Saturday, 8 December 2012

Legionella protection and energy demand

It is poignant that I write this whilst a hot debate continues in Doha, Qatar over the issue of climate change, which is more evident in some poorer parts of the world, and the proposal that the energy-guzzling West should compensate in some way. This seems to put my blog below into perspective.

I recently read the following on a brochure from a mixing valve manufacturer:
'The hot water storage tank must be kept at a temperature of 65°C (149°F) or higher in order to control any growth of legionella bacteria'.
I don't believe this is correct, nonetheless, there seems to be a growing general feeling that cylinders should be kept hotter and hotter.

The desirable temperature to ensure that hot water is 'safe', is debatable, and last week's request from MCS for evidence on the topic confirms (I am pleased to hear) that the jury is still out.

My concern here is the energy required to achieve elevated temperatures. If this is achieved using a conventional boiler system, the extra energy required may not be excessive. However, for a heat pump it is a different matter since the COP varies dramatically as temperatures rise, and I'm not sure that everyone is aware how great the change is.

The concept of periodic pasturisation is a well established method, but the necessary frequency: daily, weekly or monthly, still seems debatable.

Let's look at the energy efficiency relating to cylinder temperature. For a 'high temperature' model,(e.g. refrigerant 134A), and if a heat pump were to heat a cylinder to 50°C (122°F) with a COP of say about 2.3 (assuming evaporating at 0°C, 32F), then the energy-penalty for increasing the store temperature above 50°C for a typical heat pump, as shown by the blue line, could be 11%, 21% and 29% for store temperatures of 55,60 and 65°C respectively. (131,140 and 149°F)

(scroll compressor data including a pump load etc. Evaporating at zero C)

The blue line is bad enough, but for the conventional heat pump model (crimson), with an upper temperature limit of say around 55°C, 131°F, (heat pump water), then the resulting drop in energy-efficiency is significantly worse with elevated store temperatures, since some of the hot water will need to be delivered from an immersion heater, with a COP of only 1. If 65°C were really needed, then even a conservative estimation would halve the COP, and this is on top of the increased energy loss from the hotter cylinder (and pipes) of a hotter cylinder. All in all, the energy implication for heat pumps to comply with over stringent legionella protection could be very considerable.

I am of course being general here, and one could create quite a big spreadsheet attempting to 'model' this since there are many variables, and the percentage provided by heat pump/immersion will be affected by things like sensor height, time clock, quantity consumed and various other variables. I am mostly ignoring options of pre-heating or batch heating the water since not much of the installed ‘kit’ does this, but it can be achieved, in part, by the owner’s careful use of a time clock. There is a lot of scope to optimise the net COP here, and a lot to lose as the store temperature rises.

The current new-build requirement for mixing (safety) valves is a bit of a can-of-worms since some valves give a considerable and unnecessary 'leak past' of cold water. This forces cylinders to be maintained at unnecessarily high temperatures. Given the 2% drop in COP per degree rise, this tortures any heat pump involved.

Its worth noting here that it was not that many years back that at least one major German heat pump manufacturer suggested storing at 45°C (with occasional pasturisation).

I am yet to be convinced that the legionella risk is as high as it seems to be popularly cited.
Out of the countries millions of cylinders, I'm sure that a small (but significant) number of them are kept at a very low 'frugal' temperature. Furthermore, houses are commonly left empty, and cylinders could sit for extended periods with tepid water in them, and are not necessarily sterilised before use. Hosepipes sit out on warm summer days with water in them for for weeks. Car wiper bottles have had warm water in them since the 1960s, and I see no evidence for any significant numbers of serious legionella cases. Am I wrong? For large cooling towers, where warm water is sprayed into air it's tragically a different matter.

A couple of anomalies strike me. Why is little attention given to open header tanks in lofts (a UK habit) these open-top tanks (hopefully with cover) sit in warm lofts in summer. One might expect that if a cylinder is fed from one of these, it might requre a different steralisation regime to a mains fed cylinder, and surely there should be at least as much concern from the loft tank that there should be from the hot cylinder kept at only 50°C for example.

Of course, this is a very emotive subject. Who would dare to suggest we should 'ease off' when life it potentially at stake. On the other hand, is it too radical to consider that the added energy needed for hotter cylinder temperatures could have a wider environmental impact. I see no evidence of DECC or anyone else attempting to quantify the extra energy required. I for one think it is relevant.

I'm not suggesting to take a slack attitude to the problem, but I don't agree with the a broad-brush turn-up-the-thermostat approach given the energy penalty involved.

It's quite a difficult balancing act. One has to weigh-up local health and safety with energy costs and CO2. If we debated this at Qatar, and considered the global health and safety, I'm sure that the line would be drawn in a different place.