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Alternate Solar Thermal & Radiant Heating Combo Systems

by Chad Ludeman on May 27, 2008 · 10 comments

in Building Science,HVAC

In our last post we spoke of a Shuco Slim Line solar thermal heating package that would be used in conjunction with a closed loop radiant loop and a high-efficiency gas boiler to heat the domestic hot water and the radiant loop as needed. This system uses the solar thermal system to preheat only the domestic hot water. The gas boiler then heats all of the radiant system while also acting as a backup for the domestic hot water.

An alternate system is one in which the solar thermal system can heat both the domestic hot water and the radiant loop. A Vermont company, Radiantec, offers two variations of their solar thermal and radiant heating systems that do a good job at handling both. I spoke with Don Vance at length last week about his systems and recommendations for our homes. Don is an extremely helpful guy and provided us a quote for both a radiant system and solar thermal system size specifically for our homes a few months ago.

Radiantec Solar Option I System

Option one uses the concrete slab as well as the earth below the slab to increase the solar energy storage area.
Radiantec _ SolarOptionI
This option is unique in that it is a hybrid system that combines the best of active and passive solar thermal technologies without their respective disadvantages. Radiantec has one numerous awards for this design and has published a 28 page report for the US Dept of Energy (pdf) on their site detailing the benefits of this hybrid system over traditional setups.

While the website recommends up 7-8 solar collectors, Don sized our system at only 3 panels. This is due to our climate conditions, available solar energy and rough heating demand. We have a smaller home that is well insulated and located in a milder climate than Vermont. If we were to add more panels, we would have trouble handling the excess heat generated in the summer and would have diminishing returns on the system as a whole.

Some of the things I like about this system are that it improves the efficiency by utilizing a much larger mass for solar energy storage. This will mean that it will take the mass longer to heat up, but once it reaches the desired temperature, it will remain stable for a number of days without additional heating. Also, this setup has the ability to heat both domestic hot water and the radiant heating system. When the radiant heating system no longer requires additional heat, the system will automatically heat the domestic hot water supply instead.

This system is intriguing and seems to be more efficient than the current setup we are looking at. I still have a few questions about it that need to be resolved and of course there is cost as well.

Radiantec Solar Option II System

Option II from Radiantec is similar except that the radiant loop and domestic hot water are heated together in an open loop.

Radiantec _ SolarOptionII

The key aspect that interested me about this setup was it’s ability to cool the home for free during the summer by running the cold water intake from the city through the slab prior to delivering it to it’s point of use. In the DOE report, Radiantec claims they are able to extract over 50K BTUs per day from a 2-story, 1,400 sf house with a 720 sf slab on the ground floor. Not bad for free cooling.

Both options have the ability to contribute to both domestic and radiant water heating, but if there was a way to combine the efficiencies of Option I with the ability to cool the home during the summer, it would seem like the best way to go for us.

Check out Radiantec’s solar site – – for a wealth of information and background on these systems.

If you enjoyed reading this post I can promise you'll love our new writing over at Postgreen Homes. Yeah, we know that's the same thing your favorite band said and their new album is nowhere near as good as their early stuff, but seriously, we are actually still getting better.

There also isn't much conversation to be had here . . . at least not with us. So come on over to the Postgreen Homes Blog and tell us what you think of our new(ish) digs and crazy ideas. We will be sure to tell you what we think of your opinion.

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{ 7 comments… read them below or add one }

1 Susan Hayes May 28, 2008 at 12:15 am

Hi there,

I have been following your blog for awhile now and love the level of detail you provide on all that you are researching. This site has been an incredible resource for me and my husband as we make plans for our green, energy efficient home in northern Vermont. I just wanted to let you know that Radiantec has a fantastic reputation in our neck of the woods for providing fantastic products and customer service. If your budget allows, I would wholeheartedly recommend that you work with them. I have honestly heard nothing negative about their enterprise…somewhat rare in the construction business! Best of luck with your project and thanks again for this wonderfully helpful blog. My sister lives in Philly and is anxiously awaiting the day when she can visit the finished 100k house!


2 chad May 28, 2008 at 10:47 am

Thanks for the feedback Susan. Looks like you’ve got a great project up there in Vermont. I love the inspirational images of modern barn houses you posted. I always drool over the examples that Justin digs up at materialicious also. Added your blog to our blogroll. Keep up the good work.

P.S. – Nice choice in ceiling fans!

3 bolortuya December 3, 2008 at 5:36 am

i’m mongolian. i’m student. i study for renewable

4 tom toolbag January 5, 2009 at 10:45 pm

I have a few observations, ideas and suggestions. Suppose you have a houe with 2 bedrooms. The water heater will be sized accordingly, for 2-4 people. Now let’s call the 2nd b/r an office, does the formula change? One step further is that it is a hobby or storage room that is used sporadically, then what? My point is that I always ask how, and what and when. How was the effiency measured, and what were the conditions when it was measured? If a system is designed to be 95% efficient when running full-on, and the house is super-insulated, what would it’s efficiency be when running at 1/2 throttle so to speak and an older poorly insulated house be? I like solar thermal because it’s relatively simple and cheap over the life of a home. I’d start with 1 or 2 panels and see how it works and increase it from there. There is no such thing as underkill.

The cooling ideahas a little merit to it though, why the floor? Why not in the ceiling so the cool temp could drop into the comfort zone, because if it’s in the floor it can’t drop any lower. I think this would also give you cold feet if the floor slab was too cold, not to mention condensation issues. Why not just run tubing through the slab down into the ground under the basement slab and create a loop with a circulation pump? One big thing that affects comfort is humidity. Low humidity solves a lot of problems and creates a more confortable environment. The price of pex tubing and the possible gain doesn’t justify it in my opinion. You would be better off to use a small portable a/c-dehumidifier unit that you could move from room to room.
I hope that I don’t sound too negative, but reasoning goes a long way. I like the fact that at least they are trying to find some new alternatives.

5 Lynda April 12, 2010 at 3:37 pm

We went to a “green convention” yesterday and came out a little confused. We live in New Mexico and get sun most of the year. We have a 3700 square foot house and our hot water and heating/radiant main level and forced hot air upper level runs on propane. It is very expensive to say the least. The confusion I have is the 1st rep we talked to only sells PV systems and said it would be very expensive to get PV and a thermal system unless we converted our hot water heater and radiant heat furnace to electric and then get a PV system. Second rep said she that would be too expensive and we should just consider a Thermal solar system. The third rep introduced us to a system he installs for both the hotwater heater and radiant heat and it would use three thermal solar panels plus a tank (either a Shuco system or Stiebel system SBB 400 plus or SBB 600 plus). He said we would just have the one tank which would replace the existing hot water heater. Would this be enough to heat our radiant heat in the winter and still have hot water to take showers, plus dishwasher and laundry? We have 5 people in the house. The fourth rep said we would need at least 6 or 7 solar thermal panels the hot water tank plus a separate tank for the radiant heat (double the cost of rep 3). I really want to convert to something that is reliable and not get the wrong system. Any suggestions?

6 Tom Slaiter February 6, 2013 at 11:24 am

Great diagrams, so informative nd simple!

I love solar energy, and using it to warm your water is fantastic!

Great post, thanks

7 Santo Lezak February 13, 2013 at 8:07 pm

Certainly, but we all need to appreciate that adding Solar in their home is an asset that could increase the long term valuation of their property if / when they decide to sell. With the environment the way it is going we are unable to underestimate any system that gives zero cost power at no cost to both the client and more importantly the earth!

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