...

Changing the limit involves unplugging 2 wires and removing 2 screws in most cases-so don't be intimidated by the name.

I was wrong saying the fire goes out, fan still on. I thought the flames went out with the problem but a closer inspection shows they only get smaller and it is direct spark with no pilot.

My service tech is a good mechanic and friend. I do have one of those "twilight zone" calls and appreciate your help that saves me money and more. Today, I spent an a long time in the cramped attic waiting and watching. I did identify some parts; I saw the main limit control with the 2 wires and 2 screws and it does look easy to replace. I took the cover off the fan control board (Honeywell ST9120C).

I noticed that the pipe to the vent got very hot. I did not see the LED lights anywhere.
But the big blue flames had sometimes orange in them.

I left the attic and the heater ran perfectly for at least three hours. Then I found that the air was cool coming out of the register (original problem) and I crawled back into the attic I saw some blue flames --- BUT not like it was before.

So, I was wrong saying the fire goes out, fan still on. The fire is still on but much smaller. When the thermostat is turned to off, there is no fire at all. I will leave it off until my service tech can check it out. I'm in Houston and not uncomfortable.

I thought about just letting it run until he can come, so he could see exactly what was happening but better not. I don't smell gas but don't have CO tester. Thank you for this site!

Quote:

Originally Posted by jtuchin

I noticed that the pipe to the vent got very hot. I did not see the LED lights anywhere.
But the big blue flames had sometimes orange in them.

I left the attic and the heater ran perfectly for at least three hours. Then I found that the air was cool coming out of the register (original problem) and I crawled back into the attic I saw some blue flames --- BUT not like it was before.

So, I was wrong saying the fire goes out, fan still on. The fire is still on but much smaller.

These comments lead me to believe you need the gas pressure checked... you may have a bad regulator somewhere. Your tech will more than likely solve it for you fairly quickly.

Good Luck.

My service tech friend has not been able to find the problem. He doesn't use the Internet so I am posting.
He has checked all the bypass, safety, pressures and burners and asks, "Does the pressure switch hold a vacuum or bleed off?'