PING: All you AC/HVAC experts

Yes, ASHRAE has standards based upon several factors. Some of the factors are:

1.) Insulation value of the building 2.) Max/Min ambient temps 3.) Max humidity 4.) Sun load 5.) Household heat load 6.) Desired air changes per hour 7.) Square/cubic footage

Bigger ain't necessarily better when it comes to HVAC.

Mike

You have a link that I can look at?

Correct, I know bigger is not better in this case, but too small is not good either.

Count the doorknobs and divided by 7. About as close as anyone's guess! The only accurate way to know is to have a Manual J calc done. Other than that, a very experienced HVAC tech can give you a WAG and get pretty close to! I have seen 1600 square foot homes with 2 tons of AC that was to much, and

2200 square foot homes with 5 tons that was too small! Greg

Now that's one I won't touch!

You left out one thing Mike.

The amount of hot air Larry spew's forth on a bad day!

Roy

Yeah, but unlike the damn wabbit and his WC, I am not a cause of global warming!

In response to Greg O 's post. I thought everyone should know:

your r-value and ceiling heights go into the factor too. i was shown how to figure it out once when i was looking at house plans so i could use that as a "HAVC cost guide" when considering the building plans.

i forget how it works but mike hit all the high points best i can remember.

stuff like ceiling trays and vaults go into the factors too. that would fall under the cubic foot of air volume to be climate controlled.

Absolutely. It starts by calculating the heat gain. If you haven't been doing it for 20+ years, you need to do the calculations. That means either sitting down with paper, pencil, and Manual J, and figuring the heat gain, or getting a computer program to do it for you. Put in too little cooling, and it won't keep up with the heat gain. Put in too much, and it will drop the temperature so quickly that the system doesn't have enough time to dehumidify.

HVACcalc

is a pretty nice program, and at $50 for residential use, is pretty cheap. Try out the demo, and see what you think.

Ok let me ask this, not looking for an exact answer

1550 sq' single story 4 bed 12' celing in main rooms, 8 - 10' in rest R-11 in the wall, R-24 in the ceiling low e, dual pain windows, with additional window film to reduce as much heat gain as possible.

3 ton unit

Does this sound ok? Mind you, I maybe a little off on some of the above figure, but it is a 3 ton for sure.

Where at? Not all of us are up on locations!

3 ton seems reasonable. Are you replacing? Installing new? Existing not keeping up? Greg

Isn't R-11 a little thin for Arizona?

I would think that R-11 is a little thin anywhere, but according to the homes manufacture, that is what is used. That is a whole nother issue.

Larry... I can't give you specs, but I found that in a high humidity place like we (and you) live in, the builder could have put in smaller units..

We keep the setting pretty high for most folks, 29 or 30C or about 84 or 86F... The mini split in the shop is 2 ton and I flip it on in the morning when the shop is maybe 90.... It gets the room down to 86 so fast that the AC isn't running long enough to dry the air...

Most folks that don't plan of hanging meat in their living room prefer a smaller unit that runs more often... might be more efficient that way, too, dunno..

NOTE: Use of the "dry" cycle on the AC isn't recommended unless you rent and the landlord pays the utilities... AFAIK, ours is running both the heat and AC to dry the air and the electric meter spins really fast.. :(

mac

Please remove splinters before emailing

Down in AZ? I dunno... seems a little on the low-side to me. I had a

3-ton unit in a 900 sq.ft. ranch, all 8ft. ceilings, with approximately the same insulation... not a lot of glass, zero southern exposure, here in NJ - and it was fine.

To compare... my shop is approx. 2000 sq.ft., 14' ceilings... R-19 in the walls, R-30 in the ceilings... white roof, light-colored exterior (really makes a difference in the heat gain), and insulated doors (only 1 window in the place). It's got a 5-ton unit. That's probably a little much, but we did it for the quick recovery time when a bay gets opened and a vehicle moved in/out. I think, when I did the initial calculations, I came up with a total heat gain of approx. 36,000 BTU's (3 tons).

But - I'm certainly no expert... that's why I'd rely on the calculations instead :)

It is probably the minimum that code in your area requires and costs less. Less cost results in increased profit for the builder and higher bills for you. Without going into complicated calculations, I would say that 3 ton is about right for your area and size of your house. If you already have the unit, what do you have to lose by installing it? I have a 2.5 ton gas pack cooling and heating my first floor and will be upsizing it to a 3 ton SEER

13 or 15 myself either this fall or next spring. On 85 degree days, it works just fine but on 92 and up days it runs for excessively long periods of time (almost non-stop) and these $300 power bills are getting to be a bit much. As Tom said, the real problem with over sizing is humidity control but that might not be much of a problem where you live, especially if you are near the desert. Here in NC, it is much more of an issue but 1/2 ton is not going to reduce the run time excessively under normal days and will stop it from running non-stop when it get hot. The bigger the house, the more wiggle room you have with upsizing.

Long and the short of it is this.

Got this months elec bill and it was outrageous for a family that is very energy wise. Talk to neighbors with homes by the same builder, bills ran from 200 - 900 bucks. All the single stories have the same make/model ac, two stories has same make different model.

Homes by different builer on next block in same community have bills that max out at 180!

Trying to figure out what the hell is going on. I know there can be a lot of factors, but I have to start at group and work up

What is the SEER rating on your A/C unit? Do you have a lot of windows? Any tint or treatments used during the heat of the day? Programmable therm?

The SEER can make a big difference. For example, an 18 SEER A/C unit is

59% more efficient than a 8 SEER unit.

Different builders use different units. For example, in my last house, my builder, Coventry, used Rheem 10 SEER units. The house next door was built by Bradford. It had Trane 13 SEER units.

Sounds like you may have other things going on too ... like, many windows. I put tint, 2" blinds and programmable therms in my last house and it helped with the $400.00 per month bills those 10 SEER units were creating.

Craig C.

As you said, there are a lot of factors. What is the SEER rating of the existing unit? Does the unit have and is using a dry-air mode? As Mac stated, they re-heat the air to remove humidity and use about twice the electricity to do it. You only have R11 insulation in the walls and that in a word - SUCKS! Was the house even wrapped before the siding was installed? What kind of windows do you have? Are they at least dual-glazed? Are they low-E type glass. IF not, I would suggest at a minimum, a low-E solar film. You probably could also add another layer of insulation in that attic. I would also look for air leaks around those windows and external doors. Does this house have a crawl space, basement, or on a slab? If it has a crawl space I would check the insulation and vapor barrier under there as well. A larger AC unit will probably not save you any real money if your heat gain is way up there and the rapid cycling of the unit will shorten its life as well. Perhaps you might want to have an energy audit done.

According to the manufacture and the info I have on the unit it is a 3 ton, 13 seer. Not a lot of windows but they are large, so that makes it about the same. They are low-e rate, I have added additional low-e film to they and am blocking about 75% of the incoming heat and uv. They have window treatments over (2" blinds and curtains) them also, that stay closed 90% of the time. (Remeber I am in AZ we have a lot of heat)

Like I said earlier we are VERY energy wise, all energy saving bulbs that get used maybe an hour or two a day at night, gas vice elec dryer and range, plus the window stuff mentioned above.