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How Long Do Drainback Solar Hot Water Heaters Last?
by Karl Schultz
Dual collector drainback solar hot water heater collectors
Drainback or drain back solar hot water heaters fall under the category of indirect circulation systems. A fluid is used to transfer heat gathered in the solar collectors to potable water in a heat exchanger tank located in the home below.
While some versions of indirect circulation systems use food grade glycol, those that use distilled water are the type most commonly called drainback solar hot water heaters. Versions of the drainback solar hot water heater have been around for several decades.
How long do drainback solar hot water heaters last?
According to several solar hot water heater installers I spoke to in the Central Texas Area, who frequently come across older systems, the oldest working drainback solar hot water systems they have seen were installed in the 1970's.
The oldest still working system that one of the installers had serviced was 35 years old. I'm sure there are some older working systems out there. If you know of one, send me an e-mail at the bottom of this page.
Drainback systems last a very long time because there is very little corrosion or scale buildup in the copper pipes because distilled water is used. Maintenance consists of checking the level of distilled water periodically and adding more when needed. This may only be every few months. Most drainback solar water heater systems feature a sight glass that the homeowner checks every few weeks.
Drainback systems are more complicated than passive systems that consist of a tank or collector that directly heats the potable water, but they are more reliable overall in climates where there is the chance of freezing. The weak link is the pump, which pushes the fluid from the heat exchanger tank to the collector on the roof. The pump in a drainback solar hot water heater system may require servicing every few years, while the collector and heat exchanger tank may last decades.
Drainback solar hot water heaters will work in almost any situation, however they do require a straight "fall" for the return water, with no piping that slopes upward again in an a non insinuated area, to prevent standing water in the pipes from freezing.
Drainback solar hot water heaters last well beyond their payback period and their longevity means that they will add real value to your home long after you are finished paying for them. In almost any climate you will enjoy getting much of your hot water need met for free. The collector in the photo above is in service in often cool and cloudy New Zealand, but it provides year round hot water. In very sunny climates you can expect a drainback solar hot water heater to provide nearly 100% of your hot water needs.
How Much Energy Do Drainback Solar Water Heater Pumps Use?
One question that a lot of people interested in solar hot water heaters have is "how much energy do the circulating pumps in a drainback solar hot water heater use?".
Since the pump is only on for a few minutes each day the cost is less than two dollars a month. I recently tested a drainback solar hot water heater using a Kill a Watt power meter. If you don't have a Kill a Watt, get one so you can find out what is wasting energy in your home. You can find one at Amazon at the link below. The Kill a Watt, left plugged in, with the solar controller and pump both plugged into it, for twenty four hours on a sunny day, showed only .5 Kwh used. If your energy costs you ten cents a kilowatt hour, this means the solar water heater pump will cost you about five cents a day. Not too bad, for nearly free hot water!
For more on the different types of solar hot water heaters see: http://www1.eere.energy.gov/solar/sh_basics_water.html
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