HaloThree Posted December 5, 2019 Share Posted December 5, 2019 My whole house UFH heating requirements have been calculated at approx 15KW, I've read many times that the rule of thumb is add 3KW to your central heating requirements for hot water (system boiler for indirect unvented water cylinder). I've spoke to the manufacturer of the cylinder I'm looking at (Telford Tempest 300L Indirect), and they've told me that the coil is rated at 15KW - exact wording was "The cylinder will require roughly 15KW". Am I right in understanding that I would have to have a 30KW Boiler (15KW for central heating + 15KW hot water) in order to reheat the cylinder in the quickest time from empty whilst the heating is on full. I'm keen to ensure that if the tank is emptied it can get back up to full asap. Thoughts? Link to comment Share on other sites More sharing options...
SteamyTea Posted December 5, 2019 Share Posted December 5, 2019 15 kW used to heat the 300 lt of water, or 15 kWh stored in the 300 lt of water? I would have thought the latter. It would take about 6 hours to heat up from stone cold (around 10°C) to 60°C if it was a 3 kW input, just over an hour at 15 kW. It could be that the number just coincide here. Link to comment Share on other sites More sharing options...
A_L Posted December 5, 2019 Share Posted December 5, 2019 1 hour ago, HaloThree said: Am I right in understanding that I would have to have a 30KW Boiler (15KW for central heating + 15KW hot water) in order to reheat the cylinder in the quickest time from empty whilst the heating is on full. I'm keen to ensure that if the tank is emptied it can get back up to full asap. Yes, if you want to guarantee having 15kW available to heat the cylinder when at the external design temperature, probably -1°C/-3°C. The 3000W value for water heating is used by the Standard Assessment Procedure (SAP) boiler sizing algorithm. 1 Link to comment Share on other sites More sharing options...
Nickfromwales Posted December 5, 2019 Share Posted December 5, 2019 Get a 15kW boiler and set it up as W-plan ( DHW priority ) and the boiler will only ever do heating OR hot water, and never the two together ? edit: agree to oversize slightly and go for an 18kW unit. 1 Link to comment Share on other sites More sharing options...
Nickfromwales Posted December 5, 2019 Share Posted December 5, 2019 It’ll also heat the cylinder faster ? 1 Link to comment Share on other sites More sharing options...
Big Jimbo Posted December 5, 2019 Share Posted December 5, 2019 Don't disappear anytime soon NICKFROMWALES. I can see me having a fair few questions for you when the time comes. Great advice above. Link to comment Share on other sites More sharing options...
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