Here in New England we have significant seasonal changes in temperature and day length that affect the output of solar thermal panels. To get acceptable winter output panels are generally installed to maximize winter solar gain and reduce summer production. But despite our best efforts we may end up with more heat than we need in the summer. To protect the system components, especially the glycol antifreeze that turns acidic from over heating, we use a “heat dump” to dissipate the excess heat.
In simple terms a heat dump just gets rid of the excess heat so the tank does not get too hot. Home systems do not have the necessary safety features to store water over 180°. Dropping the tank temp can be accomplished in many different ways. One of the simplest is to use a automatic device that just let some of the hot water out of the tank; it is then replaced with cold water. Another inexpensive method is to install a piece of baseboard outside and automatically direct the hot glycol through it when necessary. With flat plate systems you are constantly loosing come heat out through the glazing and this can also help with over heating. Some controllers have a feature that turns on the circulation pump at night to drop the tank temp.
Being in the solar industry we tend to be very attached to every Btu’s we create and hate to see them wasted. This leads to endless creative ways to use your excess Btu’s.
One of the things that makes Solaris different from some of the company’s that sell “one size fits all”, prepackaged systems, is our expertise to customize your system to work with your existing equipment. In spring and fall, on sunny days, it is common to have more heat than you need. We love to dump this back into your boiler to assist with heating your home, helping your system pay for itself even faster.
Some other options include:
The important thing to remember is if you have an good quality evacuated tube systems, unless you have a very small system designed only as a pre-warm, you will have times with excess heat. Allowing the glycol to overheat will cause it to become acidic and will damage the piping if it is not replaced. In addition it is not good for the tubes and dry stagnation could void your warranty. When you are on vacation and there is no hot water use, or if the power goes out or pump fails, heat can quickly build up. You system can handle occasional stagnation but each time it wears on the components. A good quality evacuated tube is capable of generating very high temperatures; this is a good thing in the winter. A tank of 180° water will last longer than a tank of 130 water. A heat dump is almost always necessary with a quality tube. When evaluating proposals, be sure to ask your installer about it. Another way some installers handle excess heat is vapor drawback where fluid/steam is pulled into the expansion tank. This method is not approved by all manufactures and not an idea that we support. There is at least one manufacture that uses a pre-vessel to hold the hot liquid before it can hit the expansion tank. We think this pre-vessel will stop the abuse caused my hot fluid pulling into the expansion tank.
Installing a divert valve, circulator and a bit of baseboard is not expensive or complicated provided the solar installer has a reasonable understanding of plumbing. If they don't know hot to so this they they may not be the right company to install your solar hot water system.
Contact us today with your questions or to set up your free consultation.