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Author Topic: ASHP just as good as a GSHP - ?  (Read 6910 times)
JonG
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« Reply #30 on: January 20, 2013, 07:46:17 PM »

If the heat loss has been calculated to MCS standards based on geographical o/s ambient, coverage to 100% at said o/s ambient and any local factors added in and is 16kw your options are ltd.

Most single phase ASHP run to roughly 10-12kw at -3, same for GSHP. Twin compressor options exist for both, but you need to be aware of the potential max current draw, work out the diversity on the board and inform the DNO.

Same applies if you were to use 2 units.

If you have 3 phase the options expand.

You could consider bivalent as an option if you heat loss is this high.
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mpooley
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« Reply #31 on: January 20, 2013, 10:10:29 PM »

If the heat loss has been calculated to MCS standards based on geographical o/s ambient, coverage to 100% at said o/s ambient and any local factors added in and is 16kw your options are ltd.

Most single phase ASHP run to roughly 10-12kw at -3, same for GSHP. Twin compressor options exist for both, but you need to be aware of the potential max current draw, work out the diversity on the board and inform the DNO.

Same applies if you were to use 2 units.

If you have 3 phase the options expand.

You could consider bivalent as an option if you heat loss is this high.

Ah! things are not looking bright for my Heat pump ideas.
 facepalm

My calculations which I am sure are correct are for 13Kw but That's because 3 bedrooms and a dining room are never heated.
I'm sure a calculation of full coverage would be around 16Kw
I have not had a proper MCS calculation done yet. I was just about to get one done.
I will have to talk to the guy who quoted me the other day and see why he didn't mention any of this.
« Last Edit: January 20, 2013, 10:15:37 PM by mpooley » Logged

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

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JonG
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« Reply #32 on: January 21, 2013, 12:21:53 PM »

Be wary of excluding unheated zones when sizing, you may want them to be heated eventually, also unless well sealed they become cold sinks which draw heat from the heated zones potentially meaning the stat (s) don't satisfy where you want them too.

A full room by room elemental heat loss to MCS standards should confirm total heat loss, worst performing room/area, amount of watts per square metre loss in each area etc.

Depending on the model that the installer uses, it should be available in a spreadsheet which you can manipulate to show the impact of different insulation strategies

This then allows you to consider additional insulation options, heat emitter sizing to squeeze the most duty from the heat pump and bivalent possibilities.
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mpooley
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« Reply #33 on: January 21, 2013, 01:58:58 PM »

Be wary of excluding unheated zones when sizing, you may want them to be heated eventually, also unless well sealed they become cold sinks which draw heat from the heated zones potentially meaning the stat (s) don't satisfy where you want them too.

A full room by room elemental heat loss to MCS standards should confirm total heat loss, worst performing room/area, amount of watts per square metre loss in each area etc.

Depending on the model that the installer uses, it should be available in a spreadsheet which you can manipulate to show the impact of different insulation strategies

This then allows you to consider additional insulation options, heat emitter sizing to squeeze the most duty from the heat pump and bivalent possibilities.


Thanks Jon
I know what you mean but I have one room (the dining room) which is large and has 3 external walls made of 500mm thick flint. We never use it in the winter and never will as we like the flint walls in it. The rest of the house is insulated to a pretty high standard.
The reason we have a high heat loss is because its such a big house (old barn conversion) not because the insulation is poor.
I have tested the system running the water at 45c for a while now and we have never been warmer - we are retired and we have the heating on all day.
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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
titan
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« Reply #34 on: January 21, 2013, 03:27:53 PM »


I have tested the system running the water at 45c for a while now and we have never been warmer - we are retired and we have the heating on all day.


If you can record the internal and external temperatures and how much energy you are using for heating over a reasonable timescale then you will have a more accurate estimate of the size of heat pump you need than any  mcs/sap calculation.
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mpooley
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« Reply #35 on: January 21, 2013, 03:37:33 PM »


I have tested the system running the water at 45c for a while now and we have never been warmer - we are retired and we have the heating on all day.


If you can record the internal and external temperatures and how much energy you are using for heating over a reasonable timescale then you will have a more accurate estimate of the size of heat pump you need than any  mcs/sap calculation.

Yes I have a very nice system where i have records of most room temps, external temps, flow and return Temps and boiler on/off states.
every 15 minutes for over two years and I have done my own calculations.
Also double checked with my oil usage over six years it almost matches exactly.

I reckon I need a 13kw system - but when I started to do the MCS calculations it was working out much more (although I never finished all the rooms as it was such a drag)
The trouble being that I have to use their calculations rather than my own. (as far as I know).

Of course I don't suppose anyone knows the details yet?
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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
JonG
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« Reply #36 on: January 21, 2013, 04:53:27 PM »

To qualify for any grant under the current umbrella it would have to meet the MCS requirements for the heat loss which is underpinned by British Standards, CIBSE data and 30 years of geographical/meteorological data, its been in place since around last March and should be followed by any MCS installer worth dealing with.

That said it is very long winded and laborious, and when we were sizing before this came out our calcs were more conservative, just using whole house methods.

I believe you also have solid stone to contend with, which on paper looks atrocious given its U-values, but we have done quite a few units in stone properties and due to the thermal mass and continuous heating they work fine.

I will also PM you a report from Historic Scotland where they measured in situ U-values on solid walls and got some interesting results.

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titan
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« Reply #37 on: January 21, 2013, 05:28:33 PM »



Yes I have a very nice system where i have records of most room temps, external temps, flow and return Temps and boiler on/off states.
every 15 minutes for over two years and I have done my own calculations.
Also double checked with my oil usage over six years it almost matches exactly.



Is there a grant for ASHP? I am not sure but it will be no more than £1,200 and it will probably cost that or more extra for an MCS approved installation. The RHI for ASHP has not yet been agreed and is far from certain.  You know what size ASHP you need. It may be worth ringing a few HVAC installers to get a price for a non MCS installation and weighing up the pros and cons. I expect you are aware that an ASHP installation needs planning permission there is no permitted development rights as with GSHPs.
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JonG
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« Reply #38 on: January 21, 2013, 06:35:09 PM »

At the moment the grant for ASHP's is £850 plus various top ups from manufacturers as they see fit, RHI details may be out soon but who knows!

Permitted development rights subject to conditions are in place: http://www.legislation.gov.uk/uksi/2011/2056/made

I did mean to say in my post above that the 100% rule on heat pumps is draconian and detracts from decent installers who know what they are doing.

There is an argument that a heat pump sized to be most efficient at average o/s temps is a better idea because it could be smaller than its 100% counterpart and therefore cheaper and cheaper to run overall due to its smaller compressor also bigger units need bigger plant (buffers and sometimes cylinders) and can cycle more if the range it is selected from doesn't match the load closely enough (e.g. 14kw Ecodan on a 9kw load).

Clients who are forced to go to 2 units by the 100% rule generally won't invest because the cost benefit doesn't stack up, but could easily have had a well functioning system covering 85% or more of the load, with the balance on the direct immersion or an alternate bivalent source.

It all depends on the financials and the RHI if and when it comes into fruition.
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baker
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« Reply #39 on: January 23, 2013, 09:02:48 AM »


the ecodan ?
the machine can match the load all day
from 4.2 kw to 14 kw
so that issue has been resolved sometime ago
also designed for uk humid climate
less messing around with defrost cycle problems / settings
baker
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JonG
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« Reply #40 on: January 23, 2013, 10:59:21 AM »

Hi Baker, the minimum step on an Ecodan is not quite so simple to calculate in terms of its output as stating its minimum and maximum kw ratings.

I have the formula from Mitsubishi and some look up tables if you want them via a PM but to calculate at what outside temperature the heat pump hits minimum step you need:

Heat Loss
Max and min output
Delta T inside to outside at design o/s temp
Heat loss coefficient
Lowest outside operating temperature

The formula then confirms at what o/s ambient temp the compressor is at min step, if the load from the property is lower than this output cycling can be an issue.

The bigger the difference between the badge rating and the heat load, the higher the minimum step and the greater the risk of cycling.

This formula should be applied on all inverter designs but rarely is, but is more of an issue with the Ecodan range because of the big gap between the 8.5 and the 14.
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