S2D2

Most employees would be extremely happy with an 8.5% tax rate.

The hard manufacturer restriction when I fitted mine was no joints within the property, if not it could be DIY installed.

In hindsight I would have fitted mine on the ground, there is a slight noise transfer through the wall after a few years. It's only used for aircon now though so not worth bothering changing it.

Find the model number of your unit then you can find the instruction manual online. Summer bypass should run continually, not just a few hours a day. Some units increase fan speed to help purge heat but that's often disabled by default, sounds like it would help in your situation though. It will never be air con, just purging air which should get you closer to outdoor air temps. Are you unable to open windows sufficiently? Or do you live somewhere where the outdoor air temperature is in the 30s?

They are available, e.g. https://www.mazda.co.uk/cars/mazda-mx-30-r-ev/ Now that the second hand market is flooded with EVs with over 250 miles of real world range, it's not an issue for most people anymore. I took my 3 year old EV on a 600 mile round trip recently, absolutely no different to the old petrol car, stopped once each way at the same service station as always for a whopping ... 26 minutes. Newer cars will charge at least three times the speed of mine.

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.