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Author Topic: Ground Source Heatpump Power Consumption  (Read 11436 times)
marshman
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« on: April 07, 2014, 11:26:57 AM »

I'm looking at installing a Ground Source Heatpump and have a couple of basic questions. Before I ask those a bit of background info. I have a large detached house. It s old but very well insulated (loft 300mm wool, 12" [yes 12"] brick 2" cavity [filled] 5" thermal block walls, underfloor heating throughout (2" of insulation under all floors), double glazed) Also have 3.15kWpk PV fitted. House on very exposed site in S.E.

Currently heated by wood burning boiler stove (14kW to water, 8kW to room) via wet underfloor heating.

I'm now getting a bit fed up with the constant effort of maintaining the wood supply, transport, cutting, storing etc. Also the wood burner is now 30+ years old so is getting a bit near the end of its life.

I have plenty of room for the ground loops (approx 1 acre) though the ground is mainly stoney (shingle).

Questions:

1. Do GSHP run at a constant power? - i.e. if it is rated at say 15kW and has a COP of 3 so takes 5kW is this always 5kW when running  or does it throttle back. I ask because it would be nice if it could be "throttled back" to run for longer at a lower power input to better match the PV output. I am considering another 4kW of PV if the DNO will allow it.

2. Will the stoney (shingle mixed with some clay/soil) affect the efficiency of the heat source ground loops?

3. If  the wood burner with a rated output of 14kW to water is sufficient to heat the house can I assume that a GSHP rated at 14kW will be OK also. I would hope to fit a smaller one and have it running longer as we only have the fire burning in the evening for 4 or 5 hours. It's never "in" overnight or on during the day.

Thanks in advance

Roger
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3.15kWpk (15xSharp ND210)/SB3000. & 3.675kWpk (15 x Suntech 245WD)/SB4000TL, 10kW GSHP driving Wirsbo underfloor heating from 1200m ground loops. 10' x 7' solar wall (experimental). Clearview 650 Wood Burning Stove. MHRV - diy retrofit. Triple glazing.
BruceB
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« Reply #1 on: April 07, 2014, 11:50:49 AM »

Questions:

1. Do GSHP run at a constant power? - i.e. if it is rated at say 15kW and has a COP of 3 so takes 5kW is this always 5kW when running  or does it throttle back. I ask because it would be nice if it could be "throttled back" to run for longer at a lower power input to better match the PV output. I am considering another 4kW of PV if the DNO will allow it.
Generally GSHP are on or off, ie constant power.  They do not modulate like newer gas boilers or inverter driven ashp.  There may be an exception on the market now as it is an obvious development for installations with a low thermal inertia in the heat delivery system, but I am not aware of it.  For any given HP, the COP depends on the temperatures of the circulation loops

2. Will the stoney (shingle mixed with some clay/soil) affect the efficiency of the heat sourc ground loops?
Yes different ground has different heat transfer characteristics.  Some examples of different numbers for different soils in MCS022


3. If  the wood burner with a rated output of 14kW to water is sufficient to heat the house can I assume that a GSHP rated at 14kW will be OK also. I would hope to fit a smaller one and have it running longer as we only have the fire burning in the evening for 4 or 5 hours. It's never "in" overnight or on during the day.
Basically, YES, BUT if you want it MCS registered and want to claim RHI then you are in a whole new ball game which is covered by other threads, eg GDA, room by room heat loss calc, factoring in emitter sizing etc.


Thanks in advance

Roger
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JonG
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« Reply #2 on: April 07, 2014, 12:14:59 PM »

Further to the post below there are a limited number of modulating GSHP available, although they are few and far between, not sure if I can mention brands but one is an Austrian marque and the other is a Swedish one which will be introduced later this year.
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dhaslam
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« Reply #3 on: April 07, 2014, 01:05:37 PM »

Heat pumps are rated at  0C input ant 35C  output.  If the input temperature increases   the  electricity consumption increases and the  output increases.      I am not sure  what input temperature  you would  get from a ground loop, possibly about 5C  and that should give  about 15% more output than at  0C.      

It would be useful  to  measure the output  from your  boiler  before deciding  on the heat pump size.    This would mean fitting a  flow meter  on your present system.   This would allow the output to be calculated from the flow and  temperature differential.     It is  difficult to  estimate heat requirement in a well insulated house because  passive heat  is a big factor  so having an accurate  figure for normal  heat  requirement is a big help.    The aim should be  to  produce the normal heat requirement in about eight hours.   Will you replace the stove with a smaller room heater?  

I don't think that  PV will help all that much in winter except in sunny frosty weather, the rest of the time it  is cloudy, wet or dark.  From about mid March to the end of the heating season  you might get nearly 50% of input from PV but  not so much in the autumn.    In any case  you will find that electricity use is quite small.    

Kensa are bringing out a  twin compressor  6 kW model which modulates to  3 kW.  It might be large enough for your house.    I have the 3 kW version but I get  up to about 4.5 kW  output because of  higher  input temperature of  up to 15C.         
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AlanIOW
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« Reply #4 on: April 07, 2014, 01:19:11 PM »

Hi

I am in the same situation regarding a possible ASHP. I am considering a 12kW Panasonic T-Cap, which according to its spec sheet has a 2.57kW "Input Power". I am wondering if this is the maximum power it ever takes and it modulates down from there. Also when it is  modulating, does it start off at 2,57kW and drops or does it build up to that figure?

I assume I able to mention Panasonic as a trade name as its available through Navitron

I also note in their latest spec sheet that the Aquerea Heat Pump Manager has a Photovoltaic Mode to maximise the use of self generated PV, see quote below;

http://www.panasonicproclub.com/uploads/GB/catalogues/02_UK_GENERAL_14_060214.pdf   - See Page 36 - Not sure this link will work as its from the Pro Club pages.

"Panasonic has developed an innovative algorithm for its HPM (Heat Pump Manager) which
drastically improves the Heat Pumpís use of self-generated electricity from connected
Photovoltaic panels. The Heat Pump will take the electricity generation by the solar system
into consideration for the heating system and the domestic hot water production, without
reducing confort in the house.
The HPM (Heat Pump Manager) activates the heat pump based on:
∑ Energy produced by the photovoltaic system.
∑ The consumption requirement of the house, eg if a washing machine is working, the heat
pump will not draw electricity from the photovoltaic system to avoid net increases on overall
energy consumption and hence maximise efficiency.
∑ Heating demand of the house (in case of high electricity production, the house can be
overheated by 1 or 2 degrees, or reduced by 1 or 2 degrees if low production of electricity).
As the production of domestic hot water is linked to the level of electricity generated by the
solar system, if this was too low, the heat pump would start a normal process to maintain
maximum comfort in the house for a given set time (defined by the user)."

Alan
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Ted
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« Reply #5 on: April 07, 2014, 01:38:44 PM »

Roger, you could/should bed the underground pipes in sand. That's what we did here as 2m down it is all shale. We used 6 lorry loads. Avoids damage to pipes from stones and improves the thermal transfer.
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djs63
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« Reply #6 on: April 07, 2014, 01:53:40 PM »

We also bedded our underground pipes in sand and it does involve a lot of lorry loads even with narrow trenches! The surrounding soil is mostly clay and we get really good heating without using the supplementary heating elements in the GSHP (like immersion heaters?). Also the GSHP can be turned down at night (or any chosen time regularly) but we leave it on and think that the building is warmer in the mornings and overall uses less electricity in 24 hours. We have oversize radiators in one half and UFH in the other.

The external thermostat (plus room thermostat) and the ability to program the heat curve allow for fine turning.

It is amazing still to see that the return water temperature from the ground loop is plus 5 when the air temp is -19C. Six years later I still marvel at the process.
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dhaslam
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« Reply #7 on: April 07, 2014, 02:59:03 PM »

It is possible to vary  input and output of a heat pump with  a variable frequency drive.   However  if anything goes wrong  the warranty would be  compromised.    I was interested  in one  in order to allow higher  temperature input without overheating the compressor but  it will have to happen after  the warranty expires,  if at all. 

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gnarly
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« Reply #8 on: April 07, 2014, 03:56:15 PM »

Current on-off GSHP units should 'modulate' by having a few on and off periods within the hour.  The length of the on period varies depending on temperature.  The idea of having a buffer cylinder is to reduce the number of on off switching cycles so that compressor life is not reduced.

One idea for the orignal poster might be to consider a bi-modal system where GSHP provides low intensity heat but the wood burner is still used on the coldest days, it has some advantages:
* smaller GSHP install needed (eg. 3KW Shoebox plus small ground loop)
* heat emitters in house are probably sufficient and might run at 35-40C with 3KW of input heat without beefing up?
* GSHP gets used in shoulder seasons when PV generates most

Do you know what water temp your heating system runs at when the wood burner is on full tilt?

It should reduce the amount of wood needed significantly (and focus the effort on fewer very cold days)
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DonL
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« Reply #9 on: April 07, 2014, 05:19:57 PM »



One idea for the orignal poster might be to consider a bi-modal system where GSHP provides low intensity heat but the wood burner is still used on the coldest days, it has some advantages:
* smaller GSHP install needed (eg. 3KW Shoebox plus small ground loop)
* heat emitters in house are probably sufficient and might run at 35-40C with 3KW of input heat without beefing up?
* GSHP gets used in shoulder seasons when PV generates most



The bimodal system would not qualify for the RHI unless you installed a heat meter I think. Also, the RHI - which is significant for a GSHP requires design calculations be carried out on a room by room basis by your MCS installer which will establish the amount you are paid. I suggest you have a good look at https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/212089/Domestic_RHI_policy_statement.pdf.
There may be more recent documents than this but this is the one I used when I had my ASHP
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marshman
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« Reply #10 on: April 07, 2014, 09:05:26 PM »

Thanks everyone for the input.

To be honest I am looking towards the RHI to help fund the cost. So I would have to play by the rules. So that does mean the wood burner MUST be disconnected from the UFH permanently. Yes I could easily run the GSHP at a low level 24/7 and just use the wood burner to lift the temperature in the living room. To be honest I haven't a clue what temp the UFH runs at when its freezing outside and the wood burner is running flat out - At the moment the flow temp measures around 40 deg C and I don't think it ever gets much hotter. There are two thermostats in the system - 1 is a capillary room stat that slowly shuts down the flow from the boiler as the room temp goes up so reducing the temp of the water in the loops and a second water temp stat which again is in line with the flow from the boiler and responds to the flow temperature in the actual UFH loop. There is a bypass between the flow and return so that the pump keeps circulating the water even when its all up to temp and there is no demand.

The thought of having to have a GSHP sized based on the calculations based on the "rules" is a bit scary as it is a big old house and I am sure they will "not understand" certain aspects of the walls and the fact that the ceiling height is 6' 3" downstairs and only 6' up stairs and I will end up "needing" a massive output when the reality is the opposite. On the other hand if the RHI payments are based on their calculations it might work in my favour.

My first question was of course based on the fact that I was trying to maximise the use of PV to drive the heatpump. The idea of having two smaller heat pumps sounds appealing - if its allowed - the second one only kicking in if needed, that way the PV stands more chance of providing a large proportion of the power required. (obviously only when the sun is shining!) This will work better than in a lot of houses as the solar gain is far from optimised in this house as the majority of the windows face North West and most of the rest South West.

I did think of burying the coils a bit deeper at say 6' so that they sit in the water table most of the time (especially in the winter when needed). Is there a problem with that? will the water freeze around the pipe an cause problems?

Finally, I have arranged a "green Deal" assessment for tomorrow, so we will see what that says.

I'll keep you posted.

Roger
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3.15kWpk (15xSharp ND210)/SB3000. & 3.675kWpk (15 x Suntech 245WD)/SB4000TL, 10kW GSHP driving Wirsbo underfloor heating from 1200m ground loops. 10' x 7' solar wall (experimental). Clearview 650 Wood Burning Stove. MHRV - diy retrofit. Triple glazing.
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« Reply #11 on: April 07, 2014, 09:35:14 PM »

Roger,
How did you go about getting a green deal assessor and how much does it cost?

The calculations for MCS are taken on a room by room basis as if all are occupied and the temp is at 21Cinside and -2C ? outside . and your emitters (rads to most people) are up to the job ! At that point i just gave up!

DIY again?

Alan,

A really good find and shows promise as a DIY job

Ken
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marshman
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« Reply #12 on: April 07, 2014, 10:07:23 PM »

Hi Ken,

With difficulty!!!

Did the usual searches online - mainly ended up being referred to EPC type assessors who didn't know what they were talking about to be honest - one even told me that I needed an EPC BEFORE getting a Green Deal assessment. Filled in a web survey/ "application" expecting a torrent of phone calls and got just one. I also got 2 emails from another two EPC assessors who had been passed my details - neither had a clue. The phone call was from " 1 Green Place". A central "agency". Offered a Green Deal Assessment for £114 inclusive & seemed to know the "procedures". Couldn't find anything bad online - even mentioned in a good way on the Martin Lewis Money Saving website. Not entirely smooth as they changed the time/date twice because they hadn't a clue where I lived but we shall what or who turns up tomorrow.

If it all gets too much and too silly may well go the DIY route or just stick with old faithful - the wood burner.

21 deg C inside a bedroom?Huh  ouch!

Roger
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3.15kWpk (15xSharp ND210)/SB3000. & 3.675kWpk (15 x Suntech 245WD)/SB4000TL, 10kW GSHP driving Wirsbo underfloor heating from 1200m ground loops. 10' x 7' solar wall (experimental). Clearview 650 Wood Burning Stove. MHRV - diy retrofit. Triple glazing.
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« Reply #13 on: April 07, 2014, 11:14:29 PM »

Any feel for how much wood you get through in a 'typical' heating season, we can try and convert this back into kWh to size heat pump to suit your comfort levels/back of fag pack calculation.

I will say look at air source heat pumps, they will be much cheaper, modulate as you wish and have good COPs now.

You would have to run the numbers to see if air source or ground source comes out best for RHI/return.

If you are serious, talk to some MCS installers, have them out for a chat/quote, get a feel for your situation.

At the moment I would not recommend a GDA, the advice it will give you will be to fare removed from the reality, you need to size the heat pump to MCS standards anyway and this process is more fundamental than an occupancy assessment as it will determin how much capital you have to find in the first instant.



Cheers

Jon
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DonL
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« Reply #14 on: April 08, 2014, 08:53:30 AM »

The MCS design temperature for bedrooms is 18C. Have a look at MIS3005 which gives the detail on the calculation requirements.
Don
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Schuco solar hot water - 3300kWh/annum, 16 BP 4175N PV panels - 2.8kWp, log burner and back boiler and 18 Ying Li 235 PV panels - 4.2kWp, 42kW ground mount PV, 9kW Panasonic ASHP, 40kWh Nissan Leaf
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