Solar Hot Water

epsea-solar water heater

Most everyone has experience with passive solar water heating. How many times have you turned on the hose in the yard and nearly burnt yourself with hot water? While you weren’t looking, old Sol was quietly working to give you hot water, even if you didn’t want it. Well if it’s that easy, imagine what you can do if you’re actually trying to make hot water. Passive solar hot water systems are probably the oldest commercially available solar systems. At the turn of the century there were large numbers of solar water heating systems on roof tops, especially in Los Angeles and Florida. Very little has changed from the original concept. Put a water holding tank in a box, with glass on the side facing south and fill it with water. No moving parts, nothing to break down, free fuel and no pollution.

The passive solar water heater is known today by many names; PSWH, Batch Heater and Bread Box are the most common and then there is the very technical; Integrated Collector and Storage System (ICS).

The PSWH of today usually starts with a 40 gallon, glass lined tank. These tanks come disguised as ordinary electric water heaters, which are stripped of their appliance shell and insulation. Painted flat black, with high temperature engine or barbecue paint and they’re ready for solar.

 

The box should be well insulated to prevent energy loss and the amount of insulation should reflect your local climate. The typical box is constructed with 2X4s or 2X6s, using fiberglass batt insulation. The exterior siding may match that of your home, or some other material suitable for your area. The interior sheathing is often ridged insulation, preferably with a foil face facing in which works to reflect more energy onto the tank. Ridged insulation comes in various thicknesses which can help increase your insulation R-value.

The size of the box must be big enough for the tank, but also large enough to allow adequate solar gain. Typical glass sizing is 1 sq. ft. of glass for every 2 to 2 1/2 gallons of water. A standard size, double glazed, patio door replacement glass (34″X76″) is ideal for a 40 gal. water heater. Of course if you already have a piece of glass…….

A water heater has an inlet and outlet and how you attach your plumbing does make a difference. The cold water inlet has a dip tube which extends down nearly to the bottom of the tank, to deliver the cold water to the right place. The hot water outlet takes the hottest water from the top of the tank. If your design calls for the tank to lie on its side be sure that the cold inlet is at the bottom.

Be aware that the 40 gallon tank when filled with water will weigh over 350 lbs. Add to that the weight of the box/glass and it’s time to reconsider putting this monster on your mobile home. Ground mounted PSWH are very common. As always be sure the system will receive full sunshine from 9 am to 4 pm. Remember, if your installing a solar system and you’re working in the shade, there’s something wrong.

If the collector will be installed on a frame roof it’s best to attach in such a way as to spread the weight over a few rafters, and if possible, provide additional support with braces extending up to the rafters from interior walls. The ideal location is as close to the existing water heater as possible.

Shorter plumbing runs are not only more efficient, they decrease the winter freezing potential. The chances of freezing 40 gallons of water are minimal but frozen pipes are a reality. With the tank installed close to the water heater the freezing potential is minimized but not eliminated. All plumbing between the existing water heater and the PSWH is insulated, with more insulation on any pipes exposed to the outside. Also be sure to carefully insulate all plumbing in attic areas.

Plumb the system by first supplying cold water to the solar tank. From there, the hot water outlet is plumbed to the cold water inlet on your existing water heater. As long as the solar water entering your water heater is above the thermostat setting, your water heater does nothing. When the temperature of the solar water entering the water heater is less than the thermostat setting, your water heater makes up the difference.

The temperature of the water from a PSWH depends on many variables. The amount of sunshine, ambient air temperature, the amount of insulation used, the temperature of the supply water as well as the hot water demand all effect outlet temperature. Under ideal weather conditions, and no hot water used since morning, the water temperature at 5 pm can exceed 180 degrees F. You may consider installing a tempering valve which allows you to set the temperature for the water before it reaches the faucet.

How much of your hot water demand will be met by your PSWH varies depending on a number of factors. Have you installed low-flow shower heads and aerators? Have you installed a water heater blanket and set the thermostat to 120 degrees F? When do you use the most hot water? If you normally wash clothes/shower etc. in the evening, there probably won’t be any solar water in the morning. If someone is normally home during the day and clothes washing is scheduled for around solar noon, you can stretch your solar water. After normal water use in the am, the sun heats the water all morning and then that water is used for the laundry (if necessary). This schedule allows time for the water to heat up again during the afternoon.

Be sure your installation meets all local plumbing codes etc.

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