What we do is measure using air pressure loss equipment and get an accurate reading or base it on educated assessments knowing the results of many other installations or homes already tested. I think this is basically what the zero disrupt HG thing is doing, taking a very large pool of data and assessing the building being assessed against 30 others almost identical, this then makes a fast set of assumptions against proven data. 

I still think every house should be pulse tested or air tested to be accurate and then add that air loos to the calculators and say add a spare 5% max for errors, this unfortunately costs around £300 per test and there is no appetite for customers to add more cost or companies to absorbe the additional costs.

It costs about 300 I believe to do an ach measurement and it can make a big difference. Doing it every time might be going too far however.  Personally I think installers should be obliged, wherever possible to run a sense check of calculated loss vs gas consumption or some other independent measure, and if the two don't correlate suffiently well, to reconcile, if necessary by doing further measurements including ach if appropriate.  Unless and until this becomes the norm, cases like yours will continue to occur.

If you do decide to negotiate I wish you luck.  Even if you don't get any further concessions you will at least have the comfort of knowing you tried.

Mine was £265.  It would be cheaper for Octopus.  They could train up their surveyors, it needs an accurate floor plan which they are capturing anyway.  They probably need to just train up some of them and only do it on houses that are perhaps a bit bigger or of certain construction types.  I bet they could get their software to work that out with AI.  Then for those that didn't have a blower test, if it comes out over 7kW they could then do a blower test just to be sure.  I guess they will stop installing Daikin's sooner or later, but people are still coming on the facebook groups saying a big Daikin has been specified.  They also need to lobby for the regulations to be cleared up rather than hiding behind them.  They are not fit for purpose if they are going to be used against customers like this.

I am going to try to knock them down.  I've done some new break even calcs.  It would have been 6.5 years with the predicted SCOP, 8.7 with the poor SCOP and extra expenses I have incurred.  If I accept their offer it becomes 10.7 (assuming an improved SCOP), this is considerably longer than I had originally hoped. 

Oh, and do you know how much measured ACH or even ACH adjustments are used in the industry?  I get the impression they are not used much and some think that the default values are mandated.  It seems to be a huge blind spot.  Although one which Octopus, given their size and the number of issues they have had with oversized heat pumps, should have got to grips with by now.

The ACH can actually make a huge difference.  My heat loss is 5kW in reality and with the correct measured ACH and more than 7.5kW with the default pre 2000 figures.  And calculations from gas can be OK.  I read that they would be way out so I didn't do the calculations before my heat pump install.  But when the heat pump appeared to be oversized it was the only data I had at that time so I thought I would give it a go.  I didn't use the degree day method because I had a complete temperature record from inside and outside for every day.  Because I also had temperature records for all rooms I could be confident that the hall sensor was a good average of the whole house.  I plotted all the points and used a linear regression.  And also worked out the W per degree for the absolute worst gas usage day.  Scaling that to the inside/outside temperature difference at -3.2 gave me 5kW.  That's based on my boiler running at its specified efficiency of 65% - this was an assumption.  I should also add that I was not using gas for DHW at all.  The few datapoints I had for the heat pump were aligning with the gas and by mid winter it was clear that the same calculation for the heat pump was also 5kW.  Then I got a door blower test, it turns out my 1988 house is more air tight than a new build.  I used the measured ACH in heatpunk and the MCS spreadsheet models and both came out close to 5kW. 

If you think you can hold out and they have not done the correct design, job and commissioning then do so as long as you can bare it and perhaps they will show goodwill. I would imagine with their buying power that they sell so many Daikin's that they could get one for free from the supplier anyways.

I wonder if any of the employees are on the forums here? 

Unfortunately we are a company that complete a lot of Retro-fixes "coined name by Mars" but I am not so great in helping how to get their money back as I am not a consumer expert and TBH if our company completed an install to what you described I think I would give them a whole new system or a big chunk of the money back. 

We design often based on 1, it takes experience to look at a 40yr old building and see they have had improvements made over the years and now the standard air losses do not apply.  

I’m also thinking that the heatloss arising from the ACH is usually a very small amount in comparison to the overall figure.

Not so.  In my case it made the difference between 10.5 kW and 7kW.  There are a fair number of others who report similar eg op on this thread.

In any case a sense check doesn't just concern ach, it takes in every opportunity for the survey to be wildly out.  For surveys to be out  by a factor of 2 (as mine were) appears not to be uncommon and makes a vast difference to a 6kW house!

The problem in taking the gas figure is that everyone consumes differently.

Yes but (a) this makes less difference than you might first think and (b) it can be factored in to an extent with a questionnaire, or almost completely if you have half hourly readings, which many will.

The plain fact is that the current system results sometimes in errors of a factor if 2 or more.  It makes absolutely no sense not to sense check it against a measurement if you can, which will be possible for the vast majority of retrofits.

If the two correlate reasonably then proceed, if they don't then find out why before installing the wrong heat pump and ripping up floors unnecessarily to upgrade pipework.  

The current system benefits the industry by providing an ironclad get out for installers at the expense of the customer.  That is immoral.

It costs about 300 I believe to do an ach measurement and it can make a big difference. Doing it every time might be going too far however.  Personally I think installers should be obliged, wherever possible to run a sense check of calculated loss vs gas consumption or some other independent measure, and if the two don't correlate suffiently well, to reconcile, if necessary by doing further measurements including ach if appropriate.  Unless and until this becomes the norm, cases like yours will continue to occur.

If you do decide to negotiate I wish you luck.  Even if you don't get any further concessions you will at least have the comfort of knowing you tried.