Thread: If fan is "On" will compressor still come on?

Guys, I know there are exceptions and many different themostats and systems and ways of wiring them, but....in general.....If the HVAC system fan is set to "On" at the thermostat (instead of "Auto") will the compressor still come on when the room temperature reaches the thermostat setting. If for example, you have the thermostat set to keep the house at 78 degrees, and the room temperature reaches 79 degrees, the compressor would start. But if you had previously set the system fan to "On" (for general air circulation), would the compressor still come on? or would the fan setting overide the compressor? Are these two independent of each other? I am asking because I would like to keep my fan switch at "on" during the night, and have the compressor come on and off as the thermostat dictates.

What say ye experts?

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Yes. If all is as it should be, the answer is yes.

Originally Posted by Gene South

Guys, I know there are exceptions and many different themostats and systems and ways of wiring them, but....in general.....If the HVAC system fan is set to "On" at the thermostat (instead of "Auto") will the compressor still come on when the room temperature reaches the thermostat setting. If for example, you have the thermostat set to keep the house at 78 degrees, and the room temperature reaches 79 degrees, the compressor would start. But if you had previously set the system fan to "On" (for general air circulation), would the compressor still come on? or would the fan setting overide the compressor? Are these two independent of each other? I am asking because I would like to keep my fan switch at "on" during the night, and have the compressor come on and off as the thermostat dictates.

What say ye experts?

Yes.
Setting the fan switch to "on" just puts 28v on the green fan wire. When the thermostat calls for cool it puts 28v on the the yellow wire and the green wire.

During the summer months we keep our t-stat set on "ON" during the day to keep the air circulating in our home. We have done it for years.

Be aware that if you leave your fan in the on position you may be decreasing the efficiency of your unit. Many of the new variable speed air handlers have different speeds for heating and cooling, Many of them have up to 5 speeds that can be chosen with dip-switch positions. Some of these units just use high speed as a default when the fan is in the on position. Read the manuals that come with these fan coils and furnaces and make an informed decision.

Originally Posted by David Bell

Be aware that if you leave your fan in the on position you may be decreasing the efficiency of your unit. Many of the new variable speed air handlers have different speeds for heating and cooling, Many of them have up to 5 speeds that can be chosen with dip-switch positions. Some of these units just use high speed as a default when the fan is in the on position. Read the manuals that come with these fan coils and furnaces and make an informed decision.

This is a very good point. Good call.

[QUOTE= I am asking because I would like to keep my fan switch at "on" during the night, and have the compressor come on and off as the thermostat dictates.

What say ye experts?[/QUOTE]

This is not a good idea. Having the fan run after the compressor shuts off blows warm humid air (evaporating off of the now warm coil) into the ductwork, increasing the likelihood of mod growing in the ductwork, especially if your air handler is located outside the conditioned space and is sucking in humid air, which almost all do.

I have often recommended that the fan be run continuously during cooling months to keep the temperature more even throughout the living space. Especially beneficial for homes where there is no "high" return, or in split-level homes.

Don't see a problem with condensate re-evaporating off the coil causing any problem as mentioned in previous post. Cycling periods of compressor run times is usually often enough to be of no concern.

Originally Posted by Scott Patterson

During the summer months we keep our t-stat set on "ON" during the day to keep the air circulating in our home. We have done it for years.

If you do that in my neck of the woods you would just be introducing more heat into the home via convection. In the south those ducts run through the attic and draw in heat from the attic (I don't care how much insulation you put on them they will always try to equalize temperature; thus working against your A/C). That is why we can't run the fan just for ventilation, you have to have it come on when the compressor is running....

Reading some of the replies reminded me of a project I did in south Texas. The system was a small five ton unit for an office area and was insulated duct above the ceiling and below an insulated roof. I had a dedicated outside air intake duct to meet the ventilation code so any time the fan was on the OA damper opened and we pulled in outside air.

The problem was the space would cool down in the night and during the day when the humidity levels crept up, the humid air from outdoors would condense on the walls, floors and metal surfaces INSIDE of the conditioned space.

We couldn't really close the OA damper and still meet the ventilation code (the client did it anyway) but my instruction was to leave the fan switch in the "Auto" mode to lessen the intake of humid air when the compressor wasn't running.

A solution for one area does not work for all.

About the only time I advise customers to run the fan in the on position is if they have a supplemental heat source, wood stove, fireplace, gas log, etc. This keeps things fairly uniform. As far as rooftops, most economizers have enthalpy controls and, 5 to 10% minimum outside air should give you the required air changes. Continuous fan operation should not be needed.

Originally Posted by David Bell

Be aware that if you leave your fan in the on position you may be decreasing the efficiency of your unit. Many of the new variable speed air handlers have different speeds for heating and cooling, Many of them have up to 5 speeds that can be chosen with dip-switch positions. Some of these units just use high speed as a default when the fan is in the on position. Read the manuals that come with these fan coils and furnaces and make an informed decision.

Please allow me to correct this statement. If you do have a unit with a variable speed blower then yes it will run at a very low speed if the fan switch is on. This is very good for circulating air, and keeping the home a uniform temp. It also uses between 100-200 watts which is very efficient.
Here is the correction: When the compressor kicks on the control board in the unit recognizes the demand for cool and kicks the fan up to cooling speed.
I'm not too skilled on this site, so I can't figure out how to quote multiple. But some one else mentioned that running the fan would pull moisture off the coil and cause mold. Not true. It is actually more beneficial because it dries the coil and prevents mold from growing there.
Spoken from an A/C professional.

Originally Posted by ray jackson

But some one else mentioned that running the fan would pull moisture off the coil and cause mold. Not true. It is actually more beneficial because it dries the coil and prevents mold from growing there.
Spoken from an A/C professional.

Ray, I've seen this more than once in FL, which is of course, a humid climate. When the compressor shuts off, the coil and air circulating in the ductwork becomes warmer and therefore holds more moisture. (Where do you think that moisture we are "drying off the coil" is going?) We're blowing that moist air into the ductwork every time the compressor shuts off. This creates conditions conducive to the growth of mold.

It's worse if your air handler is in the attic or garage. All air handlers and all ductwork leak (unless they've been sealed and tested, then sealed to correct the leakage found during testing.) The air handler, under negative pressure, is constantly introducing humid air into the system. This increases moisture in the system and in the building. Again, this increases the possibility of mold growth. Never mind the increase in cooling load from the outdoor air you are constantly sucking into the building.

The worst case of this I've seen was a commercial building where outdoor air is intentionally brought into the system. Someone left the fan switch set in the on position, introducing warm, humid outdoor air 24/7/365. This created a mold farm in the building.

Leaving the fan run continously is never a good idea. It might not be so bad in a dry climate and if your air handler and ductwork are all located within the conditioned space, but for the rest of us, it's a bad idea. And it uses more energy.

Originally Posted by Mark Cramer

Ray, I've seen this more than once in FL, which is of course, a humid climate. When the compressor shuts off, the coil and air circulating in the ductwork becomes warmer and therefore holds more moisture. (Where do you think that moisture we are "drying off the coil" is going?) We're blowing that moist air into the ductwork every time the compressor shuts off. This creates conditions conducive to the growth of mold.

It's worse if your air handler is in the attic or garage. All air handlers and all ductwork leak (unless they've been sealed and tested, then sealed to correct the leakage found during testing.) The air handler, under negative pressure, is constantly introducing humid air into the system. This increases moisture in the system and in the building. Again, this increases the possibility of mold growth. Never mind the increase in cooling load from the outdoor air you are constantly sucking into the building.

The worst case of this I've seen was a commercial building where outdoor air is intentionally brought into the system. Someone left the fan switch set in the on position, introducing warm, humid outdoor air 24/7/365. This created a mold farm in the building.

Leaving the fan run continously is never a good idea. It might not be so bad in a dry climate and if your air handler and ductwork are all located within the conditioned space, but for the rest of us, it's a bad idea. And it uses more energy.

I may stand corrected, but not without a fight. I live and work in the dry San Joaquin Valley in CA so I don't regularly deal with high humidity. However, if the fan is running constantly the moisture left on the coil won't just sit in the duct. It will be absorbed into the air in the entire space. Most a/c systems with any logic controller (circuit board) will have a cooling fan off delay. This is designed to get whatever cold air is left in the coil into the space. This feature is more likely to assist mold growth than running the fan continuously.

As far as leaving the fan on constantly never being a good idea I disagree. With a variable speed blower it runs at about 50%. At this low speed the leakage is reduced from whatever it would be at cooling speed and may only be negligible. I do agree that circulating fan is not beneficial if there are large amounts of duct leakage. But since I am speaking hypothetically, my duct system is 100% sealed.

I would say that if you are wanting to run the fan you are covering up insulation and air sealing issues. The stack can reverse in the summer. The cool air is falling to the basement. Hot air is pulled in from the attic. The heat buildup in the attic during the day is trying to get in to the coller house---hot goes to cold. Air seal the attic floor and insulate with anything but fiberglass. Also air seal the rim joist.

For several reasons I do not see running the fan as a good idea.

Since return ducting uses wall cavities there is usally a connection to the outside when the fan is on. That is some amount of attic air is being pulled into the house. Thus you would be increasing the load on the ac.

If you have HVAC duct or equipment in the attic you are going to pull more hot air in from the attic.

Running the fans taks a lot of energy unless it is a newer ecm motor.

Originally Posted by H.G. Watson, Sr.

Yes. If all is as it should be, the answer is yes.

thank you for your expert opinions gentlemen. my friend barry owes my friend walt $100 thanks to this panel. i consulted this site via google to find out that the compressor does indeed still turn on when the fan is in the "on" position. barry thought otherwise...