How to choose the correct solar water heaters - high pressure vs. low pressure
Solar water heaters are manufactured as pressure and non-pressure models. Pressure model (also called high pressure solar heaters) means, water in the tank are under high pressure, it's equal to the pressure of tap water. Non pressure model (also called low pressure solar heaters) means, water in the tank is under low pressure, and is equal to the gravity of the water. There are some fundamental differences which you should be aware of before deciding which one works best for you and which one you should install.
LOW PRESSURE SOLAR WATER HEATERS
Typically characterized by vacuum glass tubes connected directly into a tank with a small holding tank on top of it. The low-pressure gravity system consists of a vacuum glass tube collector, an insulated storage tank and optional stand parts (make up tank). The evacuated glass tubes are filled with water and exposed to sun, thus heating up the water in the glass tubes.
- This type of solar heater is simple, with no moving parts.
- Due to its simplicity, it is usually a cheaper option (roughly 50 - 60% of high-pressure systems)
- Due to lower pressure and stress on the tank and collectors, it has a longer life than mains pressure solar geyser systems.
- This type of solar heating system is lightweight.
- Can provide hot water without external power.
- This type of solar geyser system is highly effective in converting sunlight into heat
- It has limited uses with modern plumbing systems, as it can only be used where there is small head of pressure.
- Unless it is installed at a high level above the water outlets the water pressure will be very low.
- It can cause serious scalding if not installed or used correctly.
- The entire solar heater system needs to be shut down even if one glass tube breaks.
- All the weight of the heater is carried by the roof which may need additional support
- The tank does not look very attractive when installed on top of a roof
- It breaks easily
HIGH PRESSURE SOLAR WATER HEATERS
Typically characterized by an enclosed heating unit containing copper tubes connected indirectly to a tank. The tank may be located next to heating unit or separately such as under the roof. The high-pressure Direct Pressure Solar Water Heating System features evacuated tubes and heat pipes. The evacuated tube absorbs solar energy and converts it into heat energy, this energy is then used in the process of heating water. There is no direct connection between the heating fluids in the copper tubes and the water being heated.
- The main advantage is that the water pressure can be very high.
- Works well with water pumps that have been installed
- There does not have to be an ‘unsightly’ solar heater water tank on the roof.
- They can sometimes be retrofitted to use existing storage tanks.
- The tank or collectors can be located in more accessible or appropriate locations.
- More robust than low pressure systems
- They are more expensive than low pressure solar heater systems
- Some models require power to run the pump and temperature differential controller – this can be overcome by installing a photovoltaic module.
After having worked with both types of heaters we highly recommend spending the additional amount and installing high pressure solar water heaters. Most of our house plans allow installation of high pressure solar heaters. There is nothing as annoying as trying to take a shower and the hot water only keeps dripping instead of having a strong steady flow. The difference in price may sway some people to choose the cheaper low pressure option, however, if you can afford it then always go with a high pressure system.
Good luck and enjoy your free hot bath :-)