HVAC Combination Fan Limit Switch *Revisited!*

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A much better pics and description of fan/limit for a furnace!

Category:

Education

Tags:

• hvac
• heating
• air
• conditioning
• combination
• fan
• limit
• switch
• relay
• wiring
• not
• working
• fix
• transformers
• repair
• construction
• installation

[deyenak1]

Is it possible that a bad limit could cause continuous fan. The fan comes on with a call for heat and will not shut off. Its a gas furnace and I have eliminated the thermostat and I changed the relay but still won't shut off. Could you give some pointers on what I might try next? Could The ignition module be something to check?

[mikie2501]

@deyenak1 Sorry I"m late on answering, haven't logged in for a while. Yes, depending on the system... check all of the sensors to make sure that one of them isn't open. Is the unit trying to ignite several times and failing to do so? This will cause a lockout on many furnaces and turn the blower on as well.

[lolly2cute]

I have a question you may be able to answer: How does the emergency heat work (E) in sequence with the heat in the compressor.

[mikie2501]

@lolly2cute This depends on what kind of 'emergency' heat you have, usually it it electric but sometimes it is natural gas or some other choice. Usually the heatpump (compressor) takes care of heat, but if there is a high demand and the system can't keep up or it is too cold and the heat pump is inefficient then the emergency heat or secondary heat  will kick in to help make up the difference. There is quite a bit more that I could explain in email if you need but this is the basic of it.

[cobitech]

Thanks Mike.......enjoyed your video.

[mikie2501]

@cobitech thanks for posting a reply! (and thanks to all of the others who give nice replies that I forget to answer!)

[Miketec81]

Great video. I started watching ur videos 2.5 yrs ago when I enrolled in hvac program and 2.5 yrs later as a fellow tech here in Ca I'm still watching. Thanks so much they're really educational

[yrtuag]

The pre heater f/l was used on counter flow furnaces I believe to prevent excessive heat rising into the return plenum tripping the aux. limit and maybe damaging the fan motor. On an up flow the heat rising isn't an issue since it's going into the supply duct .I believe this is the reason (or maybe among other reasons)for the pre heater.

[erd675]

this an epic video man im sure ill run into this in the field somtime

[koolaidman10]

great info

[34mclovin]

could u do a video on the actual wiring on a fan/limit switch in detail? maybe actually install one and explain the steps to wire it. starting with the line voltage to the transformer and the blower and gas valve.

[blissJesus1]

my father-in-law has a fairly new, Rheem Heat Pump; on a call for heat, on a 55 degree day, the compressor is not on, but the auxilary heat(gas), comes on. I don't know if the t-stat is wired backward, not programmed, or, what is going on?