Furnace blows cold air FAQs

Why does the furnace blow cold air when we want heat?

Why is our old furnace sometimes blowing cold air when we need heat?

Our 22 year old furnace just started having intermittent problems.

Once every few days, the furnace will kick on as normal, send out heat, but after a few minutes we’ll notice that it’s blowing cool air.

If left, it will just keep blowing cool air.

When we shut it off, then restart (sometimes up to 5 times) it resets and works properly.

We’ve replaced the flame sensor, the filter is clean. But still the same issue. Help! On 2020-12-16 by Deb

by (mod) - furnace blows cold air - what to check first

Deb:

I would start by looking at the furnace itself: is this an electric, gas or oil unit?

If gas or oil you can certainly verify whether or not the burner is shutting down when you'd expect it to keep running.

That can be caused in turn by something as simple as a dirty air filter, or there may be a need for cleaning and service of the heater.

Consider that when you shut the system off and restart it, you're re-setting the safety controls on the unit.

I suspect a bad temperature sensor (not flame sensor) or bad fan limit control.

Let us know what you find as that will help others.

More diagnostic details are at FURNACE FAN CYCLES DURING HEAT

Why does the downstairs get so cold when thermostat is 68 and way way too hot at 70?

Help, we have an up/down duplex with forced air natural gas, installed in 2010. Used to work fine and then during a repair in 2016 all the ducts were fully opened- my mistake.

Since then the downstairs apartment gets cold (like 62-64) with the thermostat set to 68. If we increase the thermostat to 70 the downstairs overheats up to 78 degrees in about 15 minutes-so we turn it down again, and then he is cold.

Should we try more duct adjustments or is this something else? As I said all was working well before the repairs as far as I know (maybe other tenants just didn't complain to me?)

We live in central NY-temps during the winter are from 30 to minus 15. Oddly he does not complain during the really cold weather. There is one furnace in the basement and one thermostat -in the upstairs apt. It seems to me if we open the ducts more fully to the thermostat area then heat will shut off sooner, and downstairs will be even colder..

If we close them the heat will stay on longer and it will get too hot. Am I missing something here?

At my request a few days ago the tenant with thermostat set the fan to be ON all the time -and assures me it's still on. But according to downstairs tenant the fan is not on.

Would it matter if the return/supply "loop" for each apt were separate somehow? I don't know if they are-just thought of it.

So why does the downstairs get so cold when thermostat is 68 and way way too hot at 70?

It seems like something else besides ducts is the problem.

Can you say what we ought to check? Two different people worked on the 2016 repair. Is it possible one made an adjustment (or work around) that he should not have?

One safety bypass he made was found during the repair and corrected. I don't remember what it was but had to do with wiring.

Any help would be appreciated. My tenant is angry and I want to fix this for him. On 2019-02-12 by Lyra

by (mod) -

Lyra

As all the ducts were "fully opened" and now you have heat distribution balancing problems, help me out here, why not simply re-adjust the duct dampers to re-balance the system;

Partly-close dampers feeding air into too-hot areas so that more heated air reaches too-cool areas.

Then be sure that the dampers supplying warm air to the too-cool areas are opened.

I would also review the work that you had done on the system to see if duct dampers were installed or if ductwork needs insulation or leak repairs.

 

Wood furnace fan control switch - fan runs when system is not hot

I have a wood furnace that the limit switch works when the thermostat is turned off but once the thermostat is turned on it runs the fan constantly even though the temperature hasn't built up or even a fire in the box.

The off, on and then high limit off doesn't work with the thermostat turned on.

I'm wondering if it could be wiring or the limit switch faulty, it hasn't worked properly since I've bought the house but all the fan functions work with the a/c.
Thanks in advance,On 2018-02-18 by Vern G

by (mod) - check the thermostat fan switch settings for a FAN-ON position

Vern,

That sounds to me as if the thermostat has a fan on switch set to the on, or always on, position.

If that's not the case then it's possible that the thermostat wires are connected incorrectly or have been shorted together.

 

Olsen HWSV9500 reverse flow furnace blows cool air into the cabin

I have an Olsen HWSV9500 reverse flow furnace. My heat pipes are under my cabin in a non usulated crawl space.

When my furnace kicks in, it blows cold air from the pipes into the cabin and actually cols it off for the first few minutes. When the heater turns off the fan keeps running for a while again blowing cooler air in the cabin.

This results in my furnace constantly turing on anf off just to keep my cabin heated to 20 degrees c. when it is 5 degrees outside. IS there some kind of setting on the furnance that will solve this? Thanks Rick Gagnon 9/5/11

Reply: Causes of furnace blowing cold air at start-up of heating cycle

Rick,

A standard warm air heating furnace fan limit switch won't turn on the blower fan until the heating plenum has reached a sufficient temperature; this design is specifically intended to avoid blowing cold air on building occupants.

But I don't think you want to set the fan cut-in to too high a temperature; I worry about overheating the plenum if you push both limits up together (cut in and cut out) and I worry about short cycling the fan if you just push up the bottom limit.

Here are two common causes of a heating furnace blowing cold-air into the room at the start of a heating cycle:

• An incorrect jumper wire setting 
  
  at the thermostat, particularly with an electric furnace. Quoting from Honeywell's RTH2300 thermostat installation guide: An incorrect setting is noticeable in a gas or oil heating system. When heating starts, you will initially feel cold air coming out of the vents as the fan is running before the furnace has enough time to heat up the air. [3]
• Heating ducts missing insulation.
  
  When heating ducts run through a cold space such as an attic, basement, crawl space or a poorly-insulated wall, the ducts may become quite cold between heating-on cycles unless the ducts are well insulated. When heat first turns on heat in air passing through the ducts is lost to the ductwork itself.

If you have not recently messed with your thermostat wiring, then iIt sounds as if the root problem is that your ducts are inadequately or completely uninsulated.

So I'd look into better insulation of the ductwork as well as wind shielding for it.

See our complete diagnostics for this question

at FURNACE BLOWS COLD AIR

Is 80 degree warm air a normal output for a heating furnace?

by (mod) - maybe, depending on your ductwork length and insulation

Tim

The normal operating temperature for a forced air furnace heating in residential use depends on where you measure it.

At the supply plenum (right at the furnace) you may see tempertures between 140F and 170F, lower than that (by about 30F) for a newer high-efficiency unit.

BUT if you measure air temperature right at the supply register in the occupied room it's going to be much lower - maybe 80F. Just how much cooler the air temperature will be at the supply register depends on your ductwork: its routing through cold spaces, the quality of its insulation, air leaks, and other factors.

Electric furnace alternates blowing hot then cool air

by (mod) -

Richard

That could be as simple as a dirty air filter causing the system to overheat.

Or in more detail: the heater elements are not staying on - if there is no air obstruction then that sounds like a control or heater failure.

Furnace blower comes on before the FAN ON temperature is reached

Our furnace blower comes on imediately before temperature set for FAN-ON on is reached - On 2015-12-20 by Len

by (mod) -

Check the settings on the fan limit switch - or replace it.

Electric furnace sometimes blows cold air, later it works OK

by (mod) -

Lee the fact that you sometimes get hot air out of the electric furnace suggests that the problem is a wiring or control failure.