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Old 10-16-13, 08:42 PM   #1
evel_knievel
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Default How to heat a passive solar house? Besides the sun! Lol

Hello Eco_folk!!
We are currently debating on building a new house and want to make it as energy efficient as possible(who doesn't). I've done a lot of research so far and pretty much have a good idea of what we want right down to the floor plan in general.
Luckily we live in the country and our lot allows us to build long east to west and the back of the house faces south. Seems logical to build something passive solar friendly.

I could use some advice on how to heat this house fairly cost effectively.

To start with, we live in Zone 5 in the open country. No natural gas runs out this far and propane is fairly expensive and the prices can be very volatile.

Some design aspects of the house are for cost savings and some are to simplify construction. It's a fine balance I know!

The basics of the construction are as follows:
4" concerete slab on grade with a foundation, not monolith. 4" of XPS foam under the entire slab as well as the perimeter. 4" XPS foam also on the outside of the foundation even below grade.

Wall construction will be 2X6 2' on center, double top plate, sealed base plate. 5/8" drywall inside with vapor barrier, dense pack cellulose insulation in the cavity, then 1/2" OSB outside that is glued(for air tightness) and screwed to the studs. Followed by a moisture barrier, then 4" XPS foam sheets(not 2 layers of 2") and then stucco using EFIS finishes.

30' 4/12 pitch trusses with a 12" energy heel to install blown cellulose to about R60 in the attic space.
Roof will be covered with 1/2" OSB then thin foam followed by raised rib steel.

As you can see by the attached floor plan, the only north windows will be over the kitchen sink and in the office. These will be high quality windows.
No windows on the west wall and no windows on the east wall as that will be against the garage.

The south wall will have four identical windows(2 in livingroom, one in each bedroom) which are 4' wide by 6' high. These will be stationary glass(except one for egress code) using Cardinal Low e glass with U-Factor of just 0.26 and an SHGC of 0.69 for allowing high solar gain with minimal heat loss.
Also there will be 2 sets of french doors (one in master bedroom, one in livingroom), hopefully with the same glass.

This should do quite well heating up the slab as we plan on using dark ceramic tile in all the south rooms.

A lot of attention will be spent on detailed sealing, etc.
So with an idea of the general house design and thermal envelope, I have no idea what to expect as far as what it's going to take to heat/cool this.

I considered running PEX in the slab and heating it, but will the sun do an adequate job of that in this scenario? If I did run the PEX what would be a cost efficient way to heat it, geothermal? electric?

If the sun is going to do a fine job on heating the slab then would a better option be a forced air geothermal? or maybe just a few mini split heat pumps? If geothermal, we have plenty of room for horizontal pipe runs.

Or is this going to be so efficient I could get away with a cheap electric furnace with the right duct design? I know I need to keep the ducts inside the conditioned envelope.

I really have no idea what we may need to handle the heat/cooling loads. Anyone have any experience with a very efficient home heating/cooling?

I'm open to suggestions but i'm not fond of heating oil or a wood burning outdoor unit because i don't want to worry about stocking wood when we're older.
We may also consider heating our hot water with solar if finances allow.
edit: there will be a wood stove in the livingroom but it doesn't run all night or all day when we are at work.

Thanks for all your help.

here is the floor plan

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Old 10-16-13, 09:10 PM   #2
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Default How to heat a passive solar house

Evel

Great project and its good your planning a home on a clean slate. I have been living with solar hot water system with a geo-thermal back-up. Its working extremely well. With the results I've had here in Canada its entirely possible to heat the home with little back-up heat.

I would suggest instead of the 2 x 6 construction investigate the foam blocks with concrete & Re-enforcement steel bar poured in the center.

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Old 10-16-13, 09:16 PM   #3
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Quote:
Originally Posted by randen View Post
Evel

Great project and its good your planning a home on a clean slate. I have been living with solar hot water system with a geo-thermal back-up. Its working extremely well. With the results I've had here in Canada its entirely possible to heat the home with little back-up heat.

I would suggest instead of the 2 x 6 construction investigate the foam blocks with concrete & Re-enforcement steel bar poured in the center.

Randen
Thanks Randen.
I did some research on wall types and a big factor in this wall construction was that I can get it framed cheaper by a framing crew without too much confusion on their part.
Also this study seems to show this wall design yeilds a fairly high R factor.
http://www.buildingscience.com/docum...t-high-r-walls
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Old 10-17-13, 06:21 PM   #4
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Evel

You are right with conventional construction there is less chance of the average framer messing up, and if he does make a mistake its easy to change. (If someone catches it) If I were to make a high performance home I would lean toward the ICF. The structure will be very well sealed by the nature of the construction. There would be no thermal bridging you get with the traditional stick construction. I like you idea of the stucco /EFIS finish. For the higher R value you may be after one can just screw rigid foam to the outside of the ICF blocks. With the ICF you start at the concrete footings and keep laying them until the truss plate no area for the smallest location for air infiltration.

If you can do most of the grunt work such as laying out the tubing for heated concrete floors, DIY your solar hot water panels, and installing the tubing for the ground source heat pump you can save lots of money there and have heat and air-conditioning for practically zero. Add in some solar PV and you probably could get to zero. Its very attainable but it isn't a free lunch.

The solar input through the windows will not be near enough. We have black tile floors and yes where the sunlight strikes the floor it becomes warm but else-were is cool and can become uncomfortable cold. Heated concrete slab is the Cadillac of space heating.

I will leave you with this: On cold mornings where wind is howling outside. -20C I can be standing, bare foot looking outside at the wind swept snow drinking my coffee. The floor is at 28C and the house temp at 23C toasty warm from the heat I had collected the day before from the solar hot water running through the concrete that has lasted through the night. And it was free!! It totally do-able

Randen
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Old 10-18-13, 07:21 AM   #5
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Evel,

That floor plan looks straight up like my parents' home. When they retired, they wanted to do the Green Acres thing. Being professionals in their fields, they had done enough corporate time.

Being a project engineer/mechanical wizard by trade, my father scrutinized a plethora of floor plans and construction methods before making a decision. They ended up with a Dutch doublewide....
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Old 10-18-13, 09:15 AM   #6
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"So with an idea of the general house design and thermal envelope, I have no idea what to expect as far as what it's going to take to heat/cool this."

ok, fine. I'll pull out my manual J and stare at the tiny numbers of your picture and try to figure out what numbers they are and do math based on the things you said about this place.

It seems for pretty much anywhere in Indiana your heating design day calc will be a single digit number that I can round down to 0. This isn't the coldest day but the temperature that you usually get down to(or close) for about the coldest 3-10 days of winter(technically 87.6 hours will be colder, on average, than design temp with the rest of the year warmer). I'll use 0 because its an even number. Do you have at least 3 days in an average winter that get this cold in the area this house is being built?

Do you know the U-values of your two non-french doors or am I factoring U0.11 for urethane or similar solid non-glass doors?
Also not sure what to factor for the north windows so I'll figure you'll buy ones that insulate better and will aim at U 0.20 since your priority isn't on SHGC on the north side unless you have something else in mind for a U-value.
Also factoring 8 foot ceilings

Last edited by MN Renovator; 10-18-13 at 09:24 AM..
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Old 10-18-13, 11:24 AM   #7
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Oh, the benefits and follies of glass. I personally hate the stuff, strongly held personal opinion of someone who rarely opens my drapes so I'd enjoy the benefit but prefer privacy and don't value natural light too much. Here is why.

Breakdown excluding slab
Walls 24.62%
Attic 22.60%
Solid Doors 3.98%
Glass 48.80% (This includes the french doors, thanks for not planning for sliders!)

Design day zero degrees on the F scale with 70 inside.
Walls 2003 BTUhr
Attic 1838 BTUhr
Solid doors 323 BTUhr
Frenchies 1529 BTUhr
South glaze 1747 BTUhr
North glaze 420 BTUhr
NrgSukRglaze 273 BTUhr
Total 8133 BTUhr (no slab, calculating heated slab is no fun)
Heated slab 2364 BTUhr
Total 10497 BTUhr

Non-heated slab loss estimate 1182 BTUhr or 9315 BTUhr

Figured you'd want solid doors for the two non-sunny doors, because efficiency.
For north glazing, figuring two 3'x5' good U 0.2 triple glazed lo-E and not the stuff you used on the sunny side.

Calculating slab losses sucks because you need to know how hot the slab is while pushing the design load into it, but then the design load changes because now the slab is losing the extra heat its holding because it is at a higher temperature.

Well, that's it for my heat load calc, took me about an hour to do it without using an Excel spreadsheet to do the math calculator plus notepad is a bit interesting because all the figures are in front of you.

Disclaimer: Based on Manual J calculation method as I understand it from reading both the 7th and 8th books(I've got my own Man J right next to me right now), my math could be off because things like that do happen but based on doing these before, this looks right to me. I also don't have any infiltration or ventilation factored in. Let's say 50CFM with a 65% standard HRV = 2457 BTUhr
I personally would probably be okay with a FV-08VKS3 set to suck 30CFM which would be 2268 BTUhr but people like to argue against that idea, so I usually don't tell them that I'd plan to turn it off on the coldest nights where we're within 10 degrees of design temp, then they really don't like me. I figure you can seal up that envelope really well but you'll still get some infiltration losses in there and if you feel like the air isn't that fresh you can always turn it up to 80CFM(or set whatever constant rate between that if you wish) until you are happy. Another option is when its during the few hours of the hottest part of the day to have a timer where it kicks it to high speed.

Another note: I'm not going to do a summer cooling calc for you but PLEASE be sure you have proper overhangs or you are going to be living in an oven during the much of the spring, all summer, and a good chunk of fall with a cooling load well over what your heat load is.

Next post: My opinion on HVAC choice and if I feel like it, I might discuss my approach and different numbers if you reduce glazing and use lower U glass.
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Old 10-18-13, 11:59 AM   #8
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Wow MN Renovator. That's a lot of good info and I greatly appreciate that.
Obviously I'm no engineer or I wouldn't be here. I've always considered myself DIY savy until I started reading some of the DIY heat pump posts by AC Hacker and Randen. Wow!

I've done quite a bit of wasted remodeling on this 100 yr old house and my fiancÚ and I decided we should persue something far more efficient. We plan on staying here the rest of our days(in our mid 40's) so I'm in a major research mode to come to an affordable solution.
I've never lived in nor have I known anyone with a high efficiency house so I am pretty much lost on what it takes to heat/ cool. Most houses here are barely built to code with 2X4 and R13 batts with an oversized HVAC.
Maybe I misunderstood just how well(or not well) these passive solar designs are with heating/cooling.
I just wasn't sure if it was going to take a big hog of a typical forced air system or be able to heat it with small ceramic space heaters. I guess I was just looking for an idea of what to expect.
We certainly want it inexpensive to heat and cool even if it takes some initial up front costs rolled into the loan.
Right now we are on an electric budget of $281 per month year round and they are suggesting we bump that up!! ouch!

Thank you for all your time and suggestions. You are spot on with the door types and ceiling height. I was planning on quality energy star doors and windows for the entries and north/west.
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Old 10-18-13, 01:51 PM   #9
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"Right now we are on an electric budget of $281 per month year round and they are suggesting we bump that up!! ouch!"

With this house, you could use two standard space heaters(1500 watts each) and it would maintain a difference of 75 degrees between the inside and outside. So if keep the house at 70 degrees, it could be 5 degrees outside. This would be if the house was very air tight and for the duration of the coldest period you had the ventilation system off. If we include ventilation and use just one space heater, it could be 70 inside and 40 outside. Of course keep one thing in mind, this is factoring no added sun and doesn't factor in the thermal mass effects of a non-carpeted concrete slab, so you could have slightly less heating and the temperature won't drop that quickly with the level of insulation you have in the house. Randen's post above regarding his solar heated slab that lasted through the night shows the potential for thermal mass heat storage. My heat load calculations don't factor that in but in general that heat has to come from somewhere and heat will then need to be replenished by solar again or supplemented by another source.

I'm not sure what your electric rates are but if you were stuck with no sun for a 24 hour period and the temperature outside was stuck at a temperature 75 degrees colder outside versus inside and your electric cost was 12 cents per kwh, it would cost $8.64 for the day. I don't suggest electric resistance heating as a primary heat source though because electric resistance heating, propane, and oil heat are the most expensive ways to heat a house!

Central air is a bit out of the question with such a low heat load, even the smallest 95+% efficient furnace(40,000 BTUhr) that runs on propane or natural gas has a heat output that is 4 times as much as what you need on your coldest day. It would fire up for only 25% of the time, 15 minutes per hour on the coldest night. ..which isn't really all that bad of a thing except you need the ductwork for about 600 CFM installed and you'd probably also end up connecting a 1.5 ton air conditioner or heat pump to it which would be oversized for your houses load and less efficient than a mini-split inverter heat pump setup that would be both cheaper to install in new construction and far more efficient.

Hydronic heating using a heat pump with solar assist or mini-split inverter heat pumps are the choices I'd personally think are the most appropriate.

Hydronic heating using an 'electric boiler' that isn't sourced by a heat pump would give you warm floors but wouldn't be cheaper to heat the house than electric baseboard or space heaters.

Hydronic heating using geothermal and assisting with solar would give the lowest electrical cost but the initial installation cost would be very expensive unless you are able to DIY a large part of it as randen suggests. I personally can't do that sort of work with geothermal design and install but if you can the high labor parts and work with a geothermal person who can help you, it would make this much cheaper than if you didn't DIY a good part of it.

Mini-split heat pumps are my personal choice. I take a different approach when I've modeled my house, which would be a retrofit of my current 2100 sq ft multi-level house. I'm looking to get my heat load from 25830 at a Minneapolis design indoor outdoor 70f/-12f 82 degree difference down to 9730. The fun thing is I've actually monitored my usage and am within 500 BTUhr between my heat load calc and actual furnace output during the coldest day in Minnesota this winter. To get down to 9730 I'd need to replace all of my windows with U 0.2 or better(R5), R60 in the attic, add an extra 4" of XPS to the exterior, and be sure that I've covered as many air leaks and thermal bridges as I possibly can get to. For good measure I'd also replace my natural gas equipment with PVC vented 90+ equipment to get rid of the sieve that is the stack effect. I'm also planning to reduce my glass area with the house, I have large picture window and two side windows that will lose 1 foot of height and be replaced with insulation because the winter sun doesn't wrap around the house enough to bring heat through them so they are terrible year round, when replaced they will be heat rejecting triple-pane for sure. I have a serious disadvantage because I don't have a single window in my house that benefits well in the winter to heat my house. The only reason why my projected retrofit uses less energy than yours is because of less window area and less thermal transfer through the glass. My current cooling load is higher than my projected heat load, with glass reduction and heat rejecting glass I should be able to shore them up and use a single 12k BTUhr inverter heat pump to cover nearly all of my heating and cooling needs. Unfortunately we drop to -20f here in MN usually once or twice a winter with -10f or lower about 5 days each winter. Below 0f, a mini-split heat pump struggles to produce the heat I'd need. Since my house was originally designed with forced air HVAC and the era of house has it, I'll keep that system installed for the sake of selling the home(with features expected from being built in the 80's) in the future and for heat when it is cheaper to heat with natural gas.

Here's a different look at solar heating in a net zero sense. If you are planning for PV, size your PV to include additional capacity to cover water heating and heating/cooling loads. With the low cost of PV and the fact that you get additional electricity fed back through solar in the summer when you don't need the solar heating, the house you could pay back the winter usage and water heating, as well as power the house. I'm actually thinking if you were looking for net zero energy use for heating and cooling the house and heating water that it would be cheaper with PV versus water. ...you don't get the benefit of warm floors with that idea though, but if you don't end up heating the slab with hydronic, its another option.

If you are looking to put down tile floors, have exposed concrete or another similar surface I think you really need to use hydronic floors to keep them comfortable. Same with if you are looking to heat the house with solar water heating, this would be the best way to do it.

Since I don't have thermal mass and retrofitting without a real hydronic option, I'm part of the super insulation camp with minimal windows. I am planning to build a solar air collector or two and duct that into the house eventually. With that I can have the glass benefit me by heating the house but when the sun isn't shining, the fan shuts off and the damper closes and I don't lose heat through that same glass at night. I also don't need to worry about summer overheating or overhang planning.
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Old 10-18-13, 01:52 PM   #10
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Quote:
Originally Posted by MN Renovator View Post
Another note: I'm not going to do a summer cooling calc for you but PLEASE be sure you have proper overhangs or you are going to be living in an oven during the much of the spring, all summer, and a good chunk of fall with a cooling load well over what your heat load is.
^^^This!^^^

Passive solar design requires solar gain in the winter and shading in the summer. I did a quick search for "calculate roof overhang passive solar" and here is a link that looks promising -

Sustainable By Design :: overhang design

FWIW,
Tim

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