In many cases, mounting the heat exchanger is one of the easiest parts of system installation, especially if you buy a pre-packaged kit; you just put the module on the storage tank and tighten a few unions. However, you may decide to construct your own heat exchanger from individual parts. In any case, the first step is to install the storage tank, then attach the exchanger.

Solar thermal storage tanks

The best type of storage tank is one designed specifically for solar thermal use. Such tanks have four ports on the top as well as a T&P valve (temperature and pressure). The ports are usually ¾-inch national pipe thread (NPT) and are clearly marked:

• Hot – the backup water heater attaches here.
• Cold – the cold supply line feeds in here.
• Solar In – attaches to the top (hot) side of the exchanger.
• Solar Out – attaches to the bottom (cold) side of the exchanger.

If you're setting up a thermosiphon heat exchanger, install the tank six inches (15cm) higher to give the convective effect a bit of a boost. In this configuration, you will also need to plug the Solar Out port on the top of the tank and attach a drain to the bottom of the heat exchanger. An automatic air vent is essential (at the high point of the pipe run) whenever a thermosiphon exchanger passes hot water to the top of the storage tank, as shown in the diagram.

Electric water heater tanks

A good alternative to dedicated solar thermal storage tanks is an electric water heater. These are cheap, easily available and of good quality, so they're a common choice. Electric heater tanks usually have ¾-inch national pipe thread (NPT) connections, except for the electric element ports (usually 1-inch machine thread), and a drain valve at the bottom. They generally have two or three ports on the top:

• Hot – open to the top of the tank, leads out to the backup water heater.
• Cold – the cold supply line comes in to a dip tube here.
• T&P Valve – for monitoring.

The obvious difficulty is that there are not enough ports to attach a heat exchanger. If you're using a new model (which is preferable), you can use the electric element port at the top on the side as the Solar In. Take the element out and you'll see that the threads are not tapered and that there is a flat faceplate at the end of the threads, where an O-ring or gasket is fitted. When the element is screwed in, the O-ring seals the tank and the faceplate touches the water heater flange.

You will need to modify this port for solar use, which requires a modified 1" by ¾" brass brushing – ask a machine shop to turn the end of the nut flange on a standard brushing flat on the inside where the threads taper into the nut. This will give you a flat surface to which you can fit an O-ring. Fit this modified brushing into the tank and you have an