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Some time ago I found my furnace (gas, forced air, downdraft) was getting stuck on, blowing cold air. I could reset it, but it would happen again after a while.

After calling out a service man, I was informed that the high limit switch was tripping. The furnace had been using a very light-weight filter. With a heavier duty filter, it overheated easily. With extra air-flow (bypassing the filter) it never overheated.

We opened more exit vents and even added an intake directly to the return passage of the furnace. This seems rather a drastic hack, and I still can't use standard paper filters, for example. Any ideas?

Update: Furnace is ~15 years; I had a service man to check high limit switch etc.

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Are you sure the high-limit switch is working correctly? I could see this happening if it's tripping at too low a temperature. –  Niall C. Jan 2 '11 at 15:50
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Check that there's not a blockage in the air flow somewhere else. –  ChrisF Jan 2 '11 at 22:01
    
How old is the furnace? –  Tester101 Jan 3 '11 at 18:09
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4 Answers

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It's common for HVAC systems to be improperly designed. It's also a common misconception that it's okay to close vents. If a regular density filter is restricting airflow (and causing the unit to heat up too much and trip a safety switch) then you need to do something about the airflow.

In general it's not a good idea to close vents because it changes the "static pressure". I don't know the details but basically you don't want the pressure in your vents to be much higher than the pressure in your house. If so the fan is working too hard and there isn't enough air passing over the heat exchanger.

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Consensus of HVAC people seems to be that the house has insufficient return air. The filter is just taking a bad situation and pushing it over the edge. –  Justin Love Feb 8 '11 at 18:24
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It could be that the air return system is blocked or insufficient, and cannot provide the proper air flow. Make sure your air return vents are not blocked by furniture or rugs, and possibly clean the return air system.

You will also want to make sure the heat vents are open and clear of obstructions, if the warm air has nowhere to go it will not flow properly.

You could also check the blower, as it might be getting old and not have the strength to move the correct amount of air. The blower can also become dirty and be blocked with dust and debris, which will cause a decrease in performance.

The final thing to check would be the high limit switch itself, as it could be on it's last leg and need to be replaced.

Remember to change your filter monthly to prevent it from clogging.

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I suppose I'd have to call a duct cleaning service to check blockage? –  Justin Love Jan 3 '11 at 18:37
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@Justin - You could check yourself if you have the proper tools, if not it's cheaper the call a duct cleaning service than to buy all the neat gear they have. You could check the vent covers yourself, to make sure they are not blocked. Removing the vent covers might also revile the problem if the blockage is close to the opening (Maybe you'll get lucky and the previous owner stashed a big bag of cash in the vent). –  Tester101 Jan 3 '11 at 19:23
    
Insufficient makeup air will cause that high limit switch to trip - which will definitely be caused by the blower fan getting weak or return ducts being blocked. Doing a D/P test across the furnace would confirm that. –  kkeilman Jan 3 '11 at 22:17
    
I had the ducts cleaned; in the processess the filter got switched out, and it overheated right off the bat. –  Justin Love Feb 8 '11 at 18:23
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Have you tested the airflow proofing switch? Typically this will be a differential pressure switch that senses an induced draft before it will allow the ignitor to light or gas valve to open.

Furnaces require sensing airflow before they will enable the ignitor/gas valve.

Additionally - just read that having gas pressure too high at the furnace will cause high limit trips.

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Take out the filters and see what happens. If it works properly, you certainly have some air flow issues. Maybe even something else. Open all vents to all rooms. It is common for people to think they are saving energy by closing the vents in unused rooms. What happens is the Furnace and A\C do not work correctly. The system is designed and sized (or should be) for all vents to all rooms to be open. You save little to nothing by not heating / cooling all of your house. Spend some money on insulation if you can. Actually, you cannot afford to not insulate the house.

I have 2 1" pleated filters in returns (just switched to a charcoal type of filter to take out paint odors, a 4" pleated filter, an electrostatic filter (Carrier - which I have had to turn off due to the slight amount of ozone that causes my wife's asthma act up), a UltraViolet Light in the Air Handler to kill possible mold and viruses. My air flow is not starved and my system works fine, even with all these filters. Insufficient air flow will cause the over temp sensor to trip out heating elements. You definitely have an air flow problem or a sensor problem. Go look yourself with a good flashlight. If it worked in the beginning, then you might can fix it cheaply yourself by looking. Look at a schematic on the internet if you are technically proficient. Check air flow at all ducts - No air flow, then you can be sure that duct has a problem.

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