Sizing a combi boiler to avoid on-off cycling

[LnP]

I'm refurbishing a Victorian coach house which will be ancillary accommodation to our main house. It's about 60 m² over 2 floors and the refurbishing has included insulating it to meet Building Regs. It has a bathroom with a shower (no bath) and a kitchen. I've used the famous @Jeremy Harrisspreadsheet to size the boiler. Heat loads vs outside air temperature are as follows:

 

     15^oC OAT - 0.7 kW      

     10^oC OAT - 1.1 kW

     5^oC OAT  - 1.6 kW

     0^oC OAT - 2.1 kW

     -5^oC OAT - 2.5 kW 

 

I've sized the radiators with the intention of running the flow temperature as low as possible. I'd prefer a combi boiler to save cost and space, but a system boiler with a DHW tank isn't out of the question. I've read on here and HeatGeek about sizing the boiler to avoid cycling. We live in Cheshire so the heating will most of the time only need to deliver about 1.0 to 1.5 kW. I'm having difficulty finding a combi boiler which modulates down to that low level of heat output.

 

Please can anybody suggest a suitable boiler or other suggestions.

And, do the heat loads I got from the spreadsheet look reasonable or are they too low?

 

Thanks

 

[MJNewton]

The Viessmann Vitodens 200-W combi (B2KB) modulates down to 1.8kW, if that's close enough? With such low heat requirements I think you'll always have to accept some level of cycling (and might be able to minimise it further through boiler configuration).

[JohnMo]

Basically you need a buffer.  The larger the minimum output of the boiler the bigger the buffer requires to be.  But operating as a single zone would help without the buffer.  The more zones you have the more likely short cycling will be, if you have TRVs on rads they need to modulate down not be on/off.