I've been wondering if the fan blower speed has any effict on an a/c's efficiency. I don't recall this aspect being covered in CR's write ups. When CR tests a/c's, do they do it at varying speeds, or just at high?

In my experience, yes the fan speed does affect the AC's cooling supply.

I'm no HVAC guy, but I can suggest that you get one of those vent thermometers, and then

you can measure your units output. The vent thermometers can probably be purchased at

just about any home supply store. Typically, they look kinda' like a meat thermometer, and

are used to measure the temperature at the return and the supply.(individually, of course).

Then, you'll need to learn how to adjust the fan speed for your unit. This is usually just done

with a jumper on the unit, but will probably require removal of at least the front panel.

 

From my measurements, I can say that a slower air flow produces a colder supply. But, I

can't say just how much colder, since I've never recorded the numbers. Traditionally, the

HVAC guys refer to this measurement as (T)emperature (D)ifference/differential.

 

Of course, although a slower air flow produces colder air, it also reduces the air volume

rate that cycles through the system in a specific period of time. So what's better, colder

air, or higher air flow?

 

 

 

When you lower the speed below what the fan charts for the system specify you lower the capacity. Also lowers the efficiency. Variable speed blowers offer a wide range of speed control. V/S blowers are more variable volume than control. We sometimes lower fan speed as a way to get more dehumidification in cooling mode, if the capacity allows it. The primary function of an a/c is dehumidification rather than cooling.

Adjusting fan speeds seems like a simple thing but isn't something for the average homeowner to try. Electrocution isn't fun. Nor is burning up a motor which is the common problem when customers have tried this own their own.

(I am referring to central A/C.)

Robert is absolutely right. DO NOT TRY TO TINKER WITH YOUR AIR CONDITIONER'S MOTOR OR ANYTHING ELSE IF YOU WANT TO BE SAFE! He is right about burning up the motor as well.

The first part of my question was "I've been wondering if the fan blower speed has any effict on an a/c's efficiency."

While I appreciate the three answers posted, none touched on efficiency.

Do the CR Testers ever look in on these posts?

Thanks to those that responded.

Fan speed does have an effect on efficiency.

There, did that answer your question?

Actually, fan speed is part of the equation for gaining efficiency, which also means matching the capacity of the combined components to the calculated heat load of the home/zone. The duct sizing, friction rates, static pressure are matched against the fan charts of the equipment and the blower speed set accordingly. If time and budget allow it, proper air balancing can be done and any adjustment can be made then.

You asked.

"Fan speed does have an effect on efficiency.

There, did that answer your question?"

GREAT, and now the scores -------

10 to 6, 3/3 tie/ , 4 to 5, and the last a rain out.

If you are talking about central ACs, you can get all the efficiency data you could possibly want from the ARI Directory.  About the size of the NY phone book, and in fine print.  Includes the ratings for about 90% of all the possible combinations for most of the major manufacturers.  Listed by model number and there are a zillion models, so it is tough for the average consumer to wade through that information.  ARI stands for the Air-conditioning and Refrigeration Institute, although they recently merged with another trade group and are now called the Air-conditioning, Heating, and Refrigerating Institute (AHRI). 

Mr. Lemley made some good comments but you may not have understood what he meant.  The cooling number that goes in the top of the efficiency number consists of two types of cooling effects, called sensible and latent.  Sensible is the cooling associated with temperature change.  Putting a thermometer in the air stream will measure only part of what the AC is doing.  Latent cooling is dehumidification since it takes energy to condense the water vapor out of the air.  Though there are minimum efficiency standards set by DOE, they don't force a manufacturer to make equipment that will provide a specified amount sensible to latent (called the sensible heat ratio or sensible heat factor).  Most manufacturers will not build a unit (or recommend fan speed combinations) that will cause the SHR to drop below 0.70 (70% sensible, 30% latent cooling) because the customers will complain about it being cold and clammy, plus possibly growing a good crop of mold on shoes, books and on the wall behind the couch.  An even lower SHR is needed for the southeast around the Gulf coast and the south Atlantic seaboard.  If you use a higher fan speed, with the resulting higher air flow rate, you will likely get a slightly higher efficiency because the indoor heat exchanger performance will be better with a higher air velocity flowing through it.  The fan motor will be pulling more power as well, so the next gain in capacity is somewhat balanced by the increased power consumption of the system.  However, your SHR will go up because you have decreased the residence time of the air in the heat exchanger, giving less time for the air to cool down to the dew point where the condensation can form.  Then the mold will start growing.  That is also why you should cycle your fan with the compressor (auto mode on the thermostat).  If you run the fan continuously (Fan mode), the condensation on the coil and in the drain pan will just re-evaporate during the off-cycle and be put right back into the house.  Two speed units typically have a good SHR in low speed condition, but may have a higher SHR in high speed, perhaps assuming that the high loads are the result of solar or temperature effects on an especially hot day and not from excessively high humidity.  Also, they will run in high speed only a small part of the time, and the manufacturers probably figure that the low speed mode will wring out the moisture during the evening and early morning hours. 

If you look at the change in system efficiency with fan speed, it follows the law of diminishing returns and the manufacturers have already pushed the operating point pretty far up the steeper part of the curve.  If you are looking for more air, it almost sounds like your duct system could stand some improvement.  It is very difficult to actually measure air flow in a field installation.  You can do it indirectly with heat pumps using the auxiliary heaters, but trying to measure the air flow rate with AC units would give you a result that might be within +/- 20% of the true value.  After the effort, you still probably would not know what you have really got.  Central units have a maximum external static pressure that their air flow rate is rated for.  If the ESP exceeds that, the air flow will be less than the rated value.  ESP is basically the backpressure that the duct system imposes on the air handler.  It can also include the vacuum pressure on the return air side, depending on how the return system is designed and how hard the fan has to work to draw the air back to it.  If an installer is going to estimate the air flow rate, they would usually check the ESP and see if it exceeds the manufacturer's maximum.  Most people don't understand that fans don't just put out the same amount of air flow under all circumstances.  A fan produces air flow according to its fan curve (pressure versus flow characteristic that is unique for each fan design).  Just like  the water faucet in your kitchen, if the flow path is wide open you get more flow, if it is restricted, you get less flow.  If you shut off dampers in rooms, you wind up choking down the fan and get less air flow through the unit.  One or two dampers closed in a house with a dozen is not a big thing, but shutting down half of what you have could shorten the life of the unit. 

This has been a pretty long winded explanation of things, but to just explain why CR probably doesn't get too complicated with varying air flow when they are doing performance tests.  It is tough to measure except under well controlled lab conditions and with the right flow equipment, and you don't want to just change air flow randomly since it can have a detrimental effect on the latent performance of the system and possibly even shorten the life of the compressor.  

WILLIAM_MURPHY - Thanks. I believe you answered what I was looking for. I will read your post again when I'm alone and can concentrate better.

Obviously I should have clarified what I was looking for. Actually, I was considering the speed effect for a window unit, but I think your explanation above covers it. 

The unit has a selectable speed control for fast, med, slow.  Last summer I ran it (as much as possible) on slow and seem to have had less mold build up.  My guess was that the slower, but more continuous, humidity removal eventually brought the humidity/condensate level down far enough that it wasn't a problem.

Again - Thanks.

Almost all window units use a single speed compressor because of first cost constraints.  The various speed conditions are simply switching the fan to different speeds while the compressor runs the same.  If you measured the sensible versus latent cooling effects at the different fan speeds, you would get the highest dehumidification rate at the low fan speed condition.  It would also be the lowest total capacity condition and thus would cause the air conditioner compressor to have to run longer to satisfy the cooling requirements of the room.  From a dehumidification perspective, that is exactly what you want.  The unit will dehumidify only when the compressor is running.  Most window units default to a "fan mode" type of operation and just cycle the compressor, probably mostly to make the noise level more uniform.  That re-evaporates some of the condensate during the compressor off-cycle to reduce the overall dehumidifying effect.  If you have the option and the variable noise level is acceptable, have the fan shut down during the off cycle and it will dehumidify better and have a better overall efficiency as well since the fan power during the off-cycle actually adds heat to the room.  If you need more air movement to feel comfortable, turn on a ceiling fan or even just a small table fan.  They will use less energy (the AC motor drives both the indoor and the outdoor fans at the same time) and can probably move the air around the room better than the window unit, and are both a lot quieter too.  If the capacity of the unit at low speed is not adequate the keep the room temperature at a comfortable condition, then turn it up a notch to get a bit more capacity (sensible) out of it.  I would run it at low speed as long as it kept the room temp comfortable since it will dehumidify better and be quieter as well.  The cost impact of the efficiency difference between low speed versus high speed is going to be really quite small, probably less than a couple of dollars over the entire summer. 

I have not looked at the CR ratings on window units in a while, but as I mentioned earlier, it is very hard to accurately measure air flow rate, and especially with a window unit.  You can't use an instrument that obstructs the air flow or you wind up changing the thing you are trying to measure.  You can use a flow system with an auxiliary fan to compensate for the obstruction of the air flow, but that gets pretty complicated.  Accurate measurement of a window air conditioner capacity is a very tough thing to do, so they probably don't want to make it even more complicated. 

Nice post(s). I am not as well versed as you mr. Murphy, but I try to bring it to the level of consumers that I deal with day to day. I and other contractors, are trying to measure as best we can in the feild within reason measure the actual airside performance of systems. Like I've said before I strongly believe that with central air conditioning and heat the airflow must be addressed in systems when designing and installing new, when servicing and before replacing. Tat is where the efficiency can be realized. The consumers want the high efficiency equipment, as high and some cases, higher than they can afford, but when i talk about their duct systems, most just don't want to hear it. I don't understand it, it generally ends up being less. Many times I have said keep your existing equipment and fix/replace your duct work and I have lost the job to someone who has just replaced their equipment.

We get called in quite a bit, especially on mass production homes to fix the problems. I make a lot of money doing it but it does not make me proud of my trade. if consumers would push local authorities to adopt and enforce standards I think it would weed some of this type of work and make contractors accountable. Again, I did say enforce. In a recent trade group meeting, several contractors stated that if they followed the codes they would be out of business. Go figure.

I sure would like to talk to you one on one some time.

Duct systems for central systems are a pet peeve of mine, as it sounds like they are for you.  Many consumers think that if they just buy the box with the best numbers on it, they will actually get that efficiency delivered to the living space.  Probably return air ducts are the biggest culprits because you can't "feel" the leaking air being drawn into the duct.  A leaking return air duct has nearly the same impact on the efficiency of the system as a leaking supply duct.  The best option is to put all ducts within the conditioned space, but most builders would never want to do that since it may require dropping down a hallway ceiling, plus they would have to actually think about how the HVAC system would be installed before they started construction.  A few years ago a local utility company started a guaranteed utility cost program for new construction where they would guarantee that the monthly electric bill would not exceed a certain level if the house was built to their standards.  One thing that the builder had to do was have a duct blaster test performed on the duct system, and it had to leak no more than a certain rate to qualify.  In virtually every case, the duct contractors knew that their work was going to be tested, yet most still had a leakage rate of up to 40% before they would have to come back and seal them up.  Since you can't see air, you don't know how much is really leaking through metal-to-metal seams (either in or out).  The real winner is when an air filter slot is cut in the return duct and put either in a crawl space or in the attic.  Regardless of what may have been used to seal the filter slot initially, eventually that often disappears and you just have a gaping 2 inch opening in the duct drawing in 140 degree attic air in the summer or cold air in the winter.  I am sure you have seen dozens of those.  Duct problems are the most insidious since the consumer is usually not aware of the problem, especially if the ducts have outside insulation that covers over the problems.  Plus, most homeowners don't relish the idea of getting up in the attic or in the crawl space.  A colleague of mine (mining engineer professor, so he knew a lot about ventilation in mines and was not afraid to get dirty) asked me to check out his HVAC system because the utility bills for his 1400 square foot town home were about four times what I pay for my 2400 square foot ranch home.  I did eventually look at his furnace and A/C, but as you would expect, I looked at the duct system first.  I'm sure you know the sorts of things that I found.  One flex duct completely disconnected at the main plenum in the attic.  A coiled up 25 foot length of flex duct used for a 4 foot run.  The filter slot wide open in the crawl space (he lived there 6 years and did not even know there was a filter in his system).  As you know, this is not really rocket science, but this engineer who worked in coal mines never bothered to go into his attic or crawl space, but did complain a lot about his utility bills.  If an engineer would not do that, we can't expect a school teacher or a little old lady to want to, or be able to, check their HVAC system.  But as you noted, the homeowner should not have to look over the shoulder of the HVAC contractor, or do a personal inspection of everything before they buy a home or rent an apartment.  Mr. Lemley, keep up the good work and word-of-mouth will keep you in business even when the construction market goes in the tank.  You will also sleep better at night because your customers are not getting ripped off.

I like the part about General Contractors actually having to think, (about where HVAC systems are installed).

But I do think this the discussions about the importance of the duct systems on the overall system's performance will at least help the consumers visiting this site, (the one that read these posts). And the funny thing is, it is possible to spend less money on improvements and save even more money on utility bills which is the whole idea anyway. I wouldn't want to have a customer install a new system and see little, if any improvement on their bills.

BTW, if you want to see an interesting thread, go to the air purifier section of this forum and look at the filter purifier one.

Bob, & Bill,

Thanks for the interesting and educational "back & forth" chatter. I found them to be good reading.

FWIW - I'm a 74 yr old electronics oriented person, with a few years in the telephone business (retired now).  I feel that people should never stop learning, and have an interest in how almost anything works.

If possible, I do as much installation and repair work myself because of the "General Contractors actually having to think" situation you mention. Most seem to take the cheapest, most profitable course available - with little consideration of their reputation.

Mr. Mellor,

Not sure if this is still an active site.  By the flow of email, you seem knowledgeable on central air conditioning systems.  We are rebuilding our home following the Southern Calif. fires.  Our contractor has two new A/C units 2 to 3 years old.  He claims the A/C technology has not changed significantly in the past few years.  It seems illogical to me.  Is this true?

Thanks, Bill

As an engineer, I try to find the limits or margins.  Consider the "extreme" cases. When the fan speed is zero, you get no cooling out and the unit will freeze over.  Conclusion: at some point, slow is bad.  At excessive high speeds you are wasting power and not getting more cooling.   The engineers who designed the unit put a good deal of thought into the problem to get a good EER rating.  The available fan speeds are probably fairly close to the best efficiency.  You are not going to find some magic trick to get a big boost.  (It would be helpful if they quoted the EER for low speed and high speed separately.)

This may be off topic a bit. Reading some of these answers, adjusting fan speeds and motors etc. Believe it or not, something as simple as cleaning the "squirrel cage" fan inside the furnace unit (Central A/AC) or inside your window unit can improve its performance to like when it was new. The catch however is that in many central systems, the squirrel cage fan is quite difficult to gain access. In a lot of units, one must remove bolts and screws and lower the fan unit which often is best left to a repair technician. This can be done by them in little time. It's amazing how this simple preventive maintenance task is neglected. Many years ago, I lived in an apartment with a 1960's vintage GE 10000 BTU window unit that blew cold but, the airflow was wimpy even on the high speed setting. The repair technician asked if I had an old butter knife or screwdriver. I let him use an old butter knife and he cleaned each blade on the cylindrical fan which ironically was easily accessible. I never thought of it. Well, five minutes later, the air conditioner performed like it was brand new out of the box. On the low cool setting, the velocity was twice as faster as when the clogged A/AC ran on high! In regard to central systems and periodic maintenances like the beginning of the cooling season, remind the technician to perform this.

The more air that you pass over the evap. coils, the more heat that will be transferred, also the greater the temperature differential the more heat will also be transferred. On lower fan speeds the air is driven to a lower temperature as the air has more time in contact with the coils, but the efficiency is less because temp. diff. is also less. there is also a lower volume and therefore less mass of cool air although it will be a bit cooler, the total number of btu's being transferred will be less. The answer is yes. Does this make sense? 

Fan speed should be set to 400 CFM per ton of air conditioning. So if your airconditioner is a three ton unit  your fan speed should be around 1200 CFM. You do need to get in the access panel, but be careful because there is 120 volt wires in gas furnaces and 240 volts in air handlers of heat pumps. The final word is... Get a reliable HVAC technician to do it for you. You may do more harm than good... To yourself or the equipment.

 

Scott