Different flow temperature for radiators and hot water tank circuits

[Gorge123]

Hi Morning, I am looking at a new boiler and hot water cylinder setup for a house I am refurbishing and am getting conflicting advice on my question: Can I use a Combi or System boiler to have a different flow temperature for the hot water tank heater and the radiators. I appreciate the hw cylinder needs a minimum of 65C, which for most days is overkill for the radiators, which I deliberately oversize to run on a lower flow temperature. Appreciate any helpful advice on here!

 

[Chuck]

Depends on the specifics of the boiler model and controls. For example, some Worcester Bosch Greenstar boilers can do what you want but they have to have an optional internal diverter valve kit fitted, which adds to the cost. If the radiators are oversized you should be able to run them with 65°C flow and achieve a return of 50°C or below but you'll need to spend a few hours making sure the system is correctly balanced. If you want an efficient system and a comfortable home, the boiler's minimum output is a key parameter that is often overlooked (the lower the better).

A competent installer should be able to take you through the options and the pros and cons of each configuration.

 

[ShaunCorbs]

System boiler and either a 4 pipe system eg Viessmann (or 3 pipe with a diverter eg Worcester) or a boiler this accepts two switch lives eg ideal vouge system and oversize rads to run at a 45 dc flow temp

 

[Enthalpy]

Depending on the boiler, you can have different flow temperatures for the hot water tank and radiators. The boiler needs more than the usual ‘switched live’ so that it controls the flow temperature according to whether the flow is going to the radiators or hot water tank.

Here is an example for Vaillant:

https://www.vaillant.co.uk/downloads/product-brochures/vaillant-controls-brochure-1955755.pdf