HDD and internal gain.

Using a standard 5° allowance for internal gain, base 65°, is a rough enough approximation from the start.  But to use it for both test-in and test-out introduces a known error.  Since the math for that -5° produces a percentage of the heat loss, the "new and improved home" results in a smaller test out allowance for internal gain.  My thoughts are, we should try to be consistent and use the same allowance for both before and after tests, adjusted for any known changes.

Here's one way.  For the test-in, use the HDD base 65 for your initial calculations to obtain the annual heat loss number.  Then repeat at base 70.  The difference between these numbers is the allowance for internal gain, but don't be shocked, it is a huge number, like 20%.  Then, for test-out, calculate the heat loss at base 70 and subtract the internal gain you determined during the test-in.  If you have changed any sources of internal gain, make an allowance for the reduction.

A quick example:  Q = U x A x HDD x 24  (credit Residential Energy)

1,500 sq ft home with an average internal surface area of 3,000 sq ft.

Average U-value = 0.2 (R=5)

Location has an HDD base 65 = 7,500 (my northern climate, Bangor Maine)

Base 70 would have an HDD = 9,000

Test-in base 65 Q = 1080 Therms

Test-in base 70 Q = 1296 Therms

Subtracting the two we get an internal gain allowance of 216 therms.

If we assume our improvements have reduced the home's average U-value to 0.1 (R=10), then our test-out base 70 would be Q = 648 Therms.  If we did not change the internal gain, then we could subtract the 216 Therms from that 648.  Our new heat loss after improvements would be Q = 432 Therms.

If we were to use the standard approach of base 65 for test-out we would think our improved heat loss (annual energy required) was Q = 540 Therms (half of the test-in base 65) and that would be 108 Therms too much.

Just a quick run through the numbers so all corrections welcome.  Plus, I have no idea how the major software packages account for internal gain.


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Here's some added thinking/questions.
Q. Does anyone have any real numbers on internal gain and what is and is not considered the source?
Q. Do the majority of EE programs use base 65 and is it used for both test-in and test-out?
Q. If base 65° is used for both test in and out, there can be a significant built in error. That error overstates the energy improvements in a warm climate and understates them in a cold climate. Of the many complaints about actual savings falling short of projected savings, does this occur more in a heating climate or a cooling climate?



  I have been collecting hourly energy use and environmental data for two homes for nearly four years.  One is superinsulated with minimal solar and internal gains (seasonal home)  and the other is a passive solar primary residence.  I have tracked UA values, which could easily be converted to U values, and am compiling a detailed report on the actual energy performance vs modeled energy use of several modeling tools. 

  The non solar home is surprisingly close to all of the modeled predictions, but, the passive solar home shows nearly 44% of the predicted energy use being supplied by solar gains and internal gains, with the solar gains providing about 40% of the offset.  We used the actual delta T, which fluctuated + or - 10 degrees inside the passive solar home, and can compare that with any baseline temperature.  The interior temperature dynamics of the passive solar home have been charted and, not surprisingly, on an average Montana winter day it is cooler in the morning (65F) and warmer in the evening (74F).   Summer temperatures follow that pattern, as the building is naturally cooled with outside air.  Outdoor night time temperatures are typically in the 50's and daytime temps will run between 80 - 95 F.   The internal temperatures of the solar home will typically start the day in the low 60"s and by sunset could reach the mid to high 70's.   Thermal mass presents its' own issues when determining energy use through modeling.  As we lower the energy use of internal devices, we will need to ask -  just how active are the occupants? 

Heating degree days are very miss used.  Here is good article on subject  http://www.energylens.com/articles/degree-days

Base temperature is when a house can not keep up with the heat loss with internal gains.  I have built close to a zero heat house and I believe my base temperature is in the high 30's or low 40's.  Here is link to house  http://cheoyleeassociation.com/family/NCHouse.html


Hi James,
The BizEE web page deals with the inaccuracies of selecting a particular base for one's HDD. What I'm pointing to is the additional error created by presuming the same base used on test-in will be appropriate for test-out. A base 65 assumption is supposed to correlate to a certain amount of internal gain. By whatever method one has decided upon their base, the use of that base is only good for the test-in calculations. Once improvements have been made, we should account for the same internal gain (adjusted for any intentional changes).

In the example above, testing in and out at base 65 implies the internal gain changed, it didn't and we end up with a 108 Therm error. That's huge.

Take any previous audit and compute the before energy lost at both base 65 and base 70. The difference is your initial internal gain allowance. Repeat the two calculations at the two bases for the after (test-out) and you will find a much different allowance for internal gain. The internal gain use for test-in should be the same as used for test-out and using a base 65 for both does NOT do that. Since the steps necessary to eliminate that error are simple, I feel it should be considered.


Using too high of a base temperature helps explain why Furnaces are almost always oversized, most by 2X-3X. Has anybody ever run into a situation where a properly performing heating system can't keep up with the load?

Hi Bob,

Failure to adjust the base temperature as improvements are made does result in higher numbers and increase an already oversized hvac choice.  Since we now have the ability to build super efficient homes it seems logical that we would be paying close attention to internal gains.  In fact I'm sure the more sophisticated software does so, it is just that all decisions are not run through that process. 



My Energy awareness is an ongoing process.The first thing I always do when trying to calculate things is convert everything to one standard unit.  In my case btu's.  When I look at homes(I'm down on the Southern Command Post in Kittery) I crunch the usual numbers of past usage, run a blower door, then get an inventory of household effects on energy.

When I first started out I strictly went by the Blower Door and defined everything from the perspective of Convective Loss.

Then I graduated to ASHRAE Audits and pent considerable time doing Calcs on Motors, Lighting, Restaurant Equipment, Gas Station Pumps, Heat Pumps and Chillers.  All great fun!

Then I got introduced to Load Calcs in the Residential HVAC world.  I've had the benefit of using Wrightsoft for some time now.  Certainly expands one's perspective.  The best part, is that you can tailor your inputs to allow for gains from, plants, pets, extra electronics, etc.  Internal gains coupled with Shell data gives a pretty darn good basis for making any decisions.


Hi Jim,

Learning how to grind out the numbers before you get locked into the software sure helps to keep the results in perspective.  I don't think many auditors today are able to use the heat loss equations, everybody simply plugs in some numbers and the software gives them the answers.  I'm still from the old school that likes to understand what the computer is doing.

I remember one very nice old lady that was embarrassed to admit that she turned off her heating system and only used it when the outside temps dropped to freezing.  Her fuel use just wasn't adding up to the house results.  After some careful coaxing she admitted what she was doing.  This was back when oil went over $4.00 a gallon.  Recalculating heat loss with a base temperature of 40° sure made the numbers look a lot better.

But testing in and testing out should be using a different base number.  You mentioned you convert everything to btus.  Have you done that for both test in and test out and subsequently used a different base?


I am curious as to which energy modeling tools have been providing accurate results for the experts in this community?  Has anyone reviewed results from Greenbuild Studio (autodesk Revits' cloud based analysis using eplus as its' energy modeling engine) ??



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