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Talk Me Out Of A GSHP


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#1 Woodgnome

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Posted 01 February 2016 - 08:56 PM

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So, planned on GSHP for the house. Built along Tony's house lines with block work and full fill Cavity etc. Close to passive U values with MVHR.
UFH pipework in slab with 300mm insulation below.
As far as I'm aware RHI should continue beyond April for GSHP. If I go MCS install, I'll get it paid for, as EPC will be based on plans, which show standard regs. (This is another area open to discussion, but let's leave that for now).
MCS installCost approx £10k.
Always intended to use GSHP due to low running costs etc. However lately I'm having doubts regarding one, especially as Jeremy gets on well with the Sunamps. My predicament is that I think I may well need a bit of heat and I'm wondering about what to use for the UFH pipes.
So I'm thinking self install PV say £2750, sunamp and then something for UFH/ DHW.
DHW will be biggest energy user as we got kids.
Suggestions?
Ta.

#2 declan52

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Posted 01 February 2016 - 09:05 PM

I take it that it's a ground loop system instead of a borehole.
If you are building to near passive u values why would you spend over £10,000 on a heating system.
Your rhi payments are based on your as built spec not what it says on your plans. They do inspect some builds randomly so you might be found out plus the building control inspector will see all that's going in and might question the difference when he reads your epc and all the recommendations saying you need extra insulation in the roof floor walls.

#3 jsharris

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Posted 01 February 2016 - 09:30 PM

I can confirm that RHI is on as-built, not design, so the payment for a well-insulated house will be pretty small. For us it was around £85 a year for seven years, IIRC. Not enough to justify the massive increase in cost involved in getting a heat pump installed with an appropriate chit.

If your house only needs very little heating, and you don't have mains gas, then a small ASHP, self-installed, makes for a pretty cost effective solution, but even then it's probably not going to pay for itself in its lifetime in terms of energy cost savings.

Edited by jsharris, 01 February 2016 - 09:30 PM.


#4 Woodgnome

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Posted 01 February 2016 - 10:36 PM

Jeremy, what about the loss of performance when it's cold. We live about 2 miles from the coast, so have some concerns.

#5 SteamyTea

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Posted 01 February 2016 - 10:41 PM

Never gets that cold down here. There are a lot of places with ASHP in Cornwall. Never heard any horror stories.

Not often that it is cold with high RH

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Edited by SteamyTea, 01 February 2016 - 11:04 PM.


#6 Triassic

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Posted 01 February 2016 - 10:50 PM

Maybe one of these would do for heating http://www.tesco.com...AQ&gclsrc=aw.ds

#7 jsharris

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Posted 02 February 2016 - 08:13 AM

View PostWoodgnome, on 01 February 2016 - 10:36 PM, said:

Jeremy, what about the loss of performance when it's cold. We live about 2 miles from the coast, so have some concerns.

This needs to be put in to perspective, as it's often quoted but quite often the reason given ("it's cold") isn't accurate.

Air at 0 deg C still has 96.4 % of the heat energy in that air at 10 deg C has. The 10 deg C drop in temperature only reduces the sensible heat in the air that can be extracted by 3.6% for that 10 deg C temperature drop. At -10 deg C, the air still has 96.3% of the heat energy in that it had at 0 deg C.

It isn't the temperature drop that causes ASHP performance to fall, it is really humidity, which increases the risk of evaporator icing and therefore increases the probability (or frequency) of defrost cycles. Every time an ASHP defrosts, it effectively reverses and pumps heat out to warm up the evaporator and melt any ice.

Older heat pump designs, and some of the cheap on/off ones from China and the like, usually have very crude defrost systems, that don't really detect the risk of ice forming, but run timed defrost cycles at a frequency that is set by the external temperature, or perhaps the temperature of the evaporator or cold side. More modern heat pumps have more sophisticated icing risk assessment, using a combination of heat pump evaporator temperature, outside air temperature and outside air humidity. This means they will only defrost when they need to and they defrost less frequently when the temperature drops below 0 deg C, because the air will be drier at this low a temperature. The worst point is a few deg above 0 deg C, when the air can be quite humid and the heat pump is being asked to work quite hard because of the low temperature. Once the air gets cooler the heat pump can work hard with less risk of icing because the air will be pretty dry.

The key thing is to size the heat pump so it doesn't need to work hard for an outside temperature range of around 0 deg C to 4 deg C. You can run it hard at -10 deg C and not really run much of a risk of icing and hence still have a good COP.

Our ASHP is inverter controlled (so can modulate its output over a fairly wide range) and has outside air temperature and humidity sensing. By experiment, I found that if I asked it to deliver a flow temperature of 45 deg C, then it would start defrosting, even at temperatures around 7 or 8 deg C. Every defrost cycle would hit the efficiency hard, as it was around 10 minutes of the heat pump running with a "negative" COP, drawing heat from the house to melt the ice on the evaporator.

When I turned the flow temperature down to 35 deg C (which is more than enough to run the heating and hot water pre-heat system) the heat pump pretty much stopped ever going into a defrost cycle. I've not seen it defrost in the past year, and I'm guessing that's because it isn't being asked to work hard through the critical region when the outside air is both cool and damp.

It's worth noting that in places like Finland, ASHPs are the normal way of heating many homes. The reason is that, despite being cold, the humidity there in winter is usually very low, so the risk of evaporator icing in the heat pump is also low.

I think that choosing the right size of heat pump and setting it up correctly are far more critical for air source pumps than for ground source pumps, and that, combined with a widespread lack of in-depth knowledge amongst installers, may well be the reason for the oft-quoted point that "ASHPs don't work well when it's cold".

Edited by jsharris, 02 February 2016 - 08:14 AM.


#8 SteamyTea

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Posted 02 February 2016 - 09:12 AM

Really down to how close to the dewpoint the air gets as is exits the heat exchanger.
Taking my data, there is a good fit between the air temperature and the dewpoint (R=0.92, for those interested).
The linear relationship is Air Temperature x 0.862 - 1.148

To make the arithmetic easy, let us round that to Air Temp - 1°C

So as long as the air exiting the heat exchanger is no more than 1°C cooler than the air entering, there should not be a problem.

With an ASHP this can only be controlled in 4 ways. If the exit temperature becomes greater than -1°C then:
Turn the unit off
Increase the coolant temperature
Increase the air flow
Heat the heat exchanger.

Increasing the air flow through the heat exchanger is the easy one, just a case of increasing the fan speed.
This does make the unit noisier, which is why correct sizing is important.

So how much air needs to be passed though the heat exchanger to keep the difference at no more than 1°C

Again, to keep the arithmetic easy I shall use mass at first and then finally convert to m3.

1 kg of air, cooled by 1°C can yield 1000 J of energy (1kJ.kg-1.K-1).
This is the same as 0.000278 kWh
So to get 1 kWH out of the air, you need to cool 9 kg of air.
This is about 12 m2.

So if your daily requirements are 30 kWh, the ASHP needs to have 360 m2 pass though it.
Sounds a lot, it is isn't really, 4.2 litres/second.

When choosing an ASHP it is important to know if the the coolant temperature can be adjusted (and how it is changed), what the mass airflow rates are and how large the surface area of the heat exchanger is.

Edited by SteamyTea, 02 February 2016 - 09:13 AM.


#9 stones

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Posted 02 February 2016 - 09:33 AM

I looked into a GSHP for our last house as a retrofit (heating and DHW requirement circa 5000kWh each). Capital / installation costs were massive and annual running costs were actually higher (5% or so) compared to an ASHP, once the cost of running ground loop pumps and DHW top up were taken into account. Salesman response when questioned 'look how much you will get back in RHI'.