Can I combine my heat pump with another boiler? Hybrid heat pumps.

In heat pump land anything is possible, a heat pump can be the only heat source or it can be combined with pretty much any other heat source, even a kettle. Its all down to the imagination of your designer.

In older, poorly insulated and very large buildings we meet 2 problems, the first being the heat load is very high so we can’t cover it with a single heat pump and the second being the homeowner does not want to replace the radiators or hot water cylinder. Installing more than one heat pump is not a problem and is a really good solution, i covered this a few weeks ago. But if you have more than one heat pump installed you always have to apply for planning permission, this can often kill the deal. So there are two separate solutions for these problems:

If the house is too big for a single heat pump, why not use a backup heat source.

System design temperature between 40 and 55C.

In this case we could use a heat pump and another heat source, either a backup boiler or an inline electric heater. In mild conditions the heat pump will be able to cover all of your heating an hot water needs, when the weather turns colder the heat pump will reach its maximum output and will need help. At this point it will ask its backup heater or boiler to assist. Both the heat pump and the backup boiler will operate at the same time.

IT is essential that the boiler / backup and the heat pump operate at exactly the same temperature. If the boiler were to run at a higher temperature the heat pump would be flooded with warm return water and it will assume the house needs less heat, it will turn off the boiler and try to do all the work itself. The boiler would cycle in and out like the fiddlers elbow. This is not a good solution.

Backup systems only work if the radiators or underfloor are designed to operate at the heat pumps outlet temperate, typically between 40 and 55C.

In the bad old days this sort of thing was common, manufacturers used to install kettles in the units (electric heaters) for this very case. It is a truly horrible solution, quick to install but massively expensive to run. I absolutory loathe any heat pump with a built in electric heater.

If the homeowner wants to keep the old radiators, a Hybrid system.

System design temperature greater than 55C.

In our industry we like to over-complicate everything with stupid names, I call these systems dual fuel or hybrids. Some people call them Bi-valent, it’s a ridiculous name but some people think calling them this makes them sound clever, it doesn’t.

A hybrid is a system which uses both a boiler and a heat pump. It’s exactly like a hybrid car, when you want to go slowly you use the electric system and when you want to go fast you use the engine.??

The idea is the homeowner can embrace the renewable heating technology, but still have the fossil fuel boiler to fall back on. Some consumers prefer this to a heat pump only solution. Hybrid systems were very popular a few years ago when people were worried about whether a heat pump would actually work, people liked the idea of having the trusty old boiler as a backup.

In a hybrid heat pump we use the heat pump to heat the radiators to 50 degrees C, this is ok in milder weather, but typically when the temperature outside falls below 3 degrees C we need the radiators to be warmer to keep the house up to temperature. In a hybrid we switch the heat pump off at 3 degrees C ambient (outdoor) temperature and start the boiler up. The boiler will run at 70 degrees C raising the output of the radiators just when you need it. In a hybrid system the boiler and heat pump NEVER heat the house at the same time. They are two different temperature systems serving the same radiators but at different times.

In most hybrid systems the heat pump only heats the house it cannot heat the hot water.

The two main types of Hybrid:

Combi hybrid, where the house has a combi boiler and no hot water cylinder.

The combi hybrid is a nice simple solution, we add a heat pump, 2 pumps and a plate heat exchanger (all high lit in the diagram below) to your existing heating system, the boiler and the rads are all left in exactly the same place they are in now.

In this system the combi boiler will help the heat pump heat the house, the combi will provide all your hot water just like it always did in the past. This means you don’t need a new hot water cylinder. The old radiators and pipework are all left in place we don’t need to change them.

System hybrid, where the house has a boiler and existing hot water cylinder.

The system hybrid is a simple solution, we add a heat pump, 2 pumps and a plate heat exchanger (all high lit in the diagram below) to your existing heating system, the boiler, hot water cylinder and the rads are all left in exactly the same place they are in now.

In this system the boiler will only help the heat pump heat the house, the boiler will provide all your hot water in the existing hot water cylinder, just like it always did in the past. This means you don’t need a new hot water cylinder. The old radiators and pipework are all left in place we don’t need to change them.

It is very difficult to get the heat pump to also heat the hot water so it is generally the norm to connect it up as per the above.?

The future of Multi-fuel heating:

Nowadays with ultra-high temperature heat pumps operating with outputs of 16kW at -10C ambient and run temperatures of up to 80 degrees C, hybrids and backup units are much less often used.?

In April 2022 the new government grant will NOT cover hybrid heat pump systems at all. If you install a hybrid the grant will be ￡0. Between 2013-2022 we used to put a heat meter into the heat pump circuit to measure how much of the work the heat pump did. The RHI grant would pay out on the reading from the heat meter.

I think we will see less and less of these systems over the years so I thought I would explain all before they became irrelevant.

thanks for reading.