The primary goal of this DIY solar hot water heater system is to pre-heat fresh well water as much as possible prior to that water going into a propane water heater.
As many folks may be aware, propane water heat is very expensive and subject to wide swings in price, primarily based on the petroleum market prices. When our house was first built, we were not aware of the price swings of propane and did what many rural folks do… install a large, fast-recovery propane water heat tank. This tank is 60 gallons in size and it has worked well for many years.
We use a LOT of hot water. Multiple showers, a jetted bath-tub, clothes washer and dishwasher all contribute to a rather high hot water use per person ratio. A solar hot water heater makes perfect sense to reduce our energy cost for hot water.
A design for seasonal variability
Our small hobby ranch is located in the high-desert of the Mountain West. We have wide temperature swings for each of the seasons. Summer can get into the 100’s and winter down into the -10’s. Spring and Fall are usually amazing with days in the 70’s and lower 80’s.
Given the seasonal variability of our location, it was important when looking at various solar hot water system designs to take into account the lowest and highest temperatures that we are likely to encounter.
As a result of the research we did, and the desire to use as simple and safe an approach as possible, we decided that the system should be able to run for 3 seasons out of the year and be dormant in the winter. This means that in the spring, we fill and charge the system and in the late fall, the system is drained in preparation for the winter freeze. In order to extend operation into the early spring and late fall, a simple freeze protection approach was built into the controller software.
The following diagram provides a visual perspective of the system design.
Active Indirect Design
The design approach chosen for this system is the actively circulated, inderect. We did not choose to use a drain-back design due to the high cost of a pump capable of meeting the panel head requirements for our installation. The Panel sits on a shop roof approximately 10 heet above the storage tank. In addition to the initial cost of a high-head ciculation pump, the run-time power requirement is also greater, which would increase the cost of a solar electric system to run the pumps.
Fresh water heat collection loop
Since this is an indirect system, it means that there are 2 “sides” or “loops” to the plumbing. The collection loop carries heat storage water from the storage tank to the panel and back to the storage tank. There is a single, 12v high temperature circulation pump which moves water from the tank, to the panel, and back. The storage tank and collection loop are pressurized to approximately 40 PSI and include an expansion tank with pressure gauge to handle thermal expansion due varying temperature of the storage water. It is amazing how much water volume can increase for a given temperature increase.
The water in the storage tank is just filtered fresh water. There is no anti-freeze in it.
Heat Exchanger Loop
In an indirect system, the heat storage fluid and the potable house water never mix, never touch and are separated by a physical barrier. A heat exchanger is used to transfer heat from the storage fluid to the potable house water. There are many designs for heat exchangers including coil, plate, submerged, coaxial and in-tank. Since I am a DIY kinda guy, I decided to take the hard-road and make my own coaxial heat exchanger. It works pretty well and was not that expensive to build, plus, it was fun. The heat exchager loop has a single, 12v high temperature circulation pump which moves water from the tank, through the heat exchanger, and back into the tank. The exchanger loop is also pressurized since it is part of the overall storage fluid system.