My gut feeling is that the heat pump is right sized.

• it does not seem to cycle
• It heats the house comfortably
• costs/usage seems reasonable - especially against predictions by EPC or MCS
• the spare capacity would probably allow us to heat all rooms to 21deg if we wanted to 

I'd be interested in other experiences - if you have the data.

Clearly heat loss calcs and EPCs depend on quality of assessors in the first instance.

I'm contemplating challenging my latest EPC - but not sure I have the "energy"!!

Any of these calculations will be an estimate. When you consider the steps an assessor would need to take to calculate it precisely, it simply wouldn't be feasible (i.e. invasive surveys). There's other factors such as air tightness/draughts etc. Then it is trying to find a U value for what you actually have in your home and the components of a particular wall/floor etc.

That said some companies who we had provide quotes used worst case (assumed no insulation). Others just guessed. Some actually asked me what was behind walls/under floorboards etc as they went around the house.

You've then estimated your heating and hot water requirements based on an approximate COP. 

I sense checked our heat loss calcs with the 'rule of thumb' method based on previous years gas usage- another estimate. EPC I largely ignored.

@benson i totally accept its an estimate but surely it should be closer to reality?

Otherwise most people could probably do better based simply on previous gas/elec/oil bills and making simple informed conversions and estimates. Maybe MCS and EPCs should use such data as a sanity check. With smart meters it would be relatively easy to collate. Also adding in information on PV solar, EVs to get even closer 

My MCS at 28k kWh

vs Latest EPC at 36k kWh 

vs consistent annual actual usage at 5800kWh with COP estimate of 3 probably equivalent to 18k kWh

The MCS (& EPC) in my case are actually misleading and would be likely to result in an oversized installation. 

@tim441 do you not see the irony that you are using your estimate, to say that the other estimations are wrong?

@benson indeed!!! But at least my estimate is based on real world.

However you cut it the worst case would be a COP of 2.5 and I am sure its not over 3.5 for a LG. Across heating and hot water.

If it was only 2.5 i think we would be using much more power - or freezing cold!

So that gives us a usage between 15k and 20k. And i stand by my estimate of 18k being realistic.

Posted by: @benson

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@tim441 do you not see the irony that you are using your estimate, to say that the other estimations are wrong?

Not sure I entirely agree, @benson. In his last post, @tim441 provided an estimate based on actual measured figures. The MCS approach has improved but it’s still based on a model, not measured figures, and that model includes a significant number of assumptions. There’s a lot of difference between an estimate based on data and an estimate based on assumptions.

Posted by: @tim441

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I'd be interested in other experiences - if you have the data.

Clearly heat loss calcs and EPCs depend on quality of assessors in the first instance.

I'm contemplating challenging my latest EPC - but not sure I have the "energy"!!

My house/experience: 

a) 2off 'professional' 3 hour surveys: 16kW.  One I paid £300 for the other was FOC, different surveyors/heat pump installers

b) DIY heat loss calculations using MCS assumptions but the correct fabric (which I made a BIG point of telling the surveyors about): 10.5kW

c) Actual measured from gas and heat pump: 7kW

a-b discrepancy accounted for by incorrect fabric assumptions (basically ignoring fabric upgrades that they couldn't see) and double counting of room to room losses

b-c discrepancy accounted for by excessive ACH used in default assumptions

Posted by: @tim441

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Maybe MCS and EPCs should use such data as a sanity check. With smart meters it would be relatively easy to collate

Absolutely they should but... the current MCS system works extremely well for its purpose, ie to protect the installer with a bomb-proof recipe that he just has to follow to comply, and which is highly likely to overestimate loss thus making it unlikely he will receive a call out because the householder is cold albeit the householder may incur significant unnecessary disruption and expense.  As soon as you introduce a 'sanity check' it weakens this defence, I cannot therefore see that MCS has any motivation at all to change the system.

As a matter of fact I did write to them a couple of years ago with observations about the current system.  Their response was that they recognised the shortfalls but 'currently have no alternative'.  In the latest iteration of MCS3005(d) (the governing document) they have protected themselves even further than previously by, for the most part, referencing a methodology provided by someone else!  Remember that MCS is funded by installers and he who pays the piper calls the tune.

Posted by: @tim441

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However you cut it the worst case would be a COP of 2.5 and I am sure its not over 3.5 for a LG. Across heating and hot water.

If it was only 2.5 i think we would be using much more power - or freezing cold!

So that gives us a usage between 15k and 20k. And i stand by my estimate of 18k being realistic.

COP of 3 perhaps a little low and you say that you don't heat unused rooms. Thus there's a few factors there which might lead to an under estimate of heating requirements for building.

To be honest I've always thought it is a tall order to come up with a heat loss calculation based on building fabric survey, particularly one that is not intrusive where the surveyor is not drilling inspection holes in everywhere. If I were an installer, I'd be erring on the side of caution as well in terms of U value assumptions. I do understand and agree with what you are saying- just playing devils advocate I guess.

Posted by: @majordennisbloodnok

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Not sure I entirely agree, @benson. In his last post, @tim441 provided an estimate based on actual measured figures. The MCS approach has improved but it’s still based on a model, not measured figures, and that model includes a significant number of assumptions. There’s a lot of difference between an estimate based on data and an estimate based on assumptions.

I've been thinking along similar lines to @tim441 from my own experiences with an Ecodan system, where I've got the benefit of reported COP data too.  I've been meaning to create a post called "Something Doesn't Add Up Somewhere" to document this for a while as I'm seeing heat pump output data which suggests a heat loss significantly lower than the consensus figures from a number of heat loss assessments, including one with a room by room template that I've been able to sense check and manipulate within Excel to apply at a sliding scale of outside temperatures.  This is what has led me to think that there's perhaps an issue with the output figures being reported by the Ecodan unit, as they look like they could be potentially understated (which in turn would provide some context around the COP scoring from the system that is niggling me).

I'm very conscious of the fact that, without providing the basis for this in some detail, it reads very much as 'man with low COP score blames the reporting' which is why I've not managed to get around to drafting the post as yet, but I have similar concerns to the OP, so this thread will hopefully be the incentive to update all the data recording and see if the conclusion remains the same before drafting the detail.  My doubts are more along the lines that the assessed consensus heat loss feels 'about right' compared to that calculated via the reported heat pump output data, and there's at least a template calculation available that I can provide details as to where the potential changes could be made to bring the heat loss calculation down into the right area.  Without that, the gap would logically be a shortfall in the output reporting. 

In the absence of the expensive heat monitoring kits being included on a system, all the heat pump output figures are essentially a hidden calculation from each manufacturer, or a calculation derived from their reported statistics.  Uniquely, where the natural tendency might be to over-report this figure, such as within a YouTube video clip I viewed recently which showed the Aira app reporting to a user that they had a COP of over 7, my instinct is that the Ecodan reporting is possibly over-cautious in this regard.