Hi All,

I have a newer well insulated passive solar home with wide open southern exposure. Winter temperatures can approach 35C on a cold sunny winter day. The person who built the home did not install much thermal mass on the main floor hence the temperature swings. The house was, however, designed with in floor hydronic heating in the basement.

I am considering running a water loop in the floor of the area that experiences temperature swings (the floor is directly exposed to sunlight most of the day). To do this, I was thinking of slate flooring with appropriate sized tubes embedded to extract the heat and transfer it to the basement slab during the day, which should moderate the temperature swings and provide excellent thermal mass for the evening hours (and also prevent peak usage of hydro).

I cannot find any info regarding such a system to size or design it properly. It seems simple enough, but I thought such a system would be somewhat common. Can anyone point me in the right direction, or offer come advice?

Regards,

Ryan

Why not simply add thermal mass to the house to moderate the temperature swings?

You could do ceramic tiled floor or a stone veneer on a wall.

Hello AngelofDebt,

When I do the calcs based on some of the 'rule of thumb' guidelines', it seems like a whole lot of ceramic or stone would be required to make a significant difference. The Southern wall of my house is almost all windows.

I'm also thinking that the basement slab would provide better storage for the heat collected since it will radiate the heat in an area that has much less exposure to cold night time windows. The temperature differential between the floor and surrounding area would be much less. The heat should radiate slower to the upper level and hopefully moderate temperatures. Another benefit would be longer term storage, once the slab is heated it stays warm for a couple days.

There may be disadvantages too though, such as maintenance of an active system rather than passive, but if designed properly it should be minimal.

I may start out as you suggested, however before I lay the tile I wanted to gather opinions on the idea of an active system since it will be too late to installed hydronic tubing after the tile is in place.

Another option I was considering for the system was 'zoned' electric floor heating between the floor joists. Every morning my wife and I enjoy the view with a cup of coffee, and I'd like to have a quick method to warm the tiles in that area. Would this be against code to have hydronic and electric heating in the same floor if the systems were seperated by plywood?

Any help would be appreciated.

Ryan

Please post some pics. I am also interested in a passive solar design. Thanks!

Hello Slenzen,

THe house is actually built already and I can send pics. These homes definately overheat in the summer without properly designed overhangs. If you are in the design phase, there is plenty of info regarding overhang (to shade the windows from the high summer sun) and also how much glass to use to acheive a good balance between winter gains and summer cooling load.
The house I purchased was overglazed, however I prefer this since the winter gains are incredible, and in the summer its just a matter of opening the patio doors to allow the breeze to push the heat out. Rarely is it uncomfortable as long as the doors are open. In my experience, most 'rules of thumb' would under glaze the south wall and some gains would be lost.

I highly recommend passive solar, it is amazing to have such abundance of free heat, and on the cold clear winter days it really warms you to the bone (and makes the winters more tolerable). If you have southern exposure and a good solar window (not too many trees shading) I would definately look into the benefits of passive solar, it doesn't cost much more to build these types of homes, and the payback of extra insulation / glazing costs is quick.

Ryan

The lack of thermal mass, that would be located in direct sunlight, is a critical flaw in any passive solar design. It is more effective to store the heat at the point where it is converted from short wave to long wave energy. At this point, it would make more sense to move the heated air, by using natural convection or forced air movement.
A properly designed overhang will prevent your overheating problem, only if is combined with thermal mass. Keep in mind, that the R-value of your large area of glass is a major contributor to your overall Heat Gain. You have stated that you have to leave the house open during the day. Without supplemental cooling, is this comfortable during the summer months?
The beginning of a good passive design is always based on your location and micro climate. Where is this home located?

Hello Dteltech,

Thanks for the reply. I am fortunate that I can still add thermal mass to the home, I'm just not sure that I can add enough to really moderate the temp swings. The house is located in Northern Ontario, east of Sudbury. The area is fairly windy and the house does remain quite comfortable with the windows open.
I have tried mechanical ventilation of the heated air and managed to transfer some heat to the basement (dropping upstairs temp by a couple degrees on average). I also have an HRV which helps out. The house has a loft with a duct running directly to the basement, so transferring larger volumes of air is possible, I just thought it would be more efficient to store the heat in the slab and let it dissipate slowly.

Thanks again for the advice.

Ryan

My brother’s house is setup for good solar gain and has radiant in floor heating on both levels. What they do is leave the circulation pump running and it does warm up the basement and some of the rooms on the east and west sides with no heat being added to the loop. When the system does call for heating it simply opens a valve to the hot water supply. I am not sure how they control it zone wise since they all need to be open to circulate the heat and then somehow close the ones not calling for heat.

But if in your case you’re not adding any external heat a simple circulation system will do the trick. It might make since to add a small tank with an ability to somehow heat it if necessary. For example using a simple dedicated 10 or 20 gallon electric water heater and only turn it on if you want to

Hi Brock,

That is exactly the type of system I had in mind. Does your brother find it worth the investment? I should have mentioned that I have an electric boiler as well, I'm just hoping to reduce its energy consumption during peak sunny hours and only use it during the night. Do you know what type of valve he uses to switch the supply on and off?

Regards,
Ryan

Ryan,
Thermal mass windows, take at look at this window system being developed for this exact prupose. I have been fortunate enough to have been working with the inventor of these windows and have the only prototype of this windows system in a home. The window being developed by Hunter Douglas, the window shade company also incorporates an exterior solar shaing system to prevent overheating in the summer. I have documented the windows and posted 8 or 9 videos explaining the windows and showing performance during different times of the year. http://www.youtube.com/user/eebuilder
These windows 150 sq/ft of south facing glazing contain over 3,000 Lbs. of water, that capture and store the suns energy, then slowly radiat it into the house at night. No pumps or mechanical systems needed to do this, a truly passive system other than the operable shading. The windows shown in the video are not the latest developement for these windows. The cost to do these ones was much to high and are being redesigned at this time to moderate costs. The newest ones are about to be put on display on the BAC/Tufts university solar decathlon http://www.livecurio.us/ project next month in DC. They have resolved some of the intial porblems like long term storage of water by replacing this with a jel like substance. Warning these windows are extremely heavy about 450lbs per linaeal ft. thermal mass in your windows who would have thunk it?

Tom Pittsley
[email protected]
www.eebt.org

My guess is that you'll have a hard time making this work.

In order for your idea to pan out, you have to be able to get some good number of BTU's into the basement slab every day, and then to pull it out at night.

A 5 inch thick slab - if that's what your basement is - will hold about 9 BTU's of energy for each degree of temperature differential. So, if your basement is 1,000 square feet, and you can tolerate 10 degrees of temperature change each day, you'll get about 90,000 BTU's out of it. Thats about $1/day right now, give or take.

One large caveat, though:

You'll need to be able to move the heat in and out, *and* remember that you can only move heat from a warm area to a cool area. So, for example, if during the evening, you need to maintain your slab at 75 degrees in order to maintain your BTU throughput in the living area, once the basement slab gets down to 75 degrees, you are finished moving heat for the night. From that, you can see that in order to get the amount of usable differential up, you'll need to heat the basement above 75 degrees (again, for example) which, if your basement is well insulated, might prove to be uncomfortable.

And then, getting the basement slab up to a higher temperature, such as 80 degrees, requires that the living room slab be at 80 degrees during the the day. That might be true, but again, you may have heating issues in the living area then.

It sounds like you are technically proficient, so I'd suggest that you make up an Excel model that tracks the temperature of the living area and the temperature of the basement, uses the heat loss numbers and the specific heat of concrete, and then tells you what the answer is.

Jeff

If you don't want to do anything to the floor right now then I would suggest placing thermal mass on the walls. I am currently planning a passive solar design in New Brunswick Canada and have gather a fair bit of info. I would be very interested in discussing with you your experience.

Ryan how large is the tank? It might be easiest to just shut off the electric heater and let it circulate and use a thermostat to turn the electric heating element back on? If I had to guess with his system off he had about a 10F temp swing in the outer / north sides of the house. So it might be 65F on the north side and 75F in the sun. When the circulation pump is on he is closer to 67.5F and 72.5F across the zones. So it's not perfect, but it does seem to even it out and seems to hold the heat longer as well.

If I get out his way again I will see exactly how he does it.

Dunno if an "active" solution will work. Your basement hydronic design will have a minimum water temperature requirement based on pex placement. If it is 90 degrees or higher, which is likely, you'll have a tough time generating those temperatures in your living room, or tolerating them if you do. 84 degrees is considered the upper limit for mass floors based on comfort and acceptable expansion.

A water wall is the quick and dirty passive fix. Water is five times more effective in heat storage than concrete, so 1500 gallons of water would sub for a 1000 sf of 5-inch concrete slab. 1500 gallons is roughly 200 cubic feet and weighs 12000 pounds. You probably wouldn't need that much. www.builditsolar.com/Projects/SpaceHeating/AWaterWallIntro.pdf

As Todd said water is better at holding the heat, so if you could leave it circulating it would tend to end up all at the same temp, even in the tank, which would even out the heat as well.

My brother uses some fancy domestic water heater with a coil in it for the in floor, so he can't continue to circulate it through that tank.

It could be great if any one provides some pictures in installing.. Looking forward for the reply.

Hi Everyone,

Thanks again for all the replies. As a quick fix this year, I'm thinking of upgrading the booster fan I have installed in the duct running from the loft to the basement. Does anyone have any info for sizing the fan properly? I am estimating that about 16 000 ft3 of air heats up to about 30 - 34C on the main floor when the sun is really shining. I have a 4'' x 10'' duct, and I am currently moving about 100 CFM to the basement all day. This increases the temperature downstairs a few degrees, but does not make a noticeable difference upstairs.

I am estimating 500 CFM would be sufficient, for about 2 air changes / hour. Does this sound reasonable? Would the duct work handle anything larger?

Thanks for the help,

Ryan

Well using Trane charts your 4 by 10 is equivalent to a 7" round duct. Pushing 500 CFM through that duct shows a velocity of ~1800 feet per minute (20 MPH).  I can get the numbers but I'm not sure if this is within normal HVAC numbers.