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September 13, 2007

Rent vs. Buy – Hot Water and Common Infrastructure


Being thoroughly middle-class, I prefer to buy rather than rent. That goes for energy as well as my house, car, phone, etc. It’s particularly more satisfying to own an energy source than to “rent” energy from the Middle East and have to contemplate what my rent payment is buying when it gets back to that unhappy region..

That’s why we have solar hot water heat at our summer place and why we just got a certificate of public good enabling us to install a 12.635 kW photovoltaic system with net metering at our home. The capital expenditures for both avoid “rental” payments for energy although neither is justified today on a purely economic basis.

Some Vermonters really live “off the grid” and are completely self-sufficient as far as their home energy use is concerned; that’s not us; we’re connected. Turns out that being connected helps makes both the solar hot water and the photovoltaic power more practical.

In the case of hot water, being connected means that the sun preheats the water that goes into our gas-fired hot water heater. During the summer, this has meant that we use practically no gas. But, when the sun refuses to shine for a couple of days in a row or later in the season, the gas kicks in as needed. If we weren’t connected – via truck in this case – to the gas distribution network, we’d have to have much more storage capacity, much more generating capacity, and still do without hot water some of the time.

The small panel you see next to the three large panels above is photovoltaic and generates direct current to run a circulator which moves a glycol solution through the big panels where it gets very warm and then through a heat exchanger in the big tank pictured below where the domestic hot water gets preheated. It’s a nice piece of engineering serendipity that, the more direct the sun light, the faster the circulator runs, which is just what you want because that’s when there’s the most heat to exchange.


In fact, there can be too much heat. You don’t want either the glycol solution or the water in the big tank to boil when heat builds up in the system. It’s not practical to solve that problem by having us run and take a shower or wash some clothes after too many hours of too much sun on the panels. So there has to be a heat dump; in this case another circulator kicks in whenever the glycol coming out of the heat exchanger gets over 180 degrees Fahrenheit; this circulator causes the flow to detour through the floor of the garage where we put radiant heat plumbing just in case someone ever planned to heat it. These pipes, which are not currently connected to furnace heat, serve as a radiator for the solar hot water system when needed. It does seem a shame to be dumping energy but don’t know any way around it. We do store a fair amount of heat in the hot water in the big tank which is just at 180 after a bright day even now at the end of summer. The heat dump is shown below.


While we are using water, it flows out of the mixing valve on the gas-heated tank (which assures that it’s not too hot). This water is replaced by pre-heated water – often at 180 degrees – from the big storage tank.  After living with it a while, a flaw in the system became apparent. When we weren’t drawing any water, the gas-heated small tank would gradually cool down. When it got too cool, the gas would kick in and warm it up again even though there was plenty of very hot water in the storage tank.

The plumber and solar installer figured out a clever workaround with yet one more circulator and a couple of thermostats. If the water in the small tank is cooler than the water in the big tank, this circulator runs water in a loop between the two tanks every once in a while to bring up the temperature of the small tank without using the gas furnace. Note that you DON’T want this circulation when the big tank is the cool one or you’d be using the gas furnace to keep it warm.

The propane gas we save by using the sun to heat domestic hot water doesn’t really come from the Middle East. It’s most likely to come from Canada which is hardly a hot bed of terrorism and is a good neighbor besides. But our do-goodism is satisfied by the knowledge that the Canadian gas we don’t use probably then goes somewhere else and eventually displaces LPG from the Middle East or oil since it is the low-cost energy alternative. Also, of course, if global warming is mainly anthropogenic, we have cut back some on carbon emissions.

Back to the point of common infrastructure enabling us to “buy” energy creation capability rather than renting it. This scheme only works because there is a distribution system for gas energy which allows us to have some whenever we need it. If we were truly off in the middle of the woods, it wouldn’t be nearly as easy to have both solar heat as a main source and a reliable supply of hot water.

Update: Giving credit where it’s due:

The plumber who helped figure out how best to integrate with domestic hot water (and did much else well for us as well) is Mark Yurcek President and owner of Radiant Works, Inc. (mark at radiantworks.net) and the solar hot water expert who patiently assured that everything worked perfectly or got replaced is Doug Wells of The Solar Specialists (www.thesolarspecialists.com). We recommend both.

Coming up, tThe story of the even more interconnected photovoltaic system we’re putting in is here.

This series of posts on rent vs. buy starts here.

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