Domestic Water Heating Systems

Posted by jondavis on Thursday, 19 of April , 2007 at 6:51 am

I have been advocating solar water heaters for domestic use for many years but lately I have been re-thinking the issue. In temperate climates such as Florida a solar water heater makes a lot of sense as the system is simple and, in Florida’s climate, no freeze protection issues are involved. In colder areas things get more complicated.

Having lived in four different homes with solar domestic water heaters, and after listening to many past clients report on theirs, I have decided what makes sense and what doesn’t. First, the flat plate collector type systems which have a freeze-proof (glycol) liquid circulating between the collector and the heat exchanger are marginal at best. The upfront costs are substantial, and maintenance is a long-term expense. Glycol systems are complicated, with all the plumbing, storage tanks, heat exchangers, the controller and so on. The glycol needs to be changed periodically, which can cost up to $300. That is if you have someone in your area who is equipped and experienced to even do the job. Disposing the old glycol is an issue as well.

The issues of energy and water conservation have competing priorities that need to be balanced in any water heating scheme. In our current home (built three years ago) we installed a flat plate collector and heat exchanger with a small storage tank. We had the storage tank tied into a gas Bosch tankless water heater which could vary the amount of heat needed depending on the incoming temperature of the water. The system worked well enough but we still had to wait for hot water to get to the various faucets in the house. In an arid climate like New Mexico, water is a huge issue and wasting it is certainly not in line with living green. Having a hot water circulating system is no answer either as they are energy hogs both in operation and heat loss. It is certainly a conundrum.

We needed a system that gives us hot water instantly (so as not to waste the water) without a tank of hot water which is constantly losing heat. It occurred to me that we could use some of the electricity that our photovoltaic (PV) system generates to help with the water heating. It is common knowledge that heating water with electricity is expensive. Installing a solar water heater is expensive as well and particularly so if it is financed. So I removed our solar water heating system and installed a small, electric instant water heaters at each sink. The bath tub, shower and washing machine are still connected to our gas-fired tankless water heater because it has more capacity. We like this system and it works well.

My intuition says that even though we are using electric for some of our hot water needs, we are using much less water and our PV system is more than covering the energy use for hot water at the sinks. I would really like to find someone who has the background to run the numbers and see what the efficiencies and trade-offs really are for water and energy use.

By the way, the best solar water heater we have ever had was a simple system we purchased in 1983 called Big Fin that was sold by ZomeWorks. In that system we had the solar heating plates inside the house under a large skylight (see photo). Since it was in a heated space, it heated water directly into a storage tank. The system was simple with no heat exchanger. The system has been working for over 25 years with no trouble. The most problematic issue was designing it into the house in a way that looked nice. We placed it in our greenhouse entry. The side of the fins that faced the sun were black and we painted the underside of the fins in multiple colors.

At the recent National Home Builders International Builders Show 2007 in Orlando, FL I saw a very nice, well engineered and low priced solar water heater for homes from FAFCO. It is a drain-back system that uses water (not glycol). The collector is not glazed and is made of a flexible material that will not break in a freeze.

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Category: Passive Solar

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