HVAC question

[LDB]

We just got a new system installed. It is all electric. When the furnace runs you can hear the thermostat click to end the cycle. It continues to run to force all the warm air out of the ducts. Good. The problem is, using incorrect but understandable terms, there is 1 gallon of warm air in the ducts. The fan blows long enough to push 5 gallons of air out. Those last 4 gallons of air are cold air from the attic, as if the a/c was running. Is it possible, and if so how, to adjust the timer controlling fan shutoff after the heating element is shut off?

 

[hrv]

The old ones had a anticipator that you were able to set for that exact issue you are talking about...but the newer electronic ones are preset...not sure if you can even mess with them anymore

 

[Creekside]

We just got a new system installed. It is all electric. When the furnace runs you can hear the thermostat click to end the cycle. It continues to run to force all the warm air out of the ducts. Good. The problem is, using incorrect but understandable terms, there is 1 gallon of warm air in the ducts. The fan blows long enough to push 5 gallons of air out. Those last 4 gallons of air are cold air from the attic, as if the a/c was running. Is it possible, and if so how, to adjust the timer controlling fan shutoff after the heating element is shut off?

Is it running to scavenge warm air or to cool the heating element? If it's a feature to cool and protect the element(s) I wouldn't adjust it.

 

[BMWTurboDzl]

I was told it does that to keep the heat exchanger from overheating.

 

[dnewton3]

The delay of fan running is to sufficienty cool the heating elements. Just because there's "5 gallons" of cold air coming through the ducts, that's not the main concern. The concern is to make sure there's no localized residual heat at the elements and they are sufficiently cool going into rest.

There is typically a way to "adjust" the delay on the control board; either direct taps or dip switches. However, I STRONGLY CAUTION you not to mess with them; leave well enough alone as they should be set at installation for the correct condition of your system. If you feel compelled and competent enough, there is probably detailed info in the installation manual as to how to set them properly.

 

[Zaedock]

FYI - I work in the building automation industry in Boston, so not with small residential systems, but some of my subcontractors do and there could be warranty issues if you adjust settings after the system has been commissioned.

 

[tmorris1]

The feature is there for a reason as others have stated. A lot of times there is a setting in the thermostat to adjust this time, but I would check with the installer before changing it so you don't void a warranty or anything.

 

[mk378]

cold air from the attic

It should be pulling return air from inside the house, so without the heater active the air still won't be colder than inside. (It may feel colder because it is moving though). Inspect the installation to be sure there are not open ducts in the attic. Ductwork both supply and return that goes through an unheated space should be insulated.

 

[Lubener]

Since it is new, I would consult your HVAC installer. There may be jumper settings on the blower control board or it is programmed.

 

[LDB]

I'm not going to do anything. I just wanted to find out if it's something to pursue before calling the a/c people about it. There are no leaks in the ducting. I just know the air is much colder as I sit directly beneath a vent so I get the gallon of cold air when it first starts and pushes that out with the heated air and then I get the several gallons of cold air at the end. I hadn't thought about it cooling the heating element, just making sure all the heated air was delivered into the house.

 

[JohnnyG]

It depends on the thermostat. The installer usually keeps the detailed operation manual to themselves, and use settings that typically ensure they get satisfactory results, and sometimes, even a callback.
The installer left the tech manual with me on my last install, and there are 18 PAGES of settings that can be manipulated in order to customize it.
You might be able to get the manual from the installer, or you might get one on the internet. I recommend getting it in hard copy so you can make notes and such. You may need to change the settings come summertime in Texas.

 

[Barkleymut]

I'm not going to do anything. I just wanted to find out if it's something to pursue before calling the a/c people about it. There are no leaks in the ducting. I just know the air is much colder as I sit directly beneath a vent so I get the gallon of cold air when it first starts and pushes that out with the heated air and then I get the several gallons of cold air at the end. I hadn't thought about it cooling the heating element, just making sure all the heated air was delivered into the house.

how long after the thermostat clicks is the system running?

 

[Fitter30]

Probably there isn't a adjustable fan delay. Have the install manual? Brand and model number?

 

[LDB]

how long after the thermostat clicks is the system running?

I've never timed it but maybe 45 seconds? I'll have to time it when it runs tonight.

 

[CDX825]

Most electric furnaces or air handlers don't have an adjustable fan off delay like on a gas furnace.

Also being in Texas I hope you got a heat pump system and not straight electric heat with cool. Much cheaper to run a heat pump than electric resistance heat.

 

[alarmguy]

We just got a new system installed. It is all electric. When the furnace runs you can hear the thermostat click to end the cycle. It continues to run to force all the warm air out of the ducts. Good. The problem is, using incorrect but understandable terms, there is 1 gallon of warm air in the ducts. The fan blows long enough to push 5 gallons of air out. Those last 4 gallons of air are cold air from the attic, as if the a/c was running. Is it possible, and if so how, to adjust the timer controlling fan shutoff after the heating element is shut off?

When you say all electric I assume you are talking heat pump?
Either way, it doesnt matter but depends on the Thermostat. Should be easy enough to look up the model # of the Thermostat and find a programing guide in the PDF format on your computer.
I would find it hard to believe that the thermostat doesn't have a programable anticipator. If you just got the system installed the installer should know. I have adjusted a number of these on Honeywell Stats as well as some from 2gig and Alarm.com (been a while cant remember the details)
Had an issue like this in a church, was blowing cold air in the winter for too long after the heat system shut off, was just a matter of going into the programming and changing the setting. That was a Honeywell. (ps we are talking modern digital thermostats)

To answer your question more directly, YES, you should be able to adjust the fan shut off timer on any modern thermostat but some might be so simplified and inexpensive, like anything, it may not have that option, I just cant imagine it not. If not, clearly if it bothers you (it would me) change it out for a different one.

 

[RhondaHonda]

Is the ductwork insulated? I can understand non-heated air may feel cooler due to movement but it shouldn’t be cold.

 

[97prizm]

I was told it does that to keep the heat exchanger from overheating.

It does. It's just like late 80's and early 90's turbocharged cars that had bushings and weren't water cooled. Many owners installed turbo time or would let them idle for 10 minutes to cool the oil down.

 

[LDB]

Thanks everyone for a lot of good information and suggestions. It gives me good info to go on.

 

[brianl703]

There is typically a way to "adjust" the delay on the control board; either direct taps or dip switches. However, I STRONGLY CAUTION you not to mess with them; leave well enough alone as they should be set at installation for the correct condition of your system. If you feel compelled and competent enough, there is probably detailed info in the installation manual as to how to set them properly.

The manuals I've seen tell you to adjust the time so that cold air isn't blown into the living space.

The HVAC installers I've seen don't bother to read those manuals...