Thursday, 20 February 2014

Potential perils of plastic pipe




A college tutor recently suggested to me that the current environment in the heating industry does not encourage thinking.   Maybe I should therefore be less amazed when I come across plumbers who think copper and plastic pipe are interchangeable. 

Environmental considerations aside, a more direct issue is that of flow rate and pressure drops.  Bore size of plastic is considerably smaller than copper, and it seems that this is often overlooked.  I am not against plastic, but if installers select plastic pipe instead of copper without checking the size correctly, this could have a negative effect on heat pump’s COP.  

(If you drop-off ½ way through this - read the scenario at the end )


With respect to the materials of plastic and copper, to my surprise I found a very informative document that discusses primary energy of these two materials in some detail, and concludes that the total energy involved in mining and manufacturing copper is far greater than the total energy (including the crude oil) to manufacture plastic pipe.  http://www.hepstore.co.uk/downloadPDF.aspx?id=840  Looking at other potentially less biased general data on copper and plastic, it seems to confirm that more oil is used processing copper than would be used to produce plastic.  

Another factor to consider, that mainly affects pipe runs to hot taps etc, is the heat capacity of the pipe material. Plastic has a relatively high specific heat, and the wall is thicker, but it’s light. The net thermal capacity of the two is fairly similar.
(The cold feel of copper is more to do with conductivity from the hand than heat capacity)

However, obvious factors aside, one of the biggest issues that could affect installations involving heat pumps relates to the internal bore diameter.  This could have a noticeable effect on the energy-efficiency of the system.

All metric pipes are measured by their outside diameter. As can be seen, with common pipe sizes (outside diameters), equivalent plastic pipes have considerably smaller internal area to copper. This has a dramatic effect on flow characteristics.

 The graphs below illustrate the relative internal dimensions of common pipes.

 (see AECB Water Standard)




A brief note about smoothness - It seems a common belief that plastic is ‘smoother’ than copper, but the inner wall ‘smoothess’ of the two is the same.  However, plastic can be one-piece with slow sweep bends. This is certainly ‘smoother’ than copper with tight elbows.   Re inner surface, we can assume the two materials are the same.

Whilst it is fairly easy to look-up the pressure drop resulting from a specific flow rate with a specific pipe, we can see from the 2nd graph at-a-glance the relative flow capacity since the cross sectional area loosely indicates flow capacity.

If a certain flow rate is required, then we can look-up the required pressure that is required across the pipe length (beginning of pipe to end of pipe length).  The internal bore must be chosen such that the circulation pump is not overly large and energy-wasteful.

This graph shows the pressure required to maintain a certain flow rate for a fixed length of pipe.  As can be seen, the pressure drop along the example pipe varies very dramatically, so the wall thickness makes very big difference. 
In this example, we can see that a 15mm copper pipe could be used with a common central heating pump (shown at 3.6m head, 36kPa). However, if plastic were chosen, then one would need almost 9m head to achieve the required flow - far beyond the capability of normal circulators. On the other hand, if 22mm plastic were chosen, the pressure requirements would be only 1m head (10kPa) which is likely to achieve very low circulation pump energy.

In real life, we tend to have a pump connected to a pipe system, and the flow rate that results is dictate by the balance between the pressure produced by the pump and the ‘restriction’ of the entire pipe work circuit.

For our final graph, we consider a pipe circulating with a fixed-pressure electronic pump (Alpha etc)


The above graphs show relative changes in flow rate that would result from a fixed pressure. If the pipe chosen were too small, then a larger circulation pump may be needed in an attempt to compensate for the extra restriction caused by the small internal bore.

The point here is that by choosing plastic instead of copper of the same nominal size, the system could potentially suffer unless the sizing is checked. There is of course no problem using plastic if it’s the right diameter.  Indeed, 28mm plastic may be an ideal choice for the connections from a heat pump simply to minimise noise transmission.  The best final solution is often a mixture of both plastic and copper for a multitude of reasons.

All this emphasises how dramatically the wall-thickness affects flow rates and pump pressure requirements, but how does this translate into reduced COPs?

Scenario (based on something I observed on a barn conversion)

Let us consider a radiator at a far distance from the heat pump.  The flow-rate relates to the pressure drop, which relates directly to the pipe-run length, and of course, the required flow-rate relates to the room size (bigger the heat demand, the more flow required).  In this instance the room is large.
The default pipe size choice would be normal 15mm (outside diameter), but if the sums are done, it may become apparent that the choice should be between either 15mm copper or 22mm plastic. 

How could a pipe with too small bore affect the COP? 

Radiators should be balanced, in general by throttling valves (lockshield)  on smaller radiators, and those with shorter pipe runs. However, this is actually quite a difficult thing to achieve with a heat pump because the temperature difference (water inlet to water outlet) may only be 5 degrees (°C).  (It’s much easier to measure and adjust when the temperature differences are larger).

It is not ideal, or easy, to have to throttle most other radiators on a system, and there is the added risk of the circulation pump not being big enough to cope.
The likely result of any ‘restrictive’ pipe-runs would be a reduced flow rate to the radiator.  This would result in a considerable area (the bottom) of the radiator being cool, and a reduced heat output to that room.
The obvious action to redress the short-fall of heat would be to increase the flow temperature by increasing the heating curve setting of the heat pump. i.e. increasing the flow temperate from say 40 to 45°C.    Now the heat pump has to heat ALL water to a 5 degrees higher level.  This is likely to reduce the COP by 10 to 12%.

This all indicates that one must never assume copper and plastic are interchangeable without considering the pressure drops and diameters.  That aside, we have also highlighted the importance radiator balancing.  Ideally the pipe runs to radiators would be laid out so that the flow rates are naturally about right without the need for much valve adjustment.  A little extra work on the design makes life much easier thereafter.

Anyone intersting in further reading on water flows - this site is very intersting
http://www.johnhearfield.com/Water/Water_in_pipes.htm


8 comments:

  1. Hello, I’m really impressed by the various blogs you have written, even though my full understanding is limited by my technical capabilities. I have a question for you that I hope you are able to answer.

    I live in a three-storey town house where the pressurised (about 2 bar) hotwater tank is installed in the loft, a 22mm plastic pipe feeds the basins, bath and shower on the 1st & 2nd floors and the kitchen sink on the ground floor. The kitchen sink is therefore the furthest from the hot water tank and the column of water in the 22mm pipe leading down from the hotwater tank is therefore significant and must be drawn off before hotwater can reach the tap. Given that the thread size into tap for the tails is 11mm, and the internal bore dimension is approximately 8mm, the amount of time to draw off the cold water is fairly lengthy. It is also a waste of water.

    My question is this: would fitting a dedicated 10mm pipe (plastic) from the hotwater tank (well, spurred off from the 22mm outlet pipe) to the kitchen tap tails deliver hotwater to the kitchen sink quicker and without loss of throughput (given that the tap tails constrict flow anyway)?

    I did pose this question to the technical team at Franke (the tap manufacturers) and their response was a disappointing ‘what are you talking about?’. My plumber’s reaction was much the same, but given he used a hammer to make a hole in the floor for a waste pipe, I’m not surprised.

    I trust that you can provide a simple to understand answer!

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  2. Hello Martin,
    The fact that your tap is constricted doesnt matter because its only a very short length. The flow rate is a result of ALL the pressure drops over the entire pipe run. If you had 1/2mile of 22mm pipe, you would have a low flow since the tiny drop over each meter adds up to a big pressure required to 'push' it through. If you have 1m pipe run from the header tank, the pipe could be maybe 8mm o.d. ?? My guess is that a 10mm pipe would be enough for your kitchen sink, but different people have different idea of 'adequate flow'. You can buy 12mm pipe as used for caravans - that might be safer.
    If you had 3 bar of pressure, I'm sure 10mm would do.
    I do discuss this along with some suggested flow rates in my book, also its on my website.

    Yes it is annoying that many people overlook this. Many new houses still have to draw off lots of water before it 'runs' hot. Passive house are the only people who seem to be embracing the issue

    Hope it works out OK

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  3. Hi John, many thanks for your prompt and simple to understand answer. I shall read your web site some more and hope to retain the information and then spread the word!
    cheers
    Martin

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  4. Bear in mind the website is desperately in need of a bit of refreshing. I need to make some time to do that.
    John

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  5. Hello John, thanks for your interesting blog. I wonder if I can bounce a further question relating to plastic piping off you? In essence, it's about whether one narrower bit of piping in the middle of the supply length acts as an absolute bottleneck (as seems intuitive), or whether its effect is proportional to how long it is (which is what I have a vague feeling that physics might say).

    I have a 5m length of 22mm plastic "Speedfit" pipe which connects my shiny new 32mm water main to my internal 22mm copper piping in the house. But it's been pointed out to me that these snap-together plastic systems use a small 15mm insert inside every connector/elbow/joint. There are probably 10 or 12 inserts in the 5m length of pipe. I'm wondering if simply having ANY constriction down to 15mm acts as a bottleneck, meaning that the maximum flow rate is instantaneously throttled back?

    Or, is it that any reduction in flow rate is proportional to the LENGTH of pipe which is of the smaller diameter, in which case I guess I've got about 12x3cm of the narrower inserts, so only about 36cm of effectively narrower pipe, which is probably not too big a problem? I see from John Hearfield's excellent webpage that this might equate to about 7x36cm of 22mm pipe, so it's like having a further 2.5m of pipework in the system, if it were all the larger diameter.

    Hope that makes sense, and thanks in advance for your thoughts.

    Steve

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  6. Hello Steve, yes, you would expect a bottleneck to reduce all the flow, but water is not like cars on a road. The pressure drop equates to the sum of the pressure drop of every bit (length) of pipe, so a short restriction does not amount to much. A long section of thinner pipe amounts to a lot. It can seem counter intuitive. I dont actually know how much the inserts affect things, but its not a great deal. That said, I often choose the stainless ones instead of the plastic ones. They are usually the same outer diameter.
    Have you looked at http://www.pressure-drop.com/Online-Calculator/
    you can type in your lengths and diameters and look up pressures. This is really useful. For the inserts, there will be an added drop due to the change in diameters, anyhow, you could simply experiment with it. (you will need to add roughness which I usually use 0.0014)
    Whilst the science of fluid dynamics can be complex, it can also be deadly simple. You simply need to use pressure drop/flow charts and make sure you know the inside diameter.
    good luck

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  7. Thank you very much John, that's reassuring - and using the calculator seems to confirm that the effect of the inserts won't be material. Much appreciate your advice.

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  8. Dear John

    I an upgrading my combination boiler (to another combination boiler) following a loft extension (new shower & bedroom) as the current boiler is not powerful enough to heat the house (4 bed 2 bathroom to 5 bed 3 bathroom following conversion). I would like the new boiler to allow the running of two showers or a bath/shower, shower/tap simultaneously which it currently cannot do.

    The current pressure is 2 bar and cold water flow rate is 12 litres per minute at the kitchen sink. There is a 4 metre length 15mm diameter lead water supply pipe attached to 15 mm copper pipes in the cellar. The flow rate at the stopcock at the edge of the property gives 25 litres per minute of water but this falls to 12 l/min by the time it gets to the kitchen sink. All internal stopcock are fully open.

    How can I increase the flow rate of the cold water by increasing the supply pipe size? Do I also need to increase the size of the 15mm copper pipes from the cellar to the new combination boiler (10 metres in length)? What size pipes do you recommend to get a flow rate of 25 litres per minute and 17 litres per minute?

    Best regards

    Michael

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