Oversize unit

[gkck]

Hello

I am converting from Evaporating colling (Swamp cooler) to a central AC unit. The local Carrier dealer stated that they started to slightly oversize the units for people so as not to have them run for vewry long. We pay .13 per/kWhr for power. We have very low humidity (35-50%) so he sated we don not need to have them run for a long duration since we are not trying to remove humidity from the air. He stated it is better to run the system fast, cool the house down, then power off in order to keep the power bill down.

Does this make sense or not?

Thanks
Greg

[air2spare]

...legitimate dealer/contractor

[Black Adder]

Ask your dealer how efficient a unit is in the first 15 minutes of run time? I guess his theory is also correct for heating. Put in a monster furnace, heat the hell out of the house so the furnace can stay off for longer periods of time. Unit is most efficient when running for longer periods of time. An oversized unit may cool the house faster but use as much or more energy than a properly sized unit running for longer periods of time. Also with the proper sized unit overall comfort will be much better.

[lakeman1234]

I personally like them slightly oversized. I turn the AC off or way up if I am gone on vacation or for a few days. I don't like having to wait for several hours for it to cool off. But they are right, it is more comfortable the closer it is sized correctly. However if you size it for 75°F on the hotest day and then you decide to go to 65° later in life, (Don't laugh some people have health problems and need it colder) it may have problems keeping it that cool.

[mark beiser]

Find a contractor that knows how to design systems and match equipment for best efficiency and comfort in a dry climate. The one you have sounds clueless.

The sensable cooling capacity of the system needs to match your peak load as closely as possible, but in a dry climate you can match up the equipment so you have much higher than the standard 400 CFM per ton. It increases the sensable capacity of the system and lowers the latent capacity.

Usually the resulting matchup would use a smaller outdoor unit and an indoor unit with a bigger blower than would be "normal".

[mark beiser]

Originally posted by lakeman1234
I personally like them slightly oversized. I turn the AC off or way up if I am gone on vacation or for a few days. I don't like having to wait for several hours for it to cool off.

Size the system correctly and get a Vision Pro thermostat!

If you are going to be gone for 7 days, put it on hold at the higher temperature for 6 days.




But they are right, it is more comfortable the closer it is sized correctly. However if you size it for 75°F on the hotest day and then you decide to go to 65° later in life, (Don't laugh some people have health problems and need it colder) it may have problems keeping it that cool.

Thats usually because someone oversized their system, or there are other problems that make it do a poor job of controlling humidity....

Nobody "needs" it that cold for health reasons. In fact, keeping it that cold could cause health problems. Not to mention that residential HVAC equipment isn't really designed to operate with return air temperatures below 72º.

[lakeman1234]

Well Mark obviously you have never been around people that have had breating problems and want it cold.

Also I like my digital thermostat and don't feel that a Vison Pro is necessary.

[Edited by lakeman1234 on 04-23-2006 at 05:14 AM]

[dan sw fl]

Originally posted by lakeman1234
But they are right, it is more comfortable the closer it is sized correctly.

and then you decide to go to 65° later in life, (Don't laugh some people have health problems and need it colder) it may have problems keeping it that cool.

NEVER heard That One before.

Seniors always like it warmer.

[beenthere]

Their turning it down to get the humidity down so they can breath easier.

Wouldn't need to if it was sized better.

[mjk_na]

Oversizing a unit too much is not advisable. You won't be able to remove moisture efficiently due to short operation periods.

Plus, compressors use more electricity during start up period. Long running (undersized acs) will cost you more money. Repeated start ups (oversized acs) will also cost you more money.

Normally contractors will use Manual J, which has some CORRECTION factors already. There is no need to put in extra Btu or KW or Ton into your ac unit.

Contractors will try to sell you larger sized units out of:

1. Doesn't know the fundamentals of air conditioner sizing, or

2. Trying to make big bucks from you :D

Best thing to do is, get the size just right for your space. Better still, tint your windows to miminise heat gain through sun's radiation. You can reduce the size requirement, and your $'s as well.

Hope that helped. :)

[aemeeich]

Originally posted by mark beiser



But they are right, it is more comfortable the closer it is sized correctly. However if you size it for 75°F on the hotest day and then you decide to go to 65° later in life, (Don't laugh some people have health problems and need it colder) it may have problems keeping it that cool.



Nobody "needs" it that cold for health reasons. In fact, keeping it that cold could cause health problems. Not to mention that residential HVAC equipment isn't really designed to operate with return air temperatures below 72º.

[/B]


You are wrong about nobody "needing" it that cold. There are in fact some people that need it cooler. My mother has has MS for 20 years. The disease has targeted her muscle control. She has been in a wheelchair for 15 years. If the temperature ever gets above 70 degrees, she becomes completely limp. She has NO muscle control at all. She can't even hold her head upright if she's sitting.

We keep our house at 67-68 degrees year round.

Last summer, she spent 6 months in a physical rehab facility to heal a large infected area on her back. They kept the rooms at 77 degrees (with 50% humidity) She couldn't even open her eyes or talk it was so bad. We had to go buy a portable AC for her room because the management didn't belive that the temperature could cause that big of a difference and wouldn't do anything about it. (After that all the nurses loved to be in her room because they found the rest of the building way too hot for comfort.)

We also can't visit at our friends/relatives houses in the summer because all of the systems in their houses are designed not to be able to cool below 75. She misses out on lots of family events because she just can't function there.

Now I'm not saying to oversize every system that's installed so it can cool below 70, but there are some circumstances where it is necessary, and you can't say NOBODY "needs" it.

Michael

[wiwbar]

Michael,
You must be very proud of her the way she keeps pushing on, as I know first hand also how hard MS can be. She must be a strong woman and sets many good examples for her family. Thanks for the good bit of information. She is in my prayers. Bruce

[thorton]

A three -ton air condtioner running for 30 minutes uses the same amount of energy as a 1.5 ton unit that runs for an hour. However, by running for twice as long, the smaller unit will reduce the humidity by twice as much.

Thorton

[citywide service]

I try to keep my house no hotter than 72 in the summer due to a real bad reaction to fresh mowed grass - it throws me into an asthma attack which can take days to recover from.

*tidbit of info for those with kids and Asthma: if you have a kid in an attack and need to calm them down so they can breathe...make them stick their head in the freezer; the cold air is easier to breathe and you can take deeper breaths and has a calming effect as they relax. Been doing it for years! Mom and dad did this to calm me enough to get me to the hospital on several occaisions.

If I keep the house cold I can come home and "recover" enough to at least function until I get some meds working.

Now that I am putting together a new system I am going a different route to hopefully help with this problem, I am trying to control humidity better than ever before and working on IAQ more. Hopefully I can do the same job this summer at a lessor utility cost! Time will tell.


Citywide.

[teddy bear]

Originally posted by thorton
A three -ton air condtioner running for 30 minutes uses the same amount of energy as a 1.5 ton unit that runs for an hour. However, by running for twice as long, the smaller unit will reduce the humidity by twice as much.

Thorton
Technically, They both remove the same amount of moisture per ton-hour. Half sized a/c removes half the water per hour. The difference is that a cooling coil takes 6-10 minutes of cooling before the coil starts removing water. The smaller unit/coil has fewer starts per day therefore slightly more water per ton hour. A couple other considerations- re-evaporation of retained moisture on the coil during the off cycle and drying time of the ducts. Another discussion is the effect using the large high SEER coils with the low speed of the much praised two speed a/cs. The larger the coil, the longer the lag before moisture is removed and the greater the amount of water retention. TB

[perel]

Michael,

I was wondering if your mother has had any success with core cooling vests? I've heard quite a few good things from MS folks about how much they help.

SteeleVest is generally considered the best core cooler for high-cooling situations, and it's definitely the one with the most research directly validating it. Basically it's a vest with a bunch of cold packs in it; you freeze the cold packs and then put them in the vest. Low-tech but highly effective.

[teddy bear]

Originally posted by thorton
A three -ton air condtioner running for 30 minutes uses the same amount of energy as a 1.5 ton unit that runs for an hour. However, by running for twice as long, the smaller unit will reduce the humidity by twice as much.
Thorton
Just to clarify a misconception. For the above example, the amount moisture removed is the same for both per btu of cooling capacity. The only difference in moisture removal is the coil down time at the start-up. Every cycle requires 3-4 minutes of cool down to the dew point of the air, during which no moisture is removed. A 3 ton cycling 3 times per hour loses (3 X 4 minutes)12 minutes of moisture removal compared to 1.5 ton unit operating continuously. This equates to a 20% reduction in moisture removal. As the moisture in the air rises, the larger unit will remove a high latent ratio, recovering part of the lost capacity. Keep in mind, all units are grossly oversized at low/no load conditions, requiring supplemental dehumidification for real humidity control during the summer in green grass climates. So, smaller unit removes slightly more moisture. The other side of the discussion with oversized a/c is the opportunity to use daily temperature set-up when the home routinely unoccupied. Temperature set-up could cut the energy cost by a significant amount. Providing supplemental dehumidification provides <50%RH reqardless of the size or on/off mode of the a/c. More homeowners are turning the a/c off when the home is unoccupied and using a whole house dehumidifier to maintain <50%RH. Measuring a north Florida home with a mold problem using a dehumidistat on a/c the previous year showed a $350 savings. The home was maintained at <55%RH throughout the summer. The dehu sales rep TB

[thorton]

Originally posted by teddy bear

Originally posted by thorton
A three -ton air condtioner running for 30 minutes uses the same amount of energy as a 1.5 ton unit that runs for an hour. However, by running for twice as long, the smaller unit will reduce the humidity by twice as much.
Thorton
Just to clarify a misconception. For the above example, the amount moisture removed is the same for both per btu of cooling capacity. The only difference in moisture removal is the coil down time at the start-up. Every cycle requires 3-4 minutes of cool down to the dew point of the air, during which no moisture is removed. A 3 ton cycling 3 times per hour loses (3 X 4 minutes)12 minutes of moisture removal compared to 1.5 ton unit operating continuously. This equates to a 20% reduction in moisture removal. As the moisture in the air rises, the larger unit will remove a high latent ratio, recovering part of the lost capacity. Keep in mind, all units are grossly oversized at low/no load conditions, requiring supplemental dehumidification for real humidity control during the summer in green grass climates. So, smaller unit removes slightly more moisture. The other side of the discussion with oversized a/c is the opportunity to use daily temperature set-up when the home routinely unoccupied. Temperature set-up could cut the energy cost by a significant amount. Providing supplemental dehumidification provides <50%RH reqardless of the size or on/off mode of the a/c. More homeowners are turning the a/c off when the home is unoccupied and using a whole house dehumidifier to maintain <50%RH. Measuring a north Florida home with a mold problem using a dehumidistat on a/c the previous year showed a $350 savings. The home was maintained at <55%RH throughout the summer. The dehu sales rep TB

Teddybear: If what you say is true about 3 to 4 minutes of cool down to dewpoint and I'm not saying it's not, then why does my Carrier YZA and FV4A VS DC with Thermidistat control only run from 1 to 10 minutes max in the Cool to Dehumidify mode to keep a pre-set dehumidification set point that is seperate from the cooling setpoint? This is a Carrier feature with this system. Does Carrier not know that it takes 3 to 4 minutes of running to reach dewpoint of the evaportator. Let me know what you think.

Thorton
__________________________
Life is not measured by the number of breaths we take,
but by the moments that take our breath away

[teddy bear]

Originally posted by thorton
Teddybear: If what you say is true about 3 to 4 minutes of cool down to dewpoint and I'm not saying it's not, then why does my Carrier YZA and FV4A VS DC with Thermidistat control only run from 1 to 10 minutes max in the Cool to Dehumidify mode to keep a pre-set dehumidification set point that is seperate from the cooling setpoint? This is a Carrier feature with this system. Does Carrier not know that it takes 3 to 4 minutes of running to reach dewpoint of the evaportator. Let me know what you think.

Thorton
__________________________
Life is not measured by the number of breaths we take,
but by the moments that take our breath away

[/B]
We have good Carrier support on this site. Answer to follow from fellow posters, I am sure. My understanding is that "the time limitation" is to avoid over-cooling the home. In a 10 minute run, 6 minutes of moisture removal is better than nothing. I feel over-cooling is not an the ideal strategy to control humidity. As you over-cool the home, moisture condenses in the exterior walls and ducts/grills. The condensation of moisture grows mold.
The real solution to controlling moisture operating a/c with a very cold coil that maintains <50%RH during hot weather. Supplement the a/c with enough dehumidification to maintain <50%RH with low/no cooling load for the wet times of the year. The results are "no dehumidification" required during hot weather because the a/c removes enough moisture. During low/no cooling load, the dehu maintains <50%RH with comfortable temperatures. A dehumidifier will maintain <50%RH during hot weather in a unoccupied home without the a/c operating for a fraction of the a/c energy. This could be real energy conservation if the home unoccupied for significant hours or days, but requires some extra a/c capacity for quick cool-down. We have many costumers using this strategy. TB

[beenthere]

Your indoor coil may have a nonbleed TXV, which will decrease the amount of time it takes to get the coil below dew point.
Plus, your air handler may also be set for a slow ramp up, which also helps.

[suptech wally]

HEY lets also add that by oversizing your equiptment you
have alot more starts therefore causing shortened life
expectentcy.Which means replacing your equiptment sooner
and pulling LRA many more times a week.

GET another contractor

IT takes more than 15 minutes before most units to start
to remove any latent heat.

[Edited by suptech wally on 07-19-2006 at 07:42 PM]

[RoBoTeq]

Originally posted by thorton
A three -ton air condtioner running for 30 minutes uses the same amount of energy as a 1.5 ton unit that runs for an hour. However, by running for twice as long, the smaller unit will reduce the humidity by twice as much.

Thorton

In the instance mentioned by the OP of this thread, humidity is not a factor.

However; by the scenario mentioned above the 3 ton unit would start twice as many times every day. The worst thing you can do to a motor is to start it, so the larger unit would wear out quicker. Also, since it does take about 15 minutes for a system to reach its peak efficiency, the larger system noted would be much less efficient more of the time.

I size right and round down. Every degree under the design temperature the system is oversized. On above design temperature days, close drapes and don't run any heat producing appliances.

[thorton]

Originally posted by beenthere
Your indoor coil may have a nonbleed TXV, which will decrease the amount of time it takes to get the coil below dew point.
Plus, your air handler may also be set for a slow ramp up, which also helps.



Beenthere: I'm not sure about the nonbleed TXV. I hope one of the Carrier guy's will let us know. Nevertheless, you are certainly barking up the right tree when you suggest that the air handler may be set for slow ramp up. In my Thermidistat Control Guide it states the following:

The cool to dehumidify selection tells the system to operate the compressor, within limits, when there is a dehumidify demand even if there is not cooling demand. the limits are that the system may overcool up to 3 degrees, but not more, while attemtping to satisfy a dehumidify demand. Within this 3 degree range, there is an additional balance between overcooling and humidity satisfaction. When overcooling must occur, the dehumidify set point is adjusted upward by 2% per degree of overcooling. During cool-to-dehumidify demand, the compressor runs a maxium of 10 minuttes on, followed by 10 minutes off. When the compressor turns off, the fan (G output) is also turned off immediately. The immediate fan shutoff prevent re-evaporation of water on the coil.

This being said there is also another section called Superdehumidify (With Cool to Dehumidify which states:

This selection only affects cool-to-dehumidify operation. It is part of the installer setup. A requirement is the use of a variable-speed indoor unit with superdehumidify capability. During cool-to-dehumidify call, it provides maximum dehumidification by reducing airdlow to minimum.

Thorton
________________________
To succeed, it is necessary to accept the world as it is and rise above it

[dash]

Yes ,it should be a non-bleed TXV,ramp up slowly,and if all is installed correctly they work great.

[thorton]

Thanks for the reply, Dash! Yes this sucker works great! I keep the temp at 77 during the day and the dehumidification set point at 55RH and I neve get overcooling, either. System doesn't run much during the day as I have a fairly hight level of insulation and just put it new low E windows that are argon filled and all casement style. I have three bay windows (small ones)and had the bays all ripped out completely and the head and seats insulated (which were not done before)and all windows are foamed including the jam extensions. It's made a big difference on my AC and comfort level in my home. All my heat loss and gain were the result of poor window installation, it seems.

Thorton
______________________
Wisdom is knowing what path to take next…
Integrity is taking it.


[Edited by thorton on 07-20-2006 at 10:30 AM]

[danglerb]

Pay little attention to the numbers, this is from memory and to illustrate the point.

Normal cooling, 400 cfm per ton.
Cool to dehumidify, 80% of normal, 320 cfm per ton.
Superdehumidify, even slower, but I forget how much, and for just like 10 minutes at a time to keep from icing the ooil.

After the 10 minutes of superdehumidify, I believe the air handler cycles to one of the higher speeds for some period, then goes back to superdehumidify. I don't know how much of the timing etc is from the air handler and how much from the thermostat.

[thorton]

Originally posted by danglerb
Pay little attention to the numbers, this is from memory and to illustrate the point.

Normal cooling, 400 cfm per ton.
Cool to dehumidify, 80% of normal, 320 cfm per ton.
Superdehumidify, even slower, but I forget how much, and for just like 10 minutes at a time to keep from icing the ooil.

After the 10 minutes of superdehumidify, I believe the air handler cycles to one of the higher speeds for some period, then goes back to superdehumidify. I don't know how much of the timing etc is from the air handler and how much from the thermostat.

Good point Danglerb! In my Thermidistat operating instructions it doesn't specifically say how much the Superdehumidification slows the blower more then normal when dehumidification demand is present. In a normal demand for dehumidification it slows the blower speed by 21%.Superdehumidification is slower and that's what prevents the overcooling. It's a great working system that Carrier has got, I think.

Thorton
_____________________
The attitude of initiative is an on-going state of exploration that is never finished…a journey that never ends.