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Our Radiant Themal Mass
After our first winter, we found that our basement could hold firm at about 50°F without any heat source except sun and the geothermal heat from the earth, so the fears set into us by friends and family about freezing pipes in the basement went away. Despite this, we decided to add a heat store to the basement for 2 reasons.
1. to make the basement more comfortable in the winter than 50°F
2. to store wasted heat from the wood stove upstairs
3. to level out the temperature fluctuation throughout the entire house
A thermal mass is a large mass that can be heated up towards the end of slowly releasing the heat over time. An example of animals taking advantage of this phenomena in nature would be snakes laying across blacktop roads at night to soak in the warmth released from a day of solar exposure. Using a thermal mass in the home is not only a renowned way to comfortably warmm your living space, but it also is a way of steading thermal fluctuation throughout the day/week.
To create our themal mass, we decided to dig down about 1 foot into the basement in a rectangular area about 12 feet by 8 feet. We chose this dimension because we had excess 4'x8' insulation panels from the building of the house and three together gave this size. Any shape really would do - and as a matter of fact, a circlular shape would be the best shape to hold heat from escaping out the sides. But for convenience, we chose an 8'x12' rectangle.
Once we had dug the hole, we leveled the bottom as best we could and removed any rocks that would puncture the foam or worse, kept it from sitting flat. Air pockets in the floor is something to be viciously prevented as they could settle later and allow the floor to crack... Which probably wouldn't be that big a deal, but none-the-less, it's a noble goal to ty to avoid such cracking later. So with the hole dug, the ground flattened and the foam laid, we set foam along the edges with rocks to prevent heat loss out the sides and began laying the tubing.
The tubing was 3/4" plastic tubing that we'd purchased at Home Depot especially for this purpose. We chose 3/4" as opposed to 1/2" to make for less stressfull pumping through the larger tube. We lay the tube in the fashion shown in the below diagram. Teh tubing was unweildy though and had a mind of it's own, so we laid it in short runs, a little at a time. We'd hold it down in place with large rocks (padded with foam as to not puncture the tube) and then set cement around it. The cement served both the purpose of holding the tube in place AND later, when hot fluid is pumped throug it, the cement will help to quickly draw off the valuable heat out into the mass.
The junction where the hot fluid comes in and the cooled fluid leaves was a bit of a trick and we wanted to thermally separate the two areas. Otherwise, the heat would be shed off the hot side and transferred right back into the cooled fluid heading out.
In this instance, our thermal mass is heated with excess heat from our wood stove, but a mass like this could also be heated with solar-heated fluid - or through direct exposure to the sun (negating the need for the tubing!) The possibilities are vast. The basic idea is simply, find a way of moving diffuse heat that you don't need or can't use, into a massive store for slow release later. This concept can even be used for cooling - removing heat from a massive store over time, and it will suck heat up slowly later - this is the reason that our basement is so cool in the summer. The walls are sucking up heat - for later release in the winter!
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