applications place special demands on a water heater.
However, there are a number of steps you can take to avoid
Don't undersize. Unless you live in the Barbados, don't even
consider a tank of less than 75 gallons. More solar problems result
from overheating and system stress than anything else. A larger tank
will give you more storage and provide a buffer for thermal stress
as it is less likely to go through extreme temperature swings. You
will also store more energy; remember, you store BTU's NOT degrees.
Also, don't get sucked into over-sizing the collector area/system
for space heating. The summer thermal stress will be a constant
service issue even with and including any heat dump. Your savings
are in domestic hot water, it is used 365 days a year.
Set it up right: The Marathon's thermostat is designed with
an upper limit of about 175F. The BOTTOM thermostat is adjustable
to this temperature, which means the entire tank can be heated to
this level. The TOP thermostat has an ECO setting of 160F for
safety reasons. During vacations and peak solar times, the tank
could easily reach this temperature. You may want to consider a high
temperature top thermostat which goes to the 190F. Just order
Special Use/Dairy Thermostat Part number SP313070.
If you are using a single upper element as back-up, upgrade to
the titanium if there are any issues. The top element is fused
to protect the tank and designed for quick heat transfer, but the
bottom element is the workhorse. Because of this, the top is copper
and the bottom is titanium. As a replacement part you can
install a titanium upper element.
Replace the boiler drain with a full port ball valve if there is
high sediment. The ¾" boiler drain on the heater actually gets
reduced to about 3⁄8" inside of the valve. If you are using the
drain as a solar port, replace the valve with a 2" brass nipple,
tee, and full port ball valve (these remain a full ¾" throughout
the body). This will allow you to open the valve quickly and blow
out any sediment in a matter of seconds. Marathon has a full port
ball valve for these applications with hose threads on the end and
is available as part number SP330000.
Do a water test. Lime (Calcium carbonate) is the enemy of
hydronic heating systems particularly where there are small orifices
involved such as flat plate heat exchangers. Inexpensive hardness
test strips are fine for this and will give you a much better idea
of what you are dealing with and how to mitigate it. Water
conditioners such as Scaltrol, and conventional salt-based conditioners are both effective in
prolonging life of the equipment.
Protect against freezing. If your temperature can drop
below freezing, you must use antifreeze in a closed loop system, or
a drainback system. The antifreeze should be propylene glycol
solution suitable for solar and at a solution strength designed for
your area. NEVER use ethylene glycol or car antifreeze — it is very
Do not go cheap on your sealants. Due to its chemical
make-up, propylene glycol will leak in places water will not.
Typically, it will seep through Teflon tape and many connections
over time. Pipe sealants such as SOS #10 (blue) or Permatex
(non-hardening) are good choices.
Expansion woes. If the system is on municipal water with a
backflow prevention valve (typical) you may need to install an
expansion tank on the water system if one is not there. As the water
is heated in a large solar tank it will expand. If there is no well
tank or expansion tank it will create pressures well in excess of
100psi and cause the T&P to weep or relieve. This is more pronounced
in solar systems due to the tank volume and temperature swings not
present in a standard water heater. Any closed loop solar system
will also have its own expansion tank.
Temper the water. Solar water heater's temperatures fluctuate
far more than any other. Avoid the risk of scalding by installing a
good quality tempering valve on your system.
Take advantage of industry experience. Use a reputable
dealer. Do they have experience with solar hot water? Do they know
how to protect against both overheating and freezing? Are they
familiar with Marathon? You have an investment that can pay
dividends for years if properly done.