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Author Topic: GSHP versus ASHP  (Read 16367 times)
titan
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« Reply #30 on: February 09, 2011, 11:34:53 AM »


   So far there has only been about two weeks that  were extremely cold and a few more weeks with temperatures down to -5C.   Once you have a way of covering those periods  then there is no difficulty.    

    The main thing with heat  pumps is that they have enough capacity to  heat the house in normal conditions  using off peak electricity only and that they only need supplementary heat for short periods.
   

What do you mean by covering those periods of extended cold. If by a woodburner are those costs taken into account for the overall heat load.Wood is not cheap.  Whenever I have calculated off peak tariffs  they have not been viable unless you use a lot of off peak electricity because of the price differential between day and off peak units
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mpooley
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« Reply #31 on: February 09, 2011, 11:39:58 AM »

Dhaslem
Do you have ASHP then?

and to use off peak I assume you need a heat bank of some sort.

The guy that came to me said he didnt see any need for one?

mike
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cornishben
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« Reply #32 on: February 09, 2011, 12:20:05 PM »

So far there has only been about two weeks that  were extremely cold and a few more weeks with temperatures down to -5C.   Once you have a way of covering those periods  then there is no difficulty.  
This is the part I'm trying to understand/convince my wife about.  Our current plan is for ~8kW ASHP for UFH/rads and we have a 10kW woodburner in the living room. The intention is that the ASHP will work well for heating over the majority of the year and the WBS can supplement it on those coldest periods.  Trouble is the WBS doesn't have a back boiler so in those coldest times we're only going to have a warm living room... which is what my wife has a problem with - spending a load of money on a heating system that doesn't keep the whole house warm at the coldest times.  However, what are the alternatives for off gas/no room for gshp/can't afford pellet £ or space..  

we're on Eco7 so my intention is to run the ASHP on that early morning to heat the UFH slab and fit the biggest buffer tank we can so that we can have a store of heat built up over the eco7 hours.
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rhys
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« Reply #33 on: February 09, 2011, 12:32:17 PM »

My understanding is that if you size the ASHP correctly ot should then be able to heat the house throughout the year. Obviously in deepest winter the COP will be much worse, than the annual average.
But it'll still work for those relatively few days. Anything the WBS can do will just impove things. UFH if the insulation is good will act as a really good heat store too. The lower the flow temperature the better. If you haven't put the UHF down yet make sure you space the pex pipes as close together as possible, much closer than for a Gas or Oil fired system, then the flow temperature can be lower.
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mpooley
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« Reply #34 on: February 09, 2011, 04:34:54 PM »

Just an extra thought.
how do you calculte the actual COP of a sytem?

You know what goes in but how do you measure the heat output?

mike
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titan
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« Reply #35 on: February 09, 2011, 05:41:30 PM »

An independent report that may be of interest  www.fuelcells.bham.ac.uk/.../review_of_domestic_heat_pump_cop.pdf
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mpooley
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« Reply #36 on: February 09, 2011, 06:02:27 PM »

Hi Titan
I am getting a "File not found"  error 404 with that link

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It's not easy having a good time. Even smiling makes my face ache.

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titan
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« Reply #37 on: February 09, 2011, 06:13:51 PM »

Hi Titan
I am getting a "File not found"  error 404 with that link



The mods don't allow editing so if you highlight  the address then right click and choose the option "search Google" it is the first hit,  I don't know why the link doesn't work it looks OK . That is with Firefox with another browser just search with Google for the address.
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mpooley
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« Reply #38 on: February 09, 2011, 06:39:19 PM »

Thanks Mate
Had a quick look at it.

I can see that the GSHP cops are significantly better - i wish i understood it all though lol Huh

I will sit down tonight and try to get my head round it,

at one point it talks of SPF . whats that?

mike

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It's not easy having a good time. Even smiling makes my face ache.

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sam123
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« Reply #39 on: February 09, 2011, 07:15:39 PM »

http://www.fuelcells.bham.ac.uk/documents/review_of_domestic_heat_pump_cop.pdf

Should work
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martin W
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« Reply #40 on: February 10, 2011, 09:54:16 AM »

thanks for the link sam (second one works).

very interesting reading.

I think in summary it seems to show that ASHP with oulet temp at around 50-55°C and air temp at -5 to 0°C have COPS at around 2-2.5. GSHP seem to have COPS of 3-4(ish) when outlet temp of 50-55°C. At least that is my view.

I wounder what the cost comparision would be over a 10 year running period (including installation) of ASHP and GSHP.

Some rough thoughts, calc.... I could be way off here
For most of us using gas for heating and hot water we can get a average yearly usage of gas consummed. Say 15,000 - 20,000 kWh of gas used to heat a average UK house. Say boiler is 75% real world efficent = 12,000 - 16,000 kWh of energy required in home p.a.

So
ASHP will use 6,000 - 8,000kWh of electricity at COP 2
GSHP will use 4,000 - 5,333kWh of electricity at COP 3

Yearly cost at £0.13 GBP per kWh (not economy 7 rate)
ASHP = £780 - £1040 p.a.
GSHP = £520 - £694 p.a.

so using my assumptions ASHP cost about £250 - £300 more per annum to run that GSHP, or you could say a GSHP about 66% the cost of an ASHP to run.


Anyone think this simplified view is somewhere near real world? We always seem to be focusing on COP's, but I've never before though about actual yealy runing costs.
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mpooley
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« Reply #41 on: February 10, 2011, 10:05:42 AM »

thanks for the link sam (second one works).

very interesting reading.

I think in summary it seems to show that ASHP with oulet temp at around 50-55°C and air temp at -5 to 0°C have COPS at around 2-2.5. GSHP seem to have COPS of 3-4(ish) when outlet temp of 50-55°C. At least that is my view.

I wounder what the cost comparision would be over a 10 year running period (including installation) of ASHP and GSHP.

Some rough thoughts, calc.... I could be way off here
For most of us using gas for heating and hot water we can get a average yearly usage of gas consummed. Say 15,000 - 20,000 kWh of gas used to heat a average UK house. Say boiler is 75% real world efficent = 12,000 - 16,000 kWh of energy required in home p.a.

So
ASHP will use 6,000 - 8,000kWh of electricity at COP 2
GSHP will use 4,000 - 5,333kWh of electricity at COP 3

Yearly cost at £0.13 GBP per kWh (not economy 7 rate)
ASHP = £780 - £1040 p.a.
GSHP = £520 - £694 p.a.

so using my assumptions ASHP cost about £250 - £300 more per annum to run that GSHP, or you could say a GSHP about 66% the cost of an ASHP to run.


Anyone think this simplified view is somewhere near real world? We always seem to be focusing on COP's, but I've never before though about actual yealy runing costs.

just two thoughts (can only manage two at my age  Wink )would a gas boiler be that inefficient ? my oil boiler is supposed to be 90% +  ?

also I have noticed here and in other forums that when people calculate electricity cost the usually seem to be in the 12 to 14p per kwh

am I incredibly lucky that I get the bulk of my leccy at 8.2p per kwh (Not economy 7)  ?

thirdly (ooh i managed another thought Cool )
I did the sums yesterday and realised that at a cop of 2 my costs would be greater than using oil  so i would need to get a lot better than that to make it worthwhile.

Mike
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martin W
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« Reply #42 on: February 10, 2011, 10:21:34 AM »

mpooley,

My gas boiler is 84% efficent (well on the label). I actually monitored the heat gained into my thermal store against the gas used the other day.... it worked out just over 45% efficent Huh outside air temperture was -5 or so. Whilst I might have some errors in my calculaitons / heat lost in pipes etc, it would be a  big stretch to think the boiler was even 60% efficent. (Oh and the boiler is working correctly).

Someone on here (ecentric?) hinted it might be a fair bit to do with how they calculate a boilers efficency  - using perfect world conditions (air temp at 20°C not minus 5).

13p per kwh is the average cost of the electric... ie 2500kwh per year at say 20p (so much a quater to cover a standing charge) and the rest at 9p or so.
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mpooley
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« Reply #43 on: February 10, 2011, 10:46:57 AM »

martin hi

I am surprised about the gas boiler  Shocked  thats quite shocking!

Ah I see what you mean about the leccy.

personally i dont think that is a fair/reasonable way to do it, as most people are already using well over the amount charged at the higher rate just for normal leccy use so any additional used is always at the cheaper price.

mike
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It's not easy having a good time. Even smiling makes my face ache.

“Science is the belief in the ignorance of experts.” Richard Feynman
qeipl
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« Reply #44 on: February 10, 2011, 12:26:03 PM »


I did the sums yesterday and realised that at a cop of 2 my costs would be greater than using oil  so i would need to get a lot better than that to make it worthwhile.


Mike,

You're trying to work out whether ASHP or GSHP will be more cost effective given your current heat demand.
Don't forget that reducing your heat demand is the first thing to tackle.

For example, I seem to recall that you were asking elsewhere about draught seals on your windows.
New windows will have zero heat loss from draughts and a U-value of 1.9 or better.

Another example would be solar gain.
My parents live in a 1980s bungalow that has electric heating and an open fire.
They built a small conservatory off the main sitting/dining room to catch the afternoon/evening sun.
The reduction in electricity and logs, over the year, was a pleasant surprise - an additional benefit that they hadn't expected.

Older houses are notorilously 'leaky' (heat and air) where the roof meets the walls. More insulation and better draught-proofing?

Apologies for banging on about this again.
You may have already done as much as you can on these fronts but I feel we get too easily absorbed in the technicalities of the equipment and miss some less exciting stuff that is more cost effective.

Malcolm
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