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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?

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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

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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.

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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.

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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! 

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