Friday, 9 November 2012

Heat Losses from Hot Water Cylinders

the solarblogger roots around in the nation’s airing cupboards
Tucked away in a cupboard, perhaps behind a pile of towels and bed linen, out of sight and out of mind.  The domestic hot water cylinder often escapes our attention, but it has a big role to play in the energy efficiency of our homes. 
Modern cylinders have a thick jacket of polyurethane foam insulation around the outside to reduce heat losses, but the connecting pipes can be just as important.  These provide a path for heat to escape from the cylinder.  To appreciate this fact you just have to touch them to feel how hot they can be, and how far from the cylinder connection they can conduct the heat.

Even in modern houses built to higher energy efficiency levels it is common to find a hot water cylinder installed without any insulation on the connecting pipe-work. The image (left) was taken at a visit to an eco-exemplar show home this year.  The home has a host of energy efficiency measures such as solar water heating, solar PV, and mechanical ventilation with heat recovery.  The only pipes in the cylinder cupboard with insulation on them are those to the solar panel. 
Presumably the plumbers had so impressed themselves with the job they’d done on covering the copper pipes with silver paint that they didn’t want to hide the beautiful craftsmanship. 
You can also see on the cylinder that the secondary return fitting is unused (just below the grey thermostat on the right hand side of the picture).  Instead of being closed off with a plug a short length of pipe is sticking out with a soldered end fitting.  This creates a completely unnecessary conducting rod to remove heat from the cylinder and transfer it to the surrounding air.
Did the energy assessors tick the box that said that all visible pipe-work was insulated?  I’d bet money that they did.  This is a failure of compliance as well as a brilliant illustration of a lack of awareness from installers. 

If a new build eco-showcase can look like this, what hope for millions of existing homes around the country, and how much impact this can have on their energy performance?

Show me the money

Research from the Energy Saving Trust demonstrates just how significant the heat losses from poorly performing hot water cylinders can be.

Heat Losses from Domestic Hot Water Cylinders

Source: Energy Saving Trust, In-situ monitoring of efficiencies of condensing boilers, June 2009

During the heating season, these cylinder heat losses are warming the house and reducing the space heating load.  For new homes with ever higher levels of insulation the heating season is becoming shorter and shorter, but for most homes you can safely assume it lasts half of the year.  So the other half of the annual loss is wasted energy, and this is shown in the last line of the table.

To put these numbers in context, the energy to heat the water demand for a four-person household is around 2,140kWh/year.  The average cylinder losses in the study add half again to the hot water load, the worst performing cylinders in the study double it.
Of course, it’s impossible to reduce the losses to zero, but what might be achievable?

Well, hot water cylinder manufacturers test and declare the heat losses from their cylinders. When you take this figure and adjust it for the fact that the cylinder doesn’t spend all day at 60C you find that a typical 210 litre modern cylinder loses around 355kWh/year, so that’s only 178kWh/year of wasted energy taking the heating season into account.  Conclusion: a modern, well-insulated cylinder saves 750kWh/year compared to  the average found in the study.
Most solar installations involve the replacement of a hot water cylinder and with upcoming changes to the MCS installation standard for solar thermal, the solar thermal installer will have to ensure that all pipes and fittings are well-insulated (not just the solar ones). 

The energy savings that come from improving the hot water cylinder as part of a solar heating installation are not acknowleged in the MCS energy estimate and do not count as renewable heat for the forthcoming Renewable Heat Incentive, but they are real and they are significant.


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  2. Very interesting. In a Passivhaus the heating season is only 3 months or so. Therefore heat losses are significant and even with the best installation these figures suggest 3/4 of 767, or 575kWh are wasted, that is 27% increase.

    What do you think about using instantaneous water heaters, which essentially remove this loss. Of course, the heat loss is not then warming the house in winter, but that is only 200kWh or so. Seems to stack up, they are peanuts.

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