HVAC system sizing
Good day everyone. I’m in the process of building a new custom home in northern NJ. I’m trying to be as energy efficient as possible within budget. It’s approximately 5000 Sqft under air, 2.5-story home with a finished basement (included in total sqft calc). I’m trying to properly estimate my HVAC system. The house has R19 insulation on the walls, and R30 in the attic. It has poured concrete foundation. There are 35 (Pella proline) windows in the house, all double-pane, along with 2 glass sliding doors. Exterior walls are 2×6 construction. Front door is R16 rated.
I was quoted a 3-ton 2stage 17 SEER unit from Trane. I’m wondering if it will be sufficient and if it needs to be sized higher as the house I’m living in now is extremely under sized. I fully understand that it’s very easy to oversize the system as well – so I’m hoping the experts can help and demystify this for me.
Many thanks in advance!
-Eugene
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Replies
R19 in the walls does not meet IRC code minimum for any NJ climate zone (zones 5A and 4A), nor does R30 in the attic. Before calculating the loads try to a LEAST bring the design up to IRC 2012 code minimums!
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm
Code-min walls are 2x6 /R20 or 2x4/R13 + R5 continuous sheathing. Attic minimum is R49.
An R19 2x6 wall sounds like "almost" code min, but in fact R19s are tested at 6.25" of loft, and only perform at R18 when compressed to 5.5" in a 2x6 wall cavity.
Basement cooling loads are negligible, but above grade loads for typical code-min run between a ton per 1000' to a ton per 1500', depending on a whole lot of particulars. With a lot of west-facing windows, insufficient south side roof overhangs and sub-code attic insulation it might be as bad as a ton per 750'. With some tweaks to the design and careful of site shading factors it's not tough to make it closer to the ton per 1500' ratio. It's reasonable in a NJ climate to specify low SHGC windows on the west facing windows (especially if there is a lot of west window), and higher SHGC windows elsewhere.
Eugene,
Dana did a good job of highlighting the important issues. The most unusual thing about your plan is that you are specifying remarkably low levels of insulation -- even below legal code minimums.
This is a green building web site, so we usually advise readers to install more than code-minimum levels of insulation, not less. You also need to focus on choosing high-performance windows and achieving a very low level of air leakage when building your home. If you follow these steps, you will (a) be more comfortable, (b) use less energy (and therefore have lower utility bills), (c) require a smaller (and sometimes less expensive) heating system and cooling system, and (d) reduce the environmental impact of your home, which is good for the planet.
If you invest in a better thermal envelope for your home, the energy savings over the life of the building will in many cases more than pay for the cost of the envelope upgrades.
To size a cooling system, the first step is to perform a cooling load calculation. This is usually done with software (for example, Manual J software). If you don't want to learn how to perform these calculations yourself, you can hire an energy consultant or an architect to perform them for you.
For more information on cooling load calculations, see these articles:
Saving Energy With Manual J and Manual D
Calculating Cooling Loads
Thanks everyone - I have to mention that I made a mistake in the original post. The wall insulation is at R21 (not 19) and attic is at R49 (as per plan). I will be reviewing this with my builder and will check into hiring an energy consultant in my area. Any idea what would be the approximate cost for an energy consultant?
Eugene,
I'm not sure how much you should budget for a Manual J calculation performed by an energy rater; GBA readers may be able to suggest a range for this cost.
If I were you, I would start by looking for an energy rater certified by RESNET or the Building Performance Institute (BPI). If you visit the websites of these organizations, you will find tools for locating certified energy raters in your geographical area.
DEFINITELY have the room-by-room Manual-J run by a disinterested third party whose business is the accuracy their calculations, and not by an HVAC contractor. In my area that would run about $400-500 on the low end, to as high as $1200 in the gold-plated 'burbs and high-rent vacation areas. But right sizing the mechanicals will save you that much more than that in up front equipment and operating costs over the lifecycle of the HVAC equipment, and offer higher comfort to boot.
Some finer points to be aware of in the calculations to discuss with whomever is running the numbers:
If you are meticulously air-sealing the place as you go, be sure to indicate that, since air infiltration defaults in some load calculation software can be ridiculously high (higher than a code-max air leakage under IRC 2012 or later) which can make a huge difference in the whole-house numbers. Code max is three air exchanges per hour at 50 pascals pressure difference (3ACH/50), but the typical air leakage numbers will be under 2ACH/50 if they're paying attention to air sealing as they go, as opposed to an afterthought. It's hard to retrofit to 2ACH/50 once the walls are insulated and wallboard is up, but between 1-2ACH/50 isn't as difficult as some seem to believe. You'll have to pick a number, but don't assume code-max.
If the ventilation number defaults in the software assume an ASHRAE 62.2 ventilation rates, that's higher than most people prefer, and also skews the numbers skyward. To avoid that error, instruct them to specify heat recovery ventilation (which is probably worth actually implementing, even at ventilation rates much lower than ASHRAE 62.2.)
You have received good advice from both Dana and Martin . There are several Resnet / Hers raters in Jersey . My experience with most all of them is that they will fudge the numbers for builders to meet requirements , you'll find most of those are unemployed Wall St guys who got trained by one of their own after they destroyed the markets.
You'll want to find someone who was involved in EE and building trades prior to 2007 . Just a heat loss calc done from an approved plan should cost approximately what Dana stated . If you cannot find someone local you can contact me as a last resort . 732-581-3833
Dana and Martin .
Unfortunately NJDCA and other agencies caved to builders groups who still don't quite get it and believe that building a good house is gonna price them out of the market or basically hurt their bottom line . While IRC and IECC are both adopted model codes here in Jersey they have been amended so R 13 is minimum wall insulation and I believe 30 is minimum ceiling / roof / crawl insulation . Building departments are also supposed to require a Man J calc and a blower door test but as of yet I have only seen a handful of towns enforce that . POLITICS is Great huh ?
Eugene,
If you have not yet built you may want to consider changing your walls to 2x4's with 2" of XPS or EPS (2 1" layers with seems staggered). This wall stackup will fit on the same foundation and taking into account the framing effect your total wall R-Value would improve from around R-15 to R19 and the air sealing is much better. The home I am building with this configuration is almost complete and the temperature has been between 68 and 70 the last month with no HVAC installed yet. This includes the several 90+ degree days we have had here in Chicago. In the winter the team used a little mushroom heater in the basement and it kept the temps in the 40's and above throughout the house.