|This section covers installing the large PEX
coil heat exchanger that is used to preheat water using heat from the
solar heat storage tank.
The cold water on the way to your regular hot water heater takes a
single pass through this very large coil of PEX to pick up heat from the
solar storage tank.
|The heat exchanger for this solar water
heater is a 300 ft coil of 1 inch diameter PEX pipe. This coil of pipe
is immersed in the storage tank. Cold water bound for the regular hot water tank
first passes through the PEX pipe coil where it picks up the stored
solar heat in the storage tank. If the water needs further heating, the
regular hot water tank tops it off.
The PEX pipe coil itself holds about 10 gallons of water. So, for each new hot water demand, there will be 10 gallons of water already in the pipe coil that has been heated up to the full temperature of the storage tank. So, for the first 10 gallons of demand, the heat exchanger is essentially 100% efficient.
If more than 10 gallons are needed, then the 300 ft pipe coil acts as
a conventional heat heat exchanger and the outlet temperature will drop
somewhat below the storage tank temperature depending on the flow rate.
I've tested the
PEX coil as a heat exchanger and it does quite well...
In some cases, people who have built the $1K solar water heating
system have used copper pipe heat exchangers. These heat
exchangers take up less room, so if space is limited in the tank they
can be a good choice. Look through the
$1K systems for examples...
Building the Heat Exchanger Support
The heat exchanger should
be supported up off the bottom of the tank to get it into the hotter
part of the tank when the tank is stratified.
To prevent any abrasion between the blocks and the pipe coil or
tank bottom, I siliconed scraps of EPDM on the top and bottom of each
An alternative to using the blocks to support the heat exchanger coil would be to hang the coil using lengths of polypropylene rope. The rope loops that support the heat exchanger could be tied to anchor eyes that are screwed into the plastic edge boards. I would use three of these eyes. The eyes should be stainless steel, as galvanized metal does not hold up well to long exposure to hot water. Normally these ropes will not be under any load, as the coil tends to float up near the top of the tank by itself, but you should have something to support the coil when the tank is drained.
The three heat exchanger support blocks finished.
Installing the PEX Pipe Coil
I kept the 300 ft PEX coil in its original coil shape. I tied it several places with polypropylene twine, and then cut the bands that held it tightly in its coiled shape. This allowed it to expand a bit, and made some space between the coils for water to circulate and transfer heat.
I installed a few CPVC pipe spacers between coils to make more space for the tank water to circulate around the coils. Unfortunately, these little pipes tend to fall out over time.
After thinking about this, I think that the best way would have been to make some T shaped pieces from CPVC such that one leg of the T could be pushed between the coils from the top -- the other two legs of the T would rest on the top of the pipe coil and keep it from falling out. This would make for vertical flow channels through the PEX coil, which is probably better for heat exchange than the horizontal channels.
Some people who have built $1K solar water heating systems have recoiled the PEX to make a more effective heat exchanger -- for example Ken's tank has a very nicely done example... I'm sure that this does make for improved heat transfer, but bear in mind that recoiling PEX is a real pain.
Note that the plumbing connections to the PEX coil are made outside the tank -- there are no heat exchanger connections immersed in the heat storage water.
The 300 ft of PEX is probably overkill for the heat exchanger for normal residential use. You could probably use 200 ft and get good results. I used the full 300 ft because that's the way is came, and I had no other use for the extra 100 ft.
The PEX coil installed in the tank.
The white pipe supports a sensor for logging
Connecting the PEX Coil Heat Exchanger
First, decide on where all of the plumbing connections for the heat exchanger and for the collector pump and floor loop pump are going to be made. The heat exchanger connections are best made on the side of the tank closest to the cold water pipe coming into the hot water tank. Peek ahead to the sections on the collector and floor loop to get an idea where you want these connections -- if there are conflicts, work out the best compromise.
Cut slots in the tank edge board for the two ends of the PEX coil to exit the tank. This can be most easily done with a small hand saw taking care not to damage the EPDM.
Before you cut a new slot, you may want to add additional screws to hold down the edge board so that its well anchored to the tank frame on both sides of the slot.
The PEX pipe must
be secured well where it goes through slots in the sill board. I used a
copper pipe clamp on the inside, and a plywood support on the outside to
keep it in position. Be sure to seal around the penetration and the pipe
clamp with silicone.
One of the connections to the PEX coil to the house
Note that there are no penetrations
of the tank liner, and there are no plumbing fittings inside the tank. That is,
the ends of the HDPE pipe are taken outside the tank where the connections to
the house hot water plumbing are made.
Since the fluid in the tank is plain old water, there should be no issue of needing a double wall heat exchanger, but this is something you might want to confirm with your local code folks.
Gary February 9, 2011