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How to Perform a Heat-Loss Calculation — Part 1

Let’s begin by discussing outdoor design temperatures and the many ways that heat can escape from a house

Posted on Apr 20 2012 by user-756436

I’m going to devote the next several blogs to a discussion of heat-loss and heat-gain calculations. These calculations are the first step in the design of a home’s heating and cooling system.

In order to address this big topic in little bites, I’ll start by discussing heat-loss calculations. I’ll get around to heat-gain calculations and cooling equipment in a future blog.

Before digging in to the topic, however, it's worth answering the question, “Why do I need to know how to perform these calculations?” If you are a homeowner or carpenter, you may not need to know anything about calculating heating loads. However, if you are a designer, architect, or builder, this knowledge will prove useful — even if you never perform the calculations yourself, but instead depend on computer software or consultants to perform the calculations.

Understanding the principles behind heat-loss calculations will help you understand how buildings work and how heating and cooling systems perform. With this knowledge, the quality of your dialogue with window suppliers, HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractors, and engineers will definitely improve.

Outdoor design temperature

If you’re performing a heat-loss calculation to size heating equipment, you need to perform the calculation for the worst-case condition: namely, the coldest night of the year. (Because the coldest condition usually occurs at night, a heat-loss calculation does not consider solar gain through windows.) The temperature on that night is referred to as the outdoor design temperature. (To be precise, the outdoor design temperature is usually defined as the temperature that is equaled or exceeded for 97.5% of the time during the three coldest months of the year. Other sources define the outdoor design temperature as the temperature that is equaled or exceeded for 99% of the year. As it turns out, most homes will remain comfortable even when the thermometer dips below the design temperature 2.5% of the time.)

Design temperatures for many locations around the world can be found in Chapter 24 of ASHRAE Fundamentals. Design temperatures for U.S. locations are posted online by the International Code Council; the design temperatures for ACCA's Manual J are posted here. Fairbanks, Alaska has a winter design temperature of -47°F, while Honolulu has a winter design temperature of 63°F.

The indoor design temperature for heating systems is usually assumed to be 70°F, although a higher design temperature may be chosen if desired. The difference between the outdoor design temperature and the indoor design temperature is the ΔT (delta-TDifference in temperature across a divider; often used to refer to the difference between indoor and outdoor temperatures.). For example, the winter design temperature in Burlington, Vermont is -7°F, so a heat-loss calculation for Burlington assumes a ΔT of 77 F°.

Obviously, the higher the ΔT, the higher the rate of heat loss. That's why it takes more fuel to stay warm in Fairbanks than it does in Miami.

There are many ways that a house loses heat

Now that we know how warm we want our interior and how cold we anticipate it will get outdoors, we can begin calculating how much heat our home will lose per hour on the coldest night of the year.

To be sure our calculations are accurate, we have to start by listing all the ways that a house loses heat:

  • Heat is transmitted to the exterior through a home’s floors, walls, ceilings, windows, doors, and penetrations.
  • Heat leaves the home when warm indoor air leaks through cracks in the home’s envelope.
  • Heat leaves the home through air that is deliberately exhausted by bathroom fans, range hoods, and clothes dryers.
  • Heat leaves the home when warm water goes down the drain and flows to municipal sewer pipes or a septic tank.
  • Heat leaves the home when combustion gases from a furnace, boiler, or water heater exit via a flue.

Some of the heat flows listed above — for example, the heat that leaves through drain pipes — are relatively minor and are therefore ignored by most heat-loss calculation methods. Others — especially heat transmission through the home's envelope and heat lost via air leakage — are so significant that they can't be ignored.

Remember, it’s called the “U-factor,” not “U-value”

To calculate how fast heat flows through a building assembly like a floor, wall, or ceiling, energy modelers need to know the building assembly’s U-factor. (U-factor is the inverse of R-value, so U=1/R and R=1/U.) The lower the U-factor, the lower the rate of heat loss. In the U.S., U-factor is defined as the number of BTUs that flow through one square foot of material in an hour for every degree Fahrenheit difference in temperature across the material (Btu/ft² • hr • F°). Thick insulation has a low U-factor, while a single sheet of glass has a relatively high U-factor. Low U-factors are good; high U-factors are bad.

Calculating heat flow through building assemblies gets complicated, because walls don’t have a uniform U-factor. In a typical wood-framed building, for example, the insulation between the studs has a certain U-factor, but the U-factor of the studs and plates is usually higher. Furthermore, the window’s U-factor differs from the U-factor of the insulation and the framing lumber; in fact, different areas of the window have different U-factors. The center of the glass has one U-factor, while the perimeter of the glass has a higher U-factor. Finally, the window frame has its own U-factor.

Each penetration has its own U-factor. For example, if an insulated ceiling is penetrated by a large brick chimney, the U-factor of the chimney is much higher than the rest of the ceiling.

Most heat-loss calculations methods avoid the hard math by making a few default assumptions: for example, calculation methods usually assign a framing factor to wood-frame walls — for example, 23%. In other words, it’s easier to assume that 23% of the wall consists of framing lumber, and 77% of the wall consists of insulation, than it is to try to calculate the actual percentage for every wall of the house.

Needless to say, heat transmission through below-grade walls and floors must be calculated differently from heat transmission through above-grade walls. Heat loss calculations must also take into account the effect of buffer spaces like crawl spaces and attics; during the winter, the temperature of these partially conditioned spaces is usually higher than the outdoor temperature and lower that the indoor temperature.

What about air leakage?

Thirty years ago, heat-loss calculation methods were fairly unsophisticated, especially when it came to air leakage. I first learned how to perform heat-loss calculations in 1975, using a pencil and a paper form called the “I=B=R Calculation Sheet.” (This method was promoted by the Institute of Boiler and Radiator Manufacturers, an organization that no longer exists.)

The handbook accompanying the calculation sheet advised that there were three levels of air tightness:

  • The tightest homes had “windows and doors weatherstripped or with storm sash.”
  • Mid-range homes had “windows and doors not weatherstripped and without storm sash.”
  • The leakiest homes had the following wall construction: “Clapboards or wood siding, studs without insulation, 1/2-inch drywall, no sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. .”

These days, some heat-loss calculation methods use a similar approach, with default values assigned to various descriptions of leakiness — for example, “quite leaky,” “average,” and “tight.” My 1993 edition of ASHRAE Fundamentals lists three levels of air leakage, and the definitions have been updated from those provided by I=B=R:

  • “Tight” homes have “close-fitting doors [and] windows ... New homes with full vapor retardant [sic], no fireplace, will-fitted windows, weatherstripped doors, one story, and less than 1,500 square feet of floor area fall into this category.”
  • “Medium” homes include “two-story frame houses or one-story houses more than 10 years old with average maintenance, a floor area greater than 1,500 square feet, average fit windows and doors, and a fireplace with damper and glass closure.”
  • “Loose” homes are “poorly constructed” residences with “poorly fitted windows and doors.”

Some software modeling programs allow users to input the results of a blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. — an approach that (at least in theory) should yield a more accurate result.

However, even when we have blower-door results for the house under consideration, we still don’t have enough information to understand how much air will leak out of the house on a cold night. That’s because when it comes to holes in a building’s thermal envelope, what matters is “location, location, location.” If the holes are evenly divided between ceiling holes and basement holes, the air leakage rate will be high; however, if the holes are mostly located near the neutral pressure plane in the middle of the house, the air leakage rate will be much lower.

The heat-loss calculation methods used by contractors and architects never take account of envelope leak location, even though it matters; instead, default assumptions prevail.

Sharpening our pencils

Now that we’ve discussed how heat leaks out of a house, we’re almost ready to begin making a few calculations. In next week’s blog, I’ll explain how to use the area of a portion of your home’s thermal envelope and its associated U-factor (a value that you can look up in a table, or calculate yourself by adding up the R-values of all the layers in the building assembly) to determine the rate of heat loss (in BTU/hour) at the design temperature for that portion of the envelope. Stay tuned!

Last week’s blog: “Heat-Pump Water Heaters Come of Age.”

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Image Credits:

  1. Dominic Alves

Apr 21, 2012 7:45 AM ET

Edited Apr 21, 2012 7:48 AM ET.

Sliding ruler
by Acobo

Good thread. I guess I need to turn off my computer, dust and oil my sliding ruler and get goin'...
I did notice that the design temperatures from ASHRAE Fundamentals differ considerably from ACCA MJ8 design temperatures. Do you know why?

Apr 21, 2012 10:10 PM ET

design temperature vs "coldest"
by watercop

I'm glad you somewhat distinguished between "the coldest night of the year" and 97.5% design temperatures, but I'd like to make the point much more emphatically. The purpose of the 97.5% design temperature is specifically to avoid specifying a system for record cold weather, which is rare and persists for only a few hours.

Apr 22, 2012 12:33 PM ET

Edited Apr 22, 2012 12:35 PM ET.

Response to Armando Cobo
by user-756436

You're right -- you'll notice that design temperature tables from different sources don't always agree. Older design temperature tables are based on older weather data; as global temperatures have warmed over the last few decades, some design temperature tables have reflected these changes by raising design temperatures.

Microclimates can make a big difference in design temperatures, especially in mountainous states like Colorado. My temperatures at 1800 feet of elevation in Wheelock, Vermont, are considerably colder than the temperatures recorded in Burlington, Vermont, so you have to use a little common sense if you are building at a location that isn't listed in the design temperature tables.

Pay attention to your thermometer for a couple of decades, and use your own local knowledge as a common-sense reality check when choosing your design temperature.

Apr 22, 2012 12:38 PM ET

Response to Curt Kinder
by user-756436

Your point about the difference between the 97.5% design temperature and "the coldest night of the year" is an important one.

I adopted a shorthand phrase ("the coldest night of the year") to make my prose more readable, at some loss of technical accuracy -- so it is, indeed, worth repeating the point that you underlined.

Apr 22, 2012 6:00 PM ET

Edited Apr 22, 2012 6:02 PM ET.

Martin,The link you provided
by Acobo

The link you provided in your article come from 1985 ASHRAE Fundamentals, where the design temps for DFW, TX are 22/100 and for ABQ, NM are 16/94. The design temps form MJ8 are 24/98 for DFW and 18/93 for ABQ. Clearly MJ8 shows DAL to be 2°F warmer in winter and 2°F cooler in summer. At the same time ABQ shows to be 2°F warmer in winter and 1°F cooler in summer.
I believe both Wrightsoft and Elite software use ACCA MJ8, which is used by most HVAC contractors. Since a two degree temperature difference can represent ½ ton more on the AC unit, I guess it could be important to know the correct design temperatures. Should ASHRAE update their tables or meet ACCA in a common ground? Any thoughts?
I’m including the ACCA MJ8 for other folks to check their design temps.

ACCA MJ Design Conditions.pdf 804 KB

Apr 22, 2012 6:39 PM ET

Response to Armando Cobo
by user-756436

Thanks -- I appreciate the link to the Manual J design conditions document.

Apr 23, 2012 1:30 AM ET

by user-1087436

And I don't mean tulips. Dry bulb? Wet bulb? These terms have long been a puzzlement. Perhaps Martin will cover them in the next installment.

Apr 23, 2012 4:05 AM ET

Response to Gordon Taylor
by user-756436

The temperatures referred to in this article are all dry bulb temperatures -- that is, the temperatures shown on an ordinary thermometer. (To determine a wet bulb temperature, you need a different type of thermometer -- one that has a dampened cloth surrounding the bulb at the bottom of the thermometer. This type of thermometer is called a "sling psychrometer," because it is designed to be slung in a circle to encourage the evaporation of the moisture held by the damp cloth.)

Fortunately, heat-loss calculations don't require consideration of wet bulb temperatures or psychrometrics.

If you're interested in learning more about psychrometrics, you can see my blog on the topic: Are Dew-Point Calculations Really Necessary?

You might also be interested in this helpful article: Dry Bulb, Wet Bulb and Dew Point Temperature.

Apr 23, 2012 12:21 PM ET

Is it common for hvac contractors to do the load calculation?
by onacurve

I'm wondering if these calculations are often done by contractors. Energy Star V3 requires it on the Checklist at Section 2.18. Air flow tests are required in Section 9. I don't see why it's not required before installing any hvac equipment.

Maybe this will qualify as a near horror story.....

The house I bought about a year ago needed a new furnace. I requested a high efficiency gas furnace and that the ducts be inspected and serviced as necessary. A new 95% gas furnace installed and I was assured that the ducts were fine.

The furnace seemed to be installed ok, except for the fact that the whole house humidifier was not reconnected when they were done and that they left large gaps in the attic penetrations. That wasn't a huge deal to me but the supply ducts in the crawlspace are another story.

There is no way they were ever inspected. The galvanized rectangular duct, resting on the dirt floor, was completely rusted out. For some years, who knows how long, there was a flexible copper drain tube from the water softener resting on the galvanized steel. It was completely corroded to the point of having a puncture in the copper, so that water drained all over the duct. The ducts are insulated internally. I guess the black stuff all over the insulation is mold. Another section of round galvanized duct was completely disconnected.

So instead of heating with my super efficient furnace, when it gets below 45F I turn the electric oven on, open it, and let the ceiling fan run to disperse the heat. It seems that my 8 year old daughter has fewer asthma attacks that way. I'm glad I work 12 hours a day and that I have 50/50 custody of my daughter so neither of us are here that often. I'm also fortunate that the house is small, about 1000 sq ft, and fairly well insulated.

My guess is that most contractors know that very few homeowners are actually going to get in the crawlspace to check out their work, especially ones that are 90+ years old, like my grandparents, the previous owners of my house. Also, I guess that few homeowner's are going to ask to see their load calculations. I bought the house from my Mom and Aunt, who inherited it after their parents died.

I was going to attach some photos but the resolution is too high for the 2MB attachment limit. I'll see if I can resize them and post them later.

Apr 23, 2012 12:44 PM ET

Response to David Martin
by user-756436

Q. "Is it common for HVAC contractors to do the load calculation?"

A. No.

For example, my brother, who lives in Boston, recently contacted three HVAC contractors for bids on a new furnace. He asked them to perform a load calculation, and not a single one was able to do one. All three contractors suggested that he install a new furnace with a rating that was three times the design heat load for his house. (I performed the heat loss calculation for him when the contractors said they couldn't.)

For more information on this problem, see Saving Energy With Manual J and Manual D .

Apr 24, 2012 1:50 PM ET

Understanding the fundamentals
by user-731553

Another great topic. Energy models would be much more accurate & meaningful if modelers truly understood heat loss calculations outside the "black box" of most modeling tools; I look forward to following this blog series.

Apr 24, 2012 2:58 PM ET

Response to David Martin & Martin Holladay
by user-1004076

On existing homes with pre-existing heating systems needing replacement, it's usually more accurate to use fuel use & burner-efficiency of the previous system against base-65F heating degree days over a mid-winter billing period, or a "K-factor" on an oil bill.

To determine the heating degree days over the billing period find a weather station near you on and download a spread sheet spanning the dates, sum the dates of the bill, and divide by the fuel use.

With fuel use per heating degree day it's simple arithmetic to convert that to the BTU output of the burner per heating degree-HOUR. Multiply that constant by the difference between the 65F and the heating design temp, and that's the whole-house load that needs to be provided by the heating system.

EG: Say a house used 238 therms of gas between January 15 and February 13 per the billing, and the total for the billing period came to 1251 heating degree days. That's ~0.19 therms/HDD, or 19000BTU/HDD. In an 82% furnace that's (19000 x 0.82=) 15,580BTU/HDD. With 24 hours in a day that's (15580/24=) 650BTU per heating degree hour.

If the 99th percentile heating design temp is +7F, that's (65-7=) 58 heating degrees, and the whole house load is about (58 x 650=) 37,700 BTU/hr.

A "K-factor" stamped on oil bills is HDD per gallon. There are 138,000BTU/ gallon, so 1/(K x 138000) = source-fuel BTU/HDD, the rest of the arithmetic flows the same as in the example. (Oil burners are typically ~85% efficient when new.)

One can quibble about the numbers being skewed by setback thermostat savings, boiler oversizing factors, burner degradation with age, or fuel used by hot water heating & cooking, solar gains offsetting fuel, etc., but in anything but a superinsulated passive solar house, all those are going to be a single-digit percentage error factor, whereas Manual-J methods are usually well into double-digits for error.

Oversizing the equipment by 15% on a fuel-use calc isn't an efficiency or comfort disaster, even with high-mass boilers, but it tells you just how ridiculously oversized most heating systems are for their actual loads. Oversizing by 15% from Manual-J can sometimes be on the edge not meeting the AFUE tested numbers for cast-iron boilers.

This approach tells you nothing about the condition of the house, or where the heat was being lost or how to improve the situation. It's not an energy use model, it's an in-situ measurement of the energy use, not how it's leaving. The rest of the story is better told by DOE2/BeOpt type modeling than a straight-ahead Manual-J or IBR heat loss calculation methods.

Apr 24, 2012 3:14 PM ET

Response to Dana Dorsett
by user-756436

The purpose of this series is to introduce readers to the principles behind heat-loss calculations. I am not recommending that readers use the old pencil-and-paper I=B=R calculation method that will be discussed here.

Thanks for sharing your tip about using historical fuel-use data to size a replacement furnace for an existing house. It's a useful method; of course, that method can't be used for sizing a furnace for a new home.

Apr 24, 2012 4:58 PM ET

The limitations are understood...
by user-1004076

Clearly historical use methods can't be applied to new homes, nor can it properly balance room-to-room heating/cooling load balance issues, but it cuts to the chase on sizing replacement units when dealing with contractors unwilling or unable to run a Manual-J (which would be most of them, in my experience.)

On a new home (or addition) a room-by-room heat loss calc is essential, yet still relatively rarely done except where mandated by law (as in California, under Title 24.) Oversizing hot air furnaces by a factor of 3 is more of a comfort issue than an efficiency issue, but with high-mass boilers or air-source heat pumps it's an efficiency disaster.

Apr 24, 2012 9:56 PM ET

Response to several
by watercop

Another wrinkle is the difference between 97.5% and 99% design temperatures. The 1 % condition is obviously more incusive but may lead to oversizing. Man J has safety factors built in, so resist the urge to add yet more.

It would have to be an awfully big house for a 2 degree delta to add a half ton of cooling load. Cooling loads are driven more by window solar gain, internal heat sorces and air infiltration than outdoor air temperatures. In a related vein, wall insulation R values are relatively inconsequential to cooling loads. Play with a load calc to prove it to yourself.

The comments about microclimates and elevations are well made. It is inappropriate to use temperature history from a nearby airport to model conditions in a nearby wooded suburban setting. Conversely urban heat island conditions must be considered. Nearby water bodies (at least until they freeze over) act to cap temperature swings.

David Martin asks if HVAC subs do load of 15 March the 2010 building code took effect in my area, mandating load calculations for system changeouts. This is eating into the margins of the low bid hacks and rewarding those of us who do perform them. If one goes to the trouble of performing a proper load calc, it is a small extra step to add in a blower door test which acts to firm up some uncertainty in the load calc.

Apr 25, 2012 10:45 AM ET

Response to curt kinder
by user-1004076

"The comments about microclimates and elevations are well made. It is inappropriate to use temperature history from a nearby airport to model conditions in a nearby wooded suburban setting. Conversely urban heat island conditions must be considered. Nearby water bodies (at least until they freeze over) act to cap temperature swings."

The ACCA Manual-J design temps are usually from local airport weather history, whereas or data can be clipped from a range of nearby weatherstations for working the fuel/HDD numbers. I've yet to meet the heating pro who uses factors in proximity a local lake into a heat load calc. Shading shading is primarily a heat gain/cooling load factor, not a heating load factor. Cooling loads are inherently more difficult to assess without understanding the occupancy duty cycle & plug loads- lots of room for error.

Still, using nearby airport weather data (rather than a more local weather station) rarely inserts an egregious error into the heating load numbers for single-family moderate density development the way it might in high-density or high rise urban centers.

If there's a significant elevation delta between the local site and the weather station/airport, using a crude 3F/1000' adiabatic temperature delta model is good enough. Even a 5F delta in the estimated vs. absolute design temp, is still only a single digit percentage error for locations in US climate zones 5 & higher.

The "small extra step" of a blower door test can nearly double the cost of a straightforward heat load calc. In even a moderately tight house the higher precision is not likely to be worth the extra expense unless the client is already committed to retrofit air sealing or hitting a particular BTU/hr or cfm number, which is beyond the scope of most furnace/boiler replacements. On energy use reduction retrofits of a broader nature, or high-R buildings where infiltration rates can dominate the heat load, sure- verification is important.

Apr 25, 2012 4:47 PM ET

Back to Design Temperatures
by KvqsrhrsHH

Sorry to take us back, and this may not be the right forum, but since you were talking about design temperatures: I have been trying to get ACCA to change the design temps for Tucson, AZ for some time. We are apparently one of few building departments that require the submittal of a manual J, and we review it. Using a high cooling design temp is one way that contractors game the calculations to increase tonnage of the AC.

The ACCA Design temps come from Appendix D of the International Plumbing Code??? I have no idea when this was last updated and it is a 2.5% temperature...apparently a standard no longer used.. ASHRAE design temps were updated in 2010 and ASHRAE now uses the 1% temperature of 103.6, at least in the table that I recieved from a local mechanical engineer that I called on for assistance. Then, I asked the local weather service to research the 1% temperature and the result was 108 (regardles of the date range of the dataset). From 102 to 108 will make a difference and how do I review a manual J for proper design when the basic design criteria can not even be established?

Apr 25, 2012 5:29 PM ET

Response to Rich
by user-1004076

The 2011 updated design temps for ACCA Manual-J lives here:

Note that the 99th percentile heating temps some areas are less than 1 degree different from other source's 97.5th percentile numbers, but the 99th percentile cooling temp listed for Tucson is 103F, not very different from the ASHRAE 103.6F design temp.

Apr 26, 2012 2:30 PM ET

UA Calculation and Temps
by user-956866

Martin, I also have been doing heat loss calculations for decades,(since 1978), and teaching students to do this for the last 20 years. I think this is very important, for them to get a gut feeling for the various contribution of different building components to the overall UA. However, I differ from your and the standard ASHRAE approach in that I think one should not bring in either Design delta T's or HDD until first having done all the UA's (in Btu/hrF). I think it is most valuable to stop and look at the different UA contributions from areas such a Abovegrade basement walls, vs. Belowgrade basement, vs Windows, vs. Infiltration, etc. One can learn a lot by doing many of these, and it is very quick to do by hand once you have the building geometry. Then you get a feeling for what parts of the building are the big contributors.
After having made a complete UA table for all Transmission and Infiltration Loads, then one can go ahead and quickly either calculate Design Loads or Seasonal Loads, using the UA. While many people think that this is the old way to do things, and that computer programs would be the better way, Michael Blasnick's work (as you recently pointed out) shows the opposite.
My students do many of these hand UA calculations every year, on existing buildings close to our campus, and by comparing results among different building types and conditions we get to where we can just do a quick walk through of a building, and with some basic R value info make a very good guess at the overall building UA. It's amazing how much you learn by doing hundreds of these over the years.

Second Point: I am surprised there is so much interest in historical weather data for Design Temps and HDD. Based on local HDD data I have for my area, going back 38 years, we have roughly 20% lower HDD in the last 10 years than we did in 25 years ago. My guess is that the Design temps are also about 8 or 10F higher now than then. Significant climate change has already happened here, and it would be a mistake for us to use the old values.

Apr 26, 2012 2:55 PM ET

Response to Peter Temple
by user-756436

Thanks for your comments.

In tomorrow's blog (Part 2 of this series), I'll discuss how to calculate heat loss at the design temperature using U-factors of the various building assemblies.

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