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  • Passive Solar Design and HVAC sizing

    I just used CoolCalc.com 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?

  • #2
    Take a look at something called TRNSYS from the Univ. of Wisconsin. Pretty detailed. I had a license until ~ 4-5 yrs. ago. It probably has what you want, but a bit pricey.

    If you're just looking for solar fenestration gain, the "Passive Solar Design Handbook" Balcomb et al. will be of some help. I used it rather extensively when I designed a passive sunspace of ~ 188 ft.^2 net glazing area about 35 yrs. ago.

    The CoolCalc is from a HVAC industry group so the focus is more of an HVAC sizing emphasis. I've not used it.

    Check out something called "Windows and Daylighting" at windows.lbl.gov for some interesting stuff on fenestration gain/loss/treatments.

    ASHRAE has some stuff as well. I've used their manual stuff in the past.

    I'm sure there are lots of others, but the TRNSYS stuff is about the best in terms of details and sophistication.

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    • #3
      J.P.M,
      Thanks for your reply. I did some digging and also found a simple calculator at http://www.borstengineeringconstruct...alculator.html, that provides an estimate of daily heat gain for any given month based on glass area, latitude, and orientation. For January, it indicates a gain of about 100000 btu per day. (This comes out to about 4000 btu/hr if it all could be used) I have also downloaded a demo copy of TRNSYS and will be checking it out.

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      • #4
        Originally posted by rman View Post
        J.P.M,
        Thanks for your reply. I did some digging and also found a simple calculator at http://www.borstengineeringconstruct...alculator.html, that provides an estimate of daily heat gain for any given month based on glass area, latitude, and orientation. For January, it indicates a gain of about 100000 btu per day. (This comes out to about 4000 btu/hr if it all could be used) I have also downloaded a demo copy of TRNSYS and will be checking it out.
        You're welcome. I've kind of get away from passive solar over the last 10 yrs. or so. I'm pretty sure my information is still good, but probably out of date.

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