Heating pipe with two different diameters

[holland141]

Hi,

We have just installed 3 extra radiators onto a branch of the heating circuit that is run off a 15mm pipe. This used to have just one rad on it.

These rads are not getting as hot as the rest of the house (even after balancing).

I've just read on this forum that a rule of thumb is 4kW max for a 15mm pipe, and indeed this the case - the combined total is more like 6kW.

My plumber has advised we can increase the pipe to 22mm for most of the circuit and this would help. By most I mean the bits in the loft and bits that come down some boxing, but it would still leave about 20% at 15mm that is in-cased in the wall.

So - is this actually true. Would increasing approximately 70-80% of the feed improve the situation, bearing in mind there will still be a 'bottleneck' in the pipe.

I'm assuming it would have some benefit (less overall resistance etc.), but I also concerned it may cause other problems - i.e. introducing noise as the flow rate (?) increases in remaining 15mm section.

Hope that makes sense as a question.

Steve

 

[doitall]

SHould help although a better option would be to drop 22mm then fit a manifold and repipe all the problem rads in 10mm.

 

[dcawkwell]

2 or 3 radiators off a 15 mm pipe max.

You need 22mm to each branch for all the radiators to have the system work well.

 

[holland141]

Hi

Thanks doitall.

Having googled what a manifold is - it seems to be a multi-point junction.

If I'm correct then this would mean we'd still need to break into the wall as the supply to the rads are buried. If this is the case then we may as well expose/replace the full 15mm pipe until it reaches the current junction.

Am I understanding you correctly?

Steve

 

[doitall]

Are the pipes protected in the screed, if not it may be a good plan.

You need to get it down to a maximum of 3 rads off 15mm (depending on size) to be sure it will work.

 

[D_Hailsham]

We have just installed 3 extra radiators onto a branch of the heating circuit that is run off a 15mm pipe. This used to have just one rad on it.

Can you post a sketch showing the layout before and after the new rads were fitted?

You only need 22mm pipe in the section which is carrying more than 6kW. So, in the example below, the section from boiler to the first rad may need to be 22mm, but the rest can be in 15mm

Untitled

• D_Hailsham
• 18 Dec 2008
• 3

 

[holland141]

Hi,

Before and after as below

View media item 55015
View media item 55016
Using this I can see that the current 15mm was just about OK @4.1 kW, but presumably not for the current set up which needs 6 kW.

I've tried to indicate where the pipes are exposed (yellow) or red (buried in wall / floor).

The plumber is recommending a 22mm pipe for the exposed section - which means there will still be a section that needs to carry 6 kW that is at 15mm. The overall branch length is (very approximately) 15 meters with the bit that would remain @ 15mm 3m.

Steve

 

[D_Hailsham]

View media item 55016
The plumber is recommending a 22mm pipe for the exposed section - which means there will still be a section that needs to carry 6 kW that is at 15mm. The overall branch length is (very approximately) 15 meters with the bit that would remain @ 15mm 3m.

The branch to the left hand rad does not need to be 22mm; 15mm will be fine. Is there any way the the whole of the pipe carrying 6kW can be upgraded to 22mm? If not, will moving the junction to between two other rads allow the whole pipe to be in 22mm. All the horizontal pipe can be 15mm; it's just the vertical which has to be 22mm.

Suddenly changing from 22mm to 15mm is not a good idea as you will get circulation noise due to the velocity increase.

 

[gas112]

Pray explain how you dont suddenly go from 22mm to 15mm ? is there some magical fitting that does this gradually ?

 

[holland141]

Thanks.

I found a very useful link to a document from the copper development association http://www.copperinfo.co.uk/plumbin...-copper-tubes-in-domestic-heating-systems.pdf that made it clear how to calculate everything.

I see now that you need to calculate the resistance of the whole branch, and workout if the pump can deliver against this. The resistance calc is based on the flow rate required, which it's self is based on the output and temp differential planned, and different flow rates in difference sized pipes give different resistances per meter.....thus length is also very important.

What this tells me is that even by increasing only a part of the pipe, the overall resistance of that branch is reduced, and thus the pump is able to deliver an increased flow rate. The key question is is this increase enough to deliver the flow rate for the rads, and without doing the detailed calculations it would be difficult to tell. However, the run that I can increase to 22 without digging out is estimated at about 10 meters, or 70 % of the main run, and looking at the figures for the decrease in resistance in a 22mm pipe this will significantly decrease the overall resistance.

What I then see is that the flow rate (and thus velocity) would be increased in the remaining 15mm section, and I fear this may introduce a noise.

This is now quite important, as the point where the junction would be is (annoyingly) close to my head when I'm sleeping, as the pipe goes down into the family area below the bedroom.

Does anyone know if noise is introduced specifically due to the junction (from 22 to 15), and in this case would a funnel shaped junction help? Or, is the noise introduced/increased due to flow rate increase in the 15mm section. I assume the latter at least - would this noise be apparent back in the 22mm section (which would be running past the bed?)

 

[D_Hailsham]

Pray explain how you don't suddenly go from 22mm to 15mm ? is there some magical fitting that does this gradually ?

I was talking about the OP's particular situation where the suggestion had been made that the pipe from boiler to the rads could be upgraded from 15mm to 22mm for the part which was accessible (The yellow bit in the OP's diagam). In this case the flow rate will be constant, but the velocity will increase when the pipe changes from 22mm to 15mm.

A change of size would normally only take place at a T-junction where part of the flow was being diverted to a radiator, so the flow rate leaving the junction is less than the flow rate entering.

Compare these two diagrams.

Untitled

• D_Hailsham
• 9 Jan 2013
• 1

 

[holland141]

That is the situation that would occur.

Would this be a significant issue? Is it going to cause excessive noise as the water gets funneled / increases velocity? Any other reasons not to do it?

 

[D_Hailsham]

What I then see is that the flow rate (and thus velocity) would be increased in the remaining 15mm section, and I fear this may introduce a noise.

The flow rate, i.e. volume of water stays the same; it's only the velocity which increases when you change from 22mm to 15mm.

You haven't mentioned which pump you have. If it's within the boiler, which boiler do you have?

 

[Agile]

Its usually possible to correct a situation like this by adding 15 mm feeds to the last rad in the trio.

That addition goes back to a 22 mm backbone somewhere convenient.

Tony