17k for air source, 25-35 for ground source?!?!

[Danny42]

Hi all,

 

I could do with a little advice from those in the know. I have just bought a house. It has about half an acre of land and I had been looking at GSHP. Between looking at the size of heat pumps people use here, the advice relating to heat loss calculations, various rules of thumb, that somewhere between a 5kw and 15kw system is likely to be the result of a proper heat loss calculation. If I assume the higher number (and I know that many will say its far too high), but if I understand correctly from the Kensa website's various rules-of-thumb, I'd need about 10m of slinky per kw, so perhaps 150m, so 3x50m or perhaps 4x40m. Now I've seen a lot of youtubes of people with far, far bigger houses than me, with 3x30m slinky trenches and being fine.

I have laid it out, following rules of 5m between slinky trenches, and we can definitely get 4x40m slinky trenches in. Thats including avoiding the 2.5 next to boundaries, and the property, enough room for turns, and that would use about 2/3 of my garden.

I've just been told by an installer that:

- air source will be about 17kw all in, installed.
- ground source from the last person I spoke to was 35k for the install.

We have a 3 bed bungalow with okayish insulation (300m loft insulation + cavity wall + double glazing) but it is an old building so not like a new one.

From all my reading, from scouring this forum, I think I can get a brand new Kensa ground source heat pump for about 8k, if I do all the trenches myself, I'm a mechanical engineer by trade I have plumbed in every house I've lived in etc. I really don't think (given lots of time) I'd have a problem doing the install myself for about 10k. Obviously if I do that I cant get back the RHI, but with air source I believe the most I'd get over 7 years would be 15k and maybe 30k for ground source, so they JUST maybe draw even. If you factor in the oportunity cost (i.e. the money I wouldn't be making on the money I'd be spending on the installation, e.g. investing in stocks), then paying someone to fit either ground source or air source is a non-starter.

They also tell me its impossible to get the system signed off by an MCS installer, even if I say paid them 3k to do the last day of install, hook everything up. They might just be lying because the want all the money, but it just doesn't make sense. Financially I mean. If I do it myself, then I break even with just the savings (from oil) from either system in about 10 years. If I do air source then I think I may well break even after just 5, as I can see heat pumps on ebay for just a few thousand.

I dunno, I'm going round in circles. I think the only thing really putting me off GSHP isn't the hard work and tearing up our garden, I'm quite happy to do that, its the risk of it not working long term. The installer I just spoke to said you need at least 2 acres for a small bungalow (what we have), otherwise the ground cant warm up enough in the summer. But that goes against the advice I've seen in papers. I can see studies which tend to agree with the Kensa advice, that you need between 3 and 5 times the floor space for the ground array (assuming horrizontal piping). Well my house is 170sqm and my garden is 2000sqm, thats more than 10 times. In winter the soil is very wet which should be good for conduction. I have access to a mini-digger, I run my own business so I have plenty of time to dedicate....

Any advice welcome. And I do get that lots of folks go for air source, and believe me, I haven't ruled that out either yet - I'm not put off by the noise and I totally get the advantages, its just that at the moment I'm leaning towards ground source - if anything as an engineering project. So while I 100% welcome any advice, including "just use AS!" I'd love it if anyone who has done a GSHP install or had one done can let me know their thoughts.

Thanks,
Danny

[PeterW]

11 minutes ago, Danny42 said:

I'm not put off by the noise

From experience, GSHP is noisier than ASHP and the unit is internal usually whereas ASHP is usually outdoors. 

[Danny42]

1 minute ago, PeterW said:

From experience, GSHP is noisier than ASHP and the unit is internal usually whereas ASHP is usually outdoors. 

In our case, air source can go on the far side of a garage, and definitely not audible from in the house and likely very very quiet even if we are outside. When I wrote that I wasn't put off by the noise, I didn't mean that I thought it was noisy and I'd cope, I meant that from a lot of research it seems that they really aren't that noisy at all and I wouldn't be put off.

The plant for our GSHP would go in the garage in a dedicated space, so I'm pretty sure that wouldn't cause any noise issues either.

[SteamyTea]

Get a proper heat loss calculation done?

 

Are you in Cornwall as you mention Kensa.

[ProDave]

I looked at ground source.  It would have taken up (and meant digging up) almost all our garden.  It was not the hard work that put me off (I had my own digger at the time) but the additional cost.  the pipe and brine to fill it would cost as much as the GSHP and then add the cost of replacing that every 5 years or so, and that would kill any savings from efficiency.

 

The noise thing.  It am always puzzled my why people detest the noise of an ASHP outside the envelope of their house but are happy to put something that makes a similar noise inside their house.

[Matt60]

I have been told that as ASHP's are far more popular (because most people don't have the space for GSHP's) than GSHP's, that all the advances in technology  and development will and are going in to ASHP. Also, because of that popularity, getting an engineer to service and repair a ASHP is much easier too. I don't know if that is true but it is logical and I think probably quite likely. 

 

For reference, I am getting a Mitsubishi ASHP and Cylinder and every inch of plumbing in the entire house including the UFH, all with quality connections and control system by a very well respected firm for £28k.

 

Now que all the people who say I could do it myself for £2k with a big hammer and an adjustable spanner... ?

[Iceverge]

7 hours ago, Danny42 said:

I have just bought a house.

 

I think you are jumping in at the wrong end here. The key to keeping a boat afloat is mending the hull, not buying an expensive bilge pump. A house with high heat loss will never be comfortable no matter how large you make the heating system. 

 

1. Ensure that the house is properly ventilated. A damp house is automatically a cold feeling house. Ideally install some kind of continuous running mechanical ventilation. This can be a matter of a few £100. 

 

2. DIY rig a depressurisation fan. Many examples on here. Methodically spends a day with caulk, expanding foam and tape sealing any and every draft you can get near. £100 will go an awful long way here.  As you are mechanically minded use the opportunity to adjust the windows correctly. Many of them are never aligned and don't seal properly. Airtightness is the single cheapest and most effective trick in improving a houses comfort and efficiency ( must be done in conjunction with proper ventilation) 

 

3. Live with the house for a while. What kind of heating system do you currently have? Buy a cheap inside and outside temperature logger and keep a track of the runtimes of the heating. This will show exactly your heating load. eg if a 20kw boiler needs to run 45 minutes out of 60 deg to keep a house at 20 deg when it is 5deg outside it is equivalent to a 15kw boiler. Extrapolate to the lowest expected temperature, say -5 deg  in this instance your maximum heat load would be 25kw. Much more accurate than any guessing about what you might need. 

 

4. Then you will have an excellent idea of what you need to do and spent very little. I expect you will come to the same conclusion most people do. If you have a very high heat demand and a high temperature heating system already installed you will stick to a gas or oil boiler. If you have a low heat demand and ideally a low temperature heating system (huge rads or UFH) an ASHP may be the best route. 

 

5. One seldom considered option is an air to air heat pump. They can have double the COP of a ASHP and have a modest cost in comparison.