Pressurized systems for space heating are very similar to drainback installations, though they have additional considerations because the solar loop is always full. Unlike a drainback system, they can't just be switched off. This puts the system at risk of overheating during the hot months when it is not in use (assuming it is not drained).

A pressurized installation's solar loop is the same as the one used in a drainback system with the slight differences outlined below. In similar fashion, a combined hot water and heating setup will need a larger array and more pipes to handle the extra energy requirement.

There's also the problem of the hot water storage tank, the same as in combined drainback systems. Because everything 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 start 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.

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.

On the other hand, if the system is only used for space heating and is not a combined hot water solar thermal installation, the antifreeze solar fluid can simply be directed straight to the heat transfer equipment (radiators, fans, baseboards and so on).

To help prevent overheating, the collector array's tilt angle should be adjusted. However, there is a serious risk of overheating because of the larger array requirement, especially in places where the climate is generally mild and there isn't much of a heating load, even in winter. If this description fits your location, we strongly encourage you to consult a professional for advice and consider just using the space heating setup as a shunt load for excess energy from your solar thermal hot water installation.

Pressurized systems often work better without the high-limit thermostat that drainback systems use. If the house gets too hot, open a window or direct the extra heat to a shunt load such as a buried pipe, external radiator or a hot tub. This will protect your solar thermal installation from overheating and, even though opening a window wastes energy, it's only a waste of renewable energy, not grid-tied fossil fuels!