Will this system work as expected

[sweating in iowa]

Built a new 3641 sq ft 2 story home in eastern Iowa. 2697 sq ft on first floor and 944 on second. First floor has a 22 X 24 Great room with a cathedral ceiling height of 22 ft at the peak . There is a 14'6" X 11'10" window facing East in this room, the contractor only installed 2 registers in this room with a large return air register 7' up on the wall. There is also a entry connected to this room that is 8 X 15 with 19'ceilings and 60 sq ft of window and door facing to the west.

The Heating and cooling contractor installed a 3.5 ton 14 Seer AC unit, with a 120,000 btu furnace. With separate zones for first and second floor.

In the middle of the summer we had a heat wave that reached aprox 95. I had the thermostat at 74 and by 6:00 PM the inside temp was 80. The unit run from sun up till almost midnight before the house was back to 74. Of course the heating contractor does not think there is a problem with this.

I have done some online calculation and come up with a heat gain of 60,000 btu. And a heat loss of 119,000 btu My question is this. Is this system properly sized for my house? My contractor swears it is, even to the point he is threatening to take me to court if I don't pay the remainder if the balance due.

Any feedback will be appreciated.

[dougfamous]

Well, if your load calc says 60,000 BTU gain (5 tons) and you have a 42000 BTU A/C (3.5 tons) then it would appear undersized. If I were you, I'd either spring for the HVAC-Calc software and do a proper load calc, or contact another contractor to do one for you.

I suspect it is too small.

[roxosix]

Im suprized that it only got up to 80 degreas, is the home in the shade? that system for that size home is very undersized. You might get by with a 5ton system if it is zoned.

[sweating in iowa]

No shade on any side of the house. New lot with no landscaping. Also black shingles on the roof and red brick on the west side of the house. I guess the only thing helping me is the white siding on the other three sides. Thanks for the reply.

[tigerdunes]

sweating

You are undersized. I would suggest upgrading to a 5 ton 2 stg unit AC or HP if your furnace is var speed.

Good LucK!

[florida pe]

Request to see your contractor's Manual J heat gain and loss calculations. It sounds as if he may have not done them or at least assumed some rather oddball design assumptions. Some jurisdictions require a heat gain/loss calc be submitted along with the permit. Might check with your local building dept. if you can't get a copy from your contractor. Otherwise, buy the HVAC calc on this site and do your own. It will only take a couple of hours, less if you have a set of plans to use.

[BaldLoonie]

Someone better check the duct system. If your contractor did as well as the new home dealers do around here, you probably don't have enough duct capacity for the unit you have let alone going bigger :(

[tigerdunes]

Good Pt by BaldLoonie!

[dan sw fl]

Originally posted by sweating in iowa

New 3,641 sq ft 2 story home in eastern Iowa.
2,697 sq ft on first floor and 944 on second.
First floor 22 X 24 Great room with a cathedral ceiling height of 22 ft at the peak.

There is a 14'6" X 11'10" window facing East in this room, the contractor only installed 2 registers in this room with a large return air register 7' up on the wall.

There is also a entry connected to this room that is
8 X 15 with 19'ceilings and 60 sq ft of window and door facing to the west.

The Heating and cooling contractor installed a 3.5 ton 14 Seer AC unit, with a 120,000 BTU furnace. With separate zones for first and second floor.

Is this system properly sized for my house?

___ No! ___

One would normally initially anticipate
using AT LEAST 6-tons.
A Manual J calculation could determine that a 5-ton
may be justified WITH zoning
if > 75'F is acceptable
on days > 90'F DB/74'F WB
(ASHRAE 1% design condition for Iowa).

Does your contract actually state that 74'F is the design indoor temperature and 95'F is the design outdoor condition?
If not, the noted 15'F difference (95' - 80'F) between indoor and outdoor temperatures would be anticipated.

"I have done some online calculation and come up with a heat gain of 60,000 btu"
(based on what indoor and outdoor conditions ?)

If a load calculation shows 60,000 SENSIBLE heat gain,
the total capacity of the equipment should be
nearly 7 tons.

60,000 BTUh __ Sensible Cooling
_ 0.74 Sensible Heat Ratio of Equipment (S.H.R.)
81,081 BTUh __ Total Cooling
21,081 BTU ___ ~ Latent Cooling Capacity of Equipment

12,000 BTUh/ ton
6.76 _ Total Tons required

NOTES:
> 18 foot ceiling increases the wall area by > factor of 2.

L A R G E window
14.5' Window Height
11.8' Window Width
171.1 Sq.Ft
50 BTU / Sq.Ft. of Window Area
8,555 BTU = Q

Dt across coil = ~ 20'F
1.08 CFM * Dt = Q
1.08 * 20 = 21.6
396 __ CFM

This one window may need a nominal 1-ton of cooling
(400 CFM/ton) in the morning (faces East).

" ... the contractor only installed 2 registers in this room with a large return air register 7' up on the wall. "

12" x 12" diffusers (vents) [2] would likely be adequate if EACH supply air duct is at least 8" diameter which would handle minimum of 250 CFM.

I suspect 600 CFM total would be a targeted air flow for this room.

[sweating in iowa]

The contractor will not so me the heat gain calculations. He is threating to take me to court if I don't pay the bill even though the system, I don't think works properly.

I'm using the the outside conditions that I found on the HVAC-CALC trial software. But doing the calculations on HEATLOAD.com it's free. I didn't want to spend the money to use the calculator, because I'm planning on having a local engineering firm do a heat load study that will be acceptable to be presented in court, should it come to that.

The diffusers in that large room are maybe 12 X 4 or possibly 12X5 but I'm sure there nowhere near 12 X 12. I will measure them tonight and also the ducts feeding them.

Thanks

[luxlock]

Stick to your guns but do your research. Was a permit needed and pulled? was heat load calculation needed?
A judge is going to side with a consumer that is expecting to pay for comfort and was short changed. Don't be intimidated but be loaded with facts!!! No heat loss calculation is not an automatic win for you especially if the contractor has done work for the judge before or is related!!!

[comfortdoc]

Originally posted by sweating in iowa
I didn't want to spend the money to use the calculator, because I'm planning on having a local engineering firm do a heat load study that will be acceptable to be presented in court, should it come to that.

That is probably your best course of action if he is threatening litigation. Make sure the engineering study includes duct sizing and evaluation. That is just as important as the system size.

[sweating in iowa]

Thanks guys, I measured the 2 diffusers in the great room and they are 12X41/2. I'm planning on measuring the ducts comming off the unit and going to each room. I will post that info when I have it.

[sweating in iowa]

Oh ya, no permit was needed since we live in the county. At least from what I understand. There was never a inspection done by any part of the house.