Help/Advice GSHP V ASHP or neither!

4 Jun 2008
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United Kingdom
Dear All,

I need help in the debate of GSHP V ASHP. I realise this topic has been covered ad infinitum, but I am still cannot decide what to do and in fairness now very confused!
The background into looking at this type of heating is twofold, firstly the environment and secondly as I have an oil fired heating system, the cost, (as I live in the UK).

The plan from the below company is to install a slinky type coil in a 900m2 area of our garden, (Our house os @300m2), the ground conditions are we border the fens arae of the UK and it is clay based soil about 1ft down but very close to the water table.

I am not a DIY enthusiast so I have asked a Company to quote me, below is their quote and charts for both GSHP and ASHP,

1. What do you experts think?
2. Those of you that have either source which would you choose (if given the option again?)
3. Is there a cheaper way of doing what I am trying to achieve?
4. Should I bother?

Here is the quote:

Your coldest day average of -3C heating requirement has been calculated to be 17.5kW.

The cost to install a 3 phase 19kW Ciat/Sofath ground source heat pump system would roughly be £15,700 + vat at 5% less grant of £1200 plus the cost of installing 3 phase electricity @ £2000. This would work with your existing hot water tank providing hot water to 45C when the heating is operated. Outside of this off peak electricity will need to be used. Note that the hot water temperature will need to be heated to 60C once a week to prevent legionella bacteria risk. This can be done with timer on the immersion circuit. Total = @£15,300 + 3 phase installation

To install a 19kW single phase twin compressor heat pump would cost an additional £2000. No grant would be available on this model so that would be an additional cost of this option would be £3200. Total @£18,500

I can also supply an 18.8kW single phase Sanyo inverter air source heat pump using an Airpac heat exchanger. Inverter heat pumps provide the highest efficiency. On the coldest day averaging -3C the output would drop to about 13kW. Your oil system could be used as a backup if required. It may be possible to automate this changeover. This heat pump system would provide most of your heating requirement as temperatures rarely fall to this level over a long period. The shortfall is just 3.5kW. A 3 kW electric backup could also be used. This option would cost about £7,500

I have estimated your heating costs for air source and ground source heating systems. These estimates are based on current electricity prices using Southern Electric Economy 10 tariff. The unit costs are 12.96p peak and 7.27p off peak. Oil price 55p /litre

Assuming that your radiators meet the specification set out in the quotation the quotation the savings is estimated to be 50% for ground source and 40% for air source. This assumes that 65% of the electricity is used at peak rates and 35% is used at off peak rates.

These estimates are based on a flow temperature of 45C which corresponds to radiators that are 2 times oversized and are based on manufacturer’s data.

Ground source COP 3.5
Air source seasonal COP 2.9
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You need an area of open ground free from shade and trees roughly 2 times the footprint of the building.

Personally I don't like slinkies if you have the ground because they can cause localized ground freezing, single 32mm pipes are far better but adds considerable to the ground work.

I have never Heard of the Ciat/Sofath gshp and would suggest you look at the Mitsubishi or Worcester gshp.

You must check with your electric supplier, depending where you are they could be looking at a local sub station and up to 20k, but argue any quote vigorously.

All GSHP work better at low temperature, we could produce 55c without any loss in efficiency, but even so underfloor heating is ideally suited.

The COP quoted is low, and in clay which is perfect for ground source, I would expect to see that as high as 5-1.

Has he said what refrigerant they use.

The purge cycle is only necessary once a month, and if you can get the cylinder up to 55c, perhaps with the aid of solar heating, it would be even better.

If you can get to interbuild at the end of October there will be 1000s of different ideas to chew over, in fact I could give you a couple of hours going through the different system if I knew what day you were going.

For the record we have fitted dozens of GSHP but sadly now retired.
In fairness, I may have been wrong with slinkies, as I suspect that he was talking about laying pipes as you suggested, as the area of ground he is looking to dig up is a paddock and he say he needs about 900m2 to lay the pipes.

I belive the make is a french company.

No chance now of putting in under floor heating.

Next door is a plant hire company with 3 phase, so I was asuming as they have it and we are so close, it wouldn't be that difficult to install?

I could make Interbuild and would love to draw on your experience, what day are you planning to go? Sunday 26th would suit me.

IF you are really lucky, your fuel savings might cover the cost of the installation and maintenance before you need a new one!

Dont hold your breath though.
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How do the economics of boreholes compare, dia? A customer in Surrey is puttnig in 2 x 70kW heat exchangers, and two deep oles.
I can make any day, in fact I may take in two days as there's so much to see.

For 19kw you need approximately 12 x 100m lengths of 32mm mdpe, spaced at 600mm centers, we connect into an manifold pit then run the the plant room via 2 x larger pipes, possibly as large as 90mm.

I'm still surprised he has quoted 45c, where 55c is nearer the mark for ground source, perhaps a typo.
How do the economics of boreholes compare, dia? A customer in Surrey is puttnig in 2 x 70kW heat exchangers, and two deep oles.

Boreholes are actually very good, but I know nothing of anything that size and would doubt very much if 2 bore holes would be suitable even at depths over 700m..

If you can give me more information I could make some inquiries

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