ASK THE EXPERTS: Overheating SHW System

Intermediate

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An Apricus heat dissipator.
An Apricus heat dissipator.

I have an evacuated tube (30-tube) solar water heater that produces more hot water in the summer than we need. I have a Steca differential control that has a setting to circulate the antifreeze to cool the tank if it gets too hot. According to the control instructions, it is supposed to circulate the fluid at night to cool the tank. It seems to turn on and circulate for hours but it loses very little, if any, heat overnight like the instructions claim. Do you have any idea what is wrong?

Bill Noyes • Winnipeg, Manitoba

The vacation mode built into some hot water controls does work—but not with evacuated tubes. The recirculation heat dissipation is effective with flat-plate collectors because they only have a single sheet of glass as insulation on the front of the collector, and the plates can readily lose the excess heat.

The vacuum insulation in evacuated tubes that provides enhanced performance in cold, cloudy weather hinders reradiation losses. In addition, most tubes sold in North America today use a heat pipe inside the tube to collect the solar energy and transfer it to the manifold at the top of the collector. The heat-transfer fluid is then circulated to a heat exchanger and into the storage tank. Heat pipes don’t work in reverse, and this prevents any reradiation in tubes of this type.

Apricus makes a heat dissipator to help prevent summer overheating in evacuated-tube systems. The heat-transfer fluid from the collector(s) is diverted to the dissipator/radiator (liquid-to-air heat exchanger) when the fluid temperature is above 170°F. The dissipator and other overheating solutions are the subject of an article in HP142.

Chuck MarkenHome Power thermal editor

Comments (3)

Paul K. Hearsey's picture

A heat dump or dissipator is not all that complicated. I have 60 evacuated tubes and plan to install fin-tube replacement cores (for baseboard heaters) in my ventilated attic to dump excess Btu's. These cores are far cheaper than entire baseboards. Go to the Slant/Fin website, pick out one of their replacement core models in 3/4 in. copper (like model C-340) and then look at the rating in Btu/hr per foot. Then go to the spec page and look at the de-rating based on fluid temperature and ambient air temperature. My setup was derated to 53% because of lower fluid temp and high attic temp. I came up with 24 ft and plan to solder three 8 ft lengths end to end, with a motorized diverter valve on a thermostat. Once fluid temp reaches some limit, probably 185 F, the fluid (glycol) will pass through the heat dump instead of the heat exchanger in my boiler room. That thermostat could be a simple click-type Klixon switch; some are adjustable. Just remember that a simple 50 ft coil of copper may not be able to dissipate enough Btu's per hour to protect the glycol from overheating and ruin.
I saw on builditsolar.com that someone used a non-electric anti-scald mixing valve and ran the glycol backwards through the unit. As the glycol got hotter, some was diverted to a heat dump. Clever.

Fred Golden's picture

Hi,

You might consider covering part of your solar system in the summer, or if practical, tilt it more upright. One system installed in New Jersey claimed that tilting to near 90 degrees was needed to enhance winter heating and prevent summer overheating (not nearly as good of sun hitting the heating surfaces). Or consider installing a heat exchanger, such as 50' coil of 1/2" copper tubing and a small pump to run when it is over 150F. This can heat your garage, or other useful purpose.

Fred Golden's picture

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