By Matt
Synopsis
If everything goes right (tightly sealed, well insulated, high solar gain) we have the potential to only use 1254 kWH of our 12,000 kWH budget to heat our home. Its more likely that heat will take up to 1/3 of our energy budget (4000 kWH) and we need to be prepared to reduce other electrical loads.
We are using the generated energy from solar panels to heat the entire house using ductless minisplits, and if the house is not constructed in an energy efficient manner than the heating load could quickly drain our entire energy budget. This means high R-value walls with minimal thermal bridging, a well insulated attic, triple pane windows and minimal infiltration (air tight). We have determined that the maximum our south facing roof will hold is a 10 kW system which will generate approximately 12,000 kWH a year, so that is our energy budget.
There are online calculators where you can plug in the specifics of your house build along with climate information to determine the number of BTUs of yearly heating your house will require. The easiest I have found is the calculator at Build it Solar here.
Here is the information that is required to plug in and the values for our build.
Design Outdoor Temperature = -20 F
The typical coldest temperature your climate sees in the winter (Find Here)
Heating Degree Days = 7416
Heating degree days are defined as the number of degrees that a day’s average temperature is below 65 F, the temperature below which buildings need to be heated. One HDD means that the temperature conditions outside the building were equivalent to being below a defined threshold comfort temperature inside the building by one degree for one day. Thus heat has to be provided inside the building to maintain thermal comfort. Values for your area can be downloaded here.
Then one has to enter a number of house size parameters and corresponding R-values
Ceiling (Area Ft2) = 920, R-Value = 90
4” open cell foam, blown fiberglass to R-90
Wall (Area) = 2120, R-Value = 40
The wall construction from outside to inside is : LP Smartside, Tyvek, 7/16 OSB Sheathing, 2 x 4 exterior wall, 2.25 air gap, 2 x 4 exterior wall, open cell spray insulation shaved to back of drywall, 6 mil poly, 1/2” sheet rock
Windows (Area) = 280, R-Value = 6.6
WASCO GENEO Triple Pane Windows
Center of Glass U=0.115, SHGC = 0.55, Whole Window U=0.15
Doors (Area) = 65.5, R-Value = 6
2 – Wasco Doors Triple Pane glazing, 1 Fiberglass Thermatru Fire rated door
Floor (Area) = 0, R-Value = 0
We have a slab on grade for most of the main floor so we are going to enter 0 here
Slab (Perimeter, Ft) = 106.5, R-Value per linear Ft = 2
Most of the heat loss through a slab is at the edges, so the program requires knowledge of the slab perimeter size and then since its no longer an area you have to click through in the program to determine what “factor” you put in as an R-value per foot of perimeter. The examples they give only go to R-5 insulation on exterior of footer/wall where we have R-10 so I will just go with their best case value…. an R of 2 per foot of perimeter.
House Size (Volume Ft3) and Infiltration Rate.
16537 Ft3 and Ach50 < 1.0
This is where super tight houses make for a complicated equation. One has to figure out, based on the blower door test whether the amount of air which infiltrates the house is sufficient and to code for the number of air exchanges per hour. Occupants need a minimal amount of air exchanges (0.35 Ach) per hour to remove stale air. In old houses this value might be as high as 5, but in super tight houses the AchNat can be almost negligible. Then a ERV (energy recovery ventilator) is used to bring in additional air while conserving energy as it exchanges the heat from the warm stale inner air to the incoming cold fresh outer air. Once we do a blower door test we will know our Ach50 which is the amount of air leakage when the house is under 50 pascals of over pressure, but the program requires AchNat. So to use the calculator properly we first have to convert our goal Ach50 to AchNat, that is the natural air exchange rate the house will achieve when not under pressure. The conversion of AchNat to Ach50 depends on the type of build (one story, two story), amount of shielding from wind, and the location in the country. The conversion factor for Ach50 to Ach(nat) can be determined from this energy star document (which has the following table, shown right). Our build is located in the southeast corner of Minnesota (Zone 1), we have 2 stories, and we will have normal shielding (some trees, and an attached garage on the north side. Our conversion factor therefore is 12.4 and our AchNat is Ach50/12.4. If we achieve a blower door test of 1.0 (Ach50) then our AchNat will be 1/12.4 or 0.08 air exchanges per hour by infiltration. Therefore we need another 0.27 air exchanges per hour to reach the code of 0.35 air exchanges per hour. Luckily those air exchanges will be going through the energy recovery ventilator where we can recapture some of outgoing air by passing it to the incoming air. To use the calculator, we first put in 0.08 as our natural infiltration value and write down the heat loss (4.2 million BTU). Then we put in the amount of make up air which will go through the ERV (0.27) and multiple the heat loss (14.3 million BTU) by 0.25 as greater than 75% of the heat loss calculated will be recovered by the ERV.
Number of House Occupants = 3
Based on the number of occupants the program calculates how much energy will be generated by the occupants and their appliances. This energy/heat can be subtracted from the yearly heat loss as this is energy that does not need to be supplied.
Hit Calculate
We hit the calculate button and see the derived yearly loss in millions of BTU for each potential source of heat loss. For us the windows and doors (9.5 million BTU), walls (9.4 million BTU) and slab (9.4 million BTU) all have nearly equivalent losses, with infiltration and ERV ventilation coming in at 4.2 and 3.6 million BTU respectively and ceiling losses at 1.8 million BTU (see graph at top). With three occupants the program calculates that we will have +11.7 million BTU in internally generated energy, which means the total yearly loss would be 26.2 million BTU which would have to be supplied by our minisplit heat pump.
Solar Gain
However, we have a lot of southern facing windows and an energy absorbing concrete slab, therefore we should get significant solar gain which should offset energy we have to input into the house. A solar gain website can be used to calculate potential solar gains based on location and type and size of windows. This website has a pulldown to find your location so that the number of sunny days and length of day can be used in the calculation. The only other input is the SHGC (solar heat gain coefficient) of the installed window which in our case is 0.55 for our Wasco Geneo triple pane windows.
The calculator assumes that there is an overhang which shields the summer sun so the peak thermal gains are actually achieved in the transition seasons between spring and summer and fall and winter when the day is relatively long and relatively low in the sky. We only will require heat from October to March so we will only include those solar gains in our calculation. We have approximately 200 square feet of south facing windows once you factor out the window jams. From October to March the calculator indicates solar gains of 88,000 BTU/Ft2 of windows. Therefore we have the potential of generating 17,600,000 BTUs of energy to offset our total.
Electric Heating Load
At the top of this page is the bar graph indicating all of the energy losses (windows, walls, floors, infiltration, etc) and gains (appliances, people, solar gain) and then the total which nets out to 8.6 million BTU / year or 4000 BTU/sq ft. This would be an amazing number if we could hit this value as this would be below both the international ( 4,755 BTU/sq ft ) and zone 1 American (7300 BTU/sq ft) passive house standards. The conversion factor for BTU to kWH is 1 kWH = 3412 BTU, so to generate 8.6 million BTU would require 2,512 kWH. Over various temperature ranges minisplit heat pumps have efficiencies that average 200 to 300%, so if we take the low average efficiency (200%) it would only take 1254 kWH to generate the 8.6 million BTU / year we would require to heat our home. Thats only 10% of our electricity budget. We think we could allocate up to 33% of our budget to heating which would be 4000 kWH or 27.3 million BTU (200% efficient heat pump). Of course all of these calculations are just estimates and are dependent on executing proper thermal and air tight building practices.
Example Up-Hill House
Up-Hill house in upper New York is a two-story home above a walk out basement, 2-bedroom 1-bath, with each floor being 600 square feet, for 1200 ft2 of living space or 1800 ft2 of conditioned space. The slab R-value is 30, Foundation wall R-value is 41, above grade wall R-value of 44 (double stud with blown cellulose), cold attic R-value is 75, their Ach50 was 0.46, and they use mechanical ERV ventilation and Mitsubishi minisplits. Windows have U-values of approximately 0.18 to 0.24. The house is in zone 5 with 6500 CDD and the equipped solar panels generate 8500 kWH of electricity from solar. They used approximately 1800 kWH in heating each year using their ductless minisplits. To convert the heating use to a relative value you divide the kWH used by the conditioned space area (1800 ft2) by the HDD to get a kWH/ft2*HDD value. This value therefore would be 1800kWH/1800ft2/6500HDD = 0.000154 kWH / Ft2-HDD. This was a very well done house where great care was utilized in sealing and insulation.
Sanity Check
Perhaps either our infiltration/ERV calculation or our passive solar gain calculation is overly optimistic. If you converted our Electric Heat Load calculated above (1254 kWH) and divided by our square footage (2100 with basement) and our HDD 7500 our thermal efficiency would be 1254 kWH / 2100 / 7500 HDD for 0.000076 kWH / Ft2-HDD. This is twice as good as the example Up-Hill House and probably pretty unlikely. Perhaps a better example would be the high performances house built by Transformations INC. These houses were either slab on grade or full basement. R-10 under slab, R-20 basement walls, R-46 open cell insulated above grade walls, R-63 cold attic, U=0.22 windows and Ach50 values between 1 and 1.5. These homes averaged 0.00013 to 0.0004 kWH/ft2/HDD, with most being approximately 0.00025 kWH/ft2/HDD when basements were included. If we back calculate our home values using the 0.00025 kWH/ft2/HDD we would use 0.00025 x 2100 x 7500 = 3975 kWH. This would be 1/3 of our energy budget and may be ok if we can keep our other uses in check.