My parents live in a mobile home in the desert. My dad says the fan switch should stay off when the A/C is running so it doesn't heat the house up after the A/C shuts off.

My mom says the fan should stay running so it circulates the cold air.

Who is correct and why?

Being that they live in a trailer in the desert... I'm surprised the a/c ever turns off! Lol

Id say have it on auto, so its only on with the a/c because, certain parts of the house may not get as cool.. therefore, circulating it will pull the warmer air in from other areas?


(the way you worded it, its like a test question in school! Lol.. if a train leaves boston traveling 70 mph... lol)

Ask this question to a dozen home owners,, and you`ll get 13 answers,, I would try it both ways,, let them agree to decide which way is more comfy,,

With the blower running constantly you will find that the outer rooms of the home will keep a moore constant temp with the center part. You may find that your A/C unit runs less from doing this and in turn saves you money. I moved into my home three years ago and the blower has been in the on position and running 24/7 with no problems.

As already stated, leaving it on will help keep even temperatures in the whole house and probably keep the AC from from running as much. Checking the filter(s) more often would be a good idea; it's likely they will accumulate dust and dirt a little faster.

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Originally Posted by Missouri Bound

As already stated, leaving it on will help keep even temperatures in the whole house and probably keep the AC from from running as much. Checking the filter(s) more often would be a good idea; it's likely they will accumulate dust and dirt a little faster.

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By leaving the bower run at high (A/C ) speed all the time you will introduce some of the moisture you just paid to remove back into your home.

The cure is to use a relay that lowers the blower motor speed to low (or the lowest speed possible) during the times when the outside unit is not running. This will prevent the moisture reintrogration in the home but still allow the benefits of air movement.

Try this below to help pervent the moisture problem.

If you have major air leaks in your supply duct I can see that. It would be pulling a negative pressure on the building your trying to cool. But moste homes won't have that big of an air lost to be introducing high amounts of humidity in to the home. High speed blower or low speed blower with out the compressor running should not make a difference of the humidity in your home from 1 or 2 %.

Incorrect.

Here are facts that have not been taken into consideration. The slower the air moves across an air conditioning coil, the colder that coil is and the more moisture it collects. The faster that air moves across the coil the warmer it becomes and the less moisture it collects. The colder the coil's surface is, the better water will stick to it. The warmer the coil is, the less water will stick to it. Combine high air speed with a warm coil (as in when the condenser outside is turned off) and you have water shooting down the ductwork from the residual on the coil itself and from the water left in the drain pan.

Energy usage/ Called Fan Laws to the engineer types out there.

Fan Laws:
CFM2 = CFM1 x RPM2 / RPM1 or CFM is directly pegged to rpm.

If RPM is cut in half then CFM is also cut in half. If 1,040 RPM produces 1,200 CFM then 520 RPM will produce 600 CFM.

HP @ S2 = HP @ S1 x (RPM2 / RPM1)2 This demonstrates that the horsepower required to turn the fan is related to the square root of the speed change. Or if the fan's speed is cut in half, then the amount of air delivered is also cut in half but the Horsepower required is only ¼ of the original Horsepower required. OR, a fully loaded ½ HP fan motor running at 1,040 RPM and producing 1,200 CFM will only require 1/8 HP to deliver 600 CFM at ½ speed of 520 RPM. This demonstrates that the power required to turn the fan reduces a lot faster than the reduction in CFM being delivered.

Watts2 = Watts1 x (RPM2/RPM1)3 or Watts2 = Watts1 (CFM2/CFM1)3 This demonstrates that the electrical power required to turn the fan drops by the cube of the speed change. Or drop the speed (RPM) in half and the Power (Watts) required is 1/8 the original power required. Our example: ½ HP fully loaded fan motor running at 1,040 RPM and delivering 1,200 CFM requires 745 watts/2 or 372.5 Watts. Cut this motor's speed in half and you cut the air delivery in half but the Wattage required is 372.5/8 = 46.6 Watts.

Therefore, a fan motor must run 8 hours at half speed to use the same amount of electricity as it would running at full speed for 1 hour.

With the high cost of electric today you can see by the above formula that a blower motor running at normal A/C (high) speed is going to use more electric than is needed to just circulate the air in the home. When you lower the speed of the indoor unit during the off cycle of the outdoor condensing unit you first accomplish quite a bit of electrical savings as compared to continue running at high speed. You will also loose the tendency to pick up any remaining coil/pan moisture that is left in the system after the outside unit shuts down.

A few more facts.

The high-speed blowers that are installed in new equipment are designed to meet an energy goal and totally neglect the comfort of the homeowner. When the air conditioner is on and the super high-speed blower strips water from the air conditioner's coil and sends it down the ductwork, all kinds of nasty things can start growing in those ducts. Especially Mold!! That dirty sock smell!! Allergies!! The only thing worse is running that blower at high speed when the outside unit is off and not cooling.

One super important aspect of air conditioning is the REMOVAL of moisture (each 10% reduction in relative humidity is the comfort equivalent of a three degree drop in temperature). These high-speed fans drastically reduce humidity removal capacity even with the A/C outside unit on. This process is called water stripping and under those conditions it just strips the water off the coil/pan and sends it down the duct. Here again you get the dirty sock syndrome that we all here about so much.

When you install a furnace or airhandler a professional will use special tools to determine the correct speed of the blower motor for both heating and cooling there by reducing the uncomfortable drafts,stoping the water stripping,and save the customer big $$$ on the utility bill.

Most non professionals will just slap the unit in and let it rip since they figured the factory set the correct blower speed for proper operation. NOT SO. The factory always sets the A/C fan to high speed and in 75% of the cases that is just to fast and causes problems.

There is much more to this subject but I am not holding class tonight.

Hey teach you have way too much time on your hands. But please do give more info.