HVAC sizing disparity relative to HERS report
My HERS rater shows heating load of 39,500 Btu/hr and a cooling load of 22,600 Btu/hr on a high performance house I plan to build. This includes 1000 sq ft of basement, 2500 sq ft on the main floor and 800 sq ft in an office/loft area. R40 ceiling, R20 walls, R5 basement and crawlspace wall insulation plus serious air sealing (3 ACH or less).
The HVAC quote shows no system in the basement (it will be finished by me after construction), 3 tons on the main floor and two tons for the loft with all duct work in the conditioned space.
I was thinking I could get by with a mini-split configuration and am seriously questioning the sizing and the system type.
Your thoughts?
Norman
CZ 3A
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Norman,
Can you post the rater's calculations? Three ACH is not super. With a little attention to detail, you should be able to get close to 1 air change per hour.
Here is the initial HERS report. We have done some tweaking since then to drop the rating below 50. I expect to get better air sealing than 3, but that is the minimum requirement for design purposes. We also expect to use better windows than he has spec'ed, so these numbers should be conservative.
Annual Load MMBtu/yr
Heating 56.1
Cooling 13.9
Water Heating 8.5
Water Heating w/out Tank Loss 7.8
Annual Consumption MMBtu/yr
Heating 20.7
Cooling 3.2
Water Heating 2.4
Lights & Appliances 28.2
Photovoltaics -0.0
Total 54.5
Annual Energy Cost $/yr
Heating 477
Cooling 74
Water Heating 55
Lights & Appliances 652
Photovoltaics -0
Service Charges 0
Total 1257
Design Loads kBtu/hr
Space Heating 39.5
Space Cooling 22.6
I do not have a manual J from the HVAC company, just a quote with some comments:
First floor: 3 Ton Lennox, Signature Variable Speed Air Handler with a Elite 16 SEER R-410A 2-Stage heat pump and a programmable thermostat. System enhancements include: High Efficiency Media Air Filtration, Aircycler Fresh Air Ventilation System, and bath fans vented with rigid ducting (as needed) and rigid duct design were applicable.
Second floor: 2 Ton Lennox, Signature Variable Speed Air Handler with a Elite 16 SEER R-410A 2-Stage heat pump and a programmable thermostat. System enhancements include: High Efficiency Media Air Filtration, Aircycler Fresh Air Ventilation System, bath fans vented with rigid ducting (as needed) and rigid duct design were applicable.
Are the R-value numbers "whole assembly" with the thermal bridging of framing factored in?
The "R40 ceiling" meets IRC code min for zone 3 as an insulation-only number, but it's not exactly a "high performance house" unless that's a whole-assembly performance number. But even then it's just barely ahead of code-min for zones 4 and higher.
The "R20 walls" are also IRC code-minimum for zone 3 as an insulation-only number, and not really considered high performance.
The "R5 basement and crawlspace wall insulation" is similarly code minimum.
The "...serious air sealing (3 ACH or less)..." isn't serious at all, it's the IRC code maximum.
I'm not intending to be really harshin' on yer mellow, but this sounds an IRC code min house, not a high performance house. See:
https://up.codes/viewer/general/int_residential_code_2015/chapter/11/re-energy-efficiency#N1102.1.2
Even so, the 39.6K heat load seems pretty high for a 2500' +800' story-and-a-half house with 1000' of basement unless your 99% outside design temp is 0F, which would be unusual for a zone 3 location, even for a code-min house. An outside design temp of +20F or higher is more likely. If the assumption was 3ACH/50 a good chunk of the load number is air leakage.
A tighter-than-code but otherwise code min house that size with heat recovery ventilation will usually come in at ~27-30,000 BTU/hr @ +20F, so if you're serious at boosting performance at a low upcharge, take the "...serious air sealing..." seriously, and cut your target maximum to 1 or 1.5ACH/50.
So, what IS your 99% outside design temperature? What did they use for design temps in their calculations?
For a bigger (but not hugely bigger) upcharge on some other factors that matter, take a peek at the zone 3 row of Table 2 p.10 in this document:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
Note, those are "whole assembly" R values.
An example of the recommended R20 whole-wall performance would be 2x6 / R20 studwall + 1" of continuous rigid polyisocyanurate on the exterior. (Or, using 1.5" ZIP-R sheathing on a 2x6/R20 wall.)
An R50 vented attic would be an 18" deep energy heel truss to be able to accomodate 15-16"+ (settled depth) cellulose (which is blown at ~17" initial depth).
The R10 basement wall could be 1.5" of continuous foil-faced polyiso strapped to the wall with 1x furring through-screwed to the foundation with the wallboard mounted to the furring, or an inch of EPS trapped to the foundation with a non-structural 2x3 studwall with R8 batts.
In zone 3 they're even recommending a continuous R5 under the slab (about a $500-600 cost adder for a 1000' basement, using virgin stock 1.5" Type-II EPS, less if using reclaimed roofing EPS.) Slab insulation will reduce the amount of mechanical dehumidfication necessary to keep mold under control in the basement- it's "worth it", even for an unfinished basement used only for storage.
Take a good look at the windows' U-factors and SGHC ratings and compare those to the Table 2 recommendations too. Any west facing windows want to be smaller, or have an even lower SGHC rating to help keep peak cooling loads bounded. A 22.6K cooling load isn't ridiculous for a house that size, but it can usually be made much lower by fine-tuning the window selection by orientation & size.
The Lennox equipment recommended is, of course at the sub-optimally oversized (read "overkill") end of the range, even for the ~40K/~23K calculated loads. It's way oversized for the likely loads after the R value and U-factor/SGHC specs have been tweaked a bit.
What jumped out at me is $55 per year for water heating. Unless you only use paper plates and bathe in the pond, I think that number is missing a decimal place.
Dana,
Thanks for you insight. I was starting with the parameters from Building America Special Research Project: High R-Value Enclosures for High Performance Residential Buildings in All Climate Zones where on page 10, they note:
CZ 3
Wall R-20
Vented attic: R 50 (my typo on the R 40)
Compact roof: R 45
Basement wall: R 10 (I was working from memory for the R 5 - it's been one of those weeks)
Slab edge: R 7.5
Sub slab: R 5
Windows: 0.30/0.30 (U/SHGC)
The HERS rater has played with the numbers and some of the basement and slab insulation barely makes his numbers move, something on the order of a few dollars a year. Where we get the biggest bang for the buck is in the windows, the HVAC efficiency and air sealing.
On the air sealing side I was leaning on my HERS rater's experience with what "normal" Energy Star houses get in my area. I am, of course, shooting for a much better number, using a variety of techniques, but wanted that as a benchmark.
All that said, the gist of my question was given what my initial HERS rating was indicating was the HVAC system grossly oversized, which you have confirmed.
Any input on the use of mini splits in this scenario?
For mini-split solutions I'd need to get better sense of the floor plan/ layout, and the 99% & 1% outside design temperatures. If the load calculation tools break it out on a room by room basis, those numbers are useful too.
Assuming the heating load drops to ~under K and the cooling load drops to ~15-20K, you'll probably do just fine with ~2- 2.5 tons of ductless or ducted mini-split /multi-split. One potential solution (among many), a single 2.5 ton 3-zone Mitsubishi MXZ-3C30NAHZ2 is good for 28.6K heating @ +17F, and can throttle all the way back to 6K in cooling mode: http://meus1.mylinkdrive.com/files/MXZ-3C30NAHZ2_ProductDataSheet.pdf
The basement slab foam isn't about moving the load numbers, it's about keeping the mold factor under control without having to resort to dedicated mechanical dehumidification in the basement. The "payback" on energy expenses is low, but the payback in having a basement that will never stink is priceless.
But tweaking the window specs a bit usually has pretty good "payback" on both energy use and comfort. It's probably overkill for zone 3, but dual-low-E glass double-panes with different coating on surfaces #2 & #4 run about U0.25, but the SGHC will vary widely depending on the coatings used for each layer. see: http://www.cardinalcorp.com/source/pdf/Technical_Glass_Guide_Web.pdf Their Loe-340 on surface #2 / Loe-i89 (#4) combination come in at U0.25, but with an SGHC of only 0.17, whereas their, Loe-180 (#2) + Loe-i89® (#4) has an SGHC of about 0.60. That's a BIG difference!
But even with single low-E windows tuning the low-E specs by orientation pays off at very low upcharge. With reasonable attention paid to south side roof overhangs you can do pretty well with higher SGHC/lower U-factor north & south facing windows than the generic recommendations there, but lower SGHC on east & west facing windows. The higher SGHC on the south windows will raise the peak cooling loads a bit (especially if not seasonally shaded) but that higher peak can usually be offset by lowering the gain on the west windows.
Stephen: Here are the water heater specs.