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.
http://www.microgenerationcertification.org/about-us/news-and-events/94-bacterial-growth-in-stored-hot-water-systems

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.

Thursday, 2 August 2012

Underfloor heating in bathrooms

A bathroom has a higher room temperature requirement, so heat requirements are generally higher, but the floor area available for underfloor heating is reduced due to the area taken up by the bath. I have recently come across a few instances of inadequate heating issues in bathrooms – not surprising. The point of this blog is to discuss the option of continuing the underfloor heating to the floor below the bath. This practice seems to be a no-no, and I’m reticent to suggest it’s a sensible approach. If the floor under the bath has wet underfloor heating, the space under the bath will be warmed, this will add heat to the room by a certain amount simply due to a slightly warmed bath and side-panel surface, and clearly a metal bath would be considerably better here than a plastic one. A roll-top, claw-foot bath even better! I have no idea if the quantity of heat is worthwhile. i.e. if the room temperature in the bathroom would be elevated by a worthwhile amount without the need to increase the underfloor water temperature (Very important when a heat pump is used). Many years ago I took a coil of micro-bore pipe, and wrapped it around the outside of my new plastic bath, and ‘bonded’ it with fibreglass. In this case, I was experimenting to see if the bath could became useful radiator area, and if the bath should stay warm whilst in use. The results seemed worse that expected, and no doubt there was little gain for a lot of effort. Given that bathroom loops are generally some of the shortest loops in the system, it strikes me that putting extra pipe below the bath would be easy, and advantageous. Is it the risk of drilling into a pipe when making a bath fixing? Is it the fear that the under-bath could overheat? Is it a daft idea with little benefit? I suppose I should lay an electric blanket under a bath and monitor some temperatures in various places to see how it performs.

Wednesday, 2 May 2012

Repairing a heat pump - How well is it carried out?


This post is a little specific, and discusses the need for care and consideration when carying out any 'major surgury' on a heat pump.  A little extra time setting the system up right will save considerable energy (and money) over its life.

Wednesday, 11 April 2012

How energy-efficient should a heat pump be?

Heat pumps have always been cited as energy saving devices. The fact that they can give out a quantity of heat several times that of the motive power to drive them is proof enough. However, the sceptics have in the past rightly pointed to the inefficiencies of the source of that motive power – electricity, and many conclude that the inefficiency of the power stations coupled with the very high efficiency of a heat pump only just cancel each other out. i.e. why not simply burn the power station’s fuel directly in the home without the complexity of heat pumps?

It is clear that the range of achievable efficiencies that a heat pump system delivers can vary greatly. This is primarily dictated by the type of application, and secondarily by the engineering details or the system.  For designers to evaluate the net-worth of a potential system, a good grasp of the environmental issues are required.

CO2 seems to be the prime consideration here, and we can make simple mathematical comparisons to see if the real advantage of a heat pump is sizeable or only marginal: worth installing or not.

On the generation side of things, market forces and other factors drive the decisions that dictate how the National Grid buy and produce electricity. The UK's generation ‘mix’ leads to a figure of how many kg of CO2 are released for every kWh of electricity produced – on average, around ½ kg for each unit of electricity. To make things complicated, this varies over the day, over the seasons, and will vary year on year.

(Note: I ignore here any ‘green’ tariffs on the grounds that we would need a great deal of it to make a notable difference to the figures nationally).

Given the UK figure of 0.5kg/kWh, and a rough notion of expected future variations, we should be able to compare CO2 figures for direct electric heaters (100% conversion), with gas and oil heating.   We should now be able to consider heat pumps with various COPs, and arrive at some figures for the efficiencies that we might like to achieve.

(Heat Pump efficiency is measured using the Coefficient of Performance (COP). COP is the ratio of useful heat output divided by the power input. Seasonal Performance Factor (SPF) is the annual useful heat divided by the annual Electrical input. This should include use of any direct electric back-up heater)


The vertical (left-hand) axis shows pollution figures for direct electric heating, gas and oil. The graph shifts to the right with increased COP or SPF of a heat pump (COP1 is the same as an electric heater).   As we can see, it’s relatively easy to work out a break-even SPFs compared to common heating methods. The more difficult question is ‘How much better should a heat pump be’?

At this point it seems important to turn our attention to what efficiency levels heat pump technology can offer, then to seek the compromise that all such designs are based upon; cost / benefit.   If very high COPs are attainable, but excessively costly to manufacture and install, they are probably of little benefit. At the other end of the spectrum; cheap low-efficiency systems may bring no carbon saving at all.  We need to look for application that are practical and affordable to install, and show a good CO2 saving.  The question to ask here is - is the particular application a good one for this technology?  Is it one where a high COP can be achieved without excessive installation cost. If not, maybe a different technology should be adopted - eg. If a boiler can be installed for 1/3 the cost, the money saved may allow some very serious insulation. The outcome might give a better net energy saving.  A holistic view is needed, and for heat pumps to compete- they need to be energy efficient.

My perception of people’s expectations for SPFs (annual COP) is that they have had a knock over the last year or so.  I had hoped that SPFs might pan-out around 4 - given some improvement in the technology over the years, however, it seems that many systems have in recent years, fallen short of  what’s-possible.

It seems to me that the industry is all too willing to go down the road of ‘mediocre’ efficiency. Indeed, for the heat pump industry to survive at all, installations must be affordable, so some are only too keen that, for example, an SPF of 3 is perceived as good – and who can blame them?.   Taking this further, if systems can be made that are cheap and easy to install, then break-even SPFs (compared to gas) might become attractive. A heat pump could possibly become a ‘convenient heating method’ with no environmental advantage.

So what efficiencies do we need?  The game on the DECC website titled ‘2050’ is worth a look. It shows how difficult it will be to achieve our carbon abatement targets by the year 2050. If we are to get even near, we need as much COP/SPF as we can get!   


The recently produced Emitter Guide  ( http://www.microgenerationcertification.org/admin/documents/MIS%203005%20Supplementary%20Information%202%20-%20Heat%20Emitter%20Guide%20v2.0.pdf)  has been developed as a ‘guide’, not a design tool (‘Emitter’ referring to ‘heat emitters’ - radiators or underfloor). If I understand it correctly, the guide was produced to assist in ‘steering’ designers and installers towards better systems.  It has a well laid-out flow chart for the installer/designer to consider both new and existing radiator, and should be commended highly for its recommendation to stand-back and consider implications of improvements to thermal insulation, or reducing ventilation losses. This is a welcome deviation from the old just-do-my-bit ways of the industry. However, as I scan down the expected SPF figures for the various options of radiator oversize or underfloor heating pipe-spacing, The question I am asking myself is – how does anyone know what SPF to aim for?  I have considered this intangible question for many years, and I am still much in the dark.

The general drop in COP expectation is not helped in my view by the range of SPFs quoted on the emitter guide which, for the air source system, has a mid-range of 2.85, and the lowest figure (2.1) relates to CO2 figures worse than gas.  If one were totally in the dark, one would probably be ‘swayed’ to thinking that an SPF of 3 was quite reasonable.   It is also important to note that the figures relate to space-heating only and since the vast majority of installations also heat hot water, and since DHW heating can occur with a relatively low COP in the region of 2 to 2.5, it is clear that some of the figures quoted will be pulled-down by DHW heating in practice.   With this in mind, the 6 star option (flow temperature only 35°C) could be viewed as ‘normal’ as opposed to the ‘exceptionally good’ that it might currently appear, being top of the list. 

I wonder at this point if those with a good handle on the environmental issues should get together with those who know what real-live SPFs are practically achievable, and give some guidance on what systems are worth pursuing, and which ones are not.   System designers need to know want level of system efficiency to aim for, otherwise there is a danger that other figures will be found, e.g. minimum standards, and these will be used as those targets.
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I have added this intesting graph sent to me by John Logan of Maine USA (East Coast).  Its interesting to see their experience. John is a pioneer and proponent of the 'Standing Column' borehole system, and their experience looks impressive.  They are experiencing average COPs of 4.5. This shows an application and a climate that match well - here we have very competitive running costs and very good SPF.

The graph also shows a 12RLS2 air-air split systems with COP almost 3 - obviously fits well in their climate.  



Home Size 1,500 sq.ft., HP COP = 4.5, 12RLS2 COP = 2.97, Electricity $0.15, Pellets $250, Oil $3.50, Propane $3.00