Simon Brooke

Air source heat pump or not?

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We are converting an old concrete block built dairy (approx. 200 square metre) which is being designed to passive house standards. We are fortunate in having three solar pv systems of 4kw which can be largely diverted to the dairy. Initial thought was underfloor heating via air source but are now wondering if it would out cheaper and simpler to have a very large thermal store (which could be located in an adjacent barn) heated with a combination of solar thermal and immersion heaters. Our thinking is that annual cost of air source assuming a 20 year life and a commissioned cost of £10k would be at least £500 which would buy a fair bit of standard rate electric. Does this argument have any merit? Any thoughts would be really welcomed. Thanks Simon

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Hi @Simon Brookeand Welcome to THE forum. You will find loads here to look at on ASHP. One of the key questions / features that seems to figure here large in highly insulated homes is the ability of the Underfloor heating (UFH) to work in cooling mode as well as heating something you cannot achieve with a pure heating system. We have added UFH to our design to take advantage of this feature as without it a passive house can probably get along without UFH. In the big picture provided you can work the ASHP at a solid Coefficient of Performance (COP) you should be able to do better than a large thermal store which brings its own problems in terms of waste heat as insulating them is very difficult. You also have the problem of heat exchange and the like. If you have PV then you can power the ASHP from it after all.

 

 

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Welcome,

 

I think that £10k for an ASHP for a PH is way too much.  Our 130m2 passive house has a 6 kW ASHP and that's a massive overkill, it just happened to be available at the right price.  It was an easy DIY install, and all told cost around £2k.  A plumber could have installed it in half a day easily, say a day to be on the safe side, so maybe £200 to £250 labour for the install, plus maybe £2.5k or so for an ASHP, less if you shop around (I bought a new one, reputable manufacturer for £1700.

Edited by JSHarris

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Welcome.

 

A couple of points: 12kW of PV sounds like a lot, but you'll generate surprisingly little during the darkest months. We have 8.5kW and this was what 2016 looked like:

 

Solar.thumb.GIF.57dc256485ee1e0406045a575e620fad.GIF

 

December only managed 146kWh, or less than 5kWh per day on average (we had a few days where we barely got above 1kWh). That's less than two hours of a standard 3kW immersion heater on full bore.

 

We have a 289mclose-to-Passivhaus Standard house with a 5kW Panasonic ASHP, which covers hot water and underfloor heating. It cost about £1800 + VAT and our electrician and plumbers installed it (monobloc ASHPs are a doddle to install) in a few hours.  The ASHP doesn't come on for probably 6 months of the year due to the PV supplying power to an immersion diverter in our unvented hot water tank.

 

The only reason I can see for such a high package cost is if you're using an RHI installer. The process does involve a lot of paperwork and keeping up certifications. That and the payback seems to have encouraged much higher prices from installers signed up to the scheme.

 

Solar thermal is expensive, requires maintenance, and tends to spend a lot of time generating no hot water or far too much hot water. Since you already have PV, you can use an immersion diverter put excess solar energy into a hot water tank. I believe it'll make hot water for a higher number of days than even the best solar thermal. Seems a no brainer if you already have the PV in place.

Edited by jack
Typo and more info

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Also worth noting that the RHI is pretty pointless for a passive house.  The additional cost of an MCS installation for an ASHP, which is required to claim the RHI, is well over £2000.  For us the RHI payments would have been around £80 a year for 7 years, so a small fraction of the premium we'd have had to pay for an MCS approved installation.  The couple of thousand we saved by not going for an MCS approved installation is more than enough to buy a complete spare ASHP, with enough change left over to get it installed.

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From a "sweat-the-asset" perspective, I think your idea has merit. Suggest you do the calculations - in our case the capital cost difference between ASHP vs electric immersion would have been about £1,100, and running costs using Economy 7 for electric immersion would be £250 lower with ASHP - so break-even would have been 4-5 years. I went with ASHP as it also has the bonus of cooling and takes up less storage space than 1,000l of water.

 

In your case running costs should be lower as you have a lot of PV. I believe a 12kWp system would be metered for export (vs assumed 50% is export) so you would lose out on the export payments if you use it.

 

If you need to install solar thermal from scratch then your capital will increase for limited benefit.

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It's down to consumption at the end of the day. A TS for storing excess Pv, backed up with the ashp for 'dark' days would be a good solution, as then you could have your DHW from a coil in said TS. Solar thermal would be pointless and expensive IMO, as it wouldn't give you the high target temps you'd need for a combined heating / DHW TS, but MAY suffice if the TS is for DHW preheat only plus space heating. You'd then need an UVC for DHW, again fed from both the ashp and excess Pv. 

How many occupants / bathrooms / what's the occupancy etc ?  

 

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Or use some of the cash saved from the £10k ASHP estimate to buy either a double Sunamp PV, or a Sunamp Stack, and get a low heat loss thermal store that takes up a great deal less space.  An ASHP, PV diverter and double Sunamp PV, installed, should cost less than £8k, so around £2k change from just the ASHP estimate, for a complete system.

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I too wonder just how you could get an ASHP up to £10K in price, I can only wonder that it must be on an RHI scheme and that includes the "MCS premium"  I have long since realised that the only ones to benefit from the RHI scheme are the installers that can get away with charging a higher price, which a lot of customers accept because they will get it back in RHI payments.  As already mentioned, a simple DIY install will cost a lot less. You can get 7KW ASHP's new on ebay under £1000, probably Chinese made so I am not saying they will be good (but they might be)

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@ProDave the list price on a Mitsubishi 8kw with controller is around £4800 - installed my local MCS installer wants £7950 for effectively a day and a halfs work. 

 

I queried the price on the unit he quoted me (a 12kw despite saying I wanted 8kw...) and it was list plus VAT... plus VAT..! So he was marking up at 20% plus any discount he would have got. 

 

That is a true MCS premium ...!

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There is a tremendous amount of profiteering going on with regard to the RHI, more than ever seemed to be the case for MCS PV installs, even when the FIT was high.  The quote we had to fit a small Panasonic ASHP was something like £6k, IIRC.  The price for the ASHP at that time was around £2.5k, so they were charging about £3.5k for perhaps a days work, at most, plus MCS certification so that the RHI could be claimed.  Allowing a generous £500 for one man's labour, plus the bits of flexible hose, Y strainer and 5 litres of antifreeze, then it seems that they were making around £3k just for providing an MCS certificate. 

 

I installed our ASHP and had it up and running in a bit over half a day, with no previous experience of installing one at all.  I did have to spend a few hours working out how to set up the command unit, but that would have been something a competent installer, with the knowledge of how it worked, could have done in half an hour or so.

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18 minutes ago, PeterW said:

@ProDave the list price on a Mitsubishi 8kw with controller is around £4800 - installed my local MCS installer wants £7950 for effectively a day and a halfs work. 

 

I queried the price on the unit he quoted me (a 12kw despite saying I wanted 8kw...) and it was list plus VAT... plus VAT..! So he was marking up at 20% plus any discount he would have got. 

 

That is a true MCS premium ...!

 

I certainly didn't pay anything like that for my 8.5kW Mitsubishi Ecodan.  £4123 + VAT inc free delivery (a bonus considering my location) for the ASHP, FTC5 controller and 300 litre pre-plumb cylinder.

 

The supplier I bought it from had similar deals / prices for Panasonic and Samsung.

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1 hour ago, Stones said:

 

I certainly didn't pay anything like that for my 8.5kW Mitsubishi Ecodan.  £4123 + VAT inc free delivery (a bonus considering my location) for the ASHP, FTC5 controller and 300 litre pre-plumb cylinder.

 

The supplier I bought it from had similar deals / prices for Panasonic and Samsung.

 

I think that's my point - MCS suppliers seem to quote full list price and keep the benefit of discounts. 

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Firstly, thank you for all the constructive and timely comments. We have done a bit of building work on the farm but i am happiest with a lump hammer and baler twine so air source is a bit of a mystery to me. We had a preliminary discussion with a supplier recommended by our architect who has used them before. The main initial attraction of an ashp was the potential to cool the house but i was immediately told that this was not possible and then the rough cost estimate put doubts in my mind. If a competent plumber could install it then that changes things substantially and having now read (if not understanding the technical bits) the JSH blog it would seem that cooling is possible! I agree with the excess cost of proprietary solar thermal but we built a veranda over the front of the house using a very basic copper tube on insulation with dark glass system and it has worked a treat and looks good too. Having said that it would seem tobe overkill if we can use most of the three pv systems. Anyway, having digested your combined wisdom and on the basis that it will both cool and heat then i think an ashp is the way forward and will speak to our plumber and gauge his reaction to him installing. I will not mention heat boxes yet he might run! I am hoping to get a final heat demand estimate from the architect by the end of June which should steer us to the size of ashp required.

By way of background the old dairy we are converting will be single storey and basically a rectangle with a room tacked on which is currently a propagating house. Total area is approx.200 square metres and from a passive viewpoint is not an ideal shape. We have thought of the enerphit standard but are aiming for passive though the insulation levels are high. A further complication is that we have to work with the existing structure so the plan is to build a house within a building. Occupancy is only going to be the two of us most of the time.

 

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All sounds very sensible.

 

If you think you have a reasonable chance of meeting the Passivhaus standard, then you shouldn't need anything very big. Ours is a 5kW Aquarea with a bigger floor area and it seems to cope perfectly well with combined UFH heating and hot water duties.

 

Our plumber was mortified when we proposed such a small capacity unit - he had assumed something like a 12kW number would be required based on the size of the house.

 

I showed him the numbers, but he was still concerned about the house cooling down during the couple of hours each morning that the ASHP is heating the tank. I pointed out that it takes a day or so for the temperature to fall by 1-2 degrees. He still didn't believe me.

 

As many of us have discovered, heating is the easy part with Passivhaus Standard levels of insulation and airtightness. Hot water becomes the dominant energy consumer, so you may wish to optimise your plans for that function.

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At the other end of the price spectrum, I have just paid £455 including delivery for a 5Kw ASHP from ebay. I'll let you know in due course if it was a bargain or not....

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i also paid about the same for my "kingspan" ASHP, from Ebay.i should find out if it works near Christmas. It was still shrinkwrapped on its original pallet from the factory so what could possibly go wrong . 

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