Air Source Heat Pump - a retro fit in an old house

  • 13 January 2020
  • 17 replies
  • 1023 views

My wife, Fiona, and I own an old Surrey cottage that dates back to 1846. Historically draughty and heated by an oil-fired boiler, the house was in need of renovation which, after a lengthy interaction with local planners, we commenced work on in May 2019.

What made you decide to change your main source of heating?

Having a keen interest in new tech, I wanted to look at different ways of making the house ready for the next 175 years of its life. Single glazed windows and lack of insulation were an obvious target to improve the heat retention but I'd always disliked the use of oil for the heat source - smelly, inefficient, and prone to running out when you hit a cold snap, let alone the increased cost combined with the likelihood of decreased availability of oil resources. I looked at solar and then became increasingly interested in Heat Pumps, particularly an Air Source Heat Pump (ASHP).

What is an Air Source Heat Pump?

Simplistically, think how a fridge works extracting heat from the air inside to make it cold, and reverse that thinking. It extracts heat from the outside air, even down to minus 4 degrees, and then turns that into useable heat, sufficient to create domestic hot water for showers etc and also for radiators and in our case underfloor heating. What you have to get your head around though is that it operates 24 hours a day at a constant but usually lower temperature than traditional oil or gas fired boilers. Key to making it efficient and successful is to insulate the house very effectively. During the renovation we installed double glazed windows throughout, took up floors and added 100mm of insulation, added insulated plasterboard, and 200mm of loft insulation whilst undertaking the major renovation. It all makes quite an impact.
Other added advantages of using an ASHP are that the Government has a scheme to incentivise the take up of alternative heat sources and gives you a grant over several years to help pay back the investment. Reports also suggest that there are significantly lower running costs once you've made that initial investment. Generally, there is no requirement for planning.

What does the equipment look like?

Think about air conditioning units - the ASHP is roughly the same sort of size; ours is a 15Kw system made by a German company called Vaillant and uses some new tech to make it even more efficient. The outside pump is actually about 1.5m tall and the fans run at 52 decibels - astonishingly quiet, less than the spoken word apparently. You do need to find a place that is right to locate the pump so it's not an eyesore. Inside the house there are a couple of other pieces of equipment but they're all wall mounted and then link into the heating pipe work. All the system was installed by Derham Ball who specialise in heating engineering systems and certainly know their stuff; in fact, the owner is also a Pure Planet customer - I introduced him and received my Amazon voucher as a result.



What are the results of the installation?

So far, brilliant. My wife 'complained' she was too hot which is a heck of a turnaround from where we used to be in years past. Obviously, it uses more electricity than we had been used to but that is offset by not forking out seasonally exorbitant prices for oil. Being a Pure Planet customer, I'm assured that I'm now a significantly more 'green' user of power and heat. I can't say I'm a green warrior, per se, but I like the fact that I'm now using a sustainable source of heat as well as power, at lower cost, and have a much warmer home as a result.

An appendix

I wrote this a few weeks back and two things have happened since then. The first is a certain frisson of excitement in the Gulf has caused oil prices to increase by around 5%. I'm now longer subject to these fluctuations in price. Secondly, I had to renew my household insurance. For the first time ever I was specifically asked whether I had oil fired heating - they explained that insurance costs now take into account oil as a heating source. Interesting.



17 replies

Hi @RoyTheBoy
Welcome to the community!
Thanks for posting this, it's fascinating. 👍
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I was just wondering what the ballpark cost of the equipment, and of the installation is. Does the Government grant cover both?

Also - and sorry if this is a daft question - how does the heat circulate around the house? Does it go via vents, just like air conditioning?
Hi Marc

The cost - how long is a piece of string - but think around the £16K mark for the equipment and installation. This is paid back over about 7 years by the grant. So for the home owner its a question of the time value of money; you're investing then get it back over time.

Not a daft question about heat circulation - the heat pump heats water which serves a mix of standard radiators and underfloor (water) heating. So just think of the ASHP as a replacement for your boiler - serves a megaflo for the hot water aswell.

R
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hi Roy
​​​​​​​What a great post!
Do you happen to know the typical output of the unit in kW?
Hi Woz

It's a 15Kw beastie. But that doesn't really answer your question.

​​​​​​​Naturally the consumption changes - at the moment the master screen is showing 41KW hours per day. But you may well be stretching my knowledge if you delve deeper with any further technical questions!
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Haha Thanks, yes that answer does pose more questions, but it's near enough to get the fag packet out and do some sums. 41kWh/day doesn't seem much when it's cold but then it depends on your insulation, and of course that could be an aggregated average including warmer days.
RoyTheBoy;46193:
Hi Woz

It's a 15Kw beastie. But that doesn't really answer your question.

​​​​​​​Naturally the consumption changes - at the moment the master screen is showing 41KW hours per day. But you may well be stretching my knowledge if you delve deeper with any further technical questions!
RoyTheBoy;46169:
Hi Marc

the heat pump heats water which serves a mix of standard radiators and underfloor (water) heating. So just think of the ASHP as a replacement for your boiler - serves a megaflo for the hot water aswell.

R


A-ha! Thanks @RoyTheBoy
That sounds excellent. 😀
Now all you need is some solar panels on the roof, and a storage battery in the garage, and you'll be laughing.

Sounds like a big job though. A complete house renovation. Your post says the work started in May 2019. It took about six months to do, then?
Still ongoing. 7 months in a caravan... back in just before Christmas 😀 But all worth it.

​​​​​​​The idea of storage battery is in the back of my mind but I'll wait until the tech is more mature and the cost comes down.
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Informative post guys, it's made me want to delve a little deeper into the technology.
Thanks 👍👍
Just a thought @RoyTheBoy, have you endured very cold weather yet? My experience with ASHP is that when you need them the most (when it’s say -6°C outside) they produce less heat as they have to pause to defrost the external unit. Does you Vaillant suffer from this convenience drawback?
Hi @25quid

The downside of an ASHP as you suggest is that it is a challenge to extract heat when it's in the minus range. Allegedly the set up works at down to -4 degrees. Whether there is a defrost function that hinders the performance I haven't yet experienced yet. But to add perspective we don't often get to the minus range and I then have two solutions - to leave the system working for longer or secondly to light up the log burner! So say that affects 5 days out of 365, I can cope with that. If we really want to get despondent though if the power grid fails then we're really stuffed, but I'm a glass half full sort of guy and we'll meet that challenge when it occurs.

​​​​​​​Hope that helps.
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but then everyone with a boiler is screwed if the power grid fails, the only reliable backup is portable calor gas heaters (or for the really well prepared an emergency generator and/or massive battery backup)
RoyTheBoy;48468:
Hi @25quid

if the power grid fails then we're really stuffed, but I'm a glass half full sort of guy and we'll meet that challenge when it occurs.

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You are, of course, absolutely right. How silly of me to write that.
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RoyTheBoy;48484:
You are, of course, absolutely right. How silly of me to write that.
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I agree with Duppy, not silly at all, it's a good point.
RoyTheBoy;48484:
You are, of course, absolutely right. How silly of me to write that.
However, a gas fire is a good supplement in the case of a power cut; no electricity needed. Just leave the internal doors open. 😉

Good luck @RoyTheBoy in the next cold snap. If you do have a defrost function, it keeps the system working periodically which is better than nothing.

Just chipping in here with our experience / thoughts.  We’ve been running an ASHP in Bath for about 6 years now in an old-ish but well-insulated 5 bed house.  It’s a 7kW Daikin split unit and heats the house via underfloor in the ‘new’ extension and oversized radiators in the ‘old’ part of the house. 

The system was part funded by an Ofgem RHI grant which amounts to a little over £7K, paid quarterly over 7 years.  The system was installed as part of a bigger refurb project which made the capital outlay easier to swallow - and we’d have needed to spend £2K+ on a new gas boiler anyway.

Doing the maths in advance, the ASHP worked out cheaper to purchase and operate over 7 years compared to a gas boiler system - but it was a marginal advantage.  In practice, that seems to have played out - but it’s impossible to A/B test these things.

One disadvantage is that because we’re all-electric now, our average electricity bill appears to be HUGE - around £200 pm, & closer to £300 in winter.  When there’s also a gas bill to pay, it reduced the electric bill and both seem a bit smaller, although probably amount to the same.

My main gripes about our ASHP are about the controls and flexibility - especially when combined with underfloor heating.

First, whatever the actual heat source, underfloor heating is particularly unresponsive and slow to heat / cool.  Ours is on a concrete base (great as a thermal battery), but it means that in winter, the only sensible thing to do is to ‘heat’ it non-stop, 24-hours a day.  The alternative is to let it cool on warmer days - but that means it’ll take the more than 24 hours to get up to temperature again on a cooler day.

I believe the heat pump operates more efficiently in this ‘always on’ way too - but again, it’s tricky to do an accurate test or A/B comparison.

In the radiator part of the house, because the output of an ASHP is a lower temperature than a typical gas boiler, it means the radiators need to be bigger and operate at a lower temperature.  Subjectively, it feels like the house takes longer to get warm - say when you come back after a 2 week holiday and the heating has been off.  If you like the instant heat from a gas boiler, then an ASHP will probably disappoint.

Our experience is that this is less of a problem than we experience with the slowness / lag of the underfloor system.

One ‘inefficiency’ inherent in when an ASHP is used for domestic hot water heating.  First, the heat pump struggles to get the water hot enough - so the back-up immersion heater kicks in to do the last bit.  This is the same kind of heating element in a normal hot water tank, so you notice it speeding up the electricity meter when it kicks in.  I have no idea how much this contributes to our electricity bill though.

Another hot water gotcha is that because the hot water stored by an ASHP system is at a slightly lower temperature than in a gas-powered system, the system has to do a boost once a week  - the legionella cycle - to kill any potential bugs that might survive at lower temperatures.  I don’t believe there’s much real risk, but this weekly hot water cycle does mean that you get surprisingly hot water for a day and it keeps the electricity meter spinning.

The real killer for an ASHP for me is the controls - they are utterly unfathomable on our Daikin unit.  It could just be Daikin, I have no experience of other heat pumps, but they couldn’t have designed a less intuitive interface for programming the unit and understanding what’s happening.

Once, one winter, when we started getting cold, we took a look at the indoor part of the Daikin unit and we couldn’t even tell if it was working or not.  Eventually, it turned out to be a blown fuse in the unit, but it really is a black box that’s been programmed by heating engineers.  No exaggeration, you need to understand Hexdecimal code to input some parameters.

Overall, I’d say the system has been reliable and worked OK, but regular plumbers won’t have anything to do with the ASHP side of things and also shy away from having anything to do with the ‘regular’ plumbing  - just in case ..

Would we do it again?  It’s a qualified ‘yes’. I don’t know how green ‘green gas’ is but I prefer the idea of separating my heating system from the burning of stuff somewhere.  I know that’s not necessarily the case with electric heating - but it’s a step in the right direction.

I’d avoid the slow / unresponsive underfloor part of the system and I’d choose a heat pump based on its ease of use / configuration.

One disadvantage is that because we’re all-electric now, our average electricity bill appears to be HUGE - around £200 pm, & closer to £300 in winter.  When there’s also a gas bill to pay, it reduced the electric bill and both seem a bit smaller, although probably amount to the same.

Thanks @martindell for your fantastic and detailed post.

I have to admit my knowledge of air source heat pumps is a bit lacking… to say the least!

But this comment about being elec-only I did find v interesting too. You’re doing more for renewables though.

And in the future I suspect it will have swung the other way, gas will be scarcer and much more expensive.

At least, that’s what I’ve seen predicted.

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