Yet Another Plumbing and Heating engineer


Pumping The Figures

From CORGI's "Installer" magazine, January 2014:

... Modern circulator pumps could ... sav[e] on average of between £63 and £89 annual savings. ... A traditional 3-speed circulator with an induction motor would require 65W of power. ... A new electronic circulator ... can ... reduce the maximum power to between 4 and 35W

£63 to £89 annual savings: that seems worth having!

Let's see how that works. This will require some simple arithmetic and a tiny bit of algebra so it may help to have a schoolkid handy.

If we use one Watt of power constantly then in a year we'll use 9 kiloWatt-hours or "Units" of electricity (1 Watt times 24 hours in a day times 365 days in a year is 8,760 Watt-hours, which is as near as dammit 9 kiloWatt-hours / Units of electricity).

Xylem Water Solutions and Gary Wilde don't say how much they assume we're paying for our electricity so let's call the rate L (for 'Leccy!). So using (or saving) one Watt for one year will cost (or save) 9 * L Pounds.

What's the most power we could save with a new pump? If the old one used 65 Watts and — let's be really optimistic — the new one runs at just 4 Watts all the time, then we'll save 61 Watts. Let's call it 60 in round numbers. Assuming the pump has been mis-wired to run constantly, 24 * 7, then the new pump will save us 60 * 9 * L pounds a year.

The pump manufacturers say we could save between £63 and £89 pounds a year. Let's take the more modest figure of £63: if this is our saving from 60 * 9 * L then:

  60 * 9 * L = 63

which we can re-arrange as:

  L = 63 / (60 * 9 ) = 0.116666...

so L – the cost per unit of our electricity – works out to be 0.116 Pounds or about 12p per unit. If we were to save the higher amount of £89 we'd have to be paying 89/(60*9) = 17p/unit. Looking at a real electricity bill these figures seem about right:

electricity bill showing rate of approx 14p per kWh

But these figures assumed our pump was running constantly. If it is wired properly and doesn't run all the time then neither the old one nor the new one will use as much energy, so we won't save so much. Let's say that during the 6 warmer months it runs for 5% of the time – a bit over an hour a day, providing hot water (assuming a non-combi system) – and during the 6 colder months it gets to run 25% of the time (6 hours a day), averaging 15% over the year. Then, to get even our £63 savings we'd have to be paying 63 / (60 * 9 * 0.15) = 79p/unit. We'd only save £89 if we were paying £1.11/unit!

At those rates we should be switching to a cheaper tariff before even thinking of swapping our pump!

And even that is with our most optimistic power-savings: the super electronic pump running at just 4 or 5 Watts, maybe in a system with an old cast-iron boiler with big waterways, and big, clean, pipework and radiators, where the pump is doing little more than helping gravity circulation along. But supposing it were driving a modern low-water-content boiler and radiators with narrow waterways and lots of 15mm or even microbore pipework and having to run at its maximum 35W: then the power saving would be 30W rather than 60W and we'd have to be paying twice as much again for our electricity to make the savings claimed – anywhere from £1.60 to £2.26 per unit!

screenshot of spreadsheet

Here's a little spreadsheet to play with these sorts of figures. Above is a screen-shot of it: click on it to download it and run it with Microsoft Excel, or OpenOffice or LibreOffice Calc. You can change the figures in the yellow boxes and see how much you have to be paying for electricity to make a given saving, or how much you can save for a given cost of electricity.

Comments? please email me

25th January 2014

Yes, but the heat is not wasted as it goes into the heating water.

It just means that the boiler will not have to do so much work and will cost less to run ( as itself ).

A more efficient pump will give an overall saving in cost by the difference between the electricity cost and say the gas cost but this obviously is less than the straight mathematical saving of pump efficiency.

What is more relevant is the plumbers' tendency to 'put in a big one' . This shifts the efficiency from the bep. at around 2/3 curve position towards cv. Small centrifs. are typically 35% at bep and fall to 25% nearer cv. That is a loss of 30% of the rated efficiency.

This is very relevant to air conditioning systems which have to take out any heat input from the pump inefficiency.

The skin temperature of the motor may be relevant wastage if the pump is in an idle area such as a basement where heating is not fitted but is again recovered energy if the pump is in an area where heating is required.

Considerations of air conditioning in California are much different to those of central heating in Northern Europe.

As the technical Director of a major pump company I have pressed the matter of efficiency to the benefit of users in many fields. There are very substantial savings to made in doing the sums and by improving the pipework by upsizing and elimination of bad fittings, but the efficiency aspect of CH circulating pumps has been largely misunderstood and perhaps pursued for its marketing benefits rather than through an appreciation of energy conversion.

Trevor Benjamin.