Posted by Steve Shantz on November 16, 2008, 2:44 am
I work for a small college in Indiana, and we have been considering a
Solar DHW system to heat water for the showers in our Rec-Fitness
center. The State of Indiana has a $5,000 matching grant program for
this type of project, but it may not be used to heat pools. To the
best of my knowledge, they have not yet given out a grant yet because
nobody has built a system yet.
The facility is used year round. We have metered our DHW consumption,
and typically use about 1200 gallons per day (gpd), with a maximum of
about 1500 gpd. The maximum sustained load is about 6 gpm for a
maximum time of about 20 minutes.
About a year ago we had 3 'contractors' spec out their systems, and
they were basically scaled up versions of residential systems, and
outrageously priced. They tried to store 500 - 1000 gallons of hot
pressurized water, and we found out that commercial tanks of that size
cost about $0 / gallon. This was one of the things that killed the
initial 3 designs.
So I stewed on this project for a long time and came up with a design
that I think would work really well. I would appreciate a critique
My design is a drain back system. Arrays of collectors (24 to 30 4' x
8' or 4' x 10') would be mounted on the roof of the pool. The roof
happens to be pitched to the west just slightly, which would
facilitate complete drainage of the collectors in shutdown mode.
Piping runs would need to be pitched appropriately for complete
drainage. The roof is standing seam construction. Any comments on
how to fasten the arrays down so they don't become expensive kites
would be appreciated. There are heavy steel beams supporting the
I plan on building a cistern in the ground right beside the building,
well insulated, and containing somewhere between 3000 and 5000 gallons
of water. The cistern would be 3 - 4 times deeper than the diameter to
help with stratification. We would pump from the bottom of the
cistern, through the collectors, and return the heated water to the
top of the cistern with some type of baffle to prevent mixing and to
preserve stratification in the cistern. This heating loop would have
either an air gap where the water returns to the cistern or suction
relief valves at the top of the collectors so that water can drain
completely when the pump stops. No shutoff valves are allowed in the
collector piping loop. Obviously the drain back system must work
robustly with no failure modes due to accidentally closed valves.
Cold water entering the existing heating system would first go through
a 120 gallon insulated storage tank before going into the existing
heating loop. Water from the bottom of the 120 gallon storage tank
would be pumped by a very small pump though a brazed plate HX,
returning to the top of the tank. Hot water from the top of the
cistern would be pumped through the other side of the HX, returning to
the bottom of the cistern. The 120 gallon tank becomes an inexpensive
buffer tank to meet the peak flow requirements. The pumps on the HX
could be switched on and off as required to meet demand requirements.
The system would be monitored and controlled by our existing energy
management system, using a VFD on the collector loop to minimize
energy consumption and maximize thermal energy collection. Our energy
management system has an outdoor light meter which can be used to
determine when the sun is shining so that pump start up and shutdown
is automatic. Or, would it be better to put a temperature probe at
the top of a collector and start the system when it gets warm?
The overall design is very simple, and rather inexpensive to build.
Critical review of the design would be appreciated.
Thank you for your comments,
Posted by Malcolm \"Mal\" Reynolds on November 16, 2008, 7:45 am
You haven't mentioned how much it currently costs to supply the hot water.
Does your college have a central plant providing hot water to any
buildings on campus?
With 6gpm as your highest load, I'd be inclined to suggest one large or
two medium tankless water heaters.
Posted by Robert Scott on November 16, 2008, 3:31 pm
You have obviously put a lot of thought into this design, and it goods basically
sound to me. Using a HX and a separate storage/collection loop from the DHW is
good for more reasons than just avoiding the cost of a large pressurized storage
vessel. It is also easier on the collectors because you can reuse the same
water over and over and avoid infusion of new dissolved minerals and corrosive
agents that attack collector metals. But check with your local codes. You may
need a double HX to keep the collector water from entering the DHW in case of a
single-point HX leak. I know it would mandatory if the collector water were
poisonous antifreeze. You cannot rely on the fact that the DHW is pressurized
and the collector fluid is not. There might be a pressure failure. But I'm
not sure if it is required if the collector fluid is just plain water that gets
reused. I assume that the code requirements for shower water is the same as for
potable drinking water. Maybe your brazed plate HX is intrinsically safe
Is it possible to integrate the HX into the cistern? That would save the
expense of the circulating pumps and the 120 gallon tank. The incoming cold
water would go through the cistern (from bottom to top) as a pre-heater to your
As for collector pump controls, temperature is a much better indicator than
light level in deciding when to turn them on.
Posted by Steve Shantz on November 17, 2008, 12:57 am
Part of the problem is that there is about 50' of run between the
closest place the cistern can be located and the inlet to the existing
DHW system. In low flow rates we may loose a substantial amount of
heat before it gets to the existing system. However, insulation is
cheap compared to the cost of the 120 gallon tank, pumps and
controls. Definitely worth thinking about. Even if we don't get
complete heating at the highest flow rate... so what. My objective
isn't to always heat all of the water, but rather to heat as much as
possible at the lowest cost.
However, I have submitted a proposal to do something like this, but
everybody else I'm working with thinks I'm crazy. What I have in mind
is a 2' x 2' air to water HX, like those used to heat furnace air when
using an outdoor wood fired water heater. I would submerge the HX
just below the top of the cistern, laying flat (horizontally). As
cold water moves through the internal tubing, it would be heated, with
the cistern water falling through the aluminum fined tubing as it is
cooled, and naturally falling to the bottom of the cistern. This cold
water convection could even be aided somewhat by building a small
chimney for it to fall down in. Several units could be stacked one on
top of the other for even better performance.
I have bought one HX, and I'm hoping to test it out this winter using
our central boiler blowdown tank as the source of heat. I'm not sure
how long a HX such as this would hold up to these operating
conditions. This configuration would be intrinsically save (I think),
as any leak in the HX would result in the water leaking into the
cistern, which can NEVER be pressurized.
Additionally, given that the cistern is in the ground, and the 120
gallon preheating storage tank is on the second floor, I've wondered
if this DHW preheat circulation loop could run without a pump, being
powered by the different densities of the cold water going down to the
cistern vs the hot water floating back up. The pressure gradient is
small, especially in marginal heating situations, so I'm not sure I
would get enough flow. I'll have to see if I can run the number on
flow rates vs pipe size, number of fittings, height, and temperature
Thanks for the feedback!
Posted by daestrom on November 16, 2008, 7:11 pm
Steve Shantz wrote:
Steve, you've obviously been thinking about this for a while now. But have
you given any thought to what happens to that hot water when it runs down
the drain? I don't know the specifics of your set up, but if you have
access to the drain water, and it has a *vertical* drop somewhere underneath
the showers, you might look into GFX technology.
It provides a high heat recovery rate but requires a *vertical*
installation. It is ideal for situations where the water runs in/out
continuously such as a shower.
I installed one in my home and have measured it's performance several times
and found it to recover quite a bit of heat.
Recovering some of this heat before it just goes 'down the drain' can save
money and reduce the size of DHW heating plant.
P.S. I have no affiliation with them whatsoever, just a satisfied user.