May 24, 2018
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Solar heat lesson from 25 years ago

By Tom Gocze, BDN Staff

In 1985 I did a solar heating system for a friend. We wanted to try storing heat in the floor of the building. The house was built on a slab. The house had two-by-six walls with R-19 fiberglass in the walls and 2 inches of plastic foam on the outside of the walls. It wasn’t superinsulated, but it was pretty decent, even by today’s standards.

We wanted to try storing heat under the slab. When using radiant heat, you get to store some heat in the slab that the building is built on. The mass of the slab has some significant heat storage capacity. This works well with solar and wood boiler systems. A 6-inch slab that covers 1,000 square feet can store about 75,000 Btu of heat for every degree Fahrenheit it is raised. A couple of degrees rise in a slab will store the equivalent of a couple of gallons of oil energy. We have to be a little careful not to overheat a slab since it could make the house too warm.

As long as the slab is insulated, this can work very well.

We wanted to store more heat than a slab might hold. I came up with the bright idea of putting 2 feet of sand underneath the slab and installing the tubing there.

The concept was to store more heat in the massive amount of capacity that 2 feet of sand might afford us. To keep the cost down, I thought that it would be enough to install the tubing just in the sand, not the slab.

The system got installed and we hooked it up to some solar collectors and a gas boiler for backup heat. Since the house was not too big, the boiler and a small solar tank were in an insulated garage. We ran the plumbing for the radiant system near an outside wall since this was the only convenient place to locate it.

The system was fired up and the first problem that surfaced was that the heating system took a very, very, very long time to heat up. The people who lived there did not like that. To remedy the problem, I installed a convector to move heat into the room before it went into the floor. That brought the room up to temperature quickly, and then heat was shunted into the floor storage system.

The system worked fairly well for a couple of years. The house was sold. My name was passed along as the person to maintain this rather unique system. The next owner had some issues with the pipes that went to the floor occasionally freezing where they passed near the outside wall. This always happened around Christmas. After some sleuthing, I realized that the pipes were actually freezing about a week before the new owners realized it. Since the plumbing was plastic, it never burst.

There was no indication of a problem until the house temperatures dropped below 65 degrees. And that took a week of cold weather at Christmastime.

To this day, I cannot figure out why it always happened then, but there were a couple of years where I got to wish the owners Christmas greetings, when probably neither of us wanted to do so.

The pipes got insulated better, and that problem went away.

In hindsight, the concept of installing radiant heating in sand underneath a slab was interesting, but it probably was not worth the extra expense.

There is a simpler way to try to store solar heat in a home, though. Building a house that is massive in its construction details above the slab makes more sense. Building mass can store heat that is gained through south-facing windows as well as heat sources such as wood stoves and solar heating systems.

If the mass is distributed around the house, the heat also will usually be distributed around the house. A local architect, J.B. Thomas IV, did this years ago in some award-winning designs by installing two layers of drywall in his buildings.

Sometimes simple things can work really well. And they usually cost less.

I was lucky enough to try a system that was able to demonstrate a different concept and now know that I would not use it again in this type of system.

And I have not spent Christmas with anyone other than my family for quite some time now and have not been thawing pipes. This makes for a very merry Christmas.

Questions for Tom Gocze should be mailed to The Home Page, Bangor Daily News, P.O. Box 1329, Bangor 04402-1329. A library of reference material and a home-project blog are at

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