Replacing a gas combination boiler with a heat pump

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This is not easy!

The name "combination boiler" shows that it is not just a heat-generator. There are pumps, pressure controllers, diverters, safety items, all sorts of bit inside. You cannot assume that any heat-pump is likewise equipped. Basic ones are only heaters, everything else has to be attached elsewhere. Other types have various integrated parts.

A small gas boiler starts about 20kW and can provide water at 65 degrees or higher. More powerful system boilers are not much bigger than the smallest wall-hung unit. A small heat pump is 3kW. A 20kW heat pump is BIG. They scale up in size as they get more powerful. Usually they are outside the living space to get their source of energy.

In a small house, bungalow or flat, a 20kW gas combi is more than adequate, so fitting one is sure to satisfy demand, even if the house has a water tank and a bath.

Designing a system to replace it with a heat pump needs a lot more knowlege and care. Heat pumps work at lower temperatures and higher primary flow rates. You have to consider insulation, solar gain, drafts, room size and whether the water tank or the bath are still needed by the occupants. Instant showers are now common and washing machines heat their own water, for example. Also there are several types of heat pump, transferring heat between different media.

See also Heat Pumps

Some examples

Much of the advice on Central heating design is still relevent, but the lower temperatures might dictate a new strategy rather than just adjusting radiator sizes.

System Design Considerations

  • Are bedrooms much smaller than the living area? Perhaps you need two separate systems!
  • Do you use the bath? Perhaps an instant shower would suit.
  • Do you have and still use a hot water cylinder? Perhaps localised instant water heating would be better.
  • Do you have a garden, a balcony or a greenhouse? You have much greater options for fitting a heat pump.
  • Do you have single-panel radiators? You can double the radiation source easily.

Changing to heat pumps will always mean needing good insulation first: walls, roof and floors too if possible. Get those done before you even think about making the change.


Having the source outside means you must transfer the heat through the walls and/or floors into the house. The usual prescription is to use the refigerant cycle because the pipes are smaller. Heat is released into a heat store of water inside the house then re-distributed as required.

Monobloc heat pumps heat the water outside. Many are used to heat swimming pools, for example. They can run under-floor heating directly if the manifiolds can be placed near the pumps. The usual single-line 22mm pipework found in boiler-based systems is only capable of handling enough low-temperature water to transfer (say) 3-5kW of heat, only enough for a small flat of annex. Bigger monobloc pumps need greater water flow so bigger piping, multiple circuits or manifold distribution.

DIY Activity

Much of the work can be a DIY project. If using a Monobloc unit, all of it can be DIY really. It's discouraged, but don't be bullied into taking a package sponsored by the big energy companies or Government. With DIY, you don't need to retire the gas boiler until the new system is in place so the work can be done at a leisurely pace. Pipes, cables, drill a few holes, get a greenhouse or conservatory if you feel like it. See also Talk:Heat Pumps