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> Is 18 kW not enough? Depends on the rest of your system. Our gas combi outputs 36kw on demand. We use high temperature rads and need no water storage. To ge

> Is 18 kW not enough? Depends on the rest of your system. Our gas combi outputs 36kw on demand. We use high temperature rads and need no water storage. To get premium performance from heat pumps you need buffet tanks to heat up water over the day and a low temperature central heating system. All achievable, but it turns a £10k boiler installation into a £30k house-wide replumb. Well before you factor in actually making the house perform better. It takes a big run up to make the sums work.

ZeroGravitas 3y ago
Your high temperature radiators are likely throwing away 20% of your heat, so you probably need to include that in your sums: https://www.theheatinghub.co.uk/why-our-condensing-boilers-do-not-condense https://www.theheatinghub.co.uk/why-our-condensing-boilers-d... > The boiler does not reach 90%+ efficiencies until the return temperatures are around 45°C. This means water must exit the boiler at a maximum of 65°C, impart 20°C of heat to the room via the radiators, and return at 45°C. edit: and if you read further they suggest that oversized boilers like yours have further efficiency losses: > When boilers fire up, they automatically ramp up to 70% of their maximum output. For a 21kW boiler (common is most homes) this would be around 15kW. Given that only 6kW is needed on a very cold day, the boiler is always going to overheat the system. When the radiators cannot get rid of enough heat the return temperatures to the boiler are too high for it to condense.
- oliwarner 3y ago
  I'm not defending our existing system, rather highlighting how incompatible it is as is with heat pumps. We'll get there one day.
  - ZeroGravitas 3y ago
    That's fine as long as you're not comparing it to some unattainably perfect fossil fuel based system, and then claiming that the heat pump alternative "needs" big radiators and or whatever when it's been recommended practice to do the same for gas heaters (new install and replacement) for two decades now.

ZeroGravitas 3y ago

Your high temperature radiators are likely throwing away 20% of your heat, so you probably need to include that in your sums: https://www.theheatinghub.co.uk/why-our-condensing-boilers-do-not-condense https://www.theheatinghub.co.uk/why-our-condensing-boilers-d... > The boiler does not reach 90%+ efficiencies until the return temperatures are around 45°C. This means water must exit the boiler at a maximum of 65°C, impart 20°C of heat to the room via the radiators, and return at 45°C. edit: and if you read further they suggest that oversized boilers like yours have further efficiency losses: > When boilers fire up, they automatically ramp up to 70% of their maximum output. For a 21kW boiler (common is most homes) this would be around 15kW. Given that only 6kW is needed on a very cold day, the boiler is always going to overheat the system. When the radiators cannot get rid of enough heat the return temperatures to the boiler are too high for it to condense.

oliwarner 3y ago
I'm not defending our existing system, rather highlighting how incompatible it is as is with heat pumps. We'll get there one day.
- ZeroGravitas 3y ago
  That's fine as long as you're not comparing it to some unattainably perfect fossil fuel based system, and then claiming that the heat pump alternative "needs" big radiators and or whatever when it's been recommended practice to do the same for gas heaters (new install and replacement) for two decades now.