Is re­new­able tech takes warmth from the air and re­pur­poses it for space heat­ing. But is it the right op­tion for your pro­ject? Our ex­pert Q&A goes be­yond the ba­sics to an­swer the in-depth ques­tions you re­ally want to ask


What should you look for to make sure your ASHP is spec­i­fied cor­rectly?

Robin Ad­der­ley: Al­ways ap­point an in­staller who is reg­is­tered with the Mi­cro Gen­er­a­tion Cer­ti­fi­ca­tion Scheme (MCS). This will en­sure you get the best ad­vice and gives you ac­cess to the Re­new­able Heat In­cen­tive (RHI).

Phil Birchenough: Your in­staller should al­ways look at the ef­fi­ciency of the house and what you need from your heat­ing so­lu­tion in or­der to of­fer you the best so­lu­tion. As with any sys­tem, up­grad­ing in­su­la­tion in ex­ist­ing homes is im­por­tant. You should also be con­fi­dent your en­gi­neer fully un­der­stands the prod­uct they are fit­ting.

An­drew Mclauch­lan: The unit should be specif­i­cally sized to ac­count for lo­cal tem­per­a­tures, too. It needs to be able to pro­vide all the warmth a prop­erty de­mands, right down to this de­sign tem­per­a­ture. If an ASHP isn’t pro­vid­ing enough heat, then it’s been sized in­cor­rectly.

In 2013, the En­ergy Sav­ing Trust (EST) re­ported that most ASHP in­stal­la­tions weren’t per­form­ing as well as ex­pected. Has much changed since then?

David Hilton: The EST re­port looked at the Sys­tem Per­for­mance Fac­tor, tak­ing into ac­count the heat losses of the whole house – it wasn’t just mea­sured across the in­let and out­let of the heat pump it­self. So one of the key things to come out of it is that we should al­ways com­pare like-with-like when we’re look­ing at things like the Sea­sonal Co­ef­fi­cient of Per­for­mance (SCOP). The other ma­jor find­ing con­cerned in­stal­la­tion qual­ity. At the time, ASHPS were of­ten be­ing fit­ted as if they were boil­ers, rather than tak­ing into ac­count the tech’s unique per­for­mance cri­te­ria. We’ve moved on quite a bit, and there are now more in­stall­ers with spe­cific heat pump ex­pe­ri­ence.

Why do some sup­pli­ers still list the COP (co­ef­fi­cient of per­for­mance) rather than the SCOP. Isn’t the lat­ter a bet­ter re­flec­tion of the ap­pli­ance’s ef­fi­ciency?

David Hilton: Sup­pli­ers will work with the data that the man­u­fac­tur­ers give them. As a con­sumer, what’s im­por­tant is that you make sure you’re com­par­ing the same per­for­mance fac­tors when look­ing at dif­fer­ent mod­els. And re­mem­ber that, while the SCOP is more de­tailed, it’s still a bench­mark that’s been de­vel­oped un­der con­trolled con­di­tions rather than a real-life sce­nario. An­drew Mclauch­lan: If you buy a car, you’re told its top speed to show its max­i­mum per­for­mance. But you also look at the miles per gal­lon to get an idea of how it’ll per­form in the real world. We pro­vide both the COP and the SCOP on our data sheets to make sure cus­tomers have all the in­for­ma­tion they need to make an in­formed de­ci­sion.

One of the crit­i­cisms of air source heat pumps has been that they’re noisy. Has any­thing been done to im­prove this?

David Hill: If you go back 10 years, you’d have had to po­si­tion your ASHP round the back of the garage – they were so noisy you wouldn’t have wanted them any­where near the house. The good man­u­fac­tur­ers have looked very hard at this as­pect, in­tro­duc­ing clever fan tech­nol­ogy with new shapes and blades. They’ve also fo­cused on how the air is drawn in to the unit and how it comes back out. It’s im­por­tant to check ex­actly what claims are be­ing made: is that 45DBA rat­ing for when it’s run­ning at full power, or does it shoot up to 60DBA? The for­mer is around the level of a wash­ing ma­chine or loud fridge – but at the 50-60DBA range, it will be a lot nois­ier. An Mcs-ap­proved in­staller should give you a cal­cu­la­tion to show how your ap­pli­ance rates – if they don’t pro­vide this, it’s a bit of a red flag.

Is it ever a good idea to fit an air source heat pump to an old boiler-fu­elled ra­di­a­tor and pipework sys­tem?

Phil Birchenough: This de­pends on the op­er­at­ing tem­per­a­ture of the sys­tem. Older emit­ters are de­signed to work with a con­ven­tional ap­pli­ance (ie a boiler) that runs at a higher flow tem­per­a­ture. Con­nect­ing these to a heat pump will re­duce their Delta T (the dif­fer­ence be­tween the room tem­per­a­ture and that of the wa­ter within the cen­tral heat­ing sys­tem). Con­se­quently, the warmth pro­duced might not be suf­fi­cient to heat the en­tire house. So if you’re con­nect­ing an ASHP to an old pipework and ra­di­a­tor setup, an ad­di­tional source of en­ergy would be needed. The most ef­fi­cient ap­proach is to fit a modern emit­ter sys­tem, such as un­der­floor heat­ing or ra­di­a­tors de­signed to work at a lower flow tem­per­a­ture.

What im­pact does the heat trans­fer fluid have on ASHP sus­tain­abil­ity?

David Hilton: Be­cause there are so many dif­fer­ent met­als in­side a heat pump and it’s a closed loop sys­tem, the trans­fer fluid usu­ally in­cludes some anti-cor­ro­sion com­po­nents. Veg­etable-based eco ver­sions are avail­able, but in my view it’s bet­ter to go with a so­lu­tion that pro­tects the sys­tem and gives it longevity – an im­por­tant part of sus­tain­abil­ity. Some trans­fer flu­ids con­tain gly­col and sim­i­lar prod­ucts that can be po­ten­tial con­tam­i­nants. The En­vi­ron­ment Agency won’t want to en­cour­age any leak­ing of these. How­ever, there are in-be­tween so­lu­tions that biode­grade and still con­tain the anti-cor­ro­sion el­e­ments needed. Your in­staller should spec­ify an op­tion that’s com­pat­i­ble with the man­u­fac­turer’s in­struc­tions and lo­cal En­vi­ron­ment Agency guide­lines.

How much guid­ance do in­stall­ers give home­own­ers on get­ting the best per­for­mance from their heat pump?

Phil Birchenough: Post-in­stal­la­tion, an en­gi­neer should do a thor­ough han­dover and ad­vise the home­owner on how to make the most of their sys­tem. They will have de­signed the setup to de­liver max­i­mum ef­fi­ciency and out­put for the re­quire­ments of that prop­erty and should show you what set­tings are best for your needs. Nor­mally, they’ll tai­lor it to your pref­er­ences and give a full run-through of how you can change the op­er­a­tional set­tings. Ide­ally, they should also take you through some ba­sic trou­bleshoot­ing tips.

Can you use a heat pump to pro­vide hot wa­ter – doesn’t the higher tem­per­a­ture needed just slash the COP?

David Hill: Dif­fer­ent mod­els of­fer dif­fer­ent ef­fi­ciency lev­els for sup­ply­ing do­mes­tic hot wa­ter (DHW). The first step is to pair the ap­pli­ance with a spe­cial cylin­der that works on a low flow tem­per­a­ture. This is fit­ted with a larger-than-usual heat ex­change coil to trans­fer enough en­ergy from the ASHP (the high­est out­put tem­per­a­ture is 55°C) to get the wa­ter in the tank up to 50°C. Some in­stall­ers might plug the ASHP into your ex­ist­ing cylin­der, which al­most cer­tainly won’t work.

A new, well-in­su­lated house might need around 20,000 kwh of heat per year for space heat­ing, but only 3,000 for DHW. So it’s a small pro­por­tion of your over­all usage. The ap­pli­ance will only run at its high­est tem­per­a­ture for about two hours per day to pro­vide this hot wa­ter. It’s au­to­mated, so a sen­sor keeps tabs on the tank all day and boosts the tem­per­a­ture if it drops be­low 45°C, be­fore switch­ing back to space heat­ing only. High qual­ity ver­sions will fea­ture two sen­sors in the tank. This is im­por­tant, as it re­duces the risk of over­heat­ing the cylin­der and wast­ing en­ergy.

There’s a lot of talk about hy­brid heat­ing sys­tems, com­bin­ing boil­ers with ASHPS – is that an ad­mis­sion that heat pumps on their own don’t work?

David Hilton: No, be­cause hy­brid sys­tems are de­signed for larger or harder-to-treat dwellings. For ex­am­ple, you might have a stone barn where an ASHP would ad­e­quately heat it un­til ex­ter­nal tem­per­a­tures got down to maybe 5°C. Ev­ery time it goes be­low that level, which might only be 10 or 20 days a year, the boiler kicks in. So we’re us­ing the re­new­able tech in or­der to cut down oil or LPG usage, for in­stance. There are also some ASHPS in­tended to work in slightly lower tem­per­a­tures. In that case, you might have a hy­brid setup where the heat pump would take care of the space heat­ing and the boiler pow­ers the hot wa­ter sup­ply. Robin Ad­der­ley: Hy­brid sys­tems have their place. How­ever, in a new build dwelling that is suf­fi­ciently well in­su­lated, an ASHP alone should pro­vide all the space heat­ing and hot wa­ter re­quire­ments. In the case of retro­fit sys­tems, pos­si­bly in older prop­er­ties that have a higher heat loss, com­bin­ing the pump with an ad­di­tional heat source may be­come more at­trac­tive in terms of cap­i­tal cost.

Above left: This NIBE F2040 16kw heat pump was fit­ted by Car­bon Legacy

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