HVAC questions for new home build in Central Texas

Texas-PatriotAugust 10, 2012

I'm going to build a 4000sf 2-story with 2x6 ext. walls and spray foam (open cell, full fill) and unvented attic. Stone/stucco exterior, front faces East, and essentially no tree shade.

Initial bids for HVAC came in as high as $36K for the best Carrier Infinity heat pump (2 units). They offered to do an American Standard 14 SEER dual unit system for $16K+.

Kept searching for other bids, convinced I was being gouged. Found a reputable installer that quoted $11,500 for 3-ton and 4-ton Amana heat pump @ 18 SEER with 2-stage compressors and variable speed handlers. Similar price for high-end Rheem.

Not a single HVAC contractor (I've spoken to 5) has talked about manual J calcs. I suspect they're all using lazy rules of thumb without regard for my particular construction features. Some bids were for dual 2.5 ton units, and the one I'm most likely to accept is calling for a 4-ton and 3-ton.

Questions:

1. Is the high-end Amana heat pump reliable?

2. Should I be concerned about oversizing?

3. Does a 2-stage compressor mitigate the risk of oversizing?

4. Do I need an ERV??? My builder is suggesting it's a waste.

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david_cary

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....

    Bookmark   August 10, 2012 at 2:12PM
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Texas-Patriot

Thanks David_cary for the thoughtful response.

I'm building in the area surrounding Austin. It's very hot most of the year. Basements are built in less than 1% of Texas homes, relegating duct work to the attic. Over the past few years the sealed spray foam attic has become the "best practice."

I've read a few things about foam being a waste in walls. Air infiltration seems to be a big part of energy inefficiency so the foam certainly performs well there. My foam bid is $10K, so perhaps foam is cheaper here than other parts of the country.

I'm pretty committed to 2x6 framing at this point, but I do see the value in rigid XPS panels in lieu of, or in addition to OSB sheathing. I'll talk with my builder about that option. I've priced them and I think it would run $.50/sf for 1" of XPS. That adds up.

You preach manual J all you want--I agree. Just telling you the reality is that it's seldom done here from what I've seen. They are using calculations on the back of a napkin.

The west side of my house has only two windows upstairs that get direct sunlight. Everything else is 100% shaded all day by porches, pergolas, etc. That was by design. The south side has only a few windows.

What about an ERV? Do I need one?

Thanks!

    Bookmark   August 10, 2012 at 3:55PM
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fsq4cw

Have you considered geothermal?

I think there's a big difference between Carrier Infinity Greenspeed and virtually everything else out there except for geothermal. The reason is variable speed heat pump as well as fan coil and a very smart thermostat.

IMO

SR

    Bookmark   August 10, 2012 at 4:43PM
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Texas-Patriot

fsq4cw,
Yes, I originally wanted an ICF home with geothermal HVAC. Then reality came knocking and I realized those things don't align with resale value, and I have no idea how long I'll be in the house.

Cheapest geothermal I could find was a non-committal quote of $5K per ton. That's a $13,500 delta, with a payback period longer than I'm willing to accept. I even inquired about direct exchange geothermal, figuring it was cheaper to drill and more efficient to run. Supposedly there's only one guy in Texas that has the equipment to dig those holes and he's out of state right now. I stopped pursuing Geothermal.

I also had heavy interest in the Maytag IQ drive. Again, couldn't find anyone in my area that specializes in that system. I love the idea of a fully modulating compressor. 15-60hz with unlimited possibilities in that range.

    Bookmark   August 10, 2012 at 5:33PM
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juliekcmo

I think you will want an ERV with a "tight house" to bring in fresh air and feel comfortable. Otherwise you may find that your load is actually so small that your home gets that stale air feeling. Don't want it to be like being in an elevator.

I also think that if your R values and wall thickness are not builders standard for your area, that you are bound to get an over sized system, and bad dehumidification because of it, unless you get a correct load calc run.

If you want another estimate, I suggest you look around for a mechanical contractor that does commercial and also custom residential. These folks will be able to lean on the distributor for help with the load if they need some help.

I ran a load this week for a project that with standard conditions would have been 1/3 more AC tonnage than what was calculated with foam insulation and high R values.

    Bookmark   August 10, 2012 at 7:50PM
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fsq4cw

Any driller can drill boreholes for DX but its total installation requires a different expertise from conventional geothermal.

The Carrier Infinity Greenspeed system will come as close as you presently can get regarding energy efficiency to geothermal - especially in heating mode.

BTW: We have a DX geothermal system in our home. It works great; next month well be in our 10th year with it; excellent economy, has never required a service call.

SR

    Bookmark   August 10, 2012 at 7:57PM
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david_cary

I had a comment about ERV but something got deleted and it made no sense.

You don't need an ERV but you might like the fresh air (as someone else has said).

Foam prices do seem cheaper elsewhere.

My complaint about foam in the rafters is that the R-value is low and the surface area is large. I have r-40 to 60 on a flat ceiling. The roof area is about twice the ceiling area so the r-30 that you might get with foam (maybe r-25) would be like me getting r-15 in my ceiling. Sure the barrier maybe slightly tighter (I have foamed/caulked all penetrations) but that is a terrible net r-value. What is your foam r-goal?

So you don't have a basement. You can put the first floor ductwork in the space between the 1st and 2nd floor - right? That is presumably the plan. Well you can create a floor between the 2nd floor and the attic and put the ductwork in there. Then throw r-60 of blown cheap insulation on it - and voila - better R, lower surface area, ducts in conditioned space. For us, that is $2000 in open web trusses and some siding instead of $10k to foam the rafters. Now your rafters maybe just 1/2 your budget so the money cost is less but the end effect is better. You also have to find space for the air handler....

Where does your 1st floor air handler go?

Sounds like your windows are taken care of. I'm not sure geothermal is worth it in a cooling climate unless it is truly extreme. Of course $5k a ton is not bad if you only need 4.

You definitely need to fight hard for the sizing. I've had all sorts of issues including HVAC contractors sneak in a larger unit than I speced (they replaced it once I realize it).

fsq4cw - I don't think a tight house in his area even needs heat - maybe $50 a year....

Now lastly - where is your hot water coming from? People can do great on the HVAC and actually get the hot water to cost more.....Obviously solar makes the most sense in your climate unless you have a good place for a heat pump.

    Bookmark   August 10, 2012 at 8:40PM
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Texas-Patriot

Hot water is coming from the AO Smith Vertex 100 that we plan to install. Looked at tankless, and I don't see a strong value proposition. Plumber I spoke with put me onto the Vertex and said he's actually finding it more efficient than tankless for most families. I had never heard of it before he mentioned it. And oh yeah, it's a lot cheaper.

@david_cary, believe it or not it can get very cold here, but only for a month out of the year. And heat pumps can struggle in very cold temps.

Sounds like there may be some fresh air options aside from an ERV, but I'm not sure what they would be. I'm skeptical that my HVAC contractor will put much thought into it.

Another concern I have is how to get supply air when exhaust venting is happening (clothes dryer, range hood, bath vent fan, etc.). I don't want a negative pressure situation. Anyone know of the proper solution for that?

    Bookmark   August 11, 2012 at 6:27PM
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david_cary

Your idea of very cold is relative..... Either way, a greenspeed would be ridiculous in your climate. People use heat pumps in Canada of course.

For fresh air, you can just introduce fresh air. It is considered inefficient but honestly an ERV is pretty inefficient anyway.

You should have a make up air system. The clothes dryer is not significant and neither is the bath vent fan. But the range hood usually is. Our hood is 1200 cfms where a bath fan is usually 80 or so. Typical hoods are usually at least 300 cfm. Of course, most people (myself included) just open a window or door when the range hood is on. Negative pressure just allows some infiltration which is what you need to exhaust anyway. The reality is that the exhaust just doesn't work as well if you don't open a window or have a make up air system. This is always inefficient and there is nothing you can do about it - an ERV does not flow enough to help.

    Bookmark   August 11, 2012 at 8:22PM
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ratflinger

Hold on there cowboy.

"Yes, I originally wanted an ICF home with geothermal HVAC. Then reality came knocking and I realized those things don't align with resale value, and I have no idea how long I'll be in the house."

Really? Around Austin? If there's anyplace in Texas that embraces green building it's around Austin. I'd build for max efficiency & not worry about resale - I don't think it will be a problem. In fact I'm waiting on a bid for an ICF home south of San Antonio. As long as the bid comes in fair I don't give a darn about resale. When electric rates double people will be bidding for your house.

BTW - for AC we're going with Mitsubishi Mr. Slims so we control every room individually.

    Bookmark   August 14, 2012 at 8:21PM
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