22mm or 28mm Flow & Return. Help please...

[Yamahayzfr6]

Just about to have a 30+ year old gas boiler replaced, and would appreciate any advice on whether I need 22mm or 28mm pipework on the flow and return....

The existing boiler is fairly central to the house, it is sited off the hall close to the bedrooms, far from ideal, so the new boiler will be sited at one end of the property, in the garage. It is a Bungalow circa 20m in length, all radiators are on one level... The boiler will be sited in the garage which is split level, below the rest of the house, so the top of the boiler is virtually level with the flow and return pipes....

The flow and return pipes will be approximately 15m in length each, with only one 90 bend in each circa 2m from the boiler....

The new boiler is going to be a Worcester Bosch 42CDI combi (unless anyone thinks otherwise), and there are 6 x large, 3 x medium, 2 x small radiators (all doubles) + an heated towel rail / small rad... Existing pipework is 22mm, but it has been suggested that I may want to consider 28mm... "for better flow"...

Is 28mm neccessary ? considering the high cost of 28mm pipe and fittings I am thinking of trying the existing 22mm and then if there where any issues, I could always fit 28mm if I had too...

Any help, very much appreciated...

 

[SimonJohns]

The connections on to that boiler are only 22mm. I would of said that if you could run it most of the way and only have two rads max per 15mm pipe you will be fine

 

[SafeGasInstall]

hi and welcome to the forum ,doesnt your plumber know ? or you are fitting it?

 

[stevetheplumber]

size of the boiler connections is irelevant what matters is the heating load 6 large double could easily be 60,000 btus on there own and theres another 7 rads so i would go with 28mm from boiler to first tees at least possibly further if the larger rads are far end
unless the boiler is close to the gas meter your probably going to need 28mm for the gas supply to a 42kw boiler

 

[Yamahayzfr6]

The connections on to that boiler are only 22mm. I would of said that if you could run it most of the way and only have two rads max per 15mm pipe you will be fine

Many thanks for the quick reply... The existing flow and return pipes are dead central and run end to end (virtually) of the house.... each radiator connects independantly directly to the flow and return, all but two are 15mm.. The two others are on "micro" micro bore, and will be changed 15mm anyway...

Yes, I knew the boiler was 22mm connection, which is why the suggestion of 28mm seemed confusing....

Thanks again...

 

[GasmanxxxR1]

As a rule of thumb if your going above 18kw output on the central heating side i would go with 28mm

 

[Halesowen1]

Only ever needed to use 28mm on 1 property out of 100s of heating systems.
If you run 22mm flow and return down the middle as the others are and then feed each one of at 15mm, you will be fine, 28mm was used on older gravity style systems and is not needed for 99% of domestic properties now.
You will deffo need 28mm gas supply for most of the run.

 

[dontknowitall]

Oil cooking stoves (Aga, Rayburn, Stanley, etc) all require 28mm for the primary flow and return. Then again, these can probably be counted in the 1 in 100 houses.

 

[Halesowen1]

I reckon the boiler will only ever be on max when the hot water is on, not for heating.