How to evaluate the performance of passive solar heating systems:

This article discusses how to make accurate measurement of the performance of passive solar heating systems, and the effect of air infiltration and the effect of incidental solar gains on passive solar systems.

References to texts and guidelines for sizing thermal mass and using thermal mass are included.

Passive Solar Heat Performance Evaluation: the Impact of Air Infiltration & Incidental Solar Gains

This article discusses how to accurately evaluate the performance of passive solar heating systems and the impact of air infiltration or incidental solar gains on passive heating performance. This material is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

The text below paraphrases, quotes-from, updates, and comments an original article, "Remember Thermal Mass?" (see links just above) from Solar Age Magazine and written by Steven Bliss.

How to make a more accurate estimate of the performance of passive solar heating systems

Question: The article entitled "The Best Passive Heating Data Yet" (Solar Age 7/83) seems to be an accurate overview of the Class B monitoring program.

However the article does not clearly explain the limitations of the method of deriving the passive solar contribution to building energy demands.

Air infiltration rate skews passive solar gain estimates:

While the auxiliary and internal gains of the passive solar heating system are directly measured, the solar contribution is arrived at indirectly by a subtractive technique. There is one potentially big source of error in this technique for estimating passive solar performance, namely the building's air infiltration rate.

See BLOWER DOORS & AIR INFILTRATION for added details about measuring a building's air infiltration rate.

Any error in estimating the air infiltration rate shows up as an error in the estimate of passive solar heat contribution.

Incidental solar gains can skew passive solar gain estimates: Incidental solar gains are another source of uncertainty in estimating the contribution of passive solar heating systems (or cooling systems) to a building's energy use. These include gains through non-south apertures (windows and doors) and the solar heating effect on conduction loads of the building's walls and roof.

For this reason, it would have been interesting to have included a few non-solar homes in the Class B program as controls. -- A.L., Madison WI.

Steve Bliss Reply:

According to Joel Swisher at SERI, the one-time air infiltration measurements made concurrently with the coheating procedure were used to separate out conductive losses from air infiltration losses to obtain the building heat loss coefficient.

The overall losses due to air infiltration over the heating season are extrapolated from the blower door and tracer gas results and corrected for average monthly wind speeds.

As for the incidental solar gains, Swisher agrees that this presents a problem but that achieving true scientific controls is not a realistic goal, particularly in inhabited homes. In the 1982-83 season Class B study, SERI monitored non-solar homes for comparison purposes.

When the subtractive methodology was applied to these homes, solar gains in the 5 to 20 percent range were found.

This would indicate that some of the poorer performing solar homes monitored are not doing much better than a non-solar home, which is likely to be the case.

Question: Commodor 64 Solar Software

Were there any heat loss and heat gain calculation programs for superinsulation design that ran on a Commodore 64 computer back in the 1980's? - Ed Bond, Washington MA

Answer

Most software for passive solar design calculations in the 1980's would work just fine for superinsulated houses. Of the 50 programs for solar calculations listed in the 1985 Spec Guide, five were heat loss/heat gain programs that would run on the Commodore 64. They were available from Compusolar (Jasper AR), and Solarcon (Ann Arbor MI).

Another possibility in that era was Canada's HOTCAN program, devised specifically for highly insulated, tightly-sealed houses. It was available from Hotcan Energy Software, Ottawa, ON, Canada.

• Solar Systems Design Software: design tool for architects familiar with passive solar energy - see www.iklimnet.com/save/solarsystemsdesignsoftware.html
• Energy-10 Passive Solar Design Software: Mother Earth News - see motherearthnews.com/.../Design-Homes-Software.aspx
• SolArch Solar Architecture Design Software (shareware) - see www.kahl.net/solarch

House Heat Loss Guesstimating by Turning Off the Heat ?

Several readers have asked why we can't just turn down the heat, wait an hour, and observe the new temperature in a building to form an estimate of the building's rate of heat loss. This is an experiment worth performing, if simply to form a quick subjective view of how quickly a building cools off on a cold day

. But there are some serious inaccuracies in the "just turn off the heat and wait" approach to estimating building heat loss.

Here are some things that would be missing from this experiment, and some of these factors are major influences on the variability of a home's rate of heat gain or heat loss. Just turning down the heat and measuring temperature loss in a building fails to measure, estimate, or account for these varying conditions:

• Wind velocity and wind direction.
  
  Wind has a very a big impact on air infiltration or exfiltration losses in buildings, and its impact may vary depending on its direction and thus which side of the building is pressurized (not all building sides may be equally leaky).
• Sun,
  
  or the absence of sun affects the extent of solar gain offsetting heat loss in a building
• Absolute temperature differences between indoors and outside at the time of the experiment.
  
  The greater the indoor and outdoor temperatures from one another, the faster the heat movement through the building.
• Building occupancy - people's activities,
  
  the number of occupants (people generate heat), whether or not people turn fans on or off, room doors are open or shut, ovens are on or off, clothes dryers, showers, and other appliances, have an impact on heat generated or consumed in a building
• Building indoor temperatures are not even -
  
  the location at which you are measuring temperature, say at the heating thermostat, is not likely to reflect temperatures throughout the building; some areas will be warmer, and some cooler than the temperature registered by the building's thermostat.
• Snow cover -
  
  presence or absence of snow, and its effect on blocking some roof vents, or in acting as an insulating material as well as a sunlight reflecting material
• Relative humidity -
  
  moisture levels impact heat transfer rates, but probably not as a major influence
• Rain
  
  wet conditions may impact heat transfer rates

Overall it makes sense to have a general idea how a house behaves, such as from the simple "turn down or off the heat" experiment, but you cannot accurately characterize a building's rate of heat loss, nor can you know just how leaky it is, nor will you know where the major sources of heat loss are, with just the simple test of turning heat off and measuring temperature change at an arbitrary time.

These difficulties lie behind other efforts to characterize homes and their energy efficiency.

When the object is to save energy in the form of heating or cooling costs, attacking the major points of un-wanted heat loss (or gain in a cooling climate) are likely to be most cost-effective.

 

Original article

• Passive Solar Heating Performance: Q&A on the Impact of Infiltration Losses & Incidental Solar Gains - PDF version, use your browser's back button to return to this page
• Q&A on Passive Solar Heat Loss & Gain Calculations - PDF version, use your browser's back button to return to this page

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

...

Continue reading at PASSIVE SOLAR HOME, LOW COST or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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Recommended Articles

• BLOWER DOORS & AIR INFILTRATION
• ENERGY SAVINGS in BUILDINGS for our collection of energy-savings articles for buildings, and also
• ENERGY SAVINGS PRIORITIES
• PASSIVE SOLAR HOME, LOW COST
• SOLAR ENERGY SYSTEMS - home
• WIND TURBINES & LIGHTNING

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