Drainback systems work very well for space heating applications without storage because, once they reach the target temperature, they can just switch off. All the solar fluid drains out of the pipes and collectors, meaning there's no risk of overheating, even during very hot periods when the system is inactive all day.

A drainback installation's solar loop is exactly the same as the one used in a drainback solar thermal hot water system. The main difference is the drainback tank, which is usually a model without an internal heat exchanger.

Things get a bit more complex if you're installing a combination system (hot water and space heating). For starters, everything needs to be a bit bigger: a larger collector array is needed for the space heating load, which means more piping and a bigger drainback tank to contain the extra solar fluid.

There's also the problem of the hot water storage tank. Because the system has been sized up to generate enough energy for space heating, the domestic hot water supply is going to get very hot, very quickly. To handle this, you can install an aquastat on the hot water tank, set it to 130°F (55°C) and have it kick off a pump when that temperature is reached. When the water gets hot enough, it can be pumped out to another load – either the space heating equipment or some other shunt load, depending on your needs.

You'll also need a thermostat, installed on the line between the aquastat and the pump. Set it to open on temperature rise, with a value of the maximum allowable temperature. When the system (and therefore the building) reaches that temperature, the thermostat turns off the pump and prevents overheating.

However, this solution creates another possible issue: you can't pump potable (drinking-quality) water just anywhere. In general, you're allowed to use it where it won't mix with another liquid and the pipes are approved for potable water, as long as the water is circulated through the radiators periodically. You'll have to check your local plumbing codes, because some never allow potable water to be used for space heating. In that case, you'd need to pump the water through another heat exchanger and transfer the energy that way.

As a rule of thumb, it's rarely a good idea to use your domestic hot water supply as a source of space heating. Use a closed loop and a heat exchanger.