Posted by: @jamespa

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It's increasingly clear that there is frequently problem with heat loss estimates and ach is one of the culprits.  There are some installers, including a few here, who will acknowledge that but most won't so far as I can tell and just stick to the safe (for them) MCS rules.  I was fortunate to find one who would assume ach 0.75 for my 1930s house and use my gas consumption as justification.  Few would do this though even though the evidence was clear.

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.

@matban I would have to check the CIBSE guide as we have a software for this but its something like 3x for Kitchen and bathroom 2x for some other areas and 1.5 across the board in other areas but is subject to age of building and structure, as far as I am aware there is no adjustments we are allowed to make out of the recommended guide by CIBSE, we are kind of bending the MCS rules if we allow for less but in most cases where we have measured or have accurate measurement's they are nearly always much lower once a home owner has installed double glazing, installed a better letter box or changed the door to one without a letter box hole and have changed the doors to updated energy efficient doors, insulated walls, got rid of chimney balancing air bricks, sealed off the open fires and so on. 

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.