Distributing hot water to tap outlets

To my mind, the whole point of having a heat pump is to save energy, so it somewhat jars with me when I see one aspect of a system that is not in-line with that aim.

A heat pump always stores Domestic Hot Water (DHW) in a cylinder. The Cylinder is, we hope, sized and optimised for heat pump use so that the COP (and lowest running cost) is the best-we-can-get.  This detail should be taken care of by the manufacturers and designers.

The detail of distributing the water to the taps is however often done using rule-of-thumb and the installer’s past experience.  Energy-efficiency is often low down on the priority list, so may be given little thought.

Hot water must of course be available at a sufficiently high flow-rate, this may lead to choosing a large-bore supply pipe.   However, this large bore pipe holds a lot of water, so when a tap is turned on, there can be a long wait before the tap runs hot.  After the tap is turned off, the heat from this volume of hot water left in the pipe is slowly lost.  This is not good for energy-efficiency and low running costs.  This may in-part usefully heat the house, but often it is simply a waste.   It is certainly a waste of water.    I recently came across a pipe run that took such a long torturous long route that I filled a whole sink before the tap stared to run warm!

For sake of energy-efficiency, the distributions to taps should be;

  1. The smallest diameter to give adequate flow rate
  2. As short as possible – the most direct route.

Normally, a single ‘main artery’ is taken from the cylinder, and run to taps in turn. This distribution pipe will normally start big and, as each ‘tee’ off to a tap, it will at some point reduce in diameter. Furthermore, if several taps are on at the same time, then one flow may affect the other. E.g. the pipe needs to be big enough in diameter to maintain a good steady flow-rate for a shower.

The choice of pipe size is often a dilemma, so playing-safe and choosing larger pipes would at least safeguard against an inadequate pipe run. This would however tend to go against saving energy.

Running separate pipes from the cylinder.

The pipe run to the bath is usually large diameter. This is where the fastest flow may be needed so that the bath fills quickly. However, a bath is not used very frequently, so the losses as described above may not be as great here as envisaged.   Kitchen sinks however tend to be used many times over the day, so total losses of heat and water could be high.

The worst scenario could be a kitchen sink at the end of a large long pipe run via a bath.

A solution to this could be to run dedicated pipes directly to sinks.

Here are some suggestions for new-builds

  1. Position the cylinder as close to the taps that are most frequently used (e.g. the kitchen sink). Ideally find a central location to all taps.
  2. Early on in the build, organise the most direct pipe run routes from the top of the cylinder to the sinks.   (the ceiling is often shorter than running in the floor)
  3. Decide the best configuration, e.g. some sinks can share, others may benefit from their own dedicated supply pipe.
  4. Identify the most suitable diameter of pipe.
  5. Insulate frequently-used pipes, BUT this may give limited advantage since it may simply take longer to lose all the heat in the pipe.

Pumped secondary return loop

Hotels and large houses often adoped a pumped circulation loop so that the taps run hot almost immediately.  This saves water and is convenient to use, however, the hot loop can waste a lot of energy, even if well-insulated.  I would try to avoid a pumped loop if possible.  It may be better to have point-of-use direct electric heaters, than to have a pumped loop that is always wasting heat.

Copper v plastic

There is a bit of a myth that suggests that plastic pipe is ‘smoother’ than copper.  The most important factor is the actual bore size. The wall material makes little difference.  However, plastic can very easily be bent with sweeps, these are better than straight pipes with tight bends. In this sense, plastic can be ‘smoother’.

Anyhow, the difference between copper and plastic is quite dramatic due to its internal bore.

It is very important to allow for the internal pipe sizes, and it is possible to use a combination of the two, e.g. run plastic through the middle of joists if it’s possible, but use long straight runs of copper along a wall.

I have put two charts at the bottom of the page here    http://heatpumps.co.uk/types-of-heat-pump/domestic-hot-water-dhw/

There are figures here to give a rough idea of what sizes might work.  It is surprising how much more restrictive 15mm plastic is compared to 15mm copper.  This info goes along with a disclaimer since it can be hard to know all the factors involved in a system.  I am a great believer in getting a sample of pipe in a coil and doing a test on site with a bucket.

You can also play with this great little pressure-drop calculator   for theoretical pipe sizes

What could go wrong?

I have successfully run a kitchen sink via a 10mm plastic pipe.  The bore is only 6.7mm which is about the diameter of a pencil.  I had no noise problem at all.   In general, more noise would come from part-open valve or tap than would come from a smooth pipe.   Remember to clip the pipe well to stop any annoying clonking that can occur with any pipe when a tap is shut-off quickly.

For further reading, see the AECB water standard



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