http://www.mnstate.edu/publications/swimmingpool.htm

Minnesota State University News Feature

Karen Branden and Dennis Jacobson by thier thermal mass swimming pool.
Capturing heat in a recreational thermal mass…A SWIMMING POOL HEATS THEIR HOUSEWhen the oil crisis hit in the early 1970s, Dennis Jacobs had a choice. Either sink into an addiction to high-energy costs, or swim against the stream in search of alternatives.
He chose swim, and now he and his wife Karen Branden are enjoying incredibly low heating costs for their 4,000-square-foot home north of Detroit Lakes with the added luxury of swimming all winter in their 16-by-32-foot heated indoor pool.
That’s because the swimming pool heats their house.
It’s called pooling your resources and resourcing your pool.
“When I first installed this heat exchange/thermal mass system 13 years ago, our heating bill for the entire year, supplemented by off-peak electricity, was $180,” he said. “Energy prices have gone up since then, so our annual heating bill now is about $350.”
It’s an ingenious contraption, using a wood stove, a small pump, a series of insulated pipes, passive solar energy and a heat exchanger to raise the temperature in their 22,000-gallon indoor pool, which doubles as a thermal mass. The energy the pool absorbs is released throughout the day to heat the house—by convection, radiation and an occasional boost from a small electric furnace.
Humidity problem? “Not at all,” Jacobs said. “We cover the pool when it’s not in use. In fact, any extra humidity we do have adds to our comfort and health during the dry winter season.”
As the Swedish moderator told Jacobs last month after his presentation on “The Use of a Swimming Pool as a Thermal Mass in Residential Heating” at the International Conference on Fluid and Thermal Energy Conversion in Jakarta, Indonesia: “You just can’t beat the American lifestyle, can you?”
Total cost for the project, including the pool installation: about $13,000, an amount he figures they recouped within the first couple of years.
For Jacobs, a specialist in energy conservation and renewable resources who teaches at Minnesota State University Moorhead’s Corrick Center for General Education, and Branden, a professor in the university’s Sociology department, it’s been one huge experiment.
It was 35 years ago when Jacobs joined the back-to-the-earth movement in the wake of the Arab oil embargo, buying 80 acres of land where his current house sits. The former Peace Corps volunteer then built a geodesic dome there from recycled materials and attached a greenhouse to grow winter herbs and vegetables.
Jacobs, kind of a hybrid Henry David Thoreau/Rube Goldberg philosopher-tinkerer, lived an ascetic lifestyle in the dome until it burned to the ground 15 years ago while he was out of the country. (A friend was using it at the time.)
Then he met Branden, and they began planning to build a modern, super-insulated home on the same site.
“The idea of using a swimming pool as a thermal mass started when I was living in the geodesic dome,” Jacobs said. “In the fall I began planting vegetables in the greenhouse, but soon found that on a sunny day it would reach temperatures above 100 degrees and at night it would drop below freezing.”
So he began looking for a way to moderate the temperatures and decided to try putting a 55-gallon water-filled drum in the greenhouse, which he figured would absorb excess heat during the day and releases it at night.
“I soon noticed the more drums
of water I added, the cooler it stayed during the day and the warmer it stayed throughout the night.”
Bingo! “It was a lesson well learned and I decided to incorporate a very large thermal mass into the design of our new house,” he said.
It took Jacobs and Branden two years to build the home themselves, and 13 years ago they began their grand energy experiment.
The design is simple enough. It starts with a wood burning stove located in a small shed about 50 feet behind their house (for insurance reasons). “We use wood scraps from a local lumber mill, so it doesn’t cost anything to fire up,” he said.
The wood stove heats the water/glycol liquid that’s forced through underground copper pipes by a small 1/12th horsepower pump to a liquid-to-liquid heat exchanger inside the house.
“Before this heated water/glycol mix approaches the heat exchanger, we switch to neopryene valves and stainless steel pipes because in the next step the heated water pipes come in contact with the pool water,” he said. “Chlorination, we’ve found, is very corrosive on copper.”
The heat exchanger, a device used to transfer heat from one fluid to another, is the heart of the operation. It works like a car radiator, where water or antifreeze transfers heat to air flowing through the radiator.
Simply put, the hot water pipe runs through the center of the pvc pipe carrying the pool water. That’s where the exchange is made.
“We pump water from the pool into the heat exchanger through two-inch pvc pipes,” Jacobs said. “At this point, we run the heated water/glycol mix through ¾-inch stainless steel pipes we’ve installed inside the pvc pipes that carry the pool water. What happens is that the heated liquid in the stainless steel pipes gives up its heat to the pool water flowing around it. I’ve also added ribbons of stainless steel inside the pipes that carry the heated water/glycol mix, which agitates the liquid to increase the heat exchange.”
By just burning wood on the weekends, Jacobs said, and taking advantage of solar heat coming from our south-facing windows, they can heat the house and keep the pool at a comfortable 70 to 80 degrees throughout the week.
It’s still a work in progress.
“We have a super-insulted house where the walls are rated about a 45 R-value and the ceiling about a 60 R-value,” he said. “But the windows are a problem, even though they’re all low-E, argon-filled and double-paned, they only rate a 3 R-value. So I’m thinking of installing some kind of moveable, insulated shutter system we can use to cut down on heat loss when the sun goes down.”
He’s also intends to tinker more with the heat exchanger and the piping system to make them more efficient. Down the line there’s even a wind turbine and an electric car in the mix.
“In the meantime, we just enjoy the house and love being in the country,” Branden said.And not surprisingly, the pool has become a very popular winter escape.