Naked Science Forum

General Science => General Science => Topic started by: Will on 27/05/2008 21:01:06

Title: Why do pipes knock when it's cold?
Post by: Will on 27/05/2008 21:01:06

Will asked the Naked Scientists:

Hi Guys,
I currently live in a century old Brooklyn apartment building. I'm
wondering why in the winter when my heat turns on, the pipes in my apartment sound like a someone forging the Sword of Damocles in my bedroom?
Thanks,
Love the Podcast!
Will from NYC

What do you think?

Title: Why do pipes knock when it's cold?
Post by: RD on 28/05/2008 02:03:45

When the flow rate in the pipe is above a certain level, the flow goes from laminar to turbulent. This sets off oscillations of the water in the pipe which produce the knocking sound. The knocking occurs when it is cold because then there is a greater demand for hot water (for heating) and the flow rate rises above the critical level to produce turbulent flow.

The fact that your building is 100 years old may be relevant: irregularities in the pipes caused by accumulated scale will lower the threshold at which turbulence (and consequent knocking) occurs. The plumbing may not have made knocking sounds when it was first installed a century ago, when the pipes were smooth and had a larger internal diameter.

Title: Why do pipes knock when it's cold?
Post by: rosalind dna on 28/05/2008 10:27:14

But I get a similar problem of the pipes making a knocking sound and the building is only 55 years old, it's walls are made from
concrete so it's hot in a good summer but cold in the winter.

Also if the heating is on then the hot pipes feel cold. Could the
reason be that the hot tank is at the top of the block that I live in?

Title: Why do pipes knock when it's cold?
Post by: RD on 28/05/2008 15:03:59

Sufficient scale to cause turbulence at normal flow rates could accumulate in a decade.

If you have a lot of scale in your hot pipes then this could account for them being "cold" to touch,
 as the scale would act as an insulator between the hot water and the metal pipe. 

Title: Why do pipes knock when it's cold?
Post by: rosalind dna on 28/05/2008 16:00:40

Quote from: RD on 28/05/2008 15:03:59

Sufficient scale to cause turbulence at normal flow rates could accumulate in a decade.

If you have a lot of scale in your hot pipes then this could account for them being "cold" to touch,
 as the scale would act as an insulator between the hot water and the metal pipe. 

RD I can't reach the hot pipes easily unless I use a stepladder
and I am scared of heights. But it is in my Airing Cupboard which is quite near to an outside wall.

BTW it was designed and built the year of the Queen's coronation
1953 !!

Title: Why do pipes knock when it's cold?
Post by: Soul Surfer on 28/05/2008 18:59:42

There are two sources for knocking in central heating pipes. 

Firstly thermal expansion as the pipes expand when the heating is turned on this can be reduced by either clamping the pipes very strongly so they can't slide or lubricating critical joints so that they slide smoothly and quietly.

Secondly if the system is an old fashioned form of "steam heating" in which the heat is distributed using superheated steam rather than hot water  this condenses into water in the radiator producing a much greater heat output than hot water heating.  When this starts up from cold there is a period of violent noise where hot steam hits the water in the radiators and condenses violently a bit like the noise made by a super expresso machine as it heats the milk.  Once the system has warmed up it just emits a quiet hissing noise.  These noises are just a part of the system and can only be reduced by making sure the flow during heating is minimised and the heating up is as gentle as possible.

I worked for many years in a room with steam heating like this.

Title: Why do pipes knock when it's cold?
Post by: RD on 28/05/2008 19:43:58

I have cold water pipes which can knock when both my neighbour and I have water taps turned on simultaneously,
(we share a cold water supply).
Knocking is not exclusively a hot water problem, it is due to laminar flow becoming turbulent when the flow exceeds a threshold.
 

Title: Why do pipes knock when it's cold?
Post by: Soul Surfer on 28/05/2008 22:55:13

That sort of knocking is standard "water hammer" assocated with the opening and closing of valves rapidly and the inertia of flowing water in the pipes.

Title: Why do pipes knock when it's cold?
Post by: RD on 28/05/2008 23:32:47

Quote from: Soul Surfer on 28/05/2008 22:55:13

That sort of knocking is standard "water hammer" associated with the opening and closing of valves rapidly and the inertia of flowing water in the pipes.

It is not necessary to "rapidly" open the tap to cause knocking: even if the tap is opened very slowly knocking occurs when a certain flow rate is attained, this is the laminar/turbulent threshold I've been banging on about.