Keep fan on after AC turns off?

[jmed999]

My Venstar T1700 has an option to run the fan after the AC has turned off. I took some data and looking for advice.

 

With my HVAC not running my register is 68 F. With the AC on, the register bottoms out at ~53 F. When I raise the setpoint to get the AC to turn off I notice the register temp slowly goes up to ~61 before it turns the airflow off.

 

So I think it cools then when I adjust the setpoint up to get the AC to turn off the compressor turns off immediately but the fan continues to run. So it's almost as if my unit already is running the fan after the compressor stops.

 

My question is should I set the Venstar to continue to run? If so how long or better yet until what register temp? ie. is it a good idea to let it run until the register temp is ~68?

 

Thoughts?

[apostolakisl]

I wouldn't. Any hvac unit that isn't ancient will continue to run the fan after the compressor is off an appropriate time to maximize energy efficiency. Running the fan after that would probably not be to your benefit.

[C Martin]

appostolakist & jmed999,

First off, I don't know of any HVAC system that doesn't control the fan mode in the "Cooling" cycle beyond what the thermostat monitors. I know that there are internal conditions for running the fan in "Heating" mode after set-point so that the unit doesn't overheat, and that has nothing to do with the thermostat.

 

Having said this, I understand what jmed999 is stating. What you are seeing is the thermal storage of your duct-work. You are smart to want take advantage of this storage, otherwise it just dissipates into the attic or other unused spaces.

 

I ran a test this morning and I noticed that by running my fan manually after reaching the HVAC set point my, I was able to run my fan for at least 5 minutes before any significant rise in my indoor temperature, therefore maximizing the use of thermo-storage in my duct-work.

 

I like your thinking and will have to think about a program to maximize this effect.

 

Any other comments?

[jmed999]

-According to my tests, my cooling system is already running the fan after the compressor turns off.

 

-I don't think running the fan until the supply air is the same temp as your house is the optimal thing to do. At some point running your fan will cost more than the energy you save by not cooling your home air. ie. if you setpoint is 75 and you run your fan until the supply air reaches 75 then the fan would run while the supply air is 74. I would guess it's cheaper to cool from 75 to 74 than running your fan at 74. I don't think you are getting much of a return at that point. I don't think running the fan while the supply air is at 74 is optimal. I would guess that running the fan past a supply air temp of ~65 isn't worth it. Of course this is just a guess.

[C Martin]

unless you have remote temperature sensors in you duct-work you system can't possibly know about thermal storage and how to take advantage of this storage.

I truly believe that if you run a test on your HVAC unit, the fan turns off immediately when your condenser shuts off.

 

Leaving your fan on to take advantage of thermal storage is indeed more economical than running both the condenser and fan together. It's just a matter of how economical it is to capture the thermal storage.

The ideal situation is to have a variable speed fan that can run at a lower speed to capture the thermal storage of your duct-work.

[apostolakisl]

Every home will have different details.

 

But the companies that make hvac systems are in a huge battle with each other to make their systems the most efficient at the best price. They all continue to run the fan after the compressor shuts down for some period of time. The have a great wealth of knowledge and research on the subject to make that decision. The price of the equipment is the same regardless of how long the fan runs after the compressor shuts down, so it would be silly to think that they didn't set it to the most efficient time.

 

Furthermore, the fan consumes electricity. Just because the air coming out the vent is still a degree or 2 colder than the air that is in the house for a few more seconds, doesn't mean that you are being more efficient. It may be that the fan electric consumption costs you more than that slightly cooler air is worth.

 

Again, the hvac manufacturers are far more versed in this stuff that you and I could ever hope to be and it just doesn't make a lick of sense to think that they didn't already optimize this simple control.

[C Martin]

apostolakisl

 

You seem to be very closed minded in your comments. You have no idea what my background is and you seem to have some misplaced loyalty to the HVAC industry as a whole.

I am involved in the industry both on the residential side as well as the commercial side. I think that the concept of thermal storage recovery is important and is under-utilized in the industry as a whole. Unless you have proven data to the contrary, I suggest that you not have such a closed mind to new ideas.

Sorry to rant about this but....

[LeeG]

I have no opinion either way. It took some energy to create the thermal storage. How much affect will reducing that thermal storage have the next time cold air is called for? It seems that some of the gain in reducing the thermal storage could be lost in replenishing the thermal storage the next cooling cycle. Could it cost more running the compressor and fan to replenish the thermal storage than saved by running the fan to siphon off the thermal storage.

[jmed999]

As a State Licensed HVAC Professional Engineer I can tell you that in industrial applications running the fan after the supply temp is above ~65 is inefficient. Now with that said, all buildings and systems are different hence the reason I asked the question. If someone with experience in home HVAC had tested this and said it was typically best to run it up ~68 I may have done so. I seriously doubt running the fan the last 2 degrees is efficient. I also have some pretty good equipment I was using for my test, like a certified laser gun that is calibrated every six months.

 

I agree the folks designing these systems know way more than I do and I'm now convinced they have the fan run the optimal amount of time generally speaking.

[jmed999]

We are not using the words "thermal storage" correctly here. When cooler air is present in the duct as opposed to the air surrounding the duct, thermal energy is not being stored in the duct. Thermal energy moves from hot to cold (2nd law of thermodynamics) so as long as you are running the AC and not heat, you aren't storing thermal energy in the duct.

[C Martin]

apostolakisl

 

I apologize for my comments. This forum is not a place for personal attacks and that is not my intention.

I believe and always will, that new ideas come from open discussions and always will.

I have experience teaching and have always told all of my students that there are no silly or stupid questions. I am an advocate of open thinking.

Innovation comes from people being able to think and explore the ways things are and are not.

[jmed999]

I have no opinion either way. It took some energy to create the thermal storage. How much affect will reducing that thermal storage have the next time cold air is called for? It seems that some of the gain in reducing the thermal storage could be lost in replenishing the thermal storage the next cooling cycle.

 

Good question.

 

Since the air in the duct will heat up while stagnate, it will have reached (or come very close to reaching) thermodynamic equilibrium with the air surrounding the duct. So the energy it will take to cool the air in the duct could be assumed to be the same if the fan ran longer than the HVAC system called for or not. I hope that make sense.

[jmed999]

apostolakisl

 

I apologize for my comments. This forum is not a place for personal attacks and that is not my intention.

I believe and always will, that new ideas come from open discussions and always will.

I have experience teaching and have always told all of my students that there are no silly or stupid questions. I am an advocate of open thinking.

Innovation comes from people being able to think and explore the ways things are and are not.

 

I agree with having a questioning attitude, that's why I asked the original question even though I have some experience in this area.

 

I'm glad we have all given our opinions here but I didn't think anyone personally attacked anyone. I'm glad bright ideas have been discussed here.

[apostolakisl]

Not exactly closed minded. Not to be argumentative, but I don't see you posting data either.

 

While no system I have seen has thermometers at the ducts measuring the outlet temp and run the math to determine when the best time for that system to shut down is, why would they have not picked the most optimized time for the average install. You speak as though the engineers at Carrier/Lenox/etc are stupid and/or don't care about efficiency and that you know more than them. If some particular install is very different than average, you may have a reason to alter their engineering.

 

Now if you have an older system that shuts down the blower the simultaneous to the condenser, then figuring in a little extra run time may help (or may be counter-productive) depending on what type of install you have.

 

1) If your ducts and your evaporator are in an unconditioned space (attic) you will want to dump as much heat from your conditioned space into the ducts and evaporator prior to the fan shutting down, otherwise it just ends up in the attic.

2) If your ducts and evaporator are inside the house (basement), you will accomplish nothing by running the fan longer. Eventually heat in your home will be sunk into the ducts/evaporator anyway. In this case, extra run time would be counterproductive (electricity needlessly used to run the fan and double counterproductive since the fan generates heat that ends up in your house).

 

Depending on how you go about altering the fan run time, you may end up damaging the equipment and/or voiding a warranty.

 

 

EDIT: CMartin. . . all's cool, I also got a bit defensive above.