SOLAR TODAY - Get Started 2009

WATER hEATIng BASIcS
Maintenance

in the house, hot water flows from the top of the stor-

age tank and is replaced with cold city water flowing

into the bottom of the storage tank.

The thermosiphon system is simple. However, it puts

the heavy storage tank high on the roof, which should

be reinforced to support it. Other solar water-heating

systems put the storage tank at ground level or in the

basement, where it’s not a structural challenge.

Both systems can be used in cooler climates if they’re

converted to a drain-down configuration. When tem-

peratures drop near freezing, valves open to drain the

collector, often into a weather-protected indoor stor-

age tank. When temperatures rise again, the collector

system is refilled, either from city water pressure or by

using an electric pump to push water back up from

All water heaters need to be flushed annually, and so do solar system storage tanks. The pumps and valves in an active system are electro-mechanical devices that will need periodic attention. Annual pressure testing can identify potential problems before they become major leaks. Long-term corrosion is an issue in any plumbing system, but a well-maintained system can go 20 years or more before replacement of major parts.

Modern evacuated-tube collectors boil propylene glycol in copper tubes; the steam then heats water in the manifold at the top of the unit.

the indoor storage tank. Because of the elaborate

automatic control systems, draindown systems are

considered active rather than passive.

The evacuated tube system is more expensive than

other passive systems, but is also much more efficient

so it takes less space on your roof. Solar heat is col-

lected in double-walled glass tubes arranged with

one end high. Each tube is built like a thermos bottle

Bu TLER SuN SOLu TIONS

liner, with a vacuum between the walls. In the center
of the tube is a copper pipe containing propylene gly-
col, a nonpoisonous antifreeze. This is the same stuff
sold in auto parts stores as pet-safe antifreeze for

your car; it’s also used in some food products. While

the outside of the glass tube remains cool, the copper

pipe turns hot, and the glycol boils. The steam rises to

the top of the pipe, where it heats a manifold. Water

flowing through the manifold is heated quickly. The

glycol steam in the copper pipe condenses back to

liquid and flows to the bottom of the tube, making a

continuous convective loop. Evacuated tube systems

work in both warm and cold climates.

Active solar water-heating systems

Active systems use an electric pump to circulate wa-

ter through the collector. In warm climates, a direct

(or open-loop) system is practical. City water goes

into an insulated storage tank, a pump draws wa-

ter out of the storage tank to pass through the solar

collector and go back into the tank. Hot water for

household use is drawn from the top of the storage

tank, sometimes passing through a booster heater.

Clean Air.

Clean Water.

Clear Conscience.

An automatic control system starts the pump when-

ever the collector is warmer than the storage tank.

In colder climates, the most common system today

is the closed-loop antifreeze heat-exchanger sys-

tem, or active indirect system. When the collector is

Find out how you can be part of the solution.

warm, a food-safe propylene glycol antifreeze solu-

tion is pumped through the collector at one end and

through a heat exchanger at the other. The heat ex-

changer heats city water for domestic use. The heat

exchanger is usually located at the bottom of an in-

sulated storage tank (sometimes the storage tank is

also the home water heater, with an electric or natu-

ral gas heating mechanism for use when the collector

is cold). A breach in the heat exchanger would leak

ISTOCKPHO TO.COM/ TOMAS BERCIC

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antifreeze into the drinking water, which is why it’s

necessary to use only food-safe propylene glycol in
these systems. r

This article is adapted from the 2008 Solar Energy Resource Guide, published by the NorCal Solar Energy Association ( norcalsolar.org), a chapter of the American Solar Energy Society.