Hot water and Radiators

[Mieleg]

Hi experts - a basic DIY’er here so forgive any naivety! I have a large house in with a vented pressurised gas central heating / hot water system with a 300l storage tank and 28 radiators. I think I understand that the same cylinder services both the hot water and the radiators. All works fine but trying to cut back on excess use. What I don’t understand is that when I set the hot water for 90 mins we get plenty hot water for the day - however, if I run the central heating the hot water disappears quickly - Isn’t it all fed from the same 300l cylinder. If the central heating system is “closed” then why does use of it impact on the amount of hot water available ? Any help/advice appreciated !

 

[John.g]

You probably have a heat store where the DHW will be provided with either a coil in the store (top, for hottest water) or a plate heat exchanger, the CH will be provided by pumping/circulating the store water through the rads. The boiler will keep the store warm by circulating water from low down in the store back up to the highest point in the store, the boiler will cut in as soon as the store thermostat (installed fairly low down) falls by ~ 8C , so as long as the boiler is big enough then should supply ample hot water, if your rads demand say 20kw then a 30/35kw boiler should provide enough hot water for both except that you have a huge continuous demand for DHW coulped to a high CH demand.

 

[ShaunCorbs]

Thermal store need to keep the cylinder hot normally around 80 there designed at