Hybrid Air-Source heating

Yes this is me talking about being ‘green’ :wink:

Comments/advice please:

We’ve been looking at purchasing an old stone/slate cottage. At the moment the heating is radiators via an old oil fired Aga. The current heating and hot water system is very old, e.g. no room stat. or even tank stat., so it will all need replacing what ever we do: A pressurised tank and new boiler system for a start.
Plan a) was compete air-source and then an lpg Rayburn. Turns out you can’t get an lpg cooking only Rayburn/Aga. Plus limitations on heating. Now I’m looking at plan b) which is a hybrid system. This means primary heating/water is via an oil Rayburn (oil because there’s a newish plastic tank there and installing lpg means ripping it all out and oil is generally cheaper as well) but air-source as a complimentary system.

Which ever system underfloor heating is out. We’re talking low ceilings, concrete floor downstairs. We would therefore have to stick with the existing wet radiator setup. I was doing some reading and it looks like using radiators is an option with a hybrid setup but you need to use alluminium rads. and double the rating, e.g. if the radiator was a single then put a double (P+ or K2), maybe even add more radiators if that’s possible.

Really I was after any comments or words of sage advice. Has anyone else gone down the air-source road? Any advice ref. costs, maintenance, ongoing costs, installation, sensible plan?



I looked into heat pumps briefly towards the end of last year: ground source simply too costly to even consider. Air source still extremely expensive compared to conventional boiler though I would still consider. I have concerns over their noise, especially if you live in a quiet neighbourhood (the planning legislation only requires less than 43 or 45 dB at the neighbouring property window. That is a very distinctly audible level in a quiet environment, and depending on house layout could be more so at your own property. Might not br a problem in reality, but I’d want to visit a house with one of thd same type installed and working in an area of similar environmental background noise before committing.

Re water heating, I’d want to combine with solar water heating. Also possibly a wood burner with boiler. Re thermal store I’d want to check that the specifications for myself: I haven’t been impressed with friends’ cylinder-based hot water supplies in modern systems, instead finding the flexibility offered by a combi boiler far more effective - but I suppose that is because I specced the combis I’ve had, whereas the cylinder systems will have been specced by the installer probably on a standard model just based on a few size of home or similar…

The only advice I can offer is, if you decide on the LPG option, check availability of LPG registered engineers in your area. When we lived in Cumbria, there were only two registered engineers in the area, and neither was taking on new customers. We could only get emergency work done. I taught myself to service the boiler and to replace the thermocouple device which seemed to burn out every few months. I think it’s illegal and no doubt dangerous for unlicensed people to service a boiler, so I’m not recommending you do what I did. Just make sure you can get on a registered engineers customer base.


In our old house there was a neighbour next door-but-one that went the air-source route; must be 10 years ago now… in fact about the same time we installed a new oil boiler. I looked then at air-source and the costs were daft. tbh I don’t think I heard mention of hybrid systems. Anyway we live in the country; no road noise, totally quiet nights. You could certainly hear when their system kicked in. I’m not saying is was objectionable but you could certainly hear it from 50 yards away and they had their fans arranged to blast upwards. Something, as you say, I’d have to look into.

Our last house was a cylinder setup and I intentionally went that way in order to run shower pumps (properly configured so the cold water comes from the main header tank to avoid fluctuations when people turn taps on) and I can’t say I found it inconvenient having to wait for the tank to heat up. tbh it only used to take 15 mins so if you were thinking of having a shower then by the time you’d fannied around getting night things out, cleaning teeth, &c. then the water was hot. However a proper pumped shower is SO much better than a thermostatic shower off a combi. Our new house is combi setup and the shower, brand new high end brand, is good but not as good as the pumped setup at our last house. Heck with the pumped setup you had a power jet mode that would pummel you like a massage! If you turned it on full power you could drain the tank in 5 mins and the drain away couldn’t cope after a couple of mins!

Next time I’d go for a pressurised cylinder… I’m told you don’t need shower pumps then.

I would look into an electric boiler-based system and installing solar panels to generate electricity to sell back to offset costs. It would be easy to replace all the radiator valves with thermostatic valves and add a thermostat to the hot water tank. Also, I think adding additional insulation, possibly on inside walls to preserve the character of the building would be worth considering.

1 Like

Well. with all due respect, we won’t be putting panels (water or electric) on the roof of or super pretty 200 year old slate roofed cottage. In fact on some of the properties we’ve looked at that DID have panels on the roof we were planning on immediately ripping them off! :wink:

Yes… TRVs and stat is first consideration but also the rads are a mish-mass of old inefficient style so they need changing. tbh this is all easy and relatively cheap. The current Aga must be 20+ years old. Things have changed in 20-30 years. It’s probably no better than 65% efficient. A new Rayburn would be 85%. I could even go for a condensing Rayburn which is 95%.

The walls are very thick stone. It would have to surface insulation. The rooms are already small (and yes I am a little concerned where to put the ATC 50 speakers!) so loosing three inches off each wall is less than desirable.

Nothing’s easy is it? :frowning:

I suspect you also need to consider that the fans will get noisier through age also

Just going green on the heating side is avoiding the issue of retaining heat in the first place. Walls can be insulated internally or externally. I used 50mm PUR fixed with baton, vapour membrane and then plasterboard for internal insulation. Reveals could only use 25mm PUR. Floors and roof also. It doesn’t have to detract from the character of an old property. It should enhance the value and have a longer life than a heating system. The heating system can then be scaled back.



Clearly our experiences are different! (And I can’t stand trickle showers.) I’ve had combi boilers in three houses now, two installed by me, and installed one for my brother in law as well, but I specify a combi boiler on the basis of hot water delivery, then they are always good enough for space heating, it is when specified by focussing on the space heating that problems tend to arise, often being inadequate for hot water demand. Mains pressure ensures good flow capability - but a thermostatic controller is not adequate: much more effective is to include a pressure balancer between hot and cold just before the shower valve. Not expensive, but seems uncommon - however the result is never a change in temperature even if another hot or cold outlet or two are turned full on suddenly. You can have two showers running at the same time, with the second slowing the second a touch when turned on but still both capable of being as fast a flow as many people set. Doing washing up as well as two showers just slows just a bit more but still very good flow. The pressure balancer also helps maximise the life of the thermostatic valve. IIRC in my present house you can have two showers running at 40C with winter mains temperature and achieve 8L/min on both (without time limit) 8L/m being my yardstick flow rate as I had previously measured the flow of a shower I really liked.

With a good combi it doesn’t matter when you want a shower, or how many visitors you have, or whether people enjoy 5 minute showers or 20 minute showers. they can all have showers as soon as a bathroom is free, ditto any other use for hot water. I’ve visited other people’s houses - including recently built brand new ones with modern cylinder systems, when there have been 12 people staying: aside from the inevitable queue to have 6 showers in each of 2 bathrooms, there is an need to for additional waiting time for the water to be warm enough, with sometimes people suffer getting in the shower and it turn cold. And in some cases the householder has to adjust settings to be ready for the visitor needs - something that can, and sometimes does, get forgotten.

I don’t ever want a less effective hot water system than I have now! That is a challenge a heat pump based setup would have to face - this doesn’t mean it is insurmountable, just what I would be looking at achieving somehow.

1 Like

Oh I don’t get a change in shower temperature… the shower controller is good enough to always maintain the correct setting. The problem is the flow drops. tbh the flow on our pumped system was 12l/min all the time… no change.

Yes one option is to use 25mm kingspan and then overboard; so some compromise on space but not too OTT. We’ll almost certainly end up getting all the rooms totally replastered - that seems the norm every time we take over a new house. Also overboard the ceilings to cover the wood chip :frowning:

Buy an electric cooker for heating your food.
Buy a Daikin or Mitsubishi ASPH for heating and DHW. DHW could be stored in a cylinder with an option to fit an electric immersion heater, so you could enjoy yourself in the shower for hours. The government would probably give to some form of financial incentive to do so. :grinning:

In commercial buildings, (big and small), ASHP’s are almost always used these days. Gas boilers are so out of fashion. :face_with_hand_over_mouth:

1 Like

You mean give up on the Rayburn…? Nooooo… :slight_smile: A Rayburn is a must have. :slight_smile: We had a Rayburn in our last but one house, so 20 years ago, it was brilliant. You have to get used to the quirkiness but once you have there is nothing better to cook with. People that have Rayburns or Agas get all evangelical about them. When we had a Rayburn my wife used to cook all the time. I admit, first year it took her three attempts to get the perfect Christmas cake but when she did it was amazing and, tbh, the other two I took to work where everyone thought these ‘rejects’ were excellent :slight_smile:
When we moved and went to a ‘normal’ cooker (so induction hob and fan oven) she refused to cook other than beans on toast or bacon and eggs (which she would burn).

We have an air source heat pump - replaced an oil boiler last summer. The ASHP was £10k (relatively cheap as we have high levels of insulation) and we will get all of that back over 7 years via renewable heat incentive. A new oil boiler would have been £5k so net capital cost of ASHP is minus £5k. And we have a share in a wind turbine so a fair bit of the energy cost is free - at least on windy days. I would be reluctant to try an ASHP unless the house was very well insulated - it could cost a fortune to run

We are just about to replace an oil fired Rayburn with an everhot (electric range oven) so completely off oil - can even get rid of the enormous oil tank. And again the wind turbine will supply some of the power. The Rayburn has been getting a bit more temperamental and the annual service with new parts is averaging over £500 pa over the last 3-4 years. No service cost on an everhot - though that won’t kick out as much heat as an aga or Rayburn

The reputation of aga and Rayburn seems to have fallen through the floor in the last ten years

We also have a wood burning stove but the back boiler had to be taken off to prevent the RHI being based on a metered payment (which would have lost us a lot of RHI I think).


Good insulation is essential - better than building regs.
ASHP provide enough HW is you use super efficient / insulated storage cylinder. Solar is a good top up option.
ASHP can provide background heating, preferably underfloor, but insulation must be top notch.
Everhots are brilliant, mine keeps the kitchen warm too.

I have ASHP in two properties; a 300 year old listed stone cottage, and 1950’s wooden kit house. Both have wood burners, essential in the cottage, never used in the kit house.


Hi, i am a heating engineer with over 35 years experience in the trade, gas safe, also been oftec (oil) for about 15years, but not any more as heat pumps have really taken over from the oil.
But air source, needs a very well insulated house, or it simply wont get your house warm, let alone hot.
Underfloor heating on the ground floor is easily the best way, as for one your floor is insulated, air source runs at around 34 degrees into the heating, ideal for underfloor, rads not so great, as they really need atleast 50 degrees to work and even better at 70 degrees.
Air source is a slow heat, very slow, and thats another reason underfloor works so well, as it retains the heat for days unlike a rad that will be cold in minutes.
Just insulation on the walls, will obviously be better than nothing, but good windows and doors, installation in the floor and good loft insulation is really required, to make it all work as it should.
Also pressurized hot water cylinder, you can not heat this up using a log burner, etc, it needs to be a controlled heat source, or this has just become a bomb under the wrong circumstances.

Any questions, then i will try to answer them


What is that? (I don’t live in UK but will be moving back to the North East possibly some time next year.)

On a new installation, you get 12 months to register it all and fill in the forms to be able to claim back money against what you use.
Your system with be electrical metered on the heat pump only and the immersion heater, these 2 meters will record the amount of units you have used.
These readings are then sent off and you then receive a payment.
This runs for 7 years and your system will have a maximum limit set to it, on what it likely to use, so you can’t just leave it on max, 24/7 and get paid for doing it.
But the normal pay back is about £6k on a 11kw air source unit over the term

1 Like


Modern ASHP maximum LTHW flow is 75C to 80C. Although with weather compensation it will only need to run this temperature at morning startup or during very cold weather.

Below is a heating schematic of a small office building I was working on afew weeks ago. If you look at the spec for the Hydrobox the max temperature is 75C.

They do similar smaller units for houses, again, with an outdoor and indoor unit.

1 Like