Loss assessments can be a very serious case of GIGO.  I had two, both coming out at 16kW following, in each case, a 3 hour survey.  I did the calculation myself and got to 10.5kW.  The measured loss is 7kW.  I can account for all the differences and sized my heat pump for 7kW, which turned out to be the right decision.  

The new rules introduced from about June last year are supposed to improve the situation, especially the significant overestimation of air changes.  As to whether they have, only time will tell!  Personally I would always recommend a sanity check against gas consumption or floor area/construction if at all possible, unless the estimated loss is 5kW or less, in which case it wont make any difference because you are fitting a 5-6kW heat pump whatever the actual loss is, because (with the exception of a couple of rarely used outliers) that's whats available!

Has anyone heard of or had a survey/design done by Apex?

There are quite a few installers I know that are called 'Apex' - can you please give me the full name? 

A company I'm currently in talks with has used them for the heat loss survey + design and there are some very questionable things in there.

One example is a hallway area (4.75m x 1.3m, 2.4m high with 1 standard size external door and roughly 2m² window area) having a supposed 1290W heat loss (all other surveys I had averaged out to about 700W).

For context they, and most others, considered a ground floor extension of 2.5m x 3.8m with 4 standard-sized velux windows, a high vaulted ceiling, fully-glazed double french doors and in addition to the velux windows another 3.64m² of windows being around 1310W heat loss. Clearly these 2 rooms are absolutely nowhere close in terms of heat loss, and yet they've got a very similar number according to Apex. 

They also disregarded some of my criteria (vertical radiator in the hallway due to size limitations) and instead listed 2 horizontal radiators. If I didn't have room for 1 horizontal radiator, where is this second one going to go??? And similarly for a bedroom I'm using as an office I requested it to be designed for 21c, they did it at 18c like all the other bedrooms.

They also seem to be really overestimating the output of my existing radiators and their proposed radiators. At a design flow of 47.5c in a room targeting 18c they've got down a P+ 600x1100 outputting 700W, but if I run the numbers (which I could be doing wrong) I only get 600W. 

Can't say I'm thrilled from what they've given and I'll be speaking to the installer about my concerns and maybe question if they should find another company to do their surveys for them? It's a small operation so I don't blame them for outsourcing the heat loss and design, but I wouldn't be happy to proceed with it myself. 

For a heatpump system that cannot respond as quickly to heat loss as a gas boiler, some would need some sort of weather anticipation feature.

That's not necessary so far as I can see.  Measuring the current weather tells the heat pump or boiler whats currently happening to the outside of house.  The inside of the house will respond much slower than the heat pump or boiler can.  

What we don't have at present is heating systems that take into account solar gain or wind factors.  Neither is that dramatic in most cases, but it is a refinement that some here have experimented with on here.  However its quite plausible that mild auto adapt of the sort featured by many boilers and heat pumps is sufficient to deal with this and if so it should probably be left at that.  Boiler/Heat pump control systems are already too complex for many, perhaps most, ordinary people to cope with, adding additional factors needs to be carefully considered!  

Anticipation might be useful for tariff optimisation, but do we really want another level of internet-driven complexity to an already complex control system.  Personally I think heat pump manufacturers should leave this sort of thing to the geeks, and instead optimise the basic controls and function for ease of use.

We had a BG badged Glow Worm boiler that was well oversized and could not condense as it heated the hot water as well as heating BUT ONLY HAD ONE TEMPERATURE DIAL for DHW and Heating. You want condensing and cooler hot water or to hell with condensing! 

I find it almost incredible that they could get away with it.. 🤔Probably when a customer is desperate, after the breakdown of a previous boiler.. Was it not BG that had some warranty that would at some point allow them to choose any boiler they wanted? 

For a heatpump system that cannot respond as quickly to heat loss as a gas boiler, some would need some sort of weather anticipation feature. 😁

I remember that my previous bosch/worcester boiler also had an external sensor that would enable it to have warmed the solid brick home to the target temperature by the set time..

As you mention, a small amount of peak rate consumption is ‘cheaper’ than investment in a second Powerwall. I didn’t fancy changing my allegiance away from Octopus as I have previously had bad experience with other suppliers but don’t have a vehicle at all. Whether an EV tariff would be cheaper in my circumstances, I am unsure but I like Octopus’s ethos and straight dealings so will stick with them. The IOF tariff is possibly my next move though (just for the summer months) then back to Cosy if it is still available. Regards, Toodles.