What's wrong with my gas furnace?

In the new house I just move into, there is a central forced air gas furnace which seems to have gone mad. It will fire up, then the blower comes on, and it runs for 20 seconds, then the blower shuts off, but the burner remains lit. After 15 seconds, the blower comes back on, and it seems to function normally for 30 seconds or so. Then every two or three minutes, the blower comes on for 10 seconds, then shuts off, with no burner lit. It repeats this cycle continuously until the room temperature drops low enough to kick on the burner again.

Is this normal for a gas furnace? I had the gas company guy out once last week, because it wouldn’t heat at all, and he reset some hidden switch that he said tripped when it overheated. He also changed the filter, but this problem persists.

Any suggestions?

There is a temperature sensor on the heat exchanger that turns the fan on and off. It sounds like the heat exchanger is being cooled off by the fan rather quickly, resulting in the sort fan run times. Hard to say what might be causing this, but one thing I would do is to check and make sure the flue is not blocked.

BTW, it should work something like this:

Thermostat calls for heat. The furnace fires the burner but the fan does not start right away. Once the heat exhanger is up to temperature, the fan comes on and blows air from the house across the heat exchanger and back into the house. When the thermostat is satisfied, the furnace turns off the burner, but the fan still runs. When the temperature of the heat exchanger cools off to a certain temperature, the fan shuts off.

Well, mine works like that. Yours could be different.

The furnance will have a place to set the temperature that the fan should start blowing at. The fan will stop blowing when the air is below that setting. You can turn down the temperature that the fan blows at. Make sure you don’t set it to low or it will blow excessively. It might be set to say 150F and you could try setting it to 120F.

The short on off is because the fan is set high and the chamber cools down below the fan run tempurature. The flame doesn’t go off and the fan will cycle like you are experiencing.

Google “short cycling” and “furnace”. While it could be one of many things, I think you’ll be looking at an investment in a heat exchanger in your near future.

Do you have a new, highly efficient furnace? The type that I am talking about uses the same pipe (actually a pipe within a pipe) for intake and exhaust. This type has an intake/exhaust on the side of the house.

The intent of this furnace, as best as I can understand, is to use the warm exhaust to preheat the outside intake, therefore saving on heating bills by not taking warm air from the house for combustion. The first cycle of this system is a purge, which is pretty much the first 20 seconds that you refer to. Then the furnace fires up, and does its thing.

The reason I ask: Several years ago I had a Carrier High Efficiency furnace installed because the gas company was offering a substantial rebate. It did seem to save me money for a few years. Last year I woke up to see my breath condensing in a very cold room. The furnace was showing the same symptoms that you describe. After a few service calls I finally got a guy who knew what he was doing. It turned out that it was defective motherboard (yeah, motherboard) sending the wrong signal, and it wouldn’t go beyond the purge cycle. Later I talked to a mechanical engineer friend; his recommendation is that homeowners should just go with the simple furnaces, and leave the high efficiency/high maintenance equipment for companies with maintenance staff and budgets.

I’m assuming that this furnace worked fine before and just recently started acting odd.

I’d check a few more items in addition to the flue.

Check the burner. Is it firing at 100% or is does it look like it is a reduced flame? If part of the burner is blocked for any reason and not firing, then this could cause the problem.

While you are at it, check the fan side. Is the filter in place? Is it clean? Is the fan blowing the same amount of air that it has in the past?

If the furnace has a multiple speed fan and/or a two stage burner, then there problem is more complicated to diagnose.

I don’t suppose you got a home-buyer’s warranty?

My first hunch is that the fan/limit switch, or its wiring is bad. This is the part Dag Otto referred to. It’s supposed to put the blower on once the furnace is hot, then shut off the blower once the burner’s gone off and the furnace has cooled. The limit portion acts to cut off the burner in case the furnace gets too hot, as would happen if the blower failed.

You say the gas guy reset some hidden whatsis that tripped because the furnace overheated? Now it’s really starting to sound like a bad fan/limit switch.

If you didin’t get a buyer’s warranty, check with some neighbors to find out who they’d recommend for a service call. If it’s just a bad fan/limit switch, that should be fairly inexpensive. If there are some electronic controls gone bad, you could be looking at something expensive.

Thanks for the suggestions guys.

The service man checked this, and found no blockage, and there appears to be adequate draft.

I could find nothing on the furnace to make a temperature adjustment. Just the gas regulator, with a two position knob (ON or PILOT) and the button to light the pilot.

Now I may be on to something. At this site I found this:

The filter the service guy put in was a “high efficiency pleated filter”, which may cause the problem. I will try to find some of the older style "open-weave filters, and see if that makes a difference.

No, it is easily 10-15 years old or more, it is a “Duomatic-Olsen” brand, which hasn’t existed since 1999, when they were bought by ECR.

Don’t know. Just moved in.

Burners look like they are firing normally.

Yes, a new filter was installed, but as noted, it is a “high efficiency” filter, which may not be suitable for this furnace.

Don’t know, just moved in.

I don’t think this is that complex of a furnace

We’ll hope not, I’ll try the easy stuff first.

Thanks everybody.

Well, of all the component problems that could happen to a furnace, a bad fan/limit switch is just about the simplest, after a broken belt on the blower motor.

Reduced airflow due to an overly dense filter would result in the burner being cycled on and off, not the blower.

Well? Any news?

I adjusted the heat anticipator on the thermostat, and it seems to have corrected the problem somewhat. The blower still comes on after the heat cycle is finished, but not as often. I may adjust the heat anticipator further to see if the cycling can be reduced further.

Our furnace was doing something similar a few weeks ago.

I started looking into it, and noticed there was a differential pressure sensor in the furnace. The sensor measures the airflow of the combustion gasses that are going up the chimney/flue; if there is airflow, there will be a differential pressure across the sensor, the sensor’s output switch will close, and the furnace will run properly. If there is no airflow, there will not be a differential pressure across the sensor, the sensor’s output switch will remain open, and the furnace will not run (or it may cycle on-and-off).
I removed the two wires going to the differential pressure sensor and shorted them together (thus fooling the system into thinking there was good air flow). The furnace worked fine. I then actuated the sensor by removing one of the flexible hoses going to the sensor and sucking on it; the furnace worked fine.

So it appeared the differential pressure sensor was working O.K. And I knew there was good airflow in the flue, because the flue was unobstructed and the exhaust fan was working properly. It was as this point that I focused on the small air ports next to the exhaust fan. (The air ports are spaced about one inch apart. Each port is connected to the differential pressure sensor via a flexible hose.) I blew into each air port, and noticed one was clogged. I cleaned it out using a small drill bit, and the furnace has worked fine ever since.

Is the exhaust fan the same as the blower? I only see one fan/blower on this furnace.

On my furnace there are two blowers. One is the main blower for circulating air throughout the house. The other is a smaller blower that pumps the hot combustion air to the flue vent/exhaust duct (and then up the chimney). I was referring to the latter (which I call the “combustion blower”).

Due to obvious safety concerns (carbon monoxide and all that stuff), the furnace should not be allowed to operate if the flue vent is clogged, and/or if the combustion blower is not working. So furnace manufacturers measure the flow of the combustion air immediately after the combustion blower. If there is no airflow, the furnace’s control system will assume the flue vent is clogged, and/or the combustion blower is not working, and it will shut down the furnace.

To measure the airflow in the combustion exhaust duct, a differential pressure sensor measures the difference in air pressure between two points in the air path in the blower housing. To do this, the blower housing has two small ports about 1 inch apart. Flexible hoses connect each port to a differential pressure sensor located about 10 inches away. When air is flowing, the sensor will “see” a difference in pressure, and will close an electrical switch located inside the sensor. This will send a signal to the controller that says, “I see airflow in the combustion exhaust duct. Continue operating.” If the controller does not get this signal, the furnace will kick on, not see flow, kick off, kick on, not see flow, kick off, etc. etc.

The flow sense system is easy to troubleshoot. Start by simply disconnecting both wires from the differential pressure sensor and connecting them together. Does the furnace operate correctly? If so, then the problem is with the flow sense system; if not, the problem lies elsewhere.

If the problem is with the flow sense system, start by disconnecting one of the hoses from the blower housing and suck or blow on it until you hear the switch inside the differential pressure sensor actuate. If you can’t get the switch inside the differential pressure sensor to switch by sucking/blowing on one of the hoses going to the sensor, then the differential pressure sensor is likely bad. If you can get it to switch, then it’s probably one of the following:

  1. The flue vent is clogged.
  2. The exhaust blower isn’t working.
  3. One or both of the small air ports are clogged.

With my furnace, I established that #1 #2 were not the case. That only left #3.

The heating anticipator on your furnace causes a small current (in the gas valve circuit) to artificially “fool” the thermostat and “satisfy” the stat a bit earlier than your setpoint. It does this by creating (via that current) a small amount of heat that the stat “sees”—essentially you’re warming up the thermostat.

The reason you do this is this: When your thermostat reaches setpoint (70° lets say) the burner shuts off. The burners. The furnace is still hot however. You paid for that heat! So, you want the blower to run another minute or 3 to disspiate the heat, and send it into the home for your comfort. (many furnaces make this time delay adjustable; from 90-180 seconds) So the blower continues to run for another minute or 2.

Also, leaving the heat in the furnace also stresses the heat exchanger. So the heating anticipator fools the stat and shuts the burners down a bit early and the residual heat is sent to the home-- by the time delayed blower-- and the house “coasts” to setpoint. Withoiut an anticipator the stat would satisfy at 70° and the residual heat dispensed by the blower would cause “overshoot”—the house may reach 72° instead of 70°. Did I explain that in a way that makes sense?

At any rate, the anticipator does not control the blower, and the conditions you’ve desribed can’t be caused or fixed by the anticipator. Any improvement is coincidental. Sorry…

From what you are describing, it sounds to me as if the settings on the fan/limit switch need to be adjusted. From this site:

Where this question is is a very good photograph of the Honeywell Combination Fan/Limit switch. (It’s about half way down the page, right below the above question. It shows the location of the fan/limit settings) It is the most common switch of that generation. (although others were made by other mfgrs)

The small fan Crafter Man refers to is called a draft inducer fan-present in 80%-85% (mid-high efficiency) gas-fired furnaces. One problem with these furnaces when first introduced to the market was a tendency for condensate to accumulate in the flexible hose between the fan and diaphragm switch. Intermittent operation or complete lockout was the typical result. Pull the hose, drain out a tiny splash of water, and the heater would run fine until more condensate accumulated.

In this type of heat unit, the run sequence is:
Thermostat calls
Draft inducer fan runs-closes draft verification switch
If standing pilot-main burner on
If intermittant spark/hot surface ignitor-ignition on-then main burner/Cad cell eye verifies main burner/will shut off if rollout flame sensor or high limit activates
Blower fan on when start run is reached
Thermostat satisfied
Burner off
Blower fan on until shutdown cutoff reached

The control boards of modern furnaces have either a set of jumpers, a series of DIP switches, or both to control fan speed in heating/cooling modes, as well as time delays. If the service/installation booklet is present, refer to the setup data. If no book is present, find one from the manufacturer.

If you’re still stuck-send me an email with model and serial-I’ll see what is out there.

One other thing-access doors have safety switches on them. If your unit doesn’t have an external filter compartment such that you have to remove a unit panel for filter replacement, make sure the panel is properly seated, or the safety switch will drive you crazy.