... Design Elements for Solar Water Heating (SWH) systems ...
This design tutorial concentrates on explaining how to design a solar
heating system for domestic potable water. Solar water heating for
space heating, and swimming pool water heating are only briefly
Solar Heating of Potable Water
Solar Water Heating can be thought of as a pre-heating system which
reduces the burden on your other (now merely a backup) water heater.
Often the sun should give you plenty of heat, and your backup system
may add no more heat to it, and indeed your system might even have to
dilute very-hot solar-heated water with cold water to keep it in the
right temperature range for showers and lavatories. But sometimes
there may not be enough sun to heat the water completely, and then the
backup can help finish the heating job. Normally the backup heater can
be much smaller and use much less energy than it would without the
solar system in place. You should be able to save a fat percentage of
your water heating bill through installing, operating, and maintaining
a properly designed solar water heating system.
Here are the design elements for such a system.
Placement, Angle, Orientation:
Place and orient the collectors to maximize collected heat value.
it to maximum perpendicular to the sun
at the time the sun produces the most valuable radiation.
should be where maximum sun is available.
An instrument including a hemi-spherical mirror and
solar-angle markings can be used to see how trees occlude
the sky from a given place; this lets you find a place where
daytime sunlight can be maximized.
- Direction or Azimuth:
Since the sun moves East to West but is highest in the sky
in the South (if you are located in the Northern hemisphere), the
collector should generally aim South. But anywhere roughly south
is approximately equivalent. Some say within 45 degrees of
South, orthers say within 15 degrees of South.
Elevation: Collector angle from the horizontal should be
close to the local latitude, which is close to the average
elevation of the noonday sun at that location. So:
- At the equator, collectors should be roughly horizontal.
- At the north pole, roughly vertical.
- In Seattle at 48 degrees latitude, the collectors
should be sloped at roughly 48 degrees from the horizontal.
where winter heating is more precious than summer heating,
the collectors should orient toward the winter sun, which is
23.5 degrees less than the latitude, and 47 degrees
lower in the sky than the summer sun.
(Earth-Sun geometry is explained here. ) Then there will be some
loss of efficiency in the summer (when there is probably too
much heat being captured) but maximum efficiency in the
winter (when the smaller amount of heat that does get captured
is more precious and useful).
- So at 48 degrees latitude in Seattle, the collectors
should be sloped at roughly 48+23.5=71.5 degrees from the
horizontal, or 18.5 degrees from the vertical plane.
But in your particular application,
if the solar collectors are undersized even in the summer,
and assuming that every BTU of backup heat
costs the same whether summer or
winter, then the slope should be equal to the latitude,
which will optimize the heat capture over more of
Evacuated tubes heat pipes, or flat plate collectors?
Flat plate collectors lose a lot more heat because they are not
vacuum-tube insulated. They can be cheaper to buy and install.
They are somewhat less efficient, but also somewhat tougher.
Hail will not break them as easily; snow will melt away from them more
quickly (because there is more heat loss!).
Evacuated tube collectors
are highly insulated by the vacuum layer.
If there would be significant heat loss from a flat-plate collector
because the collector temperature is far above ambient temperature,
then use evacuated tube collectors.
So in a very cold environment,
or where maximizing efficiency is key, use evacuated tube collectors.
In a milder environment, where wringing every bit of efficiency
out of the system not as important, use flat plate collectors.
Closed loop or open loop?
Closed loop systems run a heat transfer liquid
through the solar collectors to pick up heat, and through a heat
exchanger (a spiral of piping inside a water tank) to give the
heat off to the water.
- Freeze protection by using antifreeze (glycol) as the
heat transfer liquid. Usually the glycol used is food
grade propylene glycol rather than ethylene glycol, so
that a leaking heat exchanger won't poison someone who
drinks the heated water.
- Glycol systems must have their pH level checked
semi-annually or annually, since high temps can break
down the glycol and make the liquid acidic, which will
gradually damage the system.
- Heat exchangers may be required to have double walled piping,
with an air space between the walls which is vented to
the open atmosphere. This reduces efficiency considerably.
depends on local plumbing codes that regulate what piping and
materials are allowed to contact potable water.
- Piping need not be carefully graded since no draindown is
needed. But piping should still be drainable, avoiding
air and liquid traps.
- Automatic air vent at high points of the system, to remove
air, protected with a manual shutoff valve.
- Air separator before the pump, to capture bubbles in the stream
and remove them.
- An expansion tank is needed for a closed loop system, so
heated and expanding liquid does not burst something.
- Fill/drain valves are needed, preferably one of each
with a normally open ball valve between them. To fill
or flush the system, shut that ball valve, put a hose
from a new bucket of glycol to the (pump inlet side)
fill valve, and let the pump pull the new fluid around
through the system, pushing air or old glycol out the
drain valve. When the drain valve runs clean, stop the
pump, close the fill/drain valves, and re-open the
normally-open ball valve: the system is now charged
with fresh fluid.
- Makeup fill system for glycol, in case of loss of volume?
Open loop systems use the potable water supply itself
as the heat transfer liquid. No heat exchanger is needed, which
eliminates one source of inefficiency.
- Freeze protection by automatic draindown
- Exposed piping must be graded (1/4 inch per foot),
so that draindown is complete. Any licensed journeyman
plumber is perfectly capable of
building properly graded piping systems.
- (A demonstration system with piping, pump, tank, and
collector at within, say, 6 feet of elevation, cannot
be open loop, since the in an open loop system the
draindown air tank must be above the storage tank and
below the collectors, which would make the demonstration
system very tall.)
- Size the piping for >2fps liquid flow while pumping.
- Any air vent at the high point(s) of the system
must be protected with a manual shutoff valve, normally
closed, so that the system does not become waterlogged.
- Fill/drain valves are not needed since the water supply
itself is used to fill the solar subsystem, and since
the tank will have a drain which should be at the
bottom of the system.
- An unpressurized draindown system can use a vacuum
breaker to let air in to drain the liquid out. But an
open loop system is pressurized, so a drainback air
tank is needed (which can be the top of the storage
tank, but ideally high in the insulated part of the
building to lower the pump head and drain volume
required). The air tank should be sized to cover
expansion tank needs also, and during operation or
maintenance it should be pumped up to the pressure of
the city water supply. This tank should have an
air-pump fitting, and a sight glass to show the level
(to detect waterlogged or too-much-air conditions.
Periodic maintenance, or some type of air-admittance
valve, may be needed to adjust the air level in the
Run a separate pipe line from the top of the drainback
air tank to the high point of the system, protected
with a check valve, so that when the pump is not
pushing liquid against the check valve, the air can
come up through it and fill the system from its high
point(s), draining the liquid down into the heated part
of the building.
- Add a filter between the tank and the pump so that
scale or lime etc. doesn't get stuck up inside the
One tank or two? A separate storage tank or single, combined storage & backup-heater tank.
- A separate storage tank lets the solar system perform optimally,
without interference from the backup heater.
- A combined storage/backup water heater tank uses the water
heater as solar heat storage. If the backup heater needs to
run at all, it may be stealing opportunities from the solar
heating subsystem, heating water that could have waited for
the solar system to heat it. This reduces the solar energy
yield. Also solar efficiency is higher when the input water
is coldest, so if the backup heater has partly heated the
solar fluid, it has reduced the solar collectors' efficiency.
- I prefer a separate storage tank plus a tankless water heater
as the backup heater. The solar subsystem is optimized, and
there is no standby loss from the nonexistent water heater
tank. Often the tankless water heater won't have to turn on,
and when it does, it won't have to work nearly as hard to
heat solar preheated water as it must with the unheated, cold
water supply. Especially with the occasional heavier loads,
like two people showering at the same time, the tankless
heater's capacity is much greater when the water is solar
pre-heated. Therefore tankless and solar go well together.
A rule of thumb for domestic hot water: About 40
square feet of collector area for the first two
residents; 20sf more for each additional two
residents. A resident is an adult or a child over
Storage tank size
At least 1 gallon per square foot of collector area.
For sunny areas, up to 2.5 gal/sf.
Make a generous allowance for storage, e.g., for
a 60sf collector, use an 80 gallon storage tank.
Depends on the collector used. Some use 1/2 inch lines,
some use 3/4 inch lines, some use 1 inch lines,
and of course some are metric.
Drainback air tank sizing
Calculate the volume of the piping in the freeze-exposed space.
Then size the tank at least that size. Also apply the minimum
of the expansion tank requirement for the system (approx 1 gal
per 60 gal storage). You may need to double this if the
air tank is intended to be at most half full of air.
Circulator pump sizing
Aiming for a flow rate of 2 feet per second in 3/4 inch
tube calculates out to about 1/3 gallons per minute or
or 20 gph, which is quite small. 12V circ pumps are often used.
Gravity circulation exists but is a weak force, used in
tank-over-collector systems suitable for mountain cabins;
it is not further discussed here.
- Additional design elements.
- Check valve in the solar loop protects against heat
dumping at night, with the pump off. A floating ball check valve
can be installed in the downward return piping, which allows
downward flow past the floating ball, but stops upward
flow when the floating ball plugs up the top.
- Flexible Tubing.
Flexible tubing is incompatible with downhill grading of piping,
- Plastic tubing, such as PEX: Use no PEX tubing in the solar loop;
since stagnation temperatures can exceed 350 degrees F.
- CSST: Caleffi offers systems using corrugated stainless steel tubing, slightly over-sized
due to the corrugations which cause turbulent flow and thus pump
- required for drainback or
- recommended by the Florida
State publication, irrespective of freeze-protection
method (i.e., also for closed-loop glycol systems)
- required for maintenance on any system; otherwise the
necessary maintenance task of simply draining the system is
- What is the stagnation temperature limit? If over 400 degrees F,
soldered joints can melt, and brazed or mechanical joints may
be required. If relief valves fail, systems under pressure
could imaginably approach these temperature limits and melt apart.
- Temperature and Pressure relief valve on the storage tank,
just like a water heater tank.
- Pressure relief valve on the solar subsystem, piped to a drain.
just like a water heater tank.
- Mixing valve to reduce high-temperature solar heated water
down to the limited temperature of domestic hot water.
If the solar heated water is above the scalding limit, it should be
mixed with cold water down to below 140 degrees F (commonly to 120)
before being sent to the fixtures.
- Radiator for stagnation relief. When the system overheats,
keep the pump on and route the solar liquid to a radiator to dump
the heat out. This requires the right controller, sensors, and
a motorized diverting valve, in addition to the radiator and piping.
- Circulator pump sizing, 2fps in 3/4 inch tube is about 1/3 gpm
or 20 gph, which is quite small. 12V circ pumps can be used with
a PV panel, for off-grid power and to turn on/off with the sun's
intensity. A 110V to 12V transformer may be used instead of the
- Controller, sensors, programming, BTU metering.
The system should have sensors and program logic to handle
the following test conditions.
- Normal operating mode. All hot water demand satisfied
by the collectors. Backup heated not needed. Balanced
load & supply over a day. Temporary imbalances of
load > supply, and supply > load.
- Sub-normal operating mode. Backup heater needed.
- Normal non-operating mode. Collectors cool/cold. Pump off.
Draining, if a drainback system.
- Stagnation or overheat mode.
- Setup/charge/drain/disassemble modes.
- Insulate the piping to/from the collector; also the pump, etc.
Preferred System Summary:
This summary references part numbers in Drawing #TCV2, Dated
1/9/2009, entitled Solar Hot Water System, with a design
characterized as open loop, tankless, drainback,
responsive, and heat-dumping.
- Open Loop. Parts include:
- Solar Storage Tank (SST) (TCV2.B)
(No double-wall heat exchanger piping inside)
- collector assembly (TCV2.B) Mounted on roof with
weight-spreading, solid mechanical linkage to underlying
rafter/joist structure, roof penetration sealed against
water intrusion, elevation angle and orientation
fixed by suitable hardware.
- Air vent protected by manual valve at top of system
- Pressure relief valve at top of system (TCV2.G)
- circ pump (TCV2.H)
- pressure gages (TCV2.K)
- T and P relief valve on tank (TCV2.M)
- Drain valve on tank (TCV2.O)
- Controller (TCV2.P)
- Insulation on piping, valves, tanks, pump.
- Drainback Freeze Protection. Parts include:
- Drainback air tank (DAT), sight glass,
air-pump fill valve (TCV2.D)
- Drainback check valve (CV) (TCV2.E)
- Drainback piping from tank to DAT to CV to
top of system
- Overheat protection. Parts include:
- Radiator/heat-dump (TCV2.I)
- Controller-driven, multi-way motorized valve (TCV2.J)
- Piping from valve to radiator to storage tank.
- No-storage backup heater. Parts include:
- Tankless auxiliary heater with included tempering/
mixing valve (AUX) (TCV2.C)
- Isolation valves for aux heat to work w/o
solar preheat (TCV2.L)
- Piping from DCW and SST-output (preheated) to AUX
- Responsive storage so that the second shower is also hot and
solar heater heats the coldest water rather than mixed water
- SST option 1: horizontal baffle separating
upper half from bottom half of Solar Storage Tank
- SST option 2: three side inlet/outlets.
- SST option 3: dip tube for cold inlet to enter from
top inlet to bottom part of SST.
- Multi-way motorized valve option: add one output
path alternative to the middle of SST.
- Piping from motorized valve to middle of SST.
Solar water heating for space heating is not much
considered here because they need to be so much bigger.
Space heating systems require perhaps triple the collector size of a
DHW-only system, plus or minus a factor of three. Closed-loop, glycol
systems with efficient, single-wall heat exchangers are
code-compatible if space heating is the only application. Note,
however, the increased maintenance requirements on glycol systems
Swimming Pool Water Heating
Swimming pool heating is also barely mentioned here because pools are
relatively few in Western Washington, my location. However the
application is important and in fact ideal. In solar heating of
swimming pool water, the key fact is that the ambient temperature of
the air during the swimming months and the temperature of the pool
water are so close to each other mechanical heat losses are relatively
negligible, therefore system efficiency is very high and also
insulation is unnecessary. Every pool should use a solar pool heater.
Indeed in some jurisdictions this may be required by law.
Copyright © 2009,
Thomas C. Veatch. All rights reserved.
Modified: April 8, 2009