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You don’t require a drainback tank in this system but you will require an expansion tank. The expansion
tank should be szed to the overall volume of the ppng and collectors. For proper szng of your
expansion tank, see http://www.amtrol.com/thermxtrol.ht
m
. The expansion tank that you use should be
mounted below or besde the connecton to the ppng. Never mount t above ts connecton to the ppe.
Polypropylene-lined expansion tanks are preferred for longevity. A pressure relief valve should also be
nstalled on the system. The pressure relef valve can be mounted on the roof, n the boler room, or
anyplace n between, but you need to make sure that there are no solatng valves between the collector
and the pressure relef valve. Volate ths prncple and you are creatng a potental safety hazard.
Heat exchangers:
Many companes wll tout ther ndvdual product, whether t s the best performng
product or not (no fault there). You should understand a few concepts before you decde whch heat
exchanger makes the most sense for you. The more surface area, the shorter the heat transfer path,
and the more flow (on each side of the heat exchanger), the better the performance. There are systems
on the market today that take advantage of ndvdual peces of ths equaton tryng to convnce the
buyer that their way is better, e.g. aftermarket submerged heat exchangers. The claim is that you can
feed a spiral tube into the port of an existing tank to provide the heat exchange. This product takes
advantage of a short heat transfer path (beng mmersed n the storage tank) but sn’t able to get around
the lmtaton of surface area (you can only get so much surface area on a tube nstalled through the
existing, small, port on the top of the tank). These rules can be applied to any heat exchange process
that someone tres to sell.
While there are no exceptions to the heat exchanger rules given above, there are some other
consderatons that mght be worth notng. There are tanks on the market today that have ceramc-
coated heat exchanger coils immersed in the tank. These tanks eliminate the need to clean the heat
exchanger since scale won’t adhere to the ceramic coating. The same can be said for the heat exchange
tanks that are wrapped with a coil to provide the heat exchange. Both of these methods eliminate the
maintenance that may (not necessarily) be required on heat exchangers that don’t protect the heat
exchanger from scale. Understand, by selecting a lower maintenance option you will be sacrificing
performance.
Choosing a collector: “
Caveat Emptor
” – Buyer beware. There are (as n all ndustres) people sellng
ther product that make clams about the performance of the product that aren’t backed up wth any
data. In the solar market there s a sngle ndependent ratng agency that rates the performance and
durability of collectors the SRCC, Solar Rating and Certification Corporation. They are co-housed with
FSEC, Florda Solar Energy Corporaton. Don’t beleve the sales hype; beleve the numbers. There are
several hgh-qualty collectors on the market today as well as a number of lower qualty optons. The
SRCC provides a minimum requirement for the quality of solar collectors sold today. You can find the
lst of collectors and collector manufacturers that meet ths threshold at http://www.solar-ratng.org/.
A few ponts to consder n selectng a solar collector: You should choose a solar collector that has a
selectively coated surface to maximize the heat that your panel collects. Most manufacturers produce
both a black panted absorber model as well as a selectvely plated absorber model (generally black
chrome). The suppler wll generally charge $50-$100 more per collector for the selectvely plated model
snce t produces as much as 20% more energy. Ths s money well spent. If you look at the overall
cost of materals for your system plus the tme and money to nstall t, ths ncremental nvestment s a
small price to pay for a significant improvement in your energy savings. Among the choices you make
on your system, ths one s a no-braner.
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