Can you connect a Solar Thermal System to a Heat Pump Cylinder?

Can you connect a Solar Thermal System to a Heat Pump Cylinder? Yes you can, but it has to be a special kind of heat pump cylinder that’s been designed to take both heat sources. Typically that means using a twin coil hot water cylinder with one coil connected to the solar thermal system and another connected to the air source heat pump. The solar coil goes at the bottom of the cylinder, and the heat pump coil goes higher up. This lets you use solar energy whenever it’s available and the heat pump chips in when you need a bit more hot water.

How does a solar thermal and heat pump cylinder actually work?

A solar thermal system uses roof-mounted collectors to catch heat from the sun. There’s a heat transfer fluid that carries this energy to a special coil inside the hot water cylinder.

The heat is then transferred into the stored hot water.

An air source heat pump, on the other hand, heats up primary water which then goes into a separate coil in the same cylinder. The two systems work together to heat the same volume of hot water without needing to be directly connected.

Our air source heat pump water heater range provides different storage capacities for applications ranging from small residential properties through to commercial buildings. For the smallest applications, explore our 100 litre wall mounted heat pump water heater.

For larger homes and greater domestic hot water requirements, see our 200 litre air source heat pump water heater or 300 litre air source heat pump water heater.

For high-demand residential and commercial applications, explore our 400 litre air source heat pump water heater or 500 litre air source heat pump water heater.

Why does the solar coil go at the bottom?

The position of the solar heat exchanger is pretty important.

Solar thermal collectors work best when they’re heating up relatively cool water. So by putting the solar coil at the bottom of the cylinder, it can heat up the coldest part of the stored water. This helps make the most of the energy collected from the solar system. And it also helps with something called thermal stratification, which means the hotter water stays at the top and the cooler water stays at the bottom.

The heat pump coil can then heat up the upper part of the cylinder when you need a bit more hot water.

Do the heat pump coil need to be bigger?

Absolutely. A dedicated heat pump cylinder normally has a bigger heat exchanger surface area because heat pumps operate at lower temperatures than traditional gas or oil boilers. This is especially important in a twin coil setup.

Just having two coils doesn’t automatically make it suitable for a heat pump. The heat pump coil needs to have enough surface area and transfer capacity for the heat pump you’re using.

Which system heats the water first?

Solar thermal usually gets priority because the energy from the sun is available without having to fire up the heat pump compressor. If you’ve got enough solar energy, the collectors can contribute a lot of the cylinder’s hot water needs. And if the solar energy isn’t enough, the heat pump kicks in to top it up.

Good controls are key here. If the heat pump heats the whole cylinder early in the morning, for example, then there might be less cool water left for the solar thermal system to heat up later in the day. A well-designed control strategy can make sure you leave enough space in the cylinder for solar energy to kick in, while still making sure you’ve got enough hot water.

What about in the winter?

Solar thermal can still contribute, but its output is lower than when the sun is shining brightly. The air source heat pump becomes the main source of hot water for most of the colder months.

The best thing about combining these two systems is that the heat pump provides reliable year-round water heating, while the solar thermal system can reduce how much electricity you need when the sun is out. How much each system contributes will depend on the collector size, orientation, weather, cylinder size and how much hot water you use.

Do you still need an immersion heater?

An immersion heater is still useful to have. If the heat pump is out of action for any reason, you’ll still have hot water. And depending on the system design, it might be used for high-temperature cycles. Some modern heat pumps can get really hot using just the refrigeration cycle, so whether you need an immersion heater or not will depend on the equipment and control strategy.

Is solar thermal the same as connecting solar PV?

No way. They use solar energy in different ways.

Solar thermal collectors make heat, which is transferred into the cylinder through a dedicated hydraulic circuit and heat exchanger coil.

Solar PV panels generate electricity, which can be used to run the heat pump, immersion heater or other electrical equipment in the property.

A cylinder doesn’t need a dedicated solar coil just because you’ve got solar PV on the roof.

What kind of cylinder do you need for solar thermal and a heat pump?

For a standard indirect setup, you’ll need a purpose-designed twin-coil cylinder with one heat exchanger suitable for solar thermal and another correctly sized for the heat pump.Cylinder capacity, *coil surface area, and heat exchanger output are all key factors to look at when selecting a cylinder – don’t forget about standing heat loss, operating pressure and whether an immersion heater is even provided, and what is your hot water recovery like too.

When you get the cylinder, controls and heat exchangers matched up just right, a solar thermal system and air source heat pump can really work nicely together, with solar energy getting used when its available and the heat pump kicking in to make sure you have reliable domestic water at any time of the year.