atlantication

Variable speed is good for improving efficiency. Lowering the speed when the boiler isn't working at 100% is good to lower the pump electricity consumption and also raise the boiler efficiency due to the lower return temperature increasing condensing. However it also reduces the valve authority of your TRVs and can result in an unbalanced system and some rooms getting insufficent heat input. The problem comes from your bypass valve. In an ideal world there would never be flow through the bypass as this is pointless and reduces efficiency. However with TRVs and an incorrectly set weather compensation curve it's possible for boiler flow to reduce below the minimum without the bypass valve. Thus the bypass valve and constant speed pump. As flow reduces, differential pressure increases until the bypass valve opens assuming the pump speed and valve pressure preset are correct. Having a constant-pressure or proportional-pressure pump in principle breaks the pressure-operated bypass valve as bypass would either be constant, or increase with the flow through the rest of the system (precisely the opposite of what you want!). If I were in your position I would ensure I had weather comp set up correctly and the TRVs adjusted right (so the bypass should never bypass), then set the pump to the lowest constant pressure curve such that all the radiators get warm and the boiler is happy. I wouldn't bother with proportional pressure unless I had a badly designed system with microbore or something. OTOH, keeping it in constant speed mode with the bypass valve set to bypass at low flow will work fine too, and there isn't really that much difference in the running costs/noise levels. This whole thing sort of highlights the benefits of getting a system boiler rather than heat only - the boiler controller can control the pump to ensure optimal flow for efficiency but also that it meets its minimum requirements.

840 kWh/month, taking off 3 kWh/d for DHW equates to pretty much exactly 1 kW constant load - 730 hours per month. Assuming this is averaged over a year, and conservatively that the worst month takes 1/3 of the total heat demand. The max monthly demand would only be an average of 4 kW. With a 7 kW unit there would be more than enough headroom for intermittent use/cold snaps assuming the house has at least a little insulation. The best way to confirm would be to multiply your heat loss out by the Heating Degree Days for your location/year and see if that vaguely matches up with the electric input. That way you can be more confident in your calculations. One other caution - would be a good idea to look at the datasheet of the system, make sure the heat output at low temperatures still meets the 7 kW.

Yes the earthing will be done by the solar installers if it's for a current transformer to the mains. If it's actually to plug the inverter into the network so you can access it on the web then you probably don't need to worry - just make sure that at least one end of your cat5 cable has a clear plastic plug on it not a metal plug.

There shouldn't be any concerns about interference, if the equipment on either end is designed for it, and the solar installers wire it correctly. The rule against running mains with low voltage cables is mostly an issue with a slight buzz on analgoue signals (like old telephones) and a little bit H&S concerns if somebody working on the phone system confused a mains cable for a LV one. It's only 3m they're running together anyway, hardly a long distance! Fibre internet is a glass fiber not a copper cable (CAT5). Sounds like your fiber isn't actually coming into the house. STP cable is fine (UTP would be fine as well) but you do have to be careful with the earthing arrangements for STP or FTP cables. Make sure it's not wired up to cause an earth loop (i.e. make sure that the shield is only connected to earth on one end). You don't want any earth fault currents going through the shielding on the cable as it's not designed to carry current and could be dangerous.

Fridge isn't going to be consuming 3kW, even the most inefficient fridges will use less than a kW for defrost. It is a bit of a mystery as none of the appliances you brought up really fit the profile of the power being consumed. One thing I was wondering - do you have a PV diverter? If that's somehow malfunctioning that could explain the magnitude of the power draw, at least. Another thing to check would be what happened when the clocks went back? Did the ghost usage stay at the old schedule or update to the new?

Key safes are convenient when you want to _provide access_. Their purpose is obvious and they're intuitive to use for the people you want to let in. But they're also obvious to the people you want to keep out. And they're easy to get into without authorisation - cheap keysafes are all made of cast zinc so they'll come open after a tap with a big hammer. They're also able to be manipulated open easily with the appropriate tools/experience - to note here, I don't believe any of the fancy certifications that manufacturers promote actually test for manipulation resistance, only against brute force attacks. That's probably because the wrong'uns don't manipulate locks open so there's no sense testing for it. A good keysafe such as this (now discontinued) model will be more expensive but made out of steel with a more resistant code mechanism. But if the choice is between hiding the keys under a flowerpot and putting them in a keysafe, even a cheap one will do. At least you can change the code every so often, and it's obvious where the keys are when somebody needs them. In a low-threat environment (behind a gate in a low-risk area), I wouldn't have any problems putting one in for myself, assuming your insurance lets you have one. In fact, I'd worry more about how it looks for the WAF - most are rather ugly IMO. One other note - if your security gate has a code lock on it that looks like this then it's not actually much of a security measure, they can be opened with no tools in about the same time it takes to dial the code normally.

There is an increased chance of condensation in the internal wooden structure with a so-called hybrid roof, as moisture can travel through the mineral wool and the wood is now cold (as it's in the middle of the insulation temperature gradient). This contrasts to a warm roof where the wood is warm so there cannot be condensation, or a cold roof where there the moisture barrier should stop condensation, and it's ventilated. There would be no way for the moisture to escape your roof except the same way it got in - back to the inside in summer when the thermal gradient works the other way around. However, looking at your particular insulation thicknesses, it doesn't look like condensation is actually going to happen - see attached screenshot.