# Is 15mm Sufficient for Flow and Return

#### mcmoby69

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 ?

Does your calculation allow for a smart pump?

#### Dan Robinson

The maths at dt20 say probably. Common sense and ease of balancing say no.

You don't want a pump running at 100%. It will fail prematurely. 75% or below is the preferred level.

Wooshitter accreditation means precisely squat. Only that the installer throws enough of these sh|tboxes at the wall a year to keep the moniker.

Has the installer done the maths?

Doubt it.

Last edited:

#### Dan Robinson

Does your calculation allow for a smart pump?

Depends on what control regime the boiler uses. They have a up3, so can modulate using various metrics.

#### Biggles..

@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
Oh dear Jon!!!!

#### Biggles..

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 ?

So technically 15mm is too big as the velocity ain't right and will result in a build up of sludge.

#### fury84

Spoke to Worcester technical who said that 15mm is ok for what I have.

I think next time I'll be clearer about what pipes are being used etc but for now I am happy.

I appreciate everyone's input

#### Biggles..

Spoke to Worcester technical who said that 15mm is ok for what I have.

I think next time I'll be clearer about what pipes are being used etc but for now I am happy.

I appreciate everyone's input
Thankfully they gave the right answer, the issue is that ultimately they're not bothered about the system as long as it has no detrimental effect on their boiler.

#### Razor900

If the 22mm goes to where the system splits then 15mm does four rads each it'll be fine.

22mm pipe can only carry 11kW. Do we need to repipe every system ever fitted then??

#### Biggles..

The maths at dt20 say probably. Common sense and ease of balancing say no.

You don't want a pump running at 100%. It will fail prematurely. 75% or below is the preferred level.

Wooshitter accreditation means precisely squat. Only that the installer throws enough of these sh|tboxes at the wall a year to keep the moniker.

Has the installer done the maths?

Doubt it.

Do you really need to do maths for around 8-10kw loads?
As for your pump speed quote, come on, really?????
Feel free to quote statistics from any proven manufacturer source that says a pump will fail prematurely if it's ran above 75%!

You're turning into agile!

#### Biggles..

If the 22mm goes to where the system splits then 15mm does four rads each it'll be fine.

22mm pipe can only carry 11kW. Do we need to repipe every system ever fitted then??

Hope that's sarcasm or you might need a refresher on pipe volume, velocity and converting to KW

#### Agile

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.

Jon

As far as I recall those were the figures used for non condensing boilers with an 11 C flow/return differential.

Now that condensing boilers can operate with a differential of up to 20 C then the heat carrying capability of pipes can be doubled.

It used to be fairly normal for 11 C boilers to use 22 mm and split in 15 mm to upstairs and 15 mm to downstairs.

Tony

#### RigidRaider

Er... if 15mm is inadequate how come 10mm works just as well?

Our 4 bed house is entirely in 15mm apart from the major "trunks" to and from each floor, which are 22mm and the heating works just dandy.

#### Dan Robinson

Do you really need to do maths for around 8-10kw loads?

Nope.

As for your pump speed quote, come on, really?????

Twas what a chap that works for Grundfos told me

You're turning into agile!

Mardy a7se. I'll come round and sneeze on you

#### DP

Lee, Dan is exempt from any questioning as he can get a letter from his mum

#### Agile

Twas what a chap that works for Grundfos told me

I would be very reluctant to take something said by a salesman for a manufacturer as definitely correct.

Most pumps can work at any power up to their rating for any proportion of the time.

The only part of a pump to fail is the bearing and when turning there is no stress at all on the bearings apart from any small out of balance pressure regardless of how hard they are pumping.

The winding have exactly the same stress on them all the time and that has no effective effect on the life expectancy.

Pumps can theoretically work for ever apart from bearing wear and that is usually accelerated by dirt in the water.

Tony

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