# Calculating heat losses

| Posted in General Questions on

I am in the planning process for a pretty good house in northeastern Oregon in climate zone 5.  I have developed a simple spreadsheet to calculate approximate heat losses.  However, based on comments I see in other threads, I think I may be overestimating heat losses.

I would appreciate any thoughts on where I may have errors in my calculations.  Note that when setting this spreadsheet up over several years, I didn’t do a good job of documenting the source(s) for my formulas, so they may not make any sense to those with subject matter expertise.  For instance, I don’t recall the source of the heat loss through infiltration or ventilation formulas.

Assumptions:

>simple rectangle floor plan 26 X 54ft (outside dimensions)
>9 ft high double walls insulated to R-30 (whole wall excluding windows)
>1750 sq ft cathedral ceiling insulated to R-60
>200 sq ft of windows with U=0.25
42 sq ft of outside doors with R=4
>4″ concrete slab on foundation with R16 insulation beneath and around perimeter of slab (inside the uninsulated foundation)
>ground temperature beneath house is annual average (48F)
>HRV ventilation of 50 cfm at heat recovery efficiency of 70%
>ACH50 = 1.0
>HVAC design temperature of -5F
>Interior temperature = 70F

3100 btu/hr Heat loss through walls (minus windows) = (1240/30) X 75
3750 btu/hr Heat loss through windows = (200 X 0.25) X 75
788 btu/hr Heat loss through doors = (42/4) X 75
2188 btu/hr Heat loss through ceiling = (1750/60) X 75
1931 btu/hr heat loss through floor = (1404/16) X 22
2585 btu/hr heat loss due to infiltration = ((1404 X 9 X 1.0) X 0.018 X 75)/6.6
1229 btu/hr heat loss due to ventilation = 50 X 0.0182 X 60 X 75  X (1-(70/100))

15, 571 btu/hr total heat loss at HVAC design temperature.

Am I overestimating heat loss, and if so, where?

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### Replies

1. | | #1

Infiltration is highly dependent on wind and it's not clear what the appropriate wind speed is - design day isn't worse case, but it isn't average case either. IMO, ACH@50 * .4 is reasonable.

So I get 1404 x 9 x 1 x .018 x 75 x .4 = 6823 Btu/hr

Or use this.

Also note that this load moves around (windward/leeward), so it creates balancing problems.

2. | | #2

Stephen,

One minor point: You didn't seem to subtract the area of your doors from your wall area (but did subtract window area).

You probably already realize this...but keep in mind that you are calculating the heat losses at DESIGN temperature, not AVERAGE winter (or average January) temperature. So that design temp heating loss number can be used to design your space heating system. Your average heating needed (especially with that concrete floor mass) would be perhaps a third less (you don't mention your average winter temp).

You also may have some solar gains (south-facing windows) and heat gains from your kitchen appliances etc that would reduce your actual space heating needs. You might want to calculate your average use of electricity and expected solar gain during mid-winter. The coldest days typically are clearer skies, so solar gains might be higher than your wintertime average. (Cloudy periods tend to hold heat, vs. clear skies radiate more heat to outer space at night, bringing colder temps but also bringing solar gain during the daytime.)

Finally I think it is difficult to measure insulation losses without doing a blower-door test. Conductive heat losses are much easier to calculate reasonably accurately than infiltration losses. Attention to air sealing details and the quality of your windows and doors can result in pretty large variation in infiltration losses.

In any case your numbers do appear to give you some ballpark idea of your heat losses on the coldest winter days expected for your area.

1. | | #3

Robert,

Thank you for your response. Yes, my spreadsheet does subtract out the doors, but I didn't copy exactly from the spreadsheet because it uses some extraneous information (e.g. the amount of heat extracted by a heat pump water heater, etc) that I thought would just confuse things.

Yes, I am aware of design vs actual temperatures. For some additional information, I am using the closest NOAA station in Elgin, Oregon, a couple of miles north of the build site. The build site is ~650 ft higher in elevation, but because of topography, the effect on temperature is unpredictable. The record low in Elgin is -31F. I have been at the build site when the temperature was approx -25F. Note that a few miles south, the record low is higher than -20F and they commercially grow sweet cherries; topography is very important.

Some climate statistics:

Mean number of days with temp less than 0F = 6
Mean number of days where the max temperature is less than 32F = 16
Mean number of days with temp less than 32F = 166
Heating degree days is 6444
50% probability frost free season is 92 days
Mean snowfall is 51 inches in Elgin but probably >60 inches at the build site

Yes, there will be some heat production from appliances, occupants (people + animals), and solar heat gain, but also some heat loss from outside doors opening as people go in and out of the house. The house is designed with most of the windows facing to the south and the coldest days are usually sunny with very little wind, so there could be some significant solar heat gain. However, the coldest temps are likely to be in the early morning before the sun is shining. On the coldest days, the heat will generally be coming from a wood stove that can provide more heat than is required, but the house will need to be code compliant and stay heated without the wood stove.

3. | | #4

Stephen,
Attached is a chart from WeatherSpark.com showing percent of sunshine/cloudiness over the year for Elgin. They provide some interesting charts you might want to examine.

I'd be interested to know the percent and area of your south-facing glazing vs other directions. With your 4" slab thermal mass, you likely will have a passive solar or solar-tempered home that requires space heating only during winter after successive cloudy days. Doubt you'd need much heat in the AM if it was mostly sunny the day before. Did you design your home to be solar tempered? Or just being practical, capturing passive solar gains from south-facing windows "Pretty Good House" style? Or?

I've designed a somewhat similar 24'x44' 2BR 1BA home for coastal New England with edge-thickened 4" slab, more heavily insulated. The goal is to almost eliminate winter space heating in Zone 5 with mixed sunny/overcast winters. Forty+ years ago I designed and built a more traditional passive solar home in CO (large amount of south-facing glazing, moderately well insulated and air-sealed that required no space heating on the main floor (4" slab) and usually no auxiliary heating in upstairs bedrooms. These are more benevolent climates than yours for passive solar.

At the moment I'm pretty overloaded, renovating a home up for sale, but I'd be interested in doing some modeling to predict the temperature variations in your home, if you don't mind sharing some data on glazing area, appliance usage, etc. You can reach me at opiehere at gmail dot com or go back and forth here. I can use my spreadsheet that includes heat gains to plug in your data and maybe produce some useful numbers for you (and me, looking at other climates).

Air has little thermal mass, so people going in and out won't affect your average internal temps much, unless they keep doors or windows open, like to leave a window open a little all night for fresh air.

In any case, best of luck with your future home, it seems well-designed from an energy efficiency standpoint.

1. | | #5

Robert,

Thanks for the information. I hadn't seen the WeatherSpark graph; I think it may under-represent solar energy available especially in late winter through spring. Sometimes websites use Pendleton OR, Spokane WA, or even Portland OR for rural northeastern Oregon climate data. If you use a city, Boise ID is probably closest, although it has more sun year-round. I have used PVWatts to estimate the solar energy available each month.

Given the relatively low hours of sunshine in the winter, the goal is to take advantage of solar energy gained through windows when possible, but the design is not based on solar heat gain through windows. I do expect solar energy to provide a significant proportion of the heat required in spring and fall.

Although grid power is available, I intend to include ~4kw of PV with the goal of net zero on an annual basis. Although PV on the roof should not be shaded, the windows may have some shading in the winter due to ~8oft tall trees ~50 ft from the house on all sides.

South facing (actually facing about 15-20 degrees west of south) windows would be about ~100 sq ft. with a SHGC of ~0.68. West facing windows would be ~25 sq ft with a SHGC of ~0.4. East facing windows would be ~40 sq ft with a SHGC of ~0.4. North facing windows would be ~35 sq ft with a SHGC of ~0.4. I will also consider Passive House rated windows with U value of 0.14 and about 10-20% less SHGC; however, I think that they may not be cost effective.

If you have the chance, I would be interested to see what you model. Of course, average heat loss will be significantly less than using a -5F design temp.

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