NetTechie

Thanks @SimonD. The only information they asked for was current building elements, external walls, floors, windows and ceilings, no specific u-values were requested or given. They were provided detailed floor plans of the build and I have no idea what software they used. One thing I did notice and I called them out on it was even though I specified no radiators, the specified radiators to all the first and second floors. The ground floor, they broke down into 3 sections, front, rear and side, not sure why room based calculations weren't done for the GF, but they were for the FF and SF.

Thanks for all the responses thus far. So it seems my heat loss calculations seem pretty bad at 11.3kW, these were done by an MCS company and it did seem a little high. Our new extensions fabric will be double block wall with 120mm cavity fill with PIR boards, 40mm EWI, existing walls are double brick with 70mm cavity, which will be blown with EPS beads and then 60mm EWI applied. All windows, skylights will be triple glazed. Ground floor will be screed throughout with 150mm PIR insulation below and 150mm PIR insulation in the roof envelope. I'm not sure what else I could do to lower the heat loss, or have the company simply calculated this wrong or provided enough margin on their design?

The South facing rooms are where we will spend most of our time, this is the one plan area, I work from home so the Study is also important to me and bedrooms for night comfort, which also includes 2 loft rooms. I'm not expecting AC level cooling, but enough to keep the house comfortable in the peak summer months, so if it can deliver a delta of say 10c -15c I would be exceptionally happy.

Wow. 3.5kW at -9, could you share your build fabric details and what you did to achieve this, this sounds extremely high performance.

The property will be around 385 sqm over 3 floors. One of the companies said a 12kW would be fine as they estimate the heatloss to be around 11.3kW on the coldest day at -2c, but having a 16kW would give more headroom and they also suggested additional electric backup, whatever that is meant to mean.

Hi all, I’m in the middle of a full‑scale renovation of a very tired property — original 40‑year‑old fan boiler, vented cylinder, single‑glazed metal windows, the lot. The house is being stripped back to brick with new floors, ceilings, EWI, cavity insulation, new roof, the works. I’ve been doing the rounds at the homebuilding/renovation shows, but I’m really struggling to find installers who actually understand integrated systems. I’m an engineer, so I like to get into the detail — but the moment I mention fan coil units, I get blank stares or “we’ll have to ask someone else”. Not reassuring. My end goal for the refurbished house: ~16 kW reversible ASHP for heating and cooling - with UFH throughout + towel rads in bathrooms Fan coils for bedrooms, large open‑plan areas and south‑facing rooms MVHR with heating/cooling module (going with Zehnder Q600 + CW12, designed by Paul; armaflex radial ducting planned) DHW storage — Mixergy looks promising, but installers keep telling me I won’t see any benefit for the outlay ~7 kW solar + battery storage Where I’m stuck is tying together the ASHP, DHW cylinder, fan coils, solar and batteries into a coherent system. Heat pump thoughts The LG Therma V R290 monobloc looks excellent on paper — strong COP, good flow rates — but I cannot find a single installer who actually works with LG. I keep getting pushed toward Samsung, Vaillant or Mitsubishi. Two installers have specified Joule cylinders. I’m personally not keen on Samsung due to reliability and integration quirks. What I’d really appreciate advice on: LG ASHP installers - Has anyone found a company that actually installs LG heat pumps in the UK? Real‑world views on the LG Therma V R290 - Specs look great, but I’d love to hear from anyone who has one running. Fan coil units - This is the big one for me. I don’t want split AC with another condenser outside and messy wall units — I want everything integrated off the ASHP with concealed units in the ceilings / walls. - Anyone used Megawave Energy or other brands? How well do fan coils actually cool during those 30–35°C heatwaves? DHW cylinder choices - Is Mixergy X worth it in a heat‑pump‑led system, or should I be looking at something like Joule? Solar + battery recommendations - I’m being quoted for panels I’ve never heard of at 5× their retail cost. Most installers push Fox batteries — any real‑world experiences? If anyone has gone down a similar route — integrated ASHP + fan coils + MVHR + solar — I’d really value your insights. I’m trying to design a system that actually works as a whole rather than bolting together whatever each installer happens to sell. Oh and to throw so more complexity, control is important, I had my home automation company years ago, so can design this but the kit needs to integrate into HomeAssistant and Control4, Heatmiser is easy enough for the CH, DHW and FC's but more invested in the solar, battery / energy side of things. And how do I still get get hot water in summer when I'm running the ASHP in cooling mode? Thanks in advance.