Automatic control for passive solar overheating in winter?
Hi, I’m designing a passive solar house and attempting to gaining the most energy from the sun, but the more energy you try capture, the more likely overheating becomes a risk on those warmer, sunnier winter days. I was thinking it would be simple enough to have an ERV with a bypass that can exchange overheated indoor air (say 80°F) with the cooler outdoor winter air (say it’s 60°F) when the indoor temperature hits a upper limit setpoint (say 75°F) and the outdoor temperature is cooler.
I read a lot about ERVs that do this exact thing during the summer when the nighttime air temp drops below the indoor temp, but what I want to know is can they also operate this way during the day in the winter.
Two questions in summary:
1. Is my logic for handling passive solar overheating with an ERV bypass flawed?
2. Do you have experience with or know of an ERV that can behave the way I’m wanting it to behave?
Thanks all!
P.S. here’s some more context. If you’re thinking, “why not just make the home superinsulated and reduce the passive solar and eliminate the overheating all together?”, we have gorgeous mountain views that face south with a hillside that lends itself to a long rectangular structure where we can put a lot of south facing glazing. We will be using R-9 windows from Alpen, so the U-value will be around .12. So, based on the terrain and views and a function and form perspective, glazing on the south side makes a whole lot of sense. We will have large overhangs so summer heat from passive solar will not be an issue. Alpen windows also come with either high solar gain films or solar control films. I could very easily go the solar control route and the supplement with added heat, but I figured going with high solar gain and dumping the excess heat for the times we have it would net more energy savings.
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Replies
Sorry for the grammatical errors. I wish I was able to go back and edit my question
*I’m designing a passive solar house and I’m attempting to gain the most energy from the sun, but the more energy one tries to capture, the more likely overheating becomes a risk on those warmer, sunnier winter days.
To add to the above question: I want an automatic function so I don’t have to manually open and close windows to reduce overheating.
I designed a few passive solar houses in the Southwest, some with floor and wall reservoir, trombe walls, bead walls or combination of them. To avoid overheating, we installed fans from the solar heated rooms to the HVAC system plenum, with the idea that it would be circulated evenly throughout the house. An aircycler control or IAQ thermostat would control the timing and synchronization with the air handler. They all seem to work, as I've never heard any complaints.
Where are you located? Winter air at 60° sounds like a very warm climate, where the last thing you want is more heat from the sun.
For what it's worth, in Maine we have lots of two story high South facing windows but with a three foot overhang. It never gets hot enough that we need to open windows in winter.
Using a mechanical system to avoid opening a window occasionally seems like overkill.
Boulder, CO. 60° might be a stretch.
I got my answer from one ERV manufacturer (Zehnder America) and it looks like their ERV will engage the bypass in winter too.
ARMANDO, very interesting. We’ll also have a conditioned crawlspace, which I’ve been told isn’t compatible with passive solar. I figured we’d have to insulate between the crawlspace on first floor under the slab. The air in the crawlspace would be most definitely colder. Do you think I could circulate the air from the crawlspace to the rest of the house? Sounds like it might be a good way to take advantage of the cold air down there and also warm it up a bit.
I assume you are pursuing passive solar because you want to reduce your carbon footprint. It probably would be easier (and cheaper) to achieve that result by building a conventional net-zero house.
As my daughters like to tell me: "Boom! Do the math, Dad!"
100cfm (for example) of outside air at 15F delta-T can provide about 1,500Btu/hr of sensible cooling. On a sunny, mild, January day in Boulder, your south-facing, high-gain glass is going to let in SIGNIFICANTLY more than 1,500Btu/hr of heat gain, unless you limit it to a handful of ft2.
STEVE, Yes, I want to reduce my footprint, but I'm willing to pay more to keep the expansive mountain views and ensure a comfortable environment. If that means I need to offset more with active solar, than I'm willing to do that.
JOHN, I'm having trouble figuring out how to do all the various types of math. Do you have any good resources on the subject? I imagine that I need to calculate heat load, calculate solar gain, and also calculate thermal mass to figure it all out. I would also need to calculate the absorption rate and mass needed for thermal mass so it's timed properly with the day and night swings. I really really want to do these calculations, I just can't find a good comprehensive resource on the subject. Some other variables I can through in or reducing SHGC to .2, which would then allow me to increase window sq ft possibly by a factor of 3. Insulation would suffer, but if I can get the mass calculation right, I could store heat from passive solar to compensate and add geothermal radiant infloor heat (mostly for comfort) + a wood burning stove to handle the occasional cloudy winter days.
Sounds like you should build a Passive House, or at least a "Net Zero" design. That would solve most of your dilemas and be far more comfortable than a "passive solar" design. You can use large windows and enjoy the view. The PH developers proved that the ideal way to build depends on high levels of insulation, a tight envelope and some south glass. With your view, it should be ideal without worrying about the amount of thermal storage. The way PH design works, you balance the heating & cooling load with combinations of levels of insulation, glazing and a few other factors.
Mike,
You wrote, "I’m designing a passive solar house and attempting to gaining the most energy from the sun, but the more energy you try capture, the more likely overheating becomes a risk on those warmer, sunnier winter days."
Here is a link to a relevant article that you might want to read: Reassessing Passive Solar Design Principles.
You may achieve a breakthrough in your design process if you stop thinking of south-facing windows as heat-gathering appliances. They aren't. (To put it another way: windows certainly aren't a cost-effective way to gather space heat.)
What you want to do is to choose the orientation and size of your windows based on available views and your daylighting needs. You don't want excessive glazing. Excessive glazing brings all kinds of problems. My article quotes advice from Gary Proskiw: You should size your windows “to meet the functional and aesthetic needs of the building.”
BOB and MARTIN, thank you for your advise. I found this great site that helped me calculate heat load (http://www.sensiblehouse.org/nrg_heatloss.htm), and I realized that I need very little energy because my SIP panels are super insulating, even with all my aesthetic glazing (which will be either R-7 or R-9). During the heating season, I would need roughly So taking the passive approach to heart over the passive solar approach, I should use the .2 SHGC windows to minimize solar heat gain and skip the concrete thermal mass flooring.
I have a few follow-up questions:
1. The positive there is that I’ll be able to use radiant floor heating with the smallest geothermal and heat pump setup, and it will be very responsive, which should increase comfort. Or is my home so well insulated that geothermal radiant in floor heating wouldn’t increase comfort, and I should invest in a minisplit instead? To be clear, I want to prioritize comfort over initial expense.
2. I’ll have a 20k gallon cistern in my conditioned crawlspace. Will this act like seasonal thermal storage? Will it be my friend or will it be my foe? If foe, I could insulate the home from the crawlspace and make the crawlspace semi-conditioned.
3. Going back to the original queation: For whatever solar gain I do get, should I shield myself against it by investing in circulating fans to mix the air throughout the house or will that be unnecessary?? I plan on having a ERV with an automatic bypass that brings in cold air when the home reaches a temp above a certain setpoint (probably set to something like 72°). This should activate on either warm winter days and cold summer nights when the interior temperature exceeds the setpoint. Would this be enough of a solution? I should probably try to figure out how to calculate that...
Mike - I don't know if a sealed condition crawl space would be a good buffer zone to dump the overheated air, as I never designed any house that way, but the idea could work. However, I would assume that it won't be a controlled distribution.
Passive Solar was popular in NM when we built with adobe walls to use as reservoir and buffer, but I've never seen the Anasazi, Taos or any other pueblo build their houses with 200 sf of west facing windows either. Even though technology has improved the way we build houses, common sense still is the best design guide.
I do "get you point" about designing modern houses with oversize windows... its a thing or DNA in the mountain SW region, specially if they face west, but when the client says "that's what I want", I would always try to minimize or delay direct solar gain with a porches, canopy or even automatic shades or glass as much as I can.
I've found that installing radiant heating with a passive solar floor and wall reservoir is very difficult to get it right since it takes too long to heat up, and forever to cool off, with you opening windows and doors in the process.
We DIY designed and GC'd our house in snowy/cold NH. We have some pretty big (but appropriately sized) high SHGC Alpen windows on the south side. We did put some overhangs over the first floor windows to provide summer shade, roof overhang takes care of the second floor. South windows can provide a significant amount of heat in the winter, even in the coldest winter weather our mini split will shut down on sunny days. Sunny days in September (as the sun gets lower) can bring the house temp up a few more degrees than we'd like, but it's easy to manage.
With that said, I would design your window setup to meet your view needs, but don't go overboard, especially in sunny Boulder. Build some overhangs into your design to shade in the summer. If you keep the window area under control, hopefully you won't need to get into complicated setups to deal with excess heat.
Skip the radiant, use a ductless mini split(s) to heat/cool, and integrate solar PV into your build.
What Brian said matches our experience in Maine.
Just because I'm curious, why the huge cistern?
Mike, If you want really good automatic temperature control with passive solar you probably need to engage an HVAC engineer. That ERV may not provide enough airflow. Circulating air thru the crawl could indeed be useful. I know you said you didn't want it, but opening windows will work. In your area net zero is easily achievable for around $10k,
"We’ll also have a conditioned crawlspace, which I’ve been told isn’t compatible with passive solar." Why?
If you're really doing passive solar you probably don't want to limit the temperature to 72, you want to let it rise more than that so its still warm enough the next morning.
Martin, South facing windows may not be heat gathering appliances in some areas of the US, but they certainly are in sunny colorado.
ARMANDO, we’ll have a 6 foot overhang, so overheating in fall, spring, and summer shouldn’t be a problem. I should note that these are floor to close-to-ceiling large lift and slide doors.
BRIAN, most of our windows will be from Alpen. Which model do you have? Are you happy with them? Did you have any issues?
STEPHEN, the fire district requires it for our sprinkler system for our size home. 20k is actually still an estimate, I can be +/- 10k.
BRAD, the theory is that if the radiant floor heats the thermal mass, the mass won’t be able to absorb any of the passive solar heat, thus it will just cause overheating. That’s what I read. Makes sense. I agree on your assessment that passive solar does count as a heat gathering device in co. MARTIN, I’d love to hear your logic if you disagree.
Ultimately it sounds like I have two choices:
1. Reduce the SHGC as much as possible and use radiant infloor heat without any thermal mass. It sounds like this would use more energy but would give me more fine grained control over comfort.
2. Use passive solar and thermal mass for heat collection, reducing energy consumption, but this assumes I’m sacrificing fine grained control over comfort since temperatures would have to swing from day to night.
The energy conscious in me wants to use passive solar, but the realist in me doesn’t want to sacrifice comfort. I’m leaning towards #2, unless there’s a way to have both passive solar heat gain and comfort, but it doesn’t sound like that possible.
THANKS everyone for your engagement, this conversation has been exceptionally helpful!
The next thing I need to do is a passive solar heat gain calculation and measure that against my heat load. I’ll work on that and report my findings and if that leads me to a specific solution.
Please keep the insights, experiences, and opinions coming!
You can't store that solar energy without temperature swings.
Yes, totally agree, heated floor and passive solar don't mesh. Conditioned crawl & passive solar can work.
Perhaps you want something more along the lines of a sun tempered home. Sunny, but don't worry too much about storage. Limit the solar gain (Sqft or SHGC or window coverings) so the overheating is manageable. Have you looked at how much shading you're going to get with that 6 ft overhang? Depending on the layout, you're probably going to have a lot of window area that doesn't get any sun.
PS: Temperature swings are good for you! It keeps you closer to the natural world.
I designed and built a passive solar home in Boulder in the 1980's. For more info, see:
https://www.greenbuildingadvisor.com/homes/passive-solar-home-1980s
The first floor tiled slab floor (thermal mass) kept temperature variation to 68 to 78 degrees mid-winter most days with no supplementary heating (or cooling). House had about 150SQFT south-facing glazing for 640 SQFT on that first floor. Insulation levels, simple double-pane windows and air sealing were mediocre, barely "pretty good house" level overall. (1980's there were few products for building airtight, well-insulated homes.) Completely cloudy days would reduce the low interior temp at sunrise by 3 degrees per day. (Rarely two days completely cloudy in Boulder mid-winter.)
Upstairs 3 bedrooms had minimal solar mass ("solar tempered" not truly passive solar) so temp variations were greater, about 65 to 82. On the coldest nights we'd use radiant electric backup in bedrooms. On the warmest, we'd air out the room before going to sleep. Your design seems more similar to this second floor "solar tempered" space, since you don't seem to have sufficient thermal mass to dampen temperature swings.
It is very easy to exhaust cold air and lower interior temperatures by opening windows for 5 minutes (its winter). I don't understand why people fret about overheating mid-winter in a cold climate! Yes an ERV/HRV on bypass mode will cool the house too, as will exhaust fans with an open window.
I've been designing modified passive solar designs for climates that aren't as ideal as the Rocky Mountain Front Range. If you'd like to have me review your design (free) and give more detailed recommendations, please contact me at opiehere at gmail dot com. However I'd need to see a layout and more details than provided here.
Main impressions so far:
1. What a great lot with views, agree you should take full advantage of the views with larger south-facing windows, especially for your climate.
2. Why the crawlspace instead of slab? GBA has many articles on various slab options. You don't seem to have sufficient thermal mass to absorb heat during the day and radiate heat overnight, keeping interior temperatures more stable.
3. Boulder is an ideal location for passive solar or solar-tempered home designs. You don't need Passivhaus/PHIUS insulation levels in that climate with your south-facing windows and "pretty good house" levels of insulation (not expensive).
4. Radiant floor heating and geothermal are more expensive alternatives than you need in your climate IMHO. Minisplits or just radiant or resistance electrical heating can provide backup heat with a truly passive solar home. (Thermal mass also can keep your house cool in summer in your dry climate.)
5. Agree you need to model heat loss, solar gain before making many of the decisions mentioned here. Best if you model each space in your home separately, as well as the overall heat losses and gains. Moving air around is not a very effective method to transport heat, so best if each space is examined separately.
ROBERT, wow, thanks, I will definitely take you up on that offer after I get through all the calculations.
We can very easily put thermal mass in the home. I was originally planning on putting in 4-6" concrete floors throughout the home with the intent of thermal mall balancing out the passive solar temperature swings. I was just getting different opinions on this forum and wanted to entertain all possibilities.
1. I'm glad to get your positive feedback on that. And your personal experience gives me a lot more confidence. With today's tight homes and available tech, my amateur self thinks it will be even easier to calculate heat loss and gain.
2. The crawlspace is convenient for a few reasons, Boulder has total sqft restrictions, but crawlspace with a ceiling less than 6'8" doesn't count towards the home's sqft. The plan is to put all mechanicals down there and also it's a lot cheaper to put a pillow style cistern for 20k gallons in a conditioned crawlspace vs more rigid systems installed underground. Plus, if rules change in the future, the crawlspace could theoretically be converted into a walkout basement one day.
3. Interesting that you say I don't need Passivhaus insulation levels. I wonder if SIP panels are overkill for my case. According to my heat load calculations (I assuming I did them right), for my 4100sqft structure, a 30º outside temp requires ~15k btus/hr, 0º/25k, and -20º/32k.
4. I would install the radiant infloor heat myself, which would save a lot of money. Pex is pretty cheap, I figured I might as well run it even just to have the option in the future. Geothermal isn't terribly expensive either, I think it would be less than $20k for my heat load. It could also double as cooling in the summer (summers here seem to have been getting hotter lately, so if global warming continues, I want to be prepared for the future) and I could run pex in the ceilings as well.
5. I like your idea of modeling spaces separately, I've been wondering if that was a thing. I'll definitely do that!
Mike,
You seem willing to invest a lot of money in some features with a questionable payback -- for example, six-inch thick concrete floor throughout the house for thermal mass.
It's your house, and you should do what you want. But trying to get free heat from the sun, and then figuring out how to handle the overheating problem, takes time, energy, and money.
A simpler solution is to keep the area of the glazing moderate. Skip the thermal mass. Invest instead in air sealing measures and above-code levels of insulation. Install a simple heating system like a few ductless minisplits -- this system will cost much less than a radiant floor or ground-source heat pump.
For more information, see these two articles:
All About Radiant Floors
All About Thermal Mass
Brad,
You wrote, "South-facing windows may not be heat gathering appliances in some areas of the US, but they certainly are in sunny Colorado."
The trouble is, south-facing windows make lousy appliances, because (a) they gather heat when they feel like it, even if the house doesn't need heat, (b) they are unable to gather heat whenever heat is desired, and (c) they lose a lot of heat on cold nights. It's a window, not an appliance.
Not only that -- it's a very expensive appliance compared to other appliances designed to gather heat. For more information, see Study Shows That Expensive Windows Yield Meager Energy Returns.
"BRIAN, most of our windows will be from Alpen. Which model do you have? Are you happy with them? Did you have any issues?"
We have what they now call the Zenith Series ZR-5/525 (we were on a tight budget). We have been happy with them and there have been no issues.
I'd review the relative economics of active, PV solar. With net metering or with water storage. For example, 8KW of panels (much less with a heat pump) operating for 6 hours can heat a house insulated to passive house standards for 24 hours. With around 250 gallons of water thermal storage. No temperature swings needed.
It takes a lot of CFM to remove significant amounts of heat when it's mild outside. Ie, more like a whole house fan vs an ERV.
Mike, Martin,
Although I can't speak for Martin, having read many of his articles, I believe he's an advocate for raft slab, thickened edge slab, and similar foundations. Although you need to have an engineer review and update the plans, a 6" slab is likely unnecessary, and won't help substantially increase the effective thermal storage vs. a 4" slab. Radiant heating embedded in the slab is unlikely to be used much if your home is a well-engineered passive solar design, so is likely an unnecessary expense. I agree with Martin there. Martin didn't mention but probably agrees that a 4" slab is comparable or maybe cheaper than alternatives. The slab can serve as a foundation, main floor structural support, and (if you stain and polish the concrete) the finish floor. So its likely cheaper than a crawl space foundation plus wood joist subfloor and hardwood or carpet finish floor. Therefore the thermal mass provided by a 4" slab is essentially free (since you have to provide some type of foundation, main floor structural support, and finish floor).
Not everyone wants big windows, lots of interior sunlight during the winter, hard floor surfaces, or can tolerate ten degree indoor temperature swings. But there's also a lot of people who get depressed experiencing dark, cold winters, with thermostats at 65F (18C) or lower, or smaller or grayish tint low visual transmittance windows. Being in touch with diurnal light and temperature variations likely helps reduce sleeping problems and depression. The "optimal" American style constant indoor 73 temperature year round with minimal natural sunlight may not be healthy psychologically for some people. Sunlight provides not just heat but natural light, an important aspect of good architectural design.
Views are wonderful if you are lucky enough to have them. Planning to locate and size windows and rooms to capture those views is important, too.
Each habitable room in a house must have window or door glazing that is at least 8% or 10% of floor area. Your south-facing windows can provide the majority of that required glazing area. Since you have views to exploit, more glazing is fine so long as you DO THE MATH to figure out the consequences and make adjustments. Overheating and glare are potential problems that can be tackled. Solar control can be accomplished many ways. As you know, windows can be better insulated, reduce glare and solar gain with various treatments and window coverings. You probably have seen plenty of upscale homes with window walls. South windows are easy, its west-facing windows that are more problematic (given their highest gain occurs during the hot summer, in the hot afternoon no less). IMHO you are blessed with great solar access and views. Take advantage of them!
Martin, I'm surprised at your unsubstantiated negativity with S facing windows. Windows are a net energy gain in my climate, they also provide light and fresh air. a) Overheating with properly sized windows and overhang is not really a problem. b) Sure but not a big deal. Right now my house gets warmed up to ~75 in the day and ~65 at night, with lows around 30 or colder, both temps comfortable c) Still a net gain.
The windows don't have to be expensive, double pane Milgard for me.
Brad,
I have no negativity concerning south-facing windows. Just size them for aesthetics and practical needs, and don't think of them as a heat source.
MARTIN, I wrote "4-6 inches". You wrote "Mike, You seem willing to invest a lot of money in some features with a questionable payback -- for example, six-inch thick concrete floor throughout the house for thermal mass." Of course I don't want to invest features that have questionable payback. That's why I'm here asking for help :)
I appreciate your candor and the perspective you're conveying; I totally get what you're saying. More glazing means less super insulated walls, even if I am getting R-9 glazing, the heat gathered will not replace the heat lost when comparing solar heat gain methods/management dollar-for-dollar to more insulation and less expensive heating options. I guess it all comes down to perception of value. You value cost and simplicity over maximizing a natural view. I guess I value maximizing a natural view over cost and simplicity. So, considering my priority, my goal is to manage temperature swings as much as possible (I know it will never be as good as a passive house), and that's what I'm trying to get advice on. So if there's any advice that you can give me with those goals in mind, I'd really appreciate it.
I actually read both those articles several times over in the past. Thank you for writing them! Although, I personally disagree with your conclusion on radiant floors and comfort. We have in-floor electric radiant heat under tile with the floor temp set to 72º and it's absolutely more comfortable than a 65º tile floor. Of course this isn't an issue with super insulated homes since the floor will likely be the same temp as the rest of the environment, but that won't be true for my case, so radiant in-floor heat is still attractive to me.
JON, that's very interesting. So you're saying I can use water thermal storage heated by electricity (offset by PV) to mitigate temperature swings? Do you have any resources you can share with me? I'm quite curious about the details around how this system works. Does the PV simply maintain a specific temperature, causing the thermal mass to mute the temperature swing? I assume the thermal mass is central and different rooms away from it will get hot, unique to each room. So would the temperature be equalized by forced air?
ROBERT, thanks again for your perspective! I just completed the calculations. I'll follow up over email.
BRAD, yeah, I don't mind temp swings as much as my wife and I think I can convince her to be ok with it if it was more like 65-75 swings. I just don't want to have to "maintain" it by manually making adjustments all the time (opening/closing windows). As long as I can automate it and it's not colder than 65 in the morning and not hotter than 75 degrees by the time I go to sleep, I think it would be passable and worth the energy savings. What I liked about the ERV bypass is that it essentially automates opening/closing windows. More things like that would definitely help. I just have to check find a calculation to check that it can exchange warm with cool air quickly enough to be effective (comfortable). The only other issue I really need to understand is glare. Having solar blinds would help, but that also becomes a "maintenance" thing; I would want to automate it if possible.
It's a 11ft lift and slide door on a 12ft wall. There's about 1ft of glazing that won't get sun, but that's ok, the glazing is there for the mountain view sight line more than anything.
Ok, after talking through it with my wife, we've settled on a solution. We'll install geothermal radiant in-floor heat, keep the glazing as desired to maximize our views, and install exterior solar shades that can be drawn as a proactive measure to prevent overheating. We would set the radiant in-floor heat to say 70º and with a few home automation tools, we could potentially even auto draw the shades when passive solar heats the floor past a certain setpoint, say 75º, making it all a fully automatic system. This also will allow for specific zoning between rooms. For instance bedrooms can be set to 66º with shades auto-drawn at 71º. We can program the system however we like for each individual room and we won't have to worry about saving every passive solar BTU in some sort of thermal mass somewhere (although we could experiment with that and see if we can make it work).
The obvious downside to this, as Martin mentioned, is the expense and added complexity of geothermal, radiant in-floor heat, the extra glazing, automated solar shades, and possibly PV to offset the heat pump's electrical use, all of which wouldn't be necessary with a traditional, super-insulated passive house. But we'll have things that a passive house couldn't give us: individual temperature zoning of rooms and grand views.
That said, before we resort to geothermal heat pumps, I'll put pex in the slab floors and we'll try very hard for the passive solar angle to work in a way that we're comfortable. If we can't get it to work as desired, in goes the geothermal heat pumps.
Thanks everyone for your input! And let me know if you see any flaws in this logic.