# Is 15mm Sufficient for Flow and Return

#### fury84

Hi,

I'm in the process of having my house totally re-plumbed and with the exception of the first 1 -2 meters of pipe from the boiler, the plumber has used 15mm pipe for the entire flow and return.

The plumber tells me the size is irrelevant now the systems are pressurized.

Is this true or should it be 22mm?

The boiler is a Worcester 30i combi with 8 radiators.

Thank you in anticipation.

#### jeff the gasman

Get rid of your so called plumber promptly.

#### fury84

Get rid of your so called plumber promptly.

I feared someone might say that!

Is there anything I can challenge him with?

D

#### Doggit

The pressurisation has nothing to do with it; it's about carrying capacity on the flow and return. Someone will be able to do the calculations to prove it, but I just use a quick judgement, and with 8 rads, 22 is more sensible. Carrying capacity is why you have main roads, and side roads.

#### fury84

The pressurisation has nothing to do with it; it's about carrying capacity on the flow and return. Someone will be able to do the calculations to prove it, but I just use a quick judgement, and with 8 rads, 22 is more sensible. Carrying capacity is why you have main roads, and side roads.

I suspect he's used 15mm pipe to increase his profit margins but now I have the difficult job of challenging him over a subject I know little about!

#### Aquaheat

@fury84 As a quick rule of thumb and to keep it simple and calculation free for you to put to your plumber 15mm has a capacity of 4kw, 22mm has a capacity of 11kw and 28mm has 20kw.

If you only have 8 radiators and lets say each one is 1kw each you have a need for 8kw to pass through the system to allow for the correct temperature drop for the return to the boiler.

As you can see 15mm can only carry half of your required peak amount so you would need 22mm to ensure the correct capacity can be carried while your new boiler runs efficiently.

Like I said this is a very basic explanation and a general rule of thumb but should give you some helpful info to put to the plumber.

Jon

D

#### Doggit

If you check under the boiler, you'll find the flow and return pipes on the boiler are 22mm, not 15; so he should have continued in a likewise manner. You could try ringing Wosceter and see if the tech guys will talk to you. Have you paid him yet.

#### fury84

Thanks guys.

The plumber is a accredited Worcestet installer!!! Might try calling them tomorrow.

Haven't paid yet.

#### mcmoby69

the guy is a knob. Just shows that being an approved installer for the preferred choice of the installer doesnt guarantee a quality job.

more science involved than just throwing a white box on the wall. it will work until its paid for, then when the problems start, ring worcester then they will decline the warranty because it wasnt installed to MI's

#### fury84

the guy is a knob. Just shows that being an approved installer for the preferred choice of the installer doesnt guarantee a quality job.

more science involved than just throwing a white box on the wall. it will work until its paid for, then when the problems start, ring worcester then they will decline the warranty because it wasnt installed to MI's

Going to ring them first thing tomorrow.

#### spudkey

@fury84 As a quick rule of thumb and to keep it simple and calculation free for you to put to your plumber 15mm has a capacity of 4kw, 22mm has a capacity of 11kw and 28mm has 20kw.

If you only have 8 radiators and lets say each one is 1kw each you have a need for 8kw to pass through the system to allow for the correct temperature drop for the return to the boiler.

As you can see 15mm can only carry half of your required peak amount so you would need 22mm to ensure the correct capacity can be carried while your new boiler runs efficiently.

Like I said this is a very basic explanation and a general rule of thumb but should give you some helpful info to put to the plumber.

Jon

Absolute rubbish

8kw of rads with a DT of 20deg requires about 0.35 m3/hr in 15mm this will give a velocity of 0.55m/s and a head loss of approx 2.7m per 100m !!

please tell me why 15mm is incorrect if there is enough residual pump head in the boiler ?

#### 45yearsagasman

Absolute rubbish
That is not necessary.
I don't have the science that you seem to possess, but from my experience, if I saw 15mm flow and returns
On an 8 rad system, it would start alarm bells ringing.

#### dilalio

As said above... Call WB and ask their opinion, if they are happy to warranty it under that set up, that's all you need worry about and you can bury the hatchet with you plumber/heating engineer.

#### jeff the gasman

No more than 3 radiators on 15mm. It's not about how much heat it will carry in total, it's about making sure the radiators all work at the same time. Balancing 8 radiators on a 15mm would not be easy if not impossible. Put your calculators away and ask yourself would you do it.

#### dilalio

Personally, Jeff... I wouldn't do it!
If told to, by whoevers job it was, I do it then, but not be happy.

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