Drainback Solar Thermal System Heat Exchangers

You are here

Like any closed-loop installation, drainback solar hot water systems use a heat exchanger to pass energy from the solar fluid to the domestic water supply. However, drainback setups include a third, specialized tank – the drainback tank – which holds all the solar fluid when the system is idle. This fluid can still be very hot when circulation stops, so it makes sense to have a means of passing that energy to the domestic supply rather than waste it.

There are three main ways to get heat from the drainback tank to your domestic hot water tank, all of which use a liquid-to-liquid heat exchanger.

Submerged exchanger

Example system schematic for a double-pumped drainback solar hot water installation using a submerged heat exchangerThe first option is to use a heat exchanger that's a bit like a kettle heating element: it sits at the bottom of the drainback tank, below the minimum fluid level. Some manufacturers supply tanks with an exchanger like this as standard. They are also called "submersed" exchanger models – the two words mean the same thing.

The downside of this setup is that the exchanger loop needs a second, small pump. The pump should be a very low-power, slow model because there is very little head to overcome and because a powerful model will destratify the liquid in the drainback tank. Both pumps in a "double-pumped system" are connected to the same differential temperature controller so that they switch on and off at the same time.

In-tank exchanger

The second option is to include a heat exchanger in (or wrapped around) the drainback tank. This exchanger works just like a normal in-tank model, passing heat from the drainback tank to the domestic supply tank by means of a pipe filled with solar fluid. Since the entire setup is within the conditioned space, you can normally use simple water to carry the heat with great efficiency.

A particular advantage of this setup is that it's very simple: it does not need a second pump on the drainback tank's heat exchange loop. Installations of this type are also known as "single pumped systems".

External exchanger

A single-pumped drainback solar hot water installation using an external heat exchangerThe third choice is to install a completely separate exchanger near the storage tank. This setup works much like the submersed exchanger option, in that it needs a second pump. In this case, the same exchanger is used to transfer heat from the collectors when the system is running and from the drainback tank when the system is idle. It isn't as complicated as it sounds, as shown by the layout diagram: