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Community and Q&A

Passive solar design and HVAC sizing

rmanoman | Posted in Energy Efficiency and Durability on

I just used software (free to use online) to do a Manual J. analysis of my heating and cooling needs, since I just found out that I may need to replace my 13 year old HVAC system. One of my 2 heat pumps has a leak and needs to be recharged, and I am considering my options, so I wanted to get an accurate estimate of my heating and cooling needs. I found that the Manual J. heat loss calculations do not take into account the heat gain from the passive solar design, (except for cooling loads). The heat loss calculations remained the same even when I plugged in 6 foot overhangs into the data, which would eliminate the passive solar gains. The passive design consists of about 150 sq. ft. of south facing, double pane clear glass, and a thick masonry floor to store the heat coming through the windows. Does anyone have any ideas re: how I can evaluate whether or not, and by how much I can reduce the tonnage of the replacement system, due to the heat gained by the passive solar system?

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  1. GBA Editor
    Martin Holladay | | #1

    Manual J calculates the design heating load, which by definition is the capacity of your heating equipment needed on the coldest night of the year (more or less -- not quite the coldest night, but almost the coldest night). It has nothing to do with heating during the day, so it ignores solar gain.

    The idea is that your heating equipment should be able to handle the load at night -- or, for example, after three very cloudy days, with very dark skies.

  2. rmanoman | | #2

    Thanks for the reply. After reading your reply and your article about passive solar design ("Does Passive-solar Design Actually Save Energy?" ), it looks like it would probably not be wise to use lower capacity HVAC systems even though I do get some heat from the passive solar. I built my house in the mid 1980's, and the passive solar design seemed like a great idea at that time. At least I have been able to utilize the south facing roof. I recently installed a 6kw solar panel grid tie system which has been in operation for about a month, and I received my first power bill showing a negative 200 kilowatt hours of usage.

  3. GBA Editor
    Martin Holladay | | #3

    While south-facing windows can lower your energy bill, they are irrelevant when it comes to determining your design heating load.

  4. Expert Member
    Dana Dorsett | | #4

    Stated in other terms: Solar gain lowers the average heat load by providing supplementary heat during daylight hours, but does not affect the peak load, since the 99% outside design temperature (almost) always occurs in the pre-dawn hours.

    Solar gain definitely INCREASES the peak cooling loads though. A thorough cooling load calculation takes shading factors and location (sun angle) into account.

    Simulating the house with BeOpt (a downloadable freebie tool) would provide a better understanding of how the solar gains affect the loads.

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