Combi boiler options - min kW output

fonzie2107

I have just brought a property that needs a new central heating system.
I plan to have a complete new central heating system, inc pipework and radiators.

The correct size radiators for the rooms give a total output of around 7kW, but I have opted to oversize these slightly to give a total output of 8-8.5kW.

But... most of the combi boilers I have looked at (Worcester 34cdi and Baxi duo-tec) state that their maximum central heating output is around 30kW/34kW and minimum is 8+kW.
Obviously my system will be 8.5kW when its on full whack, but I won't have all the radiators on all the time. So the output will in fact be much lower; not meeting the minimum requirements.}
From what I can gather, this will cause the boiler to cycle on and off making it inefficient and causing premature wear on the internals.

If I opt for a combi with a lower central heating output, the domestic hot water flow rate drops substantially to <9LPM which isn't acceptable for me.

Can anybody advise me what I am best to do in regards to the combi boiler?

Thank you!

tacpot12

My experience isn't with modulating boilers so may not be as relevant as you would like, but I selected a 28KW Combi boiler for a small two-bedroomed terraced property because I wanted good flow rates on the domestic hot water. The heating output required must have been about 7KW. The boiler ran reliably for 17 years with just the fan (which ran constantly) needing replacing every 5 years and a water pressure sensor wearing out. The failure that killed it was the heat exchanger. A combi boiler with a pressed steel heat exchanger is never going to last as long as one with a cast iron heat exchanged, but I don't think you will have an issue with short cycling. Modern boiler controls will do what they can to minimise the cycling if the boiler can't modulate down to the actual demand.

I would also look at boilers from other good manufacturers; e.g. Vaillant and Viessmann. The Vaillant 30KW modulates down to 5.6KW and the Viessmann (Vitodens 200-W) down to 2.4KW.

fonzie2107

Thank you for the thorough reply.
I have read up on modulating and as you say most modern combis will modulate down to meet the demand.
The Worcester Greenstar 34cdi I have been looking at (two family members have had 38cdi for a few years and recommend them) have a minimum output of 7.7kW. I can oversize my radiators a little to meet this and from what I can gather I should also add 3kW for hot water demand giving me a total of 10kW. Meeting the demand, not causing it to cycle on and off.

southcoastrgi

Fit a boiler that can use weather compensation, neither of those can, you don't add anything for hot water on a combi

C_Mababejive

Think about this, a typical combi offers to raise the water temp by say 35 degrees.

Last week when it was very cold, i measured my cold water at 5c. My hot water was 50c. The temperature rise was therefore 45c. That happened using a 9kw input boiler. You could never do that with a combi.