icf home hvac sizing phoenix arizona

15 years ago

I am building a home in Phoenix, Arizona. It is a remodel, but most of the walls are built out of ICF Construction. The existing walls have a 4" Foam Retrofit Product. All new dual pane windows. I have also installed Radiant Barrier in the attic. I am putting radiant barrier on top of the blown in insulation and wrapping the ductwork with Radiant Barrier. There are basically no West Facing windows. The home is 2500 square feet.

I downloaded HVAC Calc Software and did a Manual J Calculation on the home. I was conservative (aka scared of undersizing) with my R Values. I did the calculations on the whole house and room by room. Both times I cam up with a 2.5 ton system. 24,000 BTUH of total heat gain including Latent Heat Gain.

HVAC guys have been trying to scare me away from going with a system this small. The last guy I spoke with is trying to push me toward two 2 ton systems (4 ton total) which seems like too much cooling to me. He is also telling me that the 2.5 ton CFM of 1000 will be too small for the home.

Just to make sure I was calculating correctly I did the same calculation on my current home. 1450 Square feet of slump block and came up with a 3 ton system which is exactly what the home has. Incidentally, most AC guys feel this system is too small, but it works fine.

Are there any experts out there that can clarify the CFM issue for me? Do I need more CFM even though I don't need the extra cooling capacity?

Thanks in advance for the help.

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Comments (8)

tk03
15 years ago
The heat loss/heat gain calculations are correct providing you put the info in properly and included everything. I am not a big proponent of DIY doing the heat losses. Most will do a good job but I have redone some and they missed something which may make a huge difference in the heat loss or heat gain. The heat and cooling appliances are too expensive to make mistakes and install the wrong size. It is also too expensive to put in a too large of a unit.
Have one of the contractors there do one and compare. If there is a difference ask him to explain what you did wrong.
ryanhughes
15 years ago
Airflow required is determined by the SHR required and total heat gain based on a room-by-room load calc, not the home's size. Aren't ICF homes especially tight? Your calculation may be correct... can't say some here. You may want to have the professionals quoting you do a load calculation (if they refuse, move on--very important for proper sizing).
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Before you spend any money on equipment I would suggest you have a Manual J Load Calculation performed. This will tell you exactly how many btu's of cooling and heating your home requires to get it to the temperature YOU desire. It will also tell you the sensible and latent numbers you NEED to meet or exceed. After these numbers are arrived at look for equipment that can handle them. Once you narrow it down to those models then you can debate name brands, color, noise and of course $$'s. By doing this you've done your best to be sure you are comfortable. Just my .02
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Waste..... Foaming the walls is a waste. Go to any green building forum and it is almost universally felt that foaming walls is a waste of money. The foam salesmen are about the only ones that disagree. In my east facing house with 2x4 fiberglass batts, the walls and attic only account for 20% of total heat gain (energy audit). You are going to spend $10k to make that 20% come down to 15%. I have a similar size house in NC and I spent about $20k on Carrier Infinity 15/16 with heat pumps and 90% NG furnace and zoning and humidifier. So I think the $36k is absolutely crap. But $11k is a bit ridiculous also given the specs. In fact- I just can't believe it. Manual J is required in NC by code - so I don't know what to tell you. It is standard around here for quotes/bids. Oversizing sucks. 2 stage helps compensate. An is an air quality thing more than anything else. It is rarely cost effective. Texas is big - what is your area and ideally what is your HDD and CDD? Here is what I would do - presuming you live in a 90% cooling area. - Build with 2x4s and sheath the house in 1 inch rigid foam. If it really hot, you could go to 2 inches of foam. - Focus on minimizing East and West windows. If you can't - then really focus on low SHGC windows. All windows are low-e nowadays but SHGCs vary considerably - there are basic windows at .2 and .45 - a rather huge difference. Overhang the windows as much as you can tolerate. Big deep porches. Do those things and you should be able to get down to 2 2ton units which you should be able to get for $10k for seer 15 or 16. Most people don't find it worthwhile to get seer 18 equipment. Usually with new construction, you can spend that $5k (or whatever) on shell improvements (windows, overhangs) and get a better permanent ROI. There is no disagreement anywhere that 2inches of rigid foam on a 2x4 wall filled with blown cellulose outperforms a 2x6 wall with spray foam (and does so at a lower cost). Lastly - why an unvented attic? Because the ductwork is there? The best practice is to get the ductwork out of the attic in the first place. It is cheaper and far more cost effective. My personal house (4000+basement) would cost $10k to spray foam the attic at a estimated annual savings of.... $70. How about that ROI....
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Well, I met with a 3rd HVAC pro yesterday.....and I am now more confused. The manual J calculation that he performed is vastly different from pro #1. Pro #3 suggests a 2-ton unit for basement, and 2 3-ton plus a 4-ton unit for the main level (12 tons total, compare to 7 tons total from pro #1) The load values vary so much between the 2 manual J reports, yet both professionals used the same software. How is the consumer supposed to sort through this information? Pro #3 accounts for infiltration values and duct loads that were much higher than Pro #1. Suggestions on where I go from here? Is there a simple way for me to understand the Manual J report, so that I will know which pro may have calculated incorrectly?
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Such a simple small house could probably get a load calc for $500 or so. Have you talked to an energy rater? Ours did a calc that the HVAC contractor used. Their fees were modest. I think it was $1000 for the overall package which included a Manual J, advice and inspection of insulation, submission for a utility rebate (we got $7500), Energy star certification which included blower door testing and duct leakage testing. Much of this is probably required with CA code - things like a blower door test and a duct leakage test. In my area, a manual J is required to be done but it is after permitting - it is required for HVAC inspection. Manual J is the typical load calculation. That determines how much tonnage you need. The complementary Manual D gives the duct sizes. (Just noticed that this is repetitive from above but different detail.) If you need an answer faster, you can download the software - last I heard available as a free trial for 7 days. Lots of inputs that you may not know. The biggest issue is solar orientation. Was that overhang designed to block the sun or more of a design element? One significant (and not often mentioned) advantage of a mini is how quiet the outdoor unit is. We have 2 trane XL15's and one mini. The mini is whisper quiet - like you feel the fan blowing on your leg rather than hear it to know that it is on. The indoor unit in our scenario is whisper quiet. Not to throw another option at you but ours is a ducted minisplit. With this the air handler is smaller than your average conventional system and also available at lower sizes.
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ryanhughes
15 years ago
Post this question on the home site of the HVAC-Talk program. http://hvac-talk.com/vbb/ -- You should receive quite a bit of help there. Just put "HVAC Calc" somewhere in the subject of the thread. :-)
energy_rater_la
15 years ago
good site to go to for advice
doesn't icf have a fourm for these type of issues?
check with icf mfg for their input.

radiant barrier should not be on attic floor
but attached (foil side facing into attic space
to undersides) of roof rafters
laying RB on attic floor will allow dust to collect
on surface and reduce reflectivity of foil
check sites like florida solar energy center (edu and info
only no selling of products) for unbiased RB info.
buildingscience.com also has good info.
best of luck
arizonajosh
Original Author
15 years ago
I have not found anyone that wants to do a Manual J Calculation. It is a little frustrating. Most HVAC guys around here use the 1 ton for every 500 sf calculation.

ICF Walls are very tight and have a very good insulation rating. I will try another forum and report back if I get any additional information.
icfspecialist
15 years ago
Try Howard Air...they actually load calc highly energy effecient homes ( no guessing).

Good Luck
dallasbill
15 years ago
Try the link below. We used them for our ICF home in 2005 and everything worked out great! Best money we ever spent!! Everyone in the ICF industry in Texas recommends them. Richard Rue is a great guy, too.

It will cost you 600 bucks, and you'll need to send them your plans. But, do you want to live in house that doesn't heat and cool properly?

Here is a link that might be useful: Energy Wise Structures
dickstermosc
6 years ago
I'm looking for a Insulated Concrete Form (ICF) contractor in Arizona. Does anyone have a recommendation???

icf home hvac sizing phoenix arizona

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icf home hvac sizing phoenix arizona

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