15mm first fixing a house run off a combi

[Riley26]

Is 15mm enough to feed the cold water services on a house run off a combi boiler. The house is feed via 25mm mdpe. Have only gone up to 22mm on unvented hot water systems.

 

[PrenticeBoyofDerry]

Normally yes on an unvented domestic system.

 

[muggles]

15mm for combis, 22mm for unvented

 

[Riley26]

Thanks so when first fixing a domestic house run of a combi 15mm is sufficient?

 

[Madrab]

Yup, assume the inlet to the combi is also 15mm? With adequate mains pressure then 15mm should flow more than enough.

 

[Riley26]

yeah inlet to combi is 15mm, does that mean if you go above 15mm with the cold there's no improvement?

 

[Terry lambert]

I would come of 25mm MDPE to 22mm then tee off in 15mm to boiler then reduce to cold first fix

 

[Riley26]

So you'd run 22mm to boiler cold then reduce to 15mm for the rest of the cold first fix?

 

[Madrab]

yeah inlet to combi is 15mm, does that mean if you go above 15mm with the cold there's no improvement?

Not necessarily, depending on the length of run then keeping 22mm till the branch to each outlet may minimise a drop in performance at other outlets when they're opened but this is dictated by the available working pressure when more than one outlet is open.
I hope though, given it's a combi going in, that the pressures and flow have been tested as suitable anyway?
Given you will have to reduce to 15mm anyway, for most fittings, flow at the outlet will be dictated by that pipe diameter as long as the pressure remains reasonably constant.

Hope that doesn't confuse too much

 

[Riley26]

So are you saying the cold possibly up to 22mm to wear it gets to the boiler, then 15mm and the hot all in 15mm?

 

[Riley26]

There's a bath and 2 showers to be feed off it. Will a combi struggle with this anyway?

 

[Madrab]

Yup, most combi's will struggle to sustain more than 1 full rate hot water outlet at a time.

 

[Terry lambert]

You don't have to run 22mm to the combi unless that is where the cold needs to tee off for the rest of the property.

25mm MDPE, 22mm, reduce 22mm tee in middle to run 15mm to combi, 22mm reduced to run rest of supply. If it wasn't far, I would consider running 22mm a bit further and reducing where the cold split for the kitchen and bathroom.

A combi is only designed for one tap operation. I would like to see at least a 28KW if it was supplying a bath

 

[Riley26]

thanks, so taking the cold up to 22mm for some of the first fix maybe an option. But I guessing running any of the hot in 22mm would be pointless as the boiler outlet is 15mm. 22mm on the hot would just give a bigger lag of cold water to draw off before the hot gets there, and give no greater flow as it is feed off a 15mm pipe.

 

[Terry lambert]

Correct