The Energy Column - Thermosiphons

The Energy Column 2002
Now in its 16^th year!
Written by: Ken Sheinkopf
Phone: 321-638-1007 Date: March 16, 2002
Fax: 321-638-1010 Column No.: 02-11
E-mail: sheinkopf@fsec.ucf.edu


Q.: "I’m interesting in having a thermosiphon solar water heater installed on my home. Should I be concerned about the size and weight of the tank mounted on the roof?"

A.: Good question. First, for those who don’t know this type of system, let me explain that a thermosiphon solar water heater is a fairly simple system that has its storage tank mounted above the collector (usually on the roof) to provide a natural gravity flow of water. Hot water rises through piping in the collector, which is mounted below the tank; heavier cold water sinks to the lowest point in the system (the collector), displacing the lighter hot water which rises to the tank. It’s pretty neat — no moving parts, and gravity does all the work.

If the system is being installed on a standard roof truss that is in good condition, there should be no problem since the weight of the system’s tank is distributed at several points. Depending on the size of the system, the collector shouldn’t weigh any more than around 100 pounds. The tank should be around the same weight empty, and when it is full of water, a 40-gallon tank would increase in weight by about 330 pounds, and an 80-gallon one will increase about 660 pounds. It sounds like a lot, but distributing the weight is the key to making it workable. Keep in mind that three grown men ought to be able to walk on a standard roof without any problems, and their weight would probably be close to this higher figure.

If you have any question about the condition of the trusses, a good solar installer should be able to look at your roof and let you know if it is in good enough condition. You might also ask a roofing contractor to inspect the trusses and see if they are sturdy enough.

By the way, the thermosiphon system is the most popular system used around the world. If you’ve visited any of the Caribbean islands, you’ve certainly seen many of these systems on the roofs of homes there. Consider that many of the homes in other countries are not as solidly built as are homes in the U.S., and they don’t have any problems with the weight of these systems.

Q.: "Our air conditioner has stopped working. I assume most homeowners get estimates from several dealers and expect that they will all recommend a system that is sized appropriately to minimize running cost and maintain proper humidity. Should the contractors perform a load analysis? How do we know they are doing a good job on this?

A.: It is usually good practice to get a minimum of three bids from qualified contractors. Each should provide you with their Manual J cooling and heating load calculations used to size the equipment. Your letter said that your home has no windows on the north and south sides and the windows on the west side are sheltered by a tall maple tree and shrubs. It is important that you make sure that the contractors note these factors or they will be over-sizing your needs. An over-sized unit will tend to cool for short periods and then turn off. This reduces air-conditioner efficiency (it is similar to start-and-stop city driving in your car vs. highway driving). It also will not lower the humidity as effectively.

Make sure each contractor gives you a paper copy of the complete cooling-load calculation using Manual J and not just a rule-of-thumb estimate based on the size of your house. And make sure they get that window shading right.

Energy Tip: A reader in Texas wrote regarding a recent column on water flow restrictors in showers. She agreed that they can make a difference in energy and water use, noting that "my experience has taught me that those folks who can ‘only wake up under the spray,’ the majority of teenagers and many people who need a longer shower to get their aching joints in motion would all do well to have the restrictors installed. But here’s an exception: many working adults have their showers down to a tight schedule, especially after 30 years of practice. The flow restrictor will only be an annoyance, adding minutes on to the routine, thus negating any water savings. Net gain — one useless new shower head and one formerly pleasant person now running late." Good point. If you’re already saving water by taking a quick shower, you probably won’t benefit much from a water flow restrictor. On the other hand, if you’ve got a few teenagers at home . . .