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Author Topic: GSHP versus ASHP  (Read 16167 times)
mpooley
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« on: February 02, 2011, 06:22:09 PM »

I have a salesman coming round tomorrow who has told me there is little difference between the efficiency of an ASHP and a GSHP.

now i dont realy believe that, but am going to listen to what he says.

what do you guys think?


mike

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BruceB
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« Reply #1 on: February 02, 2011, 06:45:42 PM »

The ASHPs at their best can have COPs as good as GSHPs.  But in a domestic setting you have to think when it is you want the HP to be working really well and it is those prolonged low temperature periods in winter when  an ASHP will struggle in my opinion.  I do not have one though; I am sure others here will.

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Bruce
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qeipl
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« Reply #2 on: February 02, 2011, 07:48:43 PM »

The thing that I didn't fully absorb when first considering heat pumps is that they are solar collectors.
All they are doing is moving solar energy from outside your house to inside.
Air source gathers solar energy directly from the air.
Ground source gathers solar energy from the ground via pipes filled with water.

The efficiency of a heat pump depends on the density of the solar energy in the medium and the cost of pumping the medium through the heat exchanger.

In winter the energy density of air will be low, so you'll have to pump lots of it to get a kWh of heat.
Ground source in winter will have a comparatively high energy density, so you'll have to pump a smaller volume of water to get your kWh.
In winter ground source will be more efficient.

In summer the energy density of air will be much higher than in winter, while that of ground will only be a little bit higher.
Air is much easier to pump than water - uses less energy.
So in summer, air source could be a lot more efficient.

Taken over 12 months the efficiencies might be comparable but, as Bruce points out, winter is when you want to heat your house, so ground source will be more appropriate.

Personally, I think ground source is an expensive way to stay warm and is only useful where a building cannot take advantage of insulation, draught-proofing and solar gain.
These three have almost no ongoing costs after installation. If done well enough they will minimise your heating requirement to such an extent that you won't need a big expensive heating unit.
Air source for DHW and background heating backed up by gas or biomass will be much cheaper to install and operate.


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titan
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« Reply #3 on: February 03, 2011, 08:47:51 AM »


Personally, I think ground source is an expensive way to stay warm

Air source for DHW and background heating backed up by gas or biomass will be much cheaper to install and operate.


You must be doing your calcs different to me. If you have mains gas then why bother with ASHP and biomass , if you don't then oil is actually cheaper than LPG but a GSHP will outperform them both. Have you looked at the price of biomass boilers installed certainly not cheap or that practical for most people with the need for pellet or chippings storage/handling.
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martin
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« Reply #4 on: February 03, 2011, 09:05:51 AM »

"I have a salesman coming round tomorrow who has told me there is little difference between the efficiency of an ASHP and a GSHP.

now i dont realy believe that, but am going to listen to what he says.

what do you guys think?"

- the guy's an eejit - cancel the appointment, or show him the door pdq! Roll Eyes
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skyewright
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« Reply #5 on: February 03, 2011, 09:50:43 AM »

Maybe your first question should be "Are you or have you ever been a double glazing salesman?".
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dhaslam
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« Reply #6 on: February 03, 2011, 10:02:14 AM »

BruceB's answer  is bang on.   You have  to have an alternative heat source  for very cold weather.  If the house is very well insulated  then off peak electricity is OK as a backup.   
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qeipl
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« Reply #7 on: February 03, 2011, 10:29:42 AM »


You must be doing your calcs different to me. If you have mains gas then why bother with ASHP and biomass , if you don't then oil is actually cheaper than LPG but a GSHP will outperform them both. Have you looked at the price of biomass boilers installed certainly not cheap or that practical for most people with the need for pellet or chippings storage/handling.


My point (inadequately explained) is that if you reduce your space heating requirement enough (insulate/draught-proof/solar gain) then you don't need to invest in big expensive heating equipment.

Solar gain is the key.

I have an ASHP cylinder that does DHW and UFH, rated at 810W.

For 50% of the year the sun does all my space heating directly (UFH doesn't run at all).
For the other 50% of the year the sun provides at least half of the space heating (so 25% of the annual total) via the ASHP and UFH.
The other 25% (in the dark days of winter when the sun isn't working) the ASHP is supplemented by a solid fuel boiler rated at 10kW.

I ran out of wood at NY (too busy building last year to get wood supply organised) and have been burning coal since then at a rate of 100kg/month = £32. Wood, when I'm organised will cost much less.
My total electricity use in January equates to £75, of which, lets say, £50 was space heating.
That's a total heating cost of £82/month for 25% of the year = £246 for the winter quarter.

Let's be generous and assume a GSHP would, in the winter quarter, have half the running costs of my ASHP/solid fuel combination and the same running costs for the other 25% that my UFH runs.
So the annual saving I'd make, in running costs, by opting for GS is around £123.

The ASHP cylinder cost £1,700, but you need a cylinder for GS which will cost c.£1k, so the net capital cost of my heat pump is £700.
The multi-fuel boiler cost £500 plus another £500 to install.
So my total capital cost is £1,700.

A quick google tells me that a 7kW GSHP (probably suitable as a replacement for my 10kW boiler) costs £4k to buy and probably another £3k? by the time I've bought the pipe, fittings, etc. and hired a man with a digger to bury it.
So that's a total capital cost of £7k - £5,300 more than my setup.

In terms of economics, the ground source system would have to run for 43 years before it caught up with my air source/solid fuel combination.

GS is a good option in some circumstances but the capital cost is high.
Using your capital to reduce the heating requirement to the point where you can use smaller, cheaper equipment seems to me to be a much more sensible approach.

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mpooley
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« Reply #8 on: February 03, 2011, 04:45:37 PM »

Thanks all for your replies.

I am only in the early stages of assessing the merits of heat pumps.
I have a modern oil boiler and to be honest if i wasn't terrified that Oil supplies are running out and prices would not shoot up above inflation I would probably stick with Oil.(no gas out in the sticks)

I know the capital cost is high but heat security is important to me.
I am getting on a bit now and I just dont fancy being one of those pensioners found dead huddled round a candle trying to keep warm.
The price hikes of this winter were truly horrifying and we had to turn the heat right down for a few weeks while we waited for the prices to fall again.  winter (still 27% higher than my last delivery though!)

Another consideration will be the renewable heat initiative (subsidy!) which understand will be available in June.
No idea what that will be yet.
so I am trying to get my head around COPs etc and wether i will have to get larger rads etc at the moment.
So I will get a few salesmen round, listen to their BS and test their claims on this forum.

BTW said salesman didn't even turn up lol he has deferred to next week.

Thanks
Mike

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sam123
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« Reply #9 on: February 03, 2011, 04:52:51 PM »

I have a salesman coming round tomorrow who has told me there is little difference between the efficiency of an ASHP and a GSHP.

now i dont realy believe that, but am going to listen to what he says.

what do you guys think?


mike



In Finnish forum there is no reports ASHP with YEAR COP 2, but 1.5-1.8 can be reached. GSHP has in same conditions regularly (floor heating) YEAR COP 3-3.5.

It is said that ASHP needs about 5 degree warmer air comparing to GSHP ground fluid.

If you get 5 degree ground fluid to GSHP, then you need 10 degree outside temperature to ASHP.

In practice situation is not so black/white, because ASHP suffers moisture near zero temperatures.

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baker
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« Reply #10 on: February 08, 2011, 01:24:45 AM »

hi
if you have football Field, and wages, UFH , damp loamy soil or a lake; which is not common.  if not you will have to spend a fortune on bore holes or dig up the whole place,
at the end of the heating season the energy bank will have run out with low cop.s
what you see today is  old technology tarted up
ASHP
have made life easier for us and have done well in the last 2 years through the  coldest season
new high/pressure temperature units are being tested at the moment
and maybe will see today's heat pumps obsolete in a year  or so
john
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mpooley
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« Reply #11 on: February 08, 2011, 03:35:39 PM »

Hi
Yes I  must admit the guy has just been and can see the potential of ASHP's .
as you say a lot easier to install.
and a lot cheaper probably.

He was talking about a 2 stage high temperature system which would mean not changing rads at least.

I will cogitate  bike
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A.L.
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« Reply #12 on: February 08, 2011, 04:36:42 PM »

hello,

Quote
Yes I  must admit the guy has just been and can see the potential of ASHP's

I must warn you that IMHO if you try and run radiators at 60-65C or higher off an ASHP your are likely to achieve an average COP no higher than 1.5 and probably considerably less.

You might as well use immersion heaters and a good electricity tariff.
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martin
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« Reply #13 on: February 08, 2011, 05:19:05 PM »

As AL says, ASHP and conventional rads is a recipe for penury! I repeat, the rep is an eejit, who gives atrociously poor advice! whistlie
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sam123
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« Reply #14 on: February 08, 2011, 06:26:24 PM »

Here you go: http://www.elisanet.fi/sanyoco2log/

ASHP probably works much better in UK than in Finland, but ASHP don´t work with good COP anywhere as long as you try to rise temperature over +35. It is almost as bad situation with GSHP, so my advice is to change big radiators or use few convectors to lower temperature. Or add some blowers to your existing radiators  stir

Sami
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