Combining gas boiler and heat-pump

[aekostas]

Hello,

Despite attic and cavity-wall insulation, my property, certainly the stick-out lounge, loses heat, so it is not ideal for heat-pump. No, I am not rebuilding it to add underfloor heating.

I saw that Vaillant do the aroTHERM hybrid system that uses tariff information and weather sensors to decide if to run air-to-water heat-pump or boiler (or both, I think) and can be combined with an existing boiler. It also reduces the glycol requirement, which I read as a concern in other threads here.

The question I have is why is it impossible/ineffective to use the heat-pump as a pre-heater to a standard boiler? I assume that that would reduce the complexity of the hybrid system, with no need for calculations of which tariff to use etc; when the boiler kicks in, it gets help from the heat-pump, that's all. Happy to assume a water tank, I have one.

Many thanks in advance for any answers.

 

[stem]

Technology might have moved on, but when I looked into heat pumps a few years ago, they couldn't match boiler output temperatures, which is why they are used with underfloor heating, which has a much lower temperature but over a far greater surface area than a radiator.

Therefore, with regard to your question, I believe it's because the return water temperature from a radiator system running under winter conditions could still be higher than the output temperature achievable from a heat pump, so the heat pump wouldn't have anything to add. With ASHP's in particular, the heat they generate drops off significantly when the outside temperature falls below about 5 degrees and they then start having to spend a lot of time using the heat they generate to defrost themselves instead of heating the property.

In mild weather, radiators don't need to get so hot to warm the house, so a heat pump might be able to manage.

 

[vulcancontinental]

While the company I work for design boilers to work in hybrid systems with solar and heat pumps controlled by a single controller we do not import or support into the UK at the moment so I can't help there.

What I will ask is what are your motivations? Are you off the natural gas grid? Do you think it will save money, do you think it will reduce CO2 emmissions by an appreciable amount?

A heat pump will cost several thousand pounds plus incorporation into the existing system which will need adaptation. Only if your fuel consumption and unit cost is extremely high and you are able to obtain subsidies does it become even remotely financially viable with NG at 3-4p/kW. Electricity is over 15p/kW.

With regard to CO2, natural gas is burned very efficiently at the boiler, perhaps even 90%. 40% of generated electricity is achieved by burning natural gas at lower efficiencies and then substantial energy losses are incurred in transmission compared to the nation gas grid which loses comparatively minuscule amounts of potential energy.

 

[aekostas]

Thanks for all the replies.

I am a proud owner of a 1987 Potterton NG boiler, using unpressurised circuits for heating and hot water (tank). I do not have solars (because Scotland ).

I think that the NG price will change to the worse and I know that I can pick an electricity supplier that uses green sources or nuclear to combat CO2. My question was mostly triggered by curiosity about the technology; I was hoping there was something to gain and clearly there isn't.

 

[Gasguru]

Heat pumps whatever the type are nothing but a cash cow for the manufacturers...study in detail the COP figures and you will see that unless installed under perfect
conditions they never achieve the claimed performance.
Now add in the huge cost of installation and the extortionate repair bills (when not under warranty or voided) and they make no sense.
Also consider that a suitable property for heat pumps (ie. with large low surface temperature radiators/underfloor loops, excellent insulation and draughtproofing) will use
so little energy for heating anyway and that makes the payback and advantages even worse.
Hot water being stored at such high temperatures has no advantage being heated by heat pumps (the COP is very low).