load calculation for central AC

[Helen]

We've never had central AC and want to have it installed before summer. We live in Denver, Colorado, where summer can be pretty miserable. The first contractor we interviewed measured the size of our rooms and windows when calculating load. He says we need a 2 ton unit for our

2000 sq ft 2-story house. He said anyone who uses just sq feet to determine what size we need is wrong. The second contractor didn't do any measurements or even look at any of the house other than the family room we sat in. He based his calculations strictly on sq feet and came up with us needing a 3 ton unit. #1 quoted us $3500 (unit, permits, installation, etc)on a 2-ton Amana 12 SEER with 10 yr warranty on parts, compressor and labor. #2 quoted us $3300 for a 3 ton, 13 SEER Carrier unit with only 5 yr warranty. I'm trying to get at least one more estimate. Which contractor should we believe? Both have excellent reputations with BBB and the local troubleshooter guru, Tom Martino. Any suggestions would be appreciated because at this point, we are a bit confused! Thanks, Helen

[m Ransley]

Id belive the one that did the load calc, and you should realise the BBB is a joke.

[Joseph Meehan]

They both may have excellent reputations, but the second one should not. You can't calculate load without doing the measurements. There are industry standard calculations for heat load, cooling load and air handling requirements. Any contractor that does not use those tools should not be considered.

You may want to ask a third out, but for now you only have one viable estimate.

[TURTLE]

This is Turtle.

Any HVAC contractor that does not run a heat load calculation on the home and expect you to use them to install your unit. RUN HIS or HER ASS OFF. Now am I clear on this.

Now I can be wrong here but I think the Amana 12 seer condenser has a life time warrenty on the compressor. They started this about

3 months ago and is still new to the dealers. I know the 13 seer has the life time warrenty but I think the 12 seer has it too. If you like I can check tomorrow and see.

TURTLE

[HRL]

Look into evaporative cooled units too. Many houses in Denver use evaporative cooling only but for various reasons many prefer refrigeration. In any case there are evaporative cooled refrigeration units that are significantly more efficient and will aid in keeping the electric bill down.

Some like the Evaporative cooling because they can keep their windows open and don't have to be concerned about using ovens etc.

[Banister Stairwell]

You should try a South Texas summer!

I would tend to agree.

An undersized system will not cool adequately and it will run up your electric bill. An oversized system won't run long enough to remove the humidity from the air. What you end up with is a house that's cool and clammy. The only way to figure out the proper size A/C system for a particular application is to do a heat load calculation.

If this first contractor took the time to calculate your heat load, chances are good that he's correct in his assessment of your A/C requirements. For where I live (So. TX), a 2-ton system would (in most cases) be inadequate for a 2000 sq. ft. 2-story house. But, you live in Denver where the latent heat removal requirements are *much* less than down here. You have a fraction of the humidity we have to deal with.

Not to discount this contractor's experience in the trade, but you might want to ask him to reassess your requirements by doing an actual "Manual J" residential heat load calculation. This shouldn't be a problem if he really wants your business. If he blows you off and says it's not necessary, blow him off and go with someone who's willing to take the time to do it correctly. You shouldn't have to live with (and most certainly not have to pay for) an improperly sized system because the contractor was too lazy to do a proper heat calc.

Getting three bids is a good idea. Just be sure that whoever you decided to go with has done his or her homework and has taken the time to do a *proper* heat load calculation that takes into account all the variables unique to

*your* home. Personally, if I were going to spend 3 or 4 grand on a new HVAC system, I would insist on it - no "rules of thumb" for that kind of money, thank you.

Hope this helps.

[RB]

I did the load calculations on my home for both heating and cooling and the results were "right on," but it took me many hours to do it properly. The answer is not necessarily intuitive, particularly if you have a lot of glass. I didn't use the ACCA Manual J because I wasn't aware of it then but today I would recommend requiring that anyone proposing a system do so. I'd also review their calculations to make sure that they considered everything. To be effective and economical to operate an air conditioning system needs to be "just right". Not too small, not too big.

Anyone who purports to properly size an air conditioner without fully evaluating the structure and its location should be shown the door.

RB

Helen wrote:

[SQLit]

Rules of thumb. 400-450 square feet per ton. This does not account for high ceilings, glass or other factors. I do know that using 400 square feet per ton with double insulated windows with out a lot of heat gain works. I was just quoted a 4 ton for 1632 sq ft. I live in Phoenix and have only one west facing window.

I would not pay the difference for an extended warranty, personally.

By the way Amana is owned by Goodman. No connotation should be taken from that statement.

[m Ransley]

SQ your rule of thumb may be for Arizona but not up north or the denver area where a super insulated house in the shade can only need 1 ton per

1000 sq ft. . It depends on windows and insulation. I hope they did the calculation right because you need a tight insulated house for that size unit.

[TURTLE]

This is Turtle.

SQ, The rule of thumb is a tool used only to see if your heat load calculations are off the wall and very much out of line. You say you have a 4 ton for 1,632 sq. ft. and every heat load calculations that I have ever done. this much tonage on a house was for the worst case that could ever be to cool a house. I remember one calculation run on a house that did call for 4 tons on 1,650 sq. ft. and here was the conditions it had. 1,650 sq. ft. with 4" of insulation in attic, off the ground 2 feet, No insulation in walls or floor, Attic not vented, Old type windows from the 1940's, In the sun all day long / no shade, and air leaks at doors and walls all over. The 4 ton would bearly keep up till they vented the attic with a motorized fan and put air in take vent around the eves. After that it did a fine job.

Your using the 400 sq. ft. per ton is in my opinion a little bit of over kill even in Phoenix and surely in Louisiana. I don't think you home is like this worst case of a heat load and you spoke of having good insulation and insulated pretty good. One thing I can say is on the hottest days. Your going to be cool as you like it but on days of say 80ºf outside your going to have to run at lower temperature to give it the run time to get the water vapor out of the house. To most people this is not a problem but it could be taken care of by running a correct heat load calculation and size accordingly.

TURTLE

[*CBHVAC*]

The ONLY rule of thumb in HVAC is that most contractors have two...

There are NO rules of thumb for sizing...ONLY manual N, J, T.....

Anyone that suggests such foolishness, is not well versed, since I have seen as little as 200SF a ton, and in the SAME area (Palm Springs CA) up to over

100SF a ton...it DEPENDS on the load calculation results based upon the structure, and area.

Also, I have lived in Colorado Springs....about 25 miles south of Denver....and AC??? LOL...maybe for one month out of the year, and if you had a good swamp cooler...you didnt need it. The last big home we worked on there, was for the owner of a large german resturant...the place was like

11,000SF, and not a single AC unit to be found...

[nicksanspam]

I'm afraid you have erred, my good man. Here's one rule of thumb:

1. Estimate the internal heat gain. E kWh/mo of electrical usage makes it 3412E/(30dx24h). For instance, E = 600 kWh makes it 2.8K Btu/h.
2. Estimate the thermal conductance of the house by dividing each external surface by its R-value, then add something for air leaks. For instance, a 32'x32' 1-story house with R20 walls and an R40 ceiling and 96 ft^2 of well-shaded R4 windows and 1 Air Change per Hour would have 96ft^2/R4 = 24 Btu/h-F of window conductance plus 928/20 = 46 for the walls plus 1024/40 = 26 for the ceiling plus about 1x32x32x8/60 = 137 for air leaks, a total conductance of 233 Btu/h-F.
3. Pick indoor and outdoor temperatures, eg 75 and 100 F, to find the AC load, eg (100-75)233 = 5825 Btu/h. Add internal heat gain and divide by 12K to find the AC load in tons, eg (5825+2.8K)/12K = 0.7 tons.

Nick

[gerry]

off the wall and very much out of line.

I actually believe experienced contractors (with local experience) can eyeball pretty well. No excuse for not doing a load calc but it is a sanity check.

Also, If I run calcs for the worst days in my area, the unit will be oversized and most of the time (AC season) will result in a cool damp home.

Usually we get dry air from the west, on rare occasion, we get very warm damp Atlantic Ocean air. Too much range for a typical AC unit.

Local experience helps.

gerry

[Lurker]

Have been seeing your posts for a couple years now, and would only say "Keep up the good advice!"

calculations are off the wall and very much out of line. You say

have ever done. this much tonage on a house was for the

run on a house that did call for 4 tons on 1,650 sq. ft.

attic, off the ground 2 feet, No insulation in walls or

day long / no shade, and air leaks at doors and walls all

motorized fan and put air in take vent around the eves. After

kill even in Phoenix and surely in

and you spoke of having good insulation and insulated

cool as you like it but on days of say 80ºf outside your

the water vapor out of the house. To most people this is

calculation and size accordingly.

[TURTLE]

This is Turtle.

Why that you there.

TURTLE