S2D2

I don't know the details but for a while they made all the smart tariffs fixed term, I remember a vague email from a while ago about they needed to to comply with the price cap when the government were giving out grants during covid. They are obliged to inform you when the fixed term comes to an end and what will happen when it does, if you do nothing then as far as I know they're entitled to switch you onto the standard tariff, someone will be along to correct if not. The link in such an email will be valid only so long as it's required to be and after that all bets are off. Perhaps the token expired so they have to treat you as a customer that hasn't logged in?

Yep. MCS will want 2-3 ACH depending on room type, it's taken as the worst case but for your average house where effort has actually been put into closing up gaping holes to outside 0.5 can be a reasonable sense check. It might actually be higher or lower, you need an air tightness test to know for sure. MVHR changes the ventilation heat loss dramatically for obvious reasons.

Yes they've done a lot of good and I've learnt a lot from their forums. I was just adding a middle ground in between what this thread started off as (very high accuracy) and what the revisit focused on (£11 aliexpress sensor)

As with everything else, the emonpi2 cost has ballooned, around £240 now. I'm using a Shelly EM Pro (~£78, got it on sale for £60something) for my heat pump as it's accurate enough to track usage and optimise performance. It comes with 2x50A CT clamps. I log via MQTT but it has the standard app for a simple view of usage so it really depends on requirements. Cheaper stuff you'll lose the factory calibration and increase error margins, but how important is that to you?

What temp did the house start at? If it held steady then yes, if there was even a small drop then a lot of uncertainty is introduced. You need the ventilation losses too which depend a lot on the house, 0.5ach is a reasonable sense check, which makes 18kW sound way out. The really, really important figure is the minimum modulation of the unit being proposed. This varies a lot with manufacturer, hence why it's hard to pin down a general rule. I'd agree with others who provide arguments for aiming higher than the bare minimum heat loss, there have also been some recent counter examples to that method as the industry has tried it in earnest. There's an urban plumber video on going back to upsize a unit because of uncertainty in the heat loss / defrost losses leading to a cooler than planned house. Vaillant also recently removed the Arotherm Plus 5kW from the compatibility chart for a 250L cylinder; I haven't had clarification on why but presumably because in the extreme case it spends too long heating the hot water, meaning the power outside that window has to be increased to compensate. So you're right to reject 18kW but maybe don't aim for the bare minimum.

Yes. 90s house but various improvements to airtightness, nothing drastic but better than average. PIV fitted and in combination with a dehumidifer for clothes drying solved the humidity/stuffiness issue. Something still didn't feel right, so got a CO2 meter, bedrooms can hit 3000ppm if the door is closed overnight. MVHR will be going in as soon as I get round to it.

+1 to internal noise being an issue with split systems.

I replaced a working 11 year old boiler to avoid this issue, waiting for it to break results in only one realistic outcome, a new combi goes in.

Disable the heater and it will be a penny a day.

Yes, this is correct.

I've had a walnut worktop in about six years, nothing but Danish oil and it looks like new unless you look extremely closely for all the small dints that come with wood. Takes a lot of time when new, it must have drunk about two dozen coats over the first few months. Now it's an hour every six months to take everything off and do a quick coat. Don't skip the reminder or you're setting yourself up for a week long job like others have said. Keep on top of standing water too, quick wipe with a tea towel is all it doesn't have to be bone dry.

I found the Bluetooth pairing to be broken/unreliable, so log onto the local WiFi it creates instead and configure it there. Then in the app just scan wifi for new devices and it'll add no problem.

From what I've heard £200-300 isn't uncommon for an official Vaillant service. Octopus have a service plan for their installs at £108 a year which is more palatable and more than offsets any efficiency differences between the units.

The point of the BUS grant is to incentivise switching to an ASHP, required for gas boiler users because if installing an ASHP costs more than just putting in another combi, the vast majority will just put in another combi. Somewhat unfortunately for resistance electric users, they already have a very strong incentive to switch to an ASHP as they could already achieve payback within a short time period. Thus, the government can leave the market to do its thing and no intervention (funding) is required. I may be in the minority but the BUS grant did exactly what it was intended to for me. 11 year old end of life boiler and the BUS grant brought the price of an ASHP below that of a new combi, so in it goes and off goes the gas connection. Big tick in the decarbonisation box for that BUS funding.

You need to check individual models for this, for the Daikin units Octopus install there are two ranges which determine what the minimum modulation is: 4-8kW and 9-14kW. There's a thead on another forum where someone got Octopus to rip out the 9kW they installed and replace it with an 8kW unit, performance increased hugely due to the minimum modulation being vastly lower on the 4-8kW units.