Air Conditioner & Heat Pump Air Flow Diagnostic FAQs

Supply air, Return air, Air flow rates, Air temperatures

A/C or Heat Pump Supply Air Too Warm FAQs

Air out of Supply Vents is Not Cool Enough

Got home today and the temp on the thermostat said 78 we have it set at 74 and it has now gone up to 79.

The air coming out of the vents is cool but not cold like usual what could be the cause of this? On 2012-07-05 by Tina -

Reply by (mod) -

Tina: regarding "air coming out of the vents is cool but not cold like usual" - if the home is very hot it is possible that it could take a while for air coming out of the supply vents to cool down - the system has to cool the whole duct system as well; It'd make sense to check for a dirty air filter or an ice-blocked cooling coil too.

But if the "cool air" supply never cools off, and if the air flow rate is not also reduced and those blockages I just named are not present, it's time for a service call.

You might want to review the more detailed advice

at LOST COOLING CAPACITY

 

Heat pump sometimes blows warm air when in COOL mode

I have a heat pump and it periodically blows warm air in AC mode, this happens several times a week.

If I turn the unit off for about 15 mi utes and then back on it works fine.

We've replaced rhe brackers and fuses, it has clean filters and the compressor is running. Please help! On 2018-06-25 by Jackie

Reply by (mod) -

if your system is switching to heating mode and you're not calling for heat I suspect there's a control board problem

Your service tech will probably first rule out a control board problem that is blowing warm air in cooling mode during a deicing cycle.

Return Air FAQs

Why is return air being taken from my garage? Garage is too hot.

Why am I getting return air for my HVAC from garage ceiling .air is so hot in garage area - On 2011-07-03 by al entez -

Reply by (mod) - block off unwanted return air but check the total return air adequacy too

Al, if your garage is not being cooled AND if there is adequate return air supply from the rest of your building, you can simply block off that garage return-air inlet. That's what I'd do.

But be sure that you've got enough return air elsewhere in the system.

See RETURN AIR, HVAC

A/C Heat Pump No Supply Air Flow FAQs

Crawlspace (where ducts run) is cold but the house above is hot

I had a new central air unit installed last summer. If it's hot outside, my new system can't keep the house at the temperature set on the thermostat. 74 degrees.

To keep the temperature from exceeding 76 degrees I have to use all of my ceiling fans, a table fan, and the central air unit

However, my crawlspace, where the duct work is located is freezing.

In addition, during routine maintenance this Spring, the technician found the unit a full pound low on refrigerant.

I have had three service calls but each time I am told that the system is fine.

I've lived in this house for 12 years and have never had this problem before. Any ideas? On 2012-07-03 by Anonymous

Reply by Mod - look for disconnected or leaky ducts in the crawlspace

Anonymous: if the crawl area where HVAC ductwork is routed is freezing cold and the occupied space of the home is too warm when your A/C is running, I suspect a supply duct leak into the crawl area;

Inspect the ductwork carefully for damage.

 

No air coming out of A/C supply registers

The A/C unit is running outside but no cool air is blowing through the ducts. We put the thermostat temperature lower and nothing happened. If the thermostat is temp is put higher the the actual room temp, the A/C unit outside does stop.

Also tried to run just the fan, but it did not work (does not start). Yesterday the A/C unit worked without the fan, after letting it off even with turning off the breakers for a while, it worked again through the night. This morning the blower stopped working again. On 2012-09-04 by Ken

Reply by (mod) -

Ken as you have no air coming out of A/C supply registers it sounds as if the indoor air handler blower fan is not running, or if it is, that something (iced coil or dirty filter or crushed or disconnected ducts) are blocking air flow.

I'd start at the top DUCT & AIR FLOW PROBLEMS and go through the diagnostic suggestions there

and at AIR FLOW TOO WEAK

Adequate air flow for air handler in a closet?

I have an indoor air handler for a heat pump installed in closet with a louvered door.

Is there enough air flow for it? On 2019-02-20 by Kenneth

Reply by Mod: Unwanted airflow restriction may added by the louvered door; safety questions

I can't say accurately, Kenneth, just how much unwanted airflow restriction is added by the louvered door in front of your air handler return opening, but here are three key points:

1. For two openings of equal size, louvers on one of them will cut the airflow there by about 33% or more depending on louver details like angle, thickness, and spacing.

So if the opening in your door do the air handler is just about the size as the opening on the air handler itself, that door is probably cutting return air flow, increasing operating cost, and in some installations may also be unsafe.

Watch out: key in understanding your question and answering it is what is the purpose of the "air flow" about which you ask.

2. IF the main or central building return air to your air handler is through that louvered door and into a return air opening in that closet then the total air opening should be at least as big as the air inlet opening on the air handler itself.

3. Combustion air safety concern:

Watch out: if your air handler is not just for cooling but also for heating, and if the heater is fueled by oil or gas, then the unit needs to have an adequate supply of outdoor air to support safe combustion - otherwise the unit could be unsafe, risking (for gas burners particularly) a fatal carbon monoxide poisoning hazard or for an oil burner, insufficient combustion air, sooty operation and a puffback explosion.

So we need to know which sort of "air supply" is in question here.

Also see these diagnostic pages

AIR FLOW WEAK at SUPPLY REGISTERS FAQs

COMBUSTION AIR REQUIREMENTS