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Passive Solar Design

mikeolder | Posted in General Questions on

Is there a passive solar home where the builders did thorough testing, and then replaced most the glazing with wall of the same R-value as the rest of the home to determine if large glazing’s reduced heating bills in cold climates?  Or two identical homes except for the large glazing’s? It would all depend how much sun and which zone so I imagine there isn’t, especially in Iowa. I keep reading PV is better, but then read recent articles about the Kenogami House, or individuals who have build passive solar collectors and piped them into the home.   Maximizing passive solar heat would requires daily attention and temperature swings, but I’m OK with that if I save money on heating bills. But if its peanuts, its not worth it.  Propane costs are through the roof on my property and I need to address that before I move there. Thanks

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  1. Expert Member
    Michael Maines | | #1

    I don't know of any physical examples but the physics behind energy modeling is pretty straightforward and accurate in most cases, especially more detailed ones like what is used for Passive House design (not to be confused with passive solar). After 40+ years of experimentation, there is nearly consensus that insulation and air-sealing saves more energy than having a lot of glazing.

  2. Expert Member
    DCcontrarian | | #2

    This article looks at passive solar gain:

    The upshot is that in most of the US, good quality glazing -- with a low U factor -- basically breaks even in cold weather if it faces due south. By "breaks even" I mean the solar heat gain balances the heat losses by having a window instead of a well-insulated wall. Windows with orientation other than due south don't come close to breaking even. In cold weather break even means they don't contribute to heating the rest of the house, they just cover themselves, although in mild weather they can contribute.

    The problem is the control mechanism. With heat, you want it where you want it when you want it. You can't control when the sun shines. Nor is there any way to turn off the heat loss of a window when you don't need it.

    Solar electric, in particular grid-tied, allows you to harvest solar energy when it's available and use it when it's needed.

  3. mikeolder | | #3

    Thanks guys. Especially the information about how larger south windows are a wash for BTU's gained in most area's. But it allows me to have as many windows on the south side as I can afford because the view is nice.
    My power company offers $400 rebates and a lower 7.7 cents per kWh winter rate if I buy ashp's AND switch to a electric water heater on the high rate 13 cent rate(2 meters)..
    But if I get Gride Tied PV , I don't get the winter rate even with ashp's, and the backfeed only pays 2.7 cents per kWh.

    My harvesting doesn't look so good, but simply switching to ashp's do for now.

    1. StephenSheehy | | #5

      The deals we get for owning solar panels differ greatly around the country. Maine has what I think is a wonderful deal for homeowners. We have simple net metering. If, over a year, my production (sent to the grid) matches my use (from the grid), all I pay is a monthly minimum charge which at the moment is $13.66.
      Currently, our grid electricity costs just over 27¢ per kwh. So in effect I'm being paid 27¢ per kwh sent to the grid. And when the rates go up, so does what I receive.

      Clearly, this isn't sustainable. If everyone had home solar, there wouldn't be enough revenue to support the grid infrastructure.

      But why everyone who can possibly afford solar doesn't get it is a mystery to me.

      1. mikeolder | | #7

        "But why everyone who can possibly afford solar doesn't get it is a mystery to me."
        Because you pay twice as much as I do, and your power company pays you back the same amount whereas I only get 3 cents per.. Your ROI is less than half the time as mine.

        1. StephenSheehy | | #10

          Mike: I meant people in Maine or places with similar net metering. What state are you in?

          1. mikeolder | | #19

            Iowa. I have a low price, but my net metering stinks.

  4. gstan | | #4

    With regard to reply #2 - "Nor is there any way to turn off the heat loss of a window when you don't need it." Not Quite! There is a way - insulated shutters will do the trick! This isn't popular but building (or having them built for you) insulated shutters and closing them at night will stop most of the heat loss through the glass and they are generally way less expensive than going with really high R-value (low U-value) windows. The downside is
    that to be really convenient they need to be on the inside. Or, if outside electrically
    controlled which is expensive and possibly unreliable. If on the inside, they may interfere. with furniture placement - depends on your design and furniture.
    This is so seldom done that it is difficult to find pictures of houses with interior insulated shutters. However I lived in three houses like this for a total of 26 years (climate zone 6) and I can guarantee that they work - my wife and I both loved
    them - they provide a feeling of security in addition to reducing the heat loss
    and also reduce sound transmission.

  5. rockies63 | | #6

    Here's some information from forensic building scientist Dr. Joe Lstiburek you might find helpful.
    (You can jump to paragraph 9 for his comments on passive solar)

    And this article in Fine Homebuilding by Martin Holladay.

    1. mikeolder | | #8

      Thanks rockies63.

      What stuck out was when Lstiburek said " Don’t bother with the passive solar. Your house will OVERHEAT in the winter. Yes, you heard that right. Even in Chicago."

      So like DC_Contrarian mentioned, "The problem is the control mechanism." but according to Lstiburek, there's enough heat energy available to bake you out of your house.

      What type of PV system with "control" could do that in Chicago?

      1. Expert Member
        Michael Maines | | #9

        He should have said that there is enough heat to bake you out of your home occasionally, during daytime, even in Chicago. On other days you won't get much insolation, and at night you'll have none. If you want to spend your time opening and closing shutters (and the view) and don't mind wide differences in indoor temperature, you are not like most US citizens.

        1. mikeolder | | #12

          It's probably because I'm Austrian and brought up by a father who the liberators tried to starve.. All food was eaten and cold showers.
          You can interpret what Joe said how you like. I read it as that he thinks there is enough energy on a cloudy day in Chicago to heat your house. But I'm biased because I don't see PV achieving that.

          1. Expert Member
            Michael Maines | | #14

            Mike, I have read most of what is published on the BSC site, I have met Joe and have been to many of his presentations. My business is designing homes that are efficient and comfortable. If you read that entire article and your takeaway is that passive solar is a good idea, you are completely missing his point. As I and others have said, on some winter days you will certainly get enough solar energy to heat your house during daytime hours. On other days you will get far too much, on other days you won't get enough, and at night you won't get any.

            If you really want to dial in, take Passivhaus training like I did and learn how to use the PHPP. There are no energy models more accurate (or more difficult to learn) and the results will tell you what you need to know. Every tiny detail is considered.

          2. Expert Member
            BILL WICHERS | | #15

            If you use minisplits, PV can get you further than you'd think in regards to heating your home. If you want enough PV output to do that in the depths of a cloudy winter, you can oversize your system sufficiently to provide the required power output on a cloudy day, you'll just have WAY more than you need on a sunny day (and a much lighter wallet too :-)

            As others have said, the issue is with CONTROLLING the heating. You'd be better off trying to offset some of your heating load with passive solar concepts and not trying to rely on passive solar for ALL of your heating needs. That way, you save some money with the sun, but you still have control over the indoor enviornment using your HVAC system.


          3. mikeolder | | #18

            To BILL WICHERS..
            My power company offers $400 rebates for ashp's and a 7.7 cent winter rate, but I have to use a electric water heater on the old 13c rate (two meters)
            If I buy and use a grid tied PV system, I don't get the winter rate and back feed is only 3 cents per.. This is why I don't want a grid tied system and I don't want off grid batteries, so ashp's and passive it is.
            Passive doesn't have to cover all my heating needs, but as much energy that's there, I'm surprised we haven't figured out how to store heat, like in a insulated sand battery the size of my truck buried under the house or in the yard so that you could extract heat when you need.

          4. Expert Member
            DCcontrarian | | #22

            Various schemes for storing heat have been proposed over the years, but they all have the same problem, they're very expensive for the amount of heat stored.

          5. Expert Member
            BILL WICHERS | | #23

            To store significant quantities of heat, you have to do a phase change of the material (usually solid to liquid and back). There are "ice chillers" that do this commercially, freezing water into ice at night on the cheaper off-peak rate, then thawing that ice during the day (when electricity costs more) to cool the building. The downside is that the current rates (at least in my area) aren't different enough between on- and off-peak times to justify the significant installation cost of the ice chiller system, which is unfortunate.

            Just heating and cooling a mass of some material doesn't store as much as if you have a material you can push through a phase change. I have designed "thermal batteries" before using large water tanks to maintain cooling when a chiller goes down until the facility generators can start, but it takes a LOT of water to store much energy if you are only working with a small number of degrees of temperature differential. Commercially I mostly work with large datacenter facilities, so my focus is always on air conditioning -- we don't need to heat these facilities regardless of the climate zone they are in.

            You can probably put in a peak shave solar system to offset your daytime electric demand without dealing with the utility, since you do NOT need net metering to use solar in a grid-tied configuration. Peak shave systems never feed power back to the grid, which usually means you don't need to deal with the utility to install such a system. Systems like this just reduce your electric needs while they're producing, so they always help you out. I usually recommend this type of system and not a net metering system trying to be "net zero" or trying to make money.


          6. Expert Member
            DCcontrarian | | #25

            This thread talks about a phase change storage system that seemed promising:

            Summary: it isn't.

          7. mikeolder | | #29

            BILL WICHERS said "You can probably put in a peak shave solar system to offset your daytime electric demand without dealing with the utility, since you do NOT need net metering to use solar in a grid-tied configuration. Peak shave systems never feed power back to the grid, which usually means you don't need to deal with the utility to install such a system. Systems like this just reduce your electric needs while they're producing, so they always help you out. I usually recommend this type of system and not a net metering system trying to be "net zero" or trying to make money. Bill"

            "peak shave solar system's"? I was under the assumption that if I install any PV that's tied to the grid or even plugged in, I can't get the low winter 7.7cent rate to run my ashp’s. Can someone post a link to a description of a peak shave solar system please?

            And a side note. is a neat site regarding passive solar.

          8. Trevor_Lambert | | #32

            I've said it before, but I'll say it again here. Joe's statement about overheating in winter is comically stupid. The only way you'll overheat in winter is if that's what you're trying to do. Every house has a simple mechanism to prevent this overheating, and it's called opening a window. It's amazing how much and how quickly you can cool down your house that way when it's cold outside. I've lived in a passive house for 5 winters, in a warmer climate than Chicago, and we've never once felt the need to even exercise this cooling option.

    2. Expert Member
      DCcontrarian | | #17

      The article by Martin makes an important point: while that south-facing window may break even or even add a little heat, windows are a lot more expensive than walls. A lot of the tradeoffs in building design have to do with cost/benefit and payback times, that money is usually better spent elsewhere.

      1. mikeolder | | #26

        Yes, but I bought the property basically without even entering the home because of this south east view.. House will be removed. I imagine someone else will think the same way when it gets sold.

        1. Expert Member
          DCcontrarian | | #33

          It's beautiful. Build the house you want to live in.

          I would recommend doing energy modeling and tweaking the design. Where you'll probably end up is good insulation, really good air sealing, good windows and doors. Most of the windows face south, but with big overhangs to keep out the direct summer sun. If you can do it without spoiling the view, plant deciduous trees to the west, the late afternoon sun in summer comes under the overhangs and is killer. In the winter the trees will let the sun in.

          A house like that will be comfortable and economical year-round.

    3. mikeolder | | #20


  6. Tim_O | | #11

    The days where it's 40 in the winter or even 60 in march are the days you'll get baked out.

    When DC says it's break even, that's an average over a large period of time. But the heat from the windows comes in less averaged.

    With that said, there are some calculators out there for solar gain. Not super easy to use, but you can dial it in so you don't over heat but still get some decent solar gain. I wouldn't plan a whole house load, but a percentage will keep you on the safe side.

    1. mikeolder | | #13

      This is great thank you. I'll spend all day pulling my hair out with it. What's odd is the instructions have more steps or data points than this calculator, but I'll figure it out. Nothing better to do since I'm recovering from shoulder surgery.

      1. Tim_O | | #16

        There are a few others on their calculators tab that are used as inputs to some of this one. It took me a while to get something out of this one.

        1. mikeolder | | #21

          The calc is asking for information I need to buy from ASHRAE but instructions also said they used the zip code, but my numbers are way off when I enter mine. When I go to the ASHRAE link, I'm not sure what to buy.. Any Idea? I also had issue with converting my Latitude. My Latitude is 41.65583 and I entered 41.9

          1. Tim_O | | #24

            I do remember searching for quite a while to find the ASHRAE info they needed. I did end up finding it, I think it's all free. But I don't remember where it was... I only saved the PDF for my city.

  7. rockies63 | | #27

    One thing I learned years ago, in addition to building as small a house as possible for your needs, is to arrange the rooms with the kitchen in the southeast corner, dining in the south middle, and living room in the southwest corner. Bedrooms can be on the northeast or northwest corners (or upstairs) and bathrooms, entry, storage spaces on the north middle.
    This arrangement not only keeps the main gathering spaces to the south for daylighting but also puts individual rooms in locations where you are most likely to be during the hours of the day.

    That said, one thing you might consider is "zoning" the interior rooms by insulating the interior walls with Roxul mineral wool - either regular or Safe and Sound. This might help keep heat in the less lit north side rooms during the day and night.

    Also, if you are considering solar PV and mini-splits and electricity usage, you could consider a Hotspot DC Solar Mini Split, which runs entirely off the panels and doesn't need batteries or an invertor (although it can be grid tied if you want to run it off AC).

    1. Expert Member
      MALCOLM TAYLOR | | #28


      "arrange the rooms with the kitchen in the southeast corner"

      I know that's the conventional wisdom, but how much time do we really spend in the kitchen in the morning - especially when for a good part of the year it is still dark out when we get up? I'd rather have a south-west orientation to take advantage of late afternoon and evening light.

  8. Expert Member
    DCcontrarian | | #30

    I couldn't even watch 90 seconds of that Youtube video. By the time he started explaining that it was solar hot air radiant floor heat my eyes were rolling so hard I couldn't see any more. I guarantee not an ounce of engineering went into that.

  9. Expert Member
    Akos | | #31

    I designed my place with a reasonable amount of passive solar (~10% south facing). The energy models did show it to save heating costs over the whole year but that it the simple story.

    In reality, on a sunny winter day, the place can get fairly hot even with the heat off, not uncommon to come home to a house that is 80F. This is generally not a problem but I do find myself opening windows in the middle of the winter to cool the place down. The bigger issue is that I have some largish west facing windows to peak house temperature happens close to the evening so bedrooms are definitely too hot and are always opening windows there.

    This extra winter time heat does come with a drawback. My cooling season is extended by about a month, with an inefficient multi split, this wipes out any cost savings from the south facing windows.

    My south facing IGUs have failed and I'm getting them replaced with low SHGC windows instead. I rather take a bit of a hit in winter time heating and limit the issues I mentioned above.

    That is a long winded way of saying less passive solar is a good thing.

  10. rockies63 | | #34

    Here's a video on the Health and Safety Benefits of High Performance Windows from Acelab for Architects you might find interesting.

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