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Why Do My Vents Blow Cold Air When My Heat is On?

This time of year, especially when it gets cold outside in the evenings, we get many calls from homeowners concerned that there is cold air blowing out of the duct registers in their home when their heat is running. We wanted to address this, as many times homeowners think this is an issue with their heating and cooling system, but if they have a heat pump, it's actually typically a very normal process.

Don't know if you have a heat pump? Check out this article, on how to tell if your unit is a heat pump or air conditioner.

 

Let's understand the basics of how a heat pump works:

In the summer, a heat pump picks up the heat in your home and dumps it outside.

 

 

 

 

 

 

In the winter, your heat pump picks up heat from the outside and dumps it into your home (yes, even when it's cold outside). Now, that may be hard to grasp, but this is the way it works.

In the summer, your indoor coil is cold (and pulls humidity out of the air, and comes in the form of water) which is drained from that PVC pipe located on your roof, or side of your house. In the winter, that same humidity is pulled out of the air, but it comes in a form of frost on the outdoor coil (condenser). If you see frost on the outdoor coil, it is natural to the process and it is not a sign of any kind of malfunction- the reason the coil is cold falls back to the second law of thermodynamics- which states that energy always moves from areas of higher concentration to areas of lower concentration.

As the frost builds up on the outdoor coil, it could potentially inhibit the ability for air to flow across that coil. Once the frost builds up, your heat pump automatically goes through what we call, a defrost cycle to keep allowing the air to flow across the coil freely (which is a necessary function of your system). You may have noticed a loud sound right before cold air blows out through the vents in your home. The sound you hear is the reversing valve switching your unit over to cooling mode. Frost on the outside coil is condensation that forms up on the coil in the form of frost. When a heat pump begins the defrost cycle, the outdoor fan shuts off and the system switches over to cooling mode. The reason for this, is that in the cooling mode, the outdoor coil actually becomes hot. This is because hot refrigerant circulates through the outdoor coil, which in turn melts the frost. The reason the fan shuts off during this process is so the system doesn't pull the cold air from the outside across the coil while the hot refrigerant circulates.

While the system is in cooling mode, the frost that had built up on your outdoor coil then turns to water, runs down the outdoor coil, and piles towards the bottom. When the defrost cycle is complete and the defrost sensor closes, the system runs in heating mode again. This is when the fan kicks back on and all of the frost that ran down to the bottom of the coil in the form of water is pushed out of the top of the unit, sometimes in the form of steam. If you see steam coming out of your outdoor coil that is a normal operating procedure for a heat pump system, so don't be alarmed.

Bottom line:

Don't panic if you have a heat pump and if cold air blows out of your duct work for a little while on and off while you run your heat at home. Don't panic if you see frost on your outside equipment's coil. Don't panic if steam comes out of the top of the outside equipment. It's all part of the process of heating your home with a heat pump!

Comments

cold air coming out of return

When my furnace is on the cold air intake sucks in air from the floor vent ( like it should) but when the furnace shuts down cold air is blowing out from the very same vent. Why is this?

RE: Cold Air Coming Out of Return

Good question and without knowing the details of your home and furnace system it hard to know 100%. I would say that the reason behind this draft is probably associated with some leaky or poorly insulated duct work. Cold air could be finding it's way back down your return after the furnace shuts down through leaks or sections of duct that have no insulation. You would not notice this when the furnace is running because your furnace is blowing out hot air.

Have you had someone out to your home to inspect your heating and cooling system? Thanks for posting on the Ideal Energy blog!

No but I'm having the vents

No but I'm having the vents cleaned in a couple weeks ill see how much extra they would charge to look at it and ill get back to you

2 vents blow cold air & 2 vents blow warm air upstairs bedrooms

I am purchasing a used home, and I had an inspection done today on the home. The inspector said that he cannot figure out why two rooms upstairs would be blowing cold air and why two vents in two bedrooms would be blowing warm air. Does anyone have any suggestions? Thank you.

It's hard to say without

It's hard to say without looking at the home, as it could be cause by a number of things. The temperature differential could be due to duct disconnects in the attic, ducts that are completely uninsulated and are picking up lots of heat, or kinked ducts. I would suggest having a professional go to your home with a blower door and velometer to figure out what the real cause of the problem is. If you're in the Phoenix metro area, we could certainly help you out! If not, look for companies that offer comprehensive energy audits or check with your utility to see if there is a utility backed program (such as home performance with ENERGY STAR) in your area.

heat blows from bathroom floor vent when air is on

this is a rental and we had a new inside unit installed the heatpump is old,when the air is on the vent in the bathroom blows heat not alot but noticeable.we had the home insulated with foam and the basement crawlspaces covered with a moisture barrier,and i dont remember it doing it before.any ideals of what it could be and maybe we can fix it,worried it could tear up the furnace. thanks kim

Possible duct leakage

Hi there,

It's hard to tell without being there to take a look. You could go and inspect where that duct connects in from the crawlspace and make sure it's not disconnected, and it's properly insulated. Where in the United States are you located? The best way to figure out what the problem is, is to just have an energy audit done or a duct leakage test done by a professional. If your area has a Home Performance with ENERGY STAR program, I would HIGHLY recommend seeking advice from a professional that is part of that program in your area.

I have a heat pump is there

I have a heat pump is there supposed to be cold air coming out of the vents when the heat is off? They are on the floor.

Cold Air Coming from Vents When Heat is Off

Hi there, no, there shouldn't be cold air coming our from your vents when the heat is turned off. This is a sign of duct leakage and insufficient duct insulation. If the vents are on the floor, most likely you have an unconditioned crawl space that the ductwork runs through. I would highly recommend getting an energy audit and the ductwork tested for leakage. This is your first starting point. APS and SRP have a great program out here that covers the testing, so a homeowner only pays $99 for the energy audit. There are also rebates available for sealing up ductwork. What state are you located in? If you're in Arizona, we'd be glad to assist you! 

cold air return

Ive read this is normal, to have cold air blowing air when the heat is on, electri heat,but it seems the cold air is nonstop and never shuts off. I havent seen my electric bill yet, but is there something I should check on the compressor? Its freezing in here and Im worried my bill will be outrageous.

cold air

cold air venting instead of worm
I set up for 25 degrees and it stays most of the time at 20 degrees

cold air return

I have an electric furnace , condo, when the heat is on (set at 68), the cold air returns are blowing cold air making the rooms cold. Is it possible the thermostat isnt working???
The cold air runs for hours , afraid my bill is going to be crazy high.

Cool air coming from vents

I just had a new unit replaced, compressor was bad. I have a heat pump. But cool air is coming from the vents, most of the time, it's not like it was before, heat ( it's cold so the heat is on) even when I kick up the temp on the thermostat it's still cool air. they have come back and checked it like 4 times to no avail, what could be the problem.

cold air thru heating vents

c? Help! Cold air is blowing from my heater vents in the house. my air conditioning unit was stolen months ago and has not yet been replaced. is that why cold air is blowing through the heating ventS

air blowing out of floor vents continuously and never shuts off

I have a heat pump and the heat is on and the fan is turned to on. I have air continuously blowing out of my floor vents and it never shuts off even when the room is at the temperature thermostat is set to

Hi, Thanks for commenting on

Hi,

Thanks for commenting on our post! You will probably need to have a professional come look at the situation to tell you what is wrong. From the sounds of it it seems that you may have a relay that is stuck and needs replacement.

What area are you located in? We are located in Tempe, AZ and serve the surrounding areas. Feel free to give us a call if you have any questions: 480-208-6330.

Thanks!

It's because you have your

It's because you have your fan turned to "on" instead of auto. The fan will continuously run when set to on. When set to auto it will cut on and off as needed

Auxillary Heat Is Always ON

So my heat pump blows out the normal cold air through our vents and the house usually drops a degree or 2 before it stops and kicks on the auxiliary heat. I am not sure what is going on but my electric bill is more than $100 what it was last month and this is just the beginning of winter.

Get a checkup

Hi Chelea,

I definitely recommend getting a fall check-up on your heat pump, especially if your bills have gone up. If you're in the Phoenix metro area, we can definitely do that for you. Just give us a call!

Just bought a house in GA

Just bought a house in GA with gas heater. Downstairs blows hot almost immediately after thermostat is turned up. Upstairs has its own thermostat but just blows cold air. Any thoughts? Got guests coming tomorrow haha and it's 44 up there now. :/

Possible Disconnected Ducts

Hi Mcvan22,

It's so hard to say without looking at it but you may have some serious duct leakage going on. The only way to find that out is to have your ductwork tested. It could be that your heater was designed like that and only serves the lower part of your home. I would highly recommend having an experienced home performance contractor that does heating and air conditioning come out and do an energy audit AND heating check-up on your furnace. Just make sure the contractor offers both services and you should be good to go. You could also attempt to get up in your attic/crawlspace and see if you can see any signs of leakage (you'll see dirt around the insulation at leakage points because it acts like a filter). Anyways, good luck!

how do I know if I have a heat pump?

Cliff notes version: 2 years ago when it was like 7 degrees outside the system on our house died (b/c the landlord doesn't do proper up keep, she still doesn't on the new one but anyhoo....). It took the heating company a week to get a new unit b/c they said they were waiting on a heat pump, which they didn't have in stock. Once it was all fixed the unit worked fine, still does, prob needs a tune up though :) but not my money ha..... Now what I am wondering is why are we are having cold air blow up out of the wall vents when the unit starts up, (it is cold outside, just so you know) then the air coming out of the vents gets warmish (we keep it to 68/69 in the house). One vent is RIGHT beside my office desk and it is COLD when it starts. So, if I am reading your site correctly we have a heat pump but here is why I wonder if we do....we had a guy come out for a free tune up from Dominion Power (our power company). He said he could only do the tune up and provide a few other free services IF WE HAD a heat pump. He took one brief look at our unit and said we didn't. So I am so confused? My landlord says we do, the heating company that installed it was waiting on a heat pump (they said) and they drug something huge under the house, so how do we tell? The free tune up guy didn't even look under the house. There is an old furnace under there that no longer works and was just left there and covered up on the inside of the house. Thoughts?

HEAT PUMP

I live on the east coast mid atlantic. My heat pump at 70 was blowing cold air so I turn it up to 75 then it starting blowing warm air. After running at 75 for about 10 mins it started blowing cold air again. I set it to 85 and it starting blowing warm air again but got cold again after running for about 10 mins. Is this normal?

Ever since I moved into my

Ever since I moved into my apartment in Ky my unit hasn't worked. I've had my landlord send out his guys out 11 times now and I keep getting "it's fine" and so on. My unit will run for 2-4 hours and the ceiling vents only blow out cold air until the aux kicks on and will never heat up to whatever the thermostat is set on at the time. The furnace is supposedly heating up to temp but the air blowing out of the vents feel like air conditioning.

During the summer they replaced something in the furnace because heat was kicking on instead of air conditioning but it didn't really help, the air coming out was still warmer than our oscelating fan. I'm tired of calling these guys without getting any results, what on earth could it be?

New heat pumps

I recently purchased an office space that has 4 heat pumps. 2 of the units where not working correctly, they would turn on but the room temp would not get within 6 or 7 degrees of the temp the thermostat was set to, and the air coming out of the vent was very cool. I contacted two separate HVAC professionals to evaluate the problem and they both advised to have the units replaced since they where old and seemed to have multiple issues. Since both technicians advised replacing them I followed their advise, but shortly after replacing both I realized the pumps where still not working correctly. I had the same problem. I had both techs come out again and take a look, and even had a third HVAC tech also examine the pumps, and all three concluded the water supply going to the pump was diminished by sediment build up in the pipes. I was advised to have the pipes supplying water to the heat pumps replaced, which I did, and I am still having the same problem. What could it be? I would think new pumps and new water lines, what else can be changed?
The other offices in the building don't have these issues so I don't think it's the outside unit. As you can imagine, this is very frustrating...any advice is appreciated!

still cold air 5 hrs after turning off heater

I got home today and the house was chilly, five degrees below the set temp. The propane tank is full, so all's good there, but the side of the heating unit has a scorch mark on it and also a big scorch mark on the ground next to the unit. It keeps blowing cold air through the floor vents hours after I turned the heat off. Why is it still running?

Return blows coad air

The return on my heater blows cold air the entire time the heater is running. My heater is in the garage which is finished and has only two small vents quite a distance from the furnace . Thank you for any help.

return vents in ceiling

Is it normal for cool air to come out of return vents when furnace is on?

Hot air from exhaust vent when heat pump is defrosting

When my heat pump is defrosting, the exhaust pipe for the combustion gasses (for when I'm on aux/propane heat) also blows out hot air. As soon as the defrost cycle ends, the hot air stops as well. Why would this be happening?

STILL COLD AIR 4 HRS AFTER TURNING OFF HEATER

I got home today and the house was chilly, five degrees below the set temp. The propane tank is full, so all's good there, but the side of the heating unit has a scorch mark on it and also a big scorch mark on the ground next to the unit. It keeps blowing cold air through the floor vents hours after I turned the heat off. Why is it still running?
So my heat pump blows out the normal cold air through our vents and the house usually drops a degree or 2 before it stops and kicks on the auxiliary heat. I am not sure what is going on but my electric bill is more than $100 what it was last month and this is just the beginning of winter.

heat Pumps

great company

Heat pump kicks on the auxiliary heat

Every day when we go to work we always lower temperature so that it stops during outside. When we came home we always increase temperate. We don't have problem for several years but now my heat pump blows out the cold air through our vents and the house usually drops by 2 degrees before it stops and kicks on the auxiliary heat. I have called the technician and have fixed the leak but still same problem. What might be the solution?

Thanks for taking this

Thanks for taking this opportunity to discuss this, I appreciate with this and if you have some more information please share it with me.

Small vents blowing hot air and big vents not blowing anything.

I purchased a used home and everything was working fine but from yesterday when I switch on my central heating system all small vents blowing hot air but two big vents on the first floor not blowing any air. Same on the 2nd floor and in my bed room it is blowing only cold air when put on heat. I have a zoning system. Thank you for your valuable reply.

Design Flaw or Disconnect

Hi there, 

I would definitely call a proffessional to check things out. It could be low velocity due to poor routing of the ductwork (kinked/crushed, etc.) or the problems could be being caused by the ductwork being disconnected in these areas. If you're in the Phoenix area, I would suggest having us out to do a duct leakage test. If not, I am sure there is someone that is well qualified where you are located. Just make sure they do a good job and offer duct leakage testing services.

Thanks!

i live on long island and

i live on long island and just had central a/c installed beginning of september. the unit is now shut off but at times notice cold air is coming out of the vents is this normal do i need to call the installing company back to check ,? thanks

Did this happen before?

Hi there,

Did this happen before the AC was installed? If not, you may have some leaks in the duct system. I would get in the attic, with the air turned on and go up to the connections where you are feeling this draft coming from. Check to see if everything is sealed with duct mastic (not duct tape!) and there are no holes. The better way of doing this would be to hire a professional to perform a duct leakage test (which usually is included in an energy audit) because if there are leaks in the duct system it will cause uncomfortable rooms and high bills. Thanks and good luck!

cold air when heat is on. i have oil burning furnace

I have an oil burning furnace. I just had the tank filled two days ago. I turned on the heat and I let it run for a while but it only kicked out cold air. What am I doing wrong?! Please help. I can't afford to pay someone to come out and look at it!!

Half the vents blow cold and half blow heat and other problem

When the heat is turned on the vents in the back of the house blow cold air while the ones in the front still blow warm air, it's almost winter in Michigan, heat would be amazing haha. AND a couple times when we've let the heat run all night, we'll wake to find the furnace off even though it still turned on

Get an Energy Audit and Furnace Tune Up

The problems you are describing are more in depth than online advice would be able to address...I would seriously consider having an energy audit done along with a heating tune up to be able to diagnose exactly what your problem is... Ecotelligent Homes is a good company located in Michigan, but I am not sure of their service area. Their phone number is 248-430-6297 Best of luck! 

No heat

I live in a mobile home in florida so we almost never use the heat. However we tried to turn on the heat this am just to warm up the inside for a few minutes & only cool air came out the vents. We didn't even smell the heat come as we usually do. Its been a yr since the heat was used last. Could it be a diy fix or do i need to call a repair person?

Central Air Issues

Hi, I'm currently having issues with my central air blowing out warm and cold air while its on auto and set for heat about 80 degrees, we had a friend of ours come out and look at the unit and he said it needed a deep cleaning which he did and cleaned out all the heater vents and its still doing the same thing and it even turned off in the middle of the night. I also know that my husband was messing with a surge breaker because the power went out. I'm worried that this maybe a major issue. can someone please help and tell me what they think it could be? we also have a cover over the air unit outside so it wont get snow on it.

I recently moved into a

I recently moved into a rental home and have a strange situation. When the thermostat is set to heat, cold air comes out, and when it is set to cool, hot air comes out. I'm hoping this is just an issue of a crossed wire somewhere but I'm not sure how to check. When I checked the wiring, all colors line up with the labels. Any suggestions?

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