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Air source heat pump with oil combi?
Question
I’m being offered an air source heat pump installation through a very good offer via my employer. My existing heating system is a new modern oil combi supplying rads, all fitted with thermostatic valves, digital room stat and no stored hot water. The heat pump being offered is an 8kw Samsung which will supplement my oil combi by providing low level baseload heat. I’m very sceptical over the figures in the survey that was completed and predicted running costs / savings and my fear is that the heat pump will be at best a cost neutral noise machine. If I had a hot water cylinder I could see potential benefit but I don’t so payback is purely via central heating and the contract states if it doesn’t provide the predicted payback to my employer I am liable to make up the shortfall. I estimate my annual oil useage to be 1200 litres for heating and hot water for a large 4 bed house and 4 adults. Anyone got any experience of such a hybrid system?
6 Replies
OMS
690 Posts
An oil combi will have a small amount of domestic hot water storage to give you service whilst waiting for the burner to purge and fire - so there may be a very small saving to be had there, but the heat pump needs to be at least pre heating this tank

As for heating, the rads will have been supplied based on the much higher flow and return temperatures of the oil boiler - so to run them at much lower temperatures needed to make the heat pump even vaguely efficient means they can only provide a relatively small proportion of your heating demand during mild, mid season conditions - so it's a balance point as to where the heat pump goes off and the oil boiler fires. If the heat pump can't output more than the oil boiler return temperature, then it's adding nothing if operating as a parallel system. The alternative is to use the heat pump down to a specific external ambient and then fire up the oil again

It can work, in this fashion, but would be unusual, and I most certainly would not get tied into making any top up payments due to lack of performance, unless the contract works is also looking at controlling the boiler flow and return temperatures and looks to swap out the heat emitters

For the capital costs involved, you'd be better off improving insulation and airtightness 

Regards

OMS
Alan17
2 Posts
As a domestic heating maintenance engineer, I rarely recommend heat pumps as a retrofit. The insulation in the property needs to be done to a very high standard for it to be even remotely cost effective. I also recommend they are used with underfloor heating rather than radiators for the lower temperature reasons OMS mentioned. Alternatively, the radiator sizes can be increased.

Going by the Sutherland Tables they are still much more expensive to run than oil. I was going to post a link but apparently they charge money to look at their data.
 
Andy C
11 Posts
What they said.
According to the latest energy cost comparison figures (July 20) kerosene is running at about half the cost of ASHP, so as OMS says spend your money on improving insulation instead.
Helios
70 Posts
mmm Interesting approach , I think for some dwellings the air source heat pump may be a good option , I suppose the best place to start is the future and new build houses , given the thermal performance that can be attained an air source heat pump may be all that you need , connected to a 10kw solar PV roof (need a large south facing roof) and collect the heat from the solar pv cells and I guess you have self sufficiency . There are some big solar thermal water storage tanks now so this negates the electric you might need for water heating , but as ever its how things go in the shortest days in the UK with solar systems .
As for your oil system , not every house is super thermal effiicienct and I know lots of people who find stuffy modern houses too much , and have the windows open with the heating on, which has always seemed a bit nuts to me . If you are able to recover the heat in your flue as part of the design , then that should be efficient , even if you dont recover the flue heat , you will need to see how it works in a few winters .
The main problem with air source heat exchangers is in hot climates , where they want cooling , but if you do it , have oil burner serviced /cleaned and try to think where you can put your monitoring points , if you have large wood beams , keep any air flows away from joints as it will dry them out .
its so unusual , you will have to think of it as a venture to look and see, the main thing with oil is that in the future it may treated differently as a fuel , there are some bio ethanol plants and if the biodiesel plant comes along that would replace oil , however undoubtably at 90% efficiency the gas boilers we have now seem to offer the best for wet circulating systems .
Helios
Simon Barker
706 Posts
I doubt solar PV would be much use for heating in the UK.  Most houses don't have enough South-facing roof space for more than about 4kWp of panels.  And in a British winter, the generation is very hit-and-miss.  On a grey cloudy day, the panels may only be generating tens of watts.  Even when it is sunny, the sun is low in the sky, so the useful generation period is quite short, and the evenings are very long.

If things start getting hot enough in the summer for the British start installing air conditioning, that's when solar PV would come into its own.
Helios
70 Posts
There is heat to be collected , but depends on heat sink on back of cell which adds weight , new graphene solar cell based architecture is claiming a 20% increase in electrical output so good on electrical output , and yes most roofs are not south orientated and 4kw is about as much as silicone cell architecture can manage , which is why I have never been happy with these sort of community generation schemes where you sell electricity to each other. shortest day in Scotland is about 6hrs and as you say you wouldnt reach peak output anyway , however in summer in Scotland 14 hrs on longest day , I guess it offers some choice in systems and the solar air conditioner for hot climates that have more solar seems a good fit , but basically I agree the performance is highly variable and nearly non existant at our latitude in winter , and solar cells as yet arnt good on the recycling. 

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