Discuss New plumbing & heating system advice in the General DIY Plumbing Forum area at Plumbers Forums


I'm installing a new central heating & plumbing system and just wanted to run through my design choices to make sure I haven't overlooked anything! It's a 2 bed, 1 bathroom semi-detached house.

These are the proposed appliances: WC, Basin, Bath, Shower, Bath, Washing Machine, Kitchen Sink, Dishwasher.

I'm looking to put in a 30kW combi boiler. The cold mains pressure is around 3 bar with 13 l/min flow rate. The cold main comes into the house at 15mm. I'll run the cold and hot pipes in 15mm to the above appliances.

Central heating wise, I'll run the main flow and return pipework in 22mm and branch off in 15mm to radiators.

Does this sound reasonable?

Any guidance is very much appreciated,


Have you calculated the Heatloss in regards to part l eg max flow temp of 55c ?
Hi Shaun,

Yes i've oversized rads to allow for this.



And converted the figures from the stock listed output to dt24 eg conversion figure of 0.385

Eg 1000w stock listed output would only output 385w at dt24
All sounds good then 15mm copper for each rad max of 2.5kw main run calculated for a max of 6kw eg 22mm copper
30kw for a 2 bed? Easily enough to heat the house.

The large KW output of a combi is for the volume of hot water produced. The bigger the combi, the more DHW you get......People like big hot showers

Yes yes yes. Heat loss calcs etc etc

The lowest output I could quickly find for a combi is 24kw. Call each rad about 2kw and that's about 12 rads. Combi's are usually only fitted in a 1 bathroom property and don't usually have more than 3 bedrooms.

Yes yes yes I know. So much wrong here and so many generalisations. I would size a system/open boiler to the property but not so much a combi.

Now, how much grief am I going to get??

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