Heat pumps and boilers?

[Graham Harrison]

Is it possible to use both a heat pump and boiler in the same system?

If it is what are the disadvantages?

[Smolley]

I have an electric boiler for DHW and anti legionella sterilisation, because the heat pump can't heat DHW hot enough and the radiator system at the samw time.

[SH]

[Theo]

Yes. Grant in particular (being an oil boiler mfr really) have support for this.

Plus: Resilience of supply (pricing arbitrage, power cuts) Possibly lower running costs if you're in very sub zero temps (Scottish mountains etc)

Minuses: Maintaining two heat sources Complexity Installation cost Space CO2 emissions

Basically it could make sense if you have a huge listed stone farmhouse with crap insulation, but it's overkill for the average suburban house.

Theo

[alan_m]

I've seen a few videos on hybrid boiler and heat pump systems. They have been in large properties such as hotels etc. where it's been the solution for piece-meal renovation. Large amounts of hot water being required in short time periods and where rooms are being renovated over a long time scale and so radiators in many rooms may not be ideal for a lower flow temperature.

[Fredxx]

Doesn't your heat pump system have the ability to just heat the DHW above 60C say once a month?

Even at 50˚C 90% of legionella dies in between 80 and 124 minutes depending on the strain

[Theo]

Electric boiler (separate box on the wall) or immersion heater (coil in the hot water tank)? Every hot water tank has an immersion heater (they are cheap), but they don't do space heating.

You can have an inline immersion in a buffer tank too, which would do space heating but an 'electric boiler' is typically much bigger (6-10kW+). People generally only fit those as a cheap install solution (why if you're paying for a heat pump, as the electric boiler has horrendous running costs) or as a backup (in a care home or somewhere the heating absolutely cannot fail).

Theo

[Smolley]

I can manually divert heat pump flow to DHW coils, but water temp doesn't exceed 45C and of course there is no sterilisation cycle.

[Fredxx]

That's a shame, 50C would be the absolute minimum. 60C is the recommended temperature.

[Tim Lamb]

I haven't done it but wonder if our thermal store gets round this issue. Mind, directly heated mains water, target 45C is going to make an uncomfortable bath:-)

[David Wade]

Personally, I would go for separate air source AC style heat pumps, then you get the benefit of being able to have cooling in the summer, but with the disadvantage you still need your original system for hot water...

... unless you add solar panels or solar water water heating....

Dave

[Theo]

Solar hot water might cover 60-70% of the year, but the piping and maintenance is a faff. PV would be easier but need more panels.

If you want cooling, an air to water HP will do that too - you just need to install fan coils rather than / in addition to radiators. Radiators are useful for heating as they're mostly silent, which forced air (fan coil or air2air HP) isn't.

Just depends on what your priorities are really...

Theo

[Smolley]

[Andrew]

If the boiler is mains gas then you will be stung for gas standing charges as well as electricity.