how much should central AC run ?

[nospam_coloradotrout]

My AC when it's 90 out will nearly run continuously. I'm just curious if that is normal or if I have been undersized by the contractor (installed a year ago).

I live in Colorado - dry climate. We get a lot of direct sun. The ranch has 1800 sq ft on the main level and a below grade bsmt of the same size. Part of bsmt is finished, but we do not occupy it often.

Mid June - end of August can have 90 degree days; seems like most days are 85-95.

We set the thermo at 76 degrees. But if it's 90 outside and I try to set below 76, it will never make it down until evening.

For mid 80s days, it's fine (remember we set our thermo at 76 though).

I'm concerned that a) the HVAC contractor has undersized my AC unit and b) if a new owner preferred 72, it could not be held during our summers.

I realize it's best not to oversize the unit, but with a dry climate to begin with, removing humidity is not much of an issue.

Thanks,

[udarrell]

Why would they want to set the TH at 72? As the temperature goes down the relative humidity tends to edge upward; i.e., cold and clammy. If you provide adequate air circulation in that dry climate you should be very comfortable at 77 to 79-F. It appears to be sized right. What tonnage is it. Check out the Human Comfort Zone. They can add room A/Cs if they want to freeze and pay the costly utility bill. - udarrell

[JimL]

Boy, you are a piece of work, udarrell - Telling the whole world what temperature they are comfortable with.

My brother-in-law just might accept 68 degrees on the high side but he usually likes it lower. Ever hear the terms about satisfying the customer or what the customer wants or the customer is always right?

[RicodJour]

I didn't think he was dictating thermostat settings, but questioning the logic. It's a question of the system design. When a system is specified design temperatures and humidities are provided. Someone who wants 68 degrees is asking a system to perform outside its design criteria.

R

[JimL]

So?

You satisfy his demands or you take a long hard look at yourself and ask if you maybe ought to get out of the way for those that can. And what the hell about 'outside its design criteria'???? The number one design criteria is - "Make the customer happy ie.. 68 degrees on a 105 degree day". What about that don't you understand?

[Joseph Meehan]

You seem to have the right idea. I will make just a couple of general observations for thought.

Most of the time the system will be designed to handle a high temperature, but not the highest likely to be encountered. So it would not be unusual for a system not to be able to held 72º on the hottest day of the year.

I would expect most well designed units to run close to full time on very hot days.

The real problem comes up when someone has it turned off or set to 85º on a very hot day and expects it to get the temp down to 72º in 45 minutes.

Most people are comfortable with a slightly higher temperature on very hot days than they would be on a less hot day. This is more so in humid areas as the humidity difference becomes greater.

If you are considering installing a new unit, do talk to the contractor about your concerns. Don't totally ignore the humidity issue, even in a dry climate. Your local professional should have a good feel for the local conditions that we really don't have.

[Grub Hoe]

"Joseph Meehan" wrote

Joe,

You missed the part that this is a new unit.

[PHIL]

JimL - first thing you need to do is chill out dude. Pun intended.

My understanding from when I had a new furnace installed a couple years ago and was discussing the option of also getting central air hooked up at the same time was this - Household central air units are designed to provide a max differential air temperature of 15 degrees.

If it's 85 degrees outside, the a/c can reduce that to 70. If it's 100, then the best it can do is drop the temp to 85.

So then if the outside temp is 90, trying to cool the house to 70 is beyond the units capability & "Outside its design criteria."

~~Phil~~

[RP]

20

hvacrmedic

[nospam_coloradotrout]

Yep, it's nearly new -- come to think of it, I may have had it installed in Aug 03. So I got about 1.5 mos. of use the 1st summer and all of last summer. But I've always questioned how much it should run on the warmer, 90+, days.

It's a 3 ton unit - both outside and in.

[RicodJour]

I understand that you must have a horrible home life to talk to people the way you do.

The HVAC system is an inanimate object and doesn't care what you want or whether you are happy. It does what it can.

An HVAC system is designed taking into account many factors, including the construction, area of windows, etc. The design criteria include what the desired ambient temperature is under the specified design temperature and humidity. Then the system is installed to meet those demands. If the temperature or humidity exceed the design specifications the sytem will not increase its capacity magically. Just because you or your brother-in-law "demands" that the system operate beyond its intended operating range, doesn't mean that it will.

R

[cowboy]

more likely that your home is "under-insulated"

[nicksanspam]

Misters might help in this case, cooling the house as water evaporates and raising the latent load and heat transfer rate at the evaporator coil, esp if the house is airtight and hot and dry to begin with.

Nick

[Joseph Meehan]

I would tend to say all homes are under-insulated. :-)

In this case with a newish unit taking a good look at insulation and possible upgrades may be a very good approach, cheaper that hardware.

[CL (dnoyeB) Gilbert]

I think the OP asked though if his unit was undersized and did not get any response to that question. Therefore to claim that the response was actually addressing the mechanics of the question if not the spirit of the question is still incorrect.

His concern was that his system runs continuously right? Was not addressed.

[cm]

My understanding from when I had a new furnace installed a couple years

This is wrong. I live in Arizona where it was about 112 degrees yesterday. That would mean the temp in my house would be 97 degrees----We keep it at 78 and our A/C unit has no problem at all. I have customers that keep their homes around 68 degrees and their units cycle just fine. A few years ago when it was 122 outside my unit didn't cycle off much.

I think the installer may have been referring to the temp split a unit operates on. The air coming out of the vents is about 15 to 20 degrees cooler than the air going into your return vent. This does not mean the the unit will only cool 15 to 20 degrees below the outside temp.

AZCAIG

[RicodJour]

Feel free to chip in if you feel there's an omission. ;)

My original response wasn't to the OP, but to the poster that felt compelled to jump all over someone who pointed out some of the salient facts about relative humidity.

If the system runs continuously on hotter than usual days, that is not unusual. If the system can't bring the house to a comfortable level, that is unacceptable. If the occupant wants 58 degrees (that's refrigeration, not air conditioning) on a 115 degree day, that's unusual but can be provided if the system is designed for it.

I worked for the construction manager when the South Street Seaport Experience was being built in NYC. It's a multi-media theater that depicted the life of whalers and such. Part of the experience was to drop the temperature 20 degrees in about five minutes to indicate the winter conditions that whalers would have to endure. On a hot summer day, sitting there eating popcorn, I envied those whalers!

R

[JimL]

Hey guys,

The only important design criteria comes directly from the customer and only from the customer.

It does not come from some moron in the news group talking about ideal humidty control or such crap. It comes from the customer who says he wants a system that provides 68 degrees on every hot day in July.

Now let me give you a clue so you are not completely clueless.

My brother-in-law did NOT say that he wanted 68 degrees and then attach a condition that it had to be a specific size unit or that it had to be efficient or that it must be limited to only one unit Half the houses in my subdivision have 2 units. So don't be a moron. Add units and tons until you satisfy the customer. I just don't understand you guys. How simple is that solution?

[udarrell]

Yes, I am a piece of work. Many factors enter into the "Human Comfort Zone Conditions;" a 72-F with little air movement and a higher %RH will normally not be as comfortable as 76-F with more air movement and a lower %RH. These findings are the result of a lot of testing resulting in the "Human Comfort Zone Chart."

I am NOT dictating anything everyone is welcome to oversize and run their A/C down to 68-F if they want; I will sell them and install whatever they want. However, I will tell them that when it is oversized and operated at around a 70-F level, the seer rating goes out the window!

They are rated at a much higher conditioned space temperature. The evaporator will not be adequately heatloaded at low temperatures and the low airflow required to get the humidity down in the comfort zone at those temps. (Coil Freeze-ups on the way.) I will also let them know how inefficient and costly it will be to run the oversized inefficient system.

I like R's Response to your comments:

I didn't think he was dictating thermostat settings, but questioning the logic. It's a question of the system design. When a system is specified design temperatures and humidities are provided. Someone who wants 68 degrees is asking a system to perform outside its design criteria. R

Why should we be concerned about the next guy that lives in our home; let them change things to suit their varied individual tastes. - udarrell

[udarrell]

What they used to say was that if your A/C could always keep the room temps 15 and later they said, 20 degrees below the outdoor ambient the occupants would feel comfortable. That goes out the window if you are in a high humidity climate and the A/C won't get the humidity level down to around 50%.

Most of Arizona is a very dry climate; therefore, the A/C is able to handle the sensible heatload rather handily. The equation is much different in a high humidity climate.

You are not wrong in what you say. Your comfort level will depend largely on where the %RH level is, and also on the air movement in your home.

The more you reduce the temperature of air, the humidity percent will go up proportionately; therefore it has to spend extra runtime wringing out that humidity from the extra degrees of colder air.

Design split between outdoor and indoor conditions --varies due to many variable factors. However, the Btu/hr and SEER ratings are taken at only one set of conditions. Changing any factors from the Design Conditions will change up or down Btu/hr and SEER. - udarrell