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temperature as the storage tank bottom, the pump automatcally turns off.
If your location experiences freezing conditions, even if it is only once every 5 years, I recommend
nstallng a freeze protected system to elmnate the concerns of havng ppes or collector freeze up
durng a "freak" cold snap that catches you off guard. There are two basc types of freeze-protected
systems: dranback and glycol. The chart below covers some of the ponts of dfference between
them.
Heat Transfer Flud:
Hgh Lmt:
Flud Lfe:
Pump Energy Requrements:
Heat Exchange Performance:
Intal Cost:
Locaton Lmtatons:
* Note: Use only buffered propylene glycol for your heat transfer fluid. Never use toxic ethylene glycol.
A freeze-protected system works by having two loops. The first loop circulates your heat transfer fluid
through the collectors and then to one side of a heat exchanger. The second loop circulates your
potable water on the other side of the heat exchanger by using either forced circulation with a pump or
passive circulation through convection. Basically, you are trying to maximize the amount of heat that
you take from your collectors and put n your storage tank.
Drainback System: With efficient heat exchange being the objective, I would recommend a
double-pumped drainback system. Water is a better heat transfer medium than the water/glycol mix
and a double pumped heat exchange is 20-60% more efficient than a single-pumped heat exchange.
On both ponts, dranback wns. In addton, dranback has the added feature of allowng the system to
shut off once you have put as much heat as you want n your storage tank. The dfferental control wll
automatcally shut your pumps off once you have reached the hgh lmt on the control. If you dd ths
wth a glycol system, the pressure would spke (openng your pressure relef valve) and the addtves n
the glycol would be destroyed, requrng t to be replaced.
A drainback system works by using a reservoir (drainback tank) that you fill with water. That reservoir
s located below the collectors. When the dfferental control says there s heat to be harvested n the
collectors, the pumps turn on and force the water from the reservoir through the heat exchanger and
Drainback
Water
Has t
Forever
Hgher
Best
More expensive system initially because of the cost
of a dranback tank
Requres gravty dranng
© SOLARHOT 2006-2009
Glycol
Propylene glycol*
Doesn't have t
3-5 years
Less
Less Effectve
Hgher nstallaton costs from glycol and
ppng requrements
Can be mounted anywhere
7
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