15mm or 22mm pipework for Central Heating

We have just replaced the aged Potterton boiler in our house with a Worcester Bosch 30CDi System boiler.

Our house is a typical Victorian end of terrace, with three bedrooms and a bathroom in the original part of the house and two further bedrooms in a rear part of the house.

We are using the existing pipework for the radiators which is all 22mm in the main part of the house and 15mm in the rear part. There are 11 rads in the main part of the house with 5 in the rear.

The rads in the rear part of the house take far longer to warm up than those in the main part of the house and I am trying to understand what differences 15mm pipework is having over the 22mm. The heating engineer says the valves on the rads at the front can be adjusted so that the rear ones come on more quickly. But I wonder if the rear radiators are being starved of hot water.

The 15mm pipework currently tees straight off from the 22mm with no reducing pipework and since the floorboards are currently up, I am wondering whether to change the pipework from the tee to 22mm and reduce down to 15mm further along.

Can anyone explain the effect that pipe sizes have on a CH system and give any suggestions?

Thank you

are all five rads fed from one 15mm flow and return tee off the 22mm or have you got two rads on one flow/return then three rads on another tee ?
how long are these 15mm runs from the tee's ?

has all the radiators been balanced as your plumber said ?

Rough rule is to feed no more then three average size rads from 15 mm or a total of 4 kW.

It woukld usually be possible to balance the system but replacing the pipework would be better.

Tony

Thanks to HSPH and Agile for your replies.

The 15mm run is all running off one flow and return. Its a long length - the three upstairs rads alone are probably on a 12 metre run which then goes on to the two rads downstairs. The last one on the run takes an age to warm up.

The plumber didn't balance them but told me to how to. Basically seems to involve starting up the boiler and running round the house to see which rads come on first.

Roger

i would opt for the 22mm option on that sort of run.

I would replace with 22mm at least as far as the drop to downstairs!!

If the downstairs rads are large or excessive pipework is involved then replace this to first rad!!

As a rough guide 18000btu on a 6m run is satisfactory!

Your plumber should be able to advise!!

Was the system thoroughly flushed? Why did your installer not balance the system? Your new boiler probably has an output between 50 and 100 percent more than your old one, how did the rads heat up before?

Thanks to you all for your replies.
In reply to Bengasman, yes the system was thoroughly flushed but I do not know why the system wasn't balanced.
Roger