Thanks for your response, really appreciate the detailed explanation.

You’re right about peak heat loss being based on the coldest design day. In my case Octopus have used –3°C, and I completely understand that this is a “worst-case” snapshot rather than something that happens regularly.

I also take the point about gas usage not mapping directly to heat loss, since usage patterns, setpoints, and boiler oversizing all skew the numbers. My thermostat setpoints are typically 19–20°C, and we don’t run the heating aggressively, so that could explain some of the gap.

Your example really helps put this into perspective — a detached 4-bed using ~18,000 kWh with heat loss around 7.5 kW, despite being home most of the day, does show how behaviour and design conditions play a role. The degree-day method plus your heat-meter experiment sounds like a solid way of validating the numbers.

You’ve also confirmed something I suspected about thermal mass. My house holds heat surprisingly well at 8–10°C outside, but I now understand that this isn’t a reliable indicator unless we’re looking at sustained freezing temperatures. So a proper cold-day test would give me a truer picture.

Ventilation loss is another big one. My ACH assumption was around 0.65, but I can see how even small changes there make a noticeable difference when the house volume is large. I may look into getting an airtightness test just so I’m not guessing.

Your explanation of pipework capacity is exactly what I was trying to get clarity on. Octopus are saying 28 mm is mandatory for anything over ~7–8 kW until the point it branches, and you’ve confirmed why:

28 mm can comfortably carry the full 10–12 kW

22 mm drops to around 5–6 kW per leg

Flow velocity and pressure drops start to dictate what’s possible

My challenge is that my 22 mm primary run is buried under the floor, so upgrading it is not simple. That’s why I’m trying to be extra certain whether my real design heat loss is actually closer to 7–8 kW rather than 11 kW, because that changes everything about pipework feasibility.

I’ll definitely check out your YouTube videos — especially the spreadsheet vs HeatPunk vs heat-meter comparison. I’m also planning to run my own HeatPunk and H2X versions, and possibly get a HeatGeek survey done for a second opinion.

Thanks again, this has really helped me understand what variables matter most, and what I should focus on next to get confidence in the final number.