Monday, August 22, 2011

Swimming Pool Heater

Yesterday, I was at a friend's house, and we talked about a swimming pool heater. His 24FT pool is cold, even in July. We talked about the idea of laying a 300FT coil of black irriguation pipe on the deck around the pool to heat it.
Doing some research, I realize this is not the best way to do it. One important requirement is that the pool heater must sustain freezing temperatures without damage. The simplest way to do that is to use a drainback system. A coil of pipe is not going to properly drain.
With that requirement, the simplest pool heater I found is based on the Thomason trickle collector, described [here].

The size of the solar collector should be at least half of the surface of the swimming pool, so for a 25FT circular pool, that is 75/2 ~ 40sqft.
The tilt angle should be optimized for the swimming season, so May through September. Ideal angle for September 1rst at noon is 40°, so the tilt should be at least that. We will use a 50° tilt angle.
The tilt angle, optimized for September, will reduce the effective size of the collector in June-August period, so the size of the collector should be increased to compensate for the increased tilt. Also, half of the surface of the swimming pool rule assumes June-August swimming season, September being cooler, it makes sense to increase collector size.

If we use standard Home Depot 6FT x 2FT corrugated panels, 5 panels will provide 60sqft of collector area, for a 6FT x 10FT dimension, which seems reasonable.
Since the pool is 24FT in size, depending on budget, we could consider a 6FT x 24FT collector, and increase the tilt, to further extend the swimming season.
A 6FT x 24FT collector would require 12 sheets of corrugated metal roofing. The frame supporting the roofing will be made of 2" x 2" x 8FT cedar studs. To make the building process easier, the collector can be build in sections of 6FT x 8FT. That way, we can decide to build one section now, and the rest later. One section will provide 6FT x 2FT x 4 = 48sqft.

6FT x 2FT corrugated roofing, 4 sheets at $12 per sheet = $48.
2 x 2 x 8 cedar studs, 7 studs for the frame, plus 4 studs to set the tilt, 11 x $3 = $36.
Black Rust-Oleum oil-based paint, 1 quart. $15
Rust-Oleum primer for Aluminum. $20
Galvanized outdoor screws.
Vinyl tubing.
Pipe brackets.
1/2" PVC pipe.
1/2" PVC end cap.
1/2" PVC Tee.
PVC cement.
[Pump]. $20

So far I am at $140 while missing some prices, so the total cost should come at less that $200.

Thursday, August 11, 2011

Heat Pump bid

We received a first bid on a air source heat pump: $5500, installation included, 5 years labor guarantee, lifetime compressor guarantee. It is a Reem heat pump, but I am still waiting for a part numbers.
The air source heat pump limits how much we can use solar energy in our heating system, but I have being thinking on a some options.
The heat pump will not heat the water for hot water usage, so we will install a solar hot water system with electric backup. The storage tank will be made as big as can fit in the hot water closet. The South wall of the garage will be covered with solar collectors. We can install three 3'*6.5' panels, for about 60sqft total. If we buy ready made panels, that will cost $1130 plus shipping. If we buy the collector plates only, it will cost $600. It makes sense to build them ourselves, since they will be integrated in the wall of the garage. They will be protected from the weather by the roof overhang.
During the months when we do not use all the hot water, the excess hot water will be circulated through a heat exchanger installed in the air intake grids of the forced air system, giving some boost to the heat pump. I will have to design a system that will prevent drawing too much heat from the hot water tank for space heating, which would displace energy normally used by the heat pump, to the electric water heater, producing efficiency loss. Since the heat exchangers will only work at high temperature, it makes sense to use the excess heat from the solar hot water system to boost the heat pump. This system should provide a good efficiency during the Spring and Falls, but I don't expect the months of December and January to see much of an improvement from either the solar hot water and the heat pump.
I will detail the design in the coming weeks.