Pex pipe-boiler: aerial distance

[phloaw]

I am adding radiators to my 2-pipe pumped, vented central heating (UK). The whole piping is copper 22mm and microbore, and I'm extending it with 15mm pex barrier pipe graded for central heating.
I know that the first bit of pipes connected to the boiler must be copper, which is the case.
However, I couldn't find whether there is a minimum distance between the boiler and plastic pipes merely passing near to it, without being directly connected to it.
Is there such a thing?

Any other comment on what I'm trying to do is welcome.

 

[Madrab]

No, there isn't a distance requirement after the 1st meter.

When you say extending, are you just taking new feeds from the 22mm pipe branching off to each new rad?

 

[phloaw]

No, there isn't a distance requirement after the 1st meter.

When you say extending, are you just taking new feeds from the 22mm pipe branching off to each new rad?

My plan is to place two reducing (22 -> 15mm) tees on the 22mm pipe, one on the feed and one on the return. The new 15mm branches will feed two new radiators via more teeing, and I plan to make them with barrier pex.
But the reducing tee on the feed, in particular, will be positioned in a portion of the 22mm pipe running close to the boiler (around 30cm), though of course the linear distance measured along the piping (as opposed to aerial) from the boiler is much bigger (more than 100cm).

I should have given more details in the first place, maybe.

 

[Madrab]

You need to be a little more careful then if you're feeding more than one radiator from a 15mm pipe where the rest of the system is 22mm to 10mm, especially if they are larger rads.

The system will also need to be carefully balanced to ensure all the rads heat up properly and uniformly.

 

[phloaw]

You need to be a little more careful then if you're feeding more than one radiator from a 15mm pipe where the rest of the system is 22mm to 10mm, especially if they are larger rads.

The system will also need to be carefully balanced to ensure all the rads heat up properly and uniformly.

I am aware about the balancing need, and will additionally put valves on the 15mm to reduce the flow if needed, besides fiddling with radiator valves.
Could you please give a bit more detail about being "a little more careful"? Does this refer to balancing the radiators or to something else? Thank you.

 

[Madrab]

Depends on the size of your system I guess but normally you wouldn't want to run any more than 2 radiators off of 15mm main feeds, especially if the original main runs are 22mm and the other rad feeds are 10mm.
The more you mix up the pipe sizes the easier it is to run into issues with radiators not heating up properly etc. Water will take the easiest path of resistance and you are adding another size of pipe into the equation.

What is you systems just now, boiler and rads nos/size?

 

[phloaw]

Depends on the size of your system I guess but normally you wouldn't want to run any more than 2 radiators off of 15mm main feeds, especially if the original main runs are 22mm and the other rad feeds are 10mm.
The more you mix up the pipe sizes the easier it is to run into issues with radiators not heating up properly etc. Water will take the easiest path of resistance and you are adding another size of pipe into the equation.

What is you systems just now, boiler and rads nos/size?

Thanks for asking.
An old natural gas boiler (output to water min 8.79, max 11.72 kW) feeding HW and 6 radiators (2 big, 2 medium and 2 smallish), all currently in microbore, of which I heard many complaints, therefore I'm adding 2 rads using 15mm.