Sunday, 29 April 2012

How to save on hot water

One litre of hot water requires 0.05 kWh of energy to heat it, assuming your boiler is 90% efficient. This means a 3 minute shower at 12 litres/minute  (25 litres of hot water, allowing mixing in some cold) takes 1.25 kWh. At the rate I am paying for gas, that would cost 4.4p (1). Our old boiler was only 70% efficient so the hot water takes more energy to heat it and would have cost a little more: say 5.4p.

The Energy Saving Trust (EST) collected data from hot water usage in 107 households (2) and they found a huge range in water consumption even by households of the same size. For example the bulk of 2 person households used between 30 and 120 litres/day. There were also more extreme cases but even in the ‘normal’ range some people are using four times as much hot water as others. (That ‘normal’ range, would cost me between £19 and £77 per year.)

Clearly, you won’t get many baths per week for 30 litres/day. One bath is at least 80 litres, say 60 litres of hot water. You won’t even get a whole ‘typical’ shower each though you could both have a ‘navy’ shower - get wet then turn the water off, soap yourself, rinse,  apparently doable with just 11 litres!

After bath and shower, the next highest use of hot water is usually the kitchen sink. The trouble is, when you turn the tap on it takes a little while to actually get some hot water because the water that was sitting in the pipes has got cold and that has to run through first. I measured how much water flows in my sink before I start getting hot coming through. It was 4 litres. For a friend of mine it was 6 litres. 

Most of the time, I don’t bother to wait for the hot water to come through, in which case, if I am thinking about it, I turn the mixer to cold and use cold. That way I don’t waste hot water unless I am prepared to wait and actually get hot water. If I am doing washing up and I really want hot then I will use the cold water running through to rinse off the saucepans and get the worst of the dirt off so it isn’t a total waste.

Despite what many people think, washing your hands in hot water is no better than cold from the point of view of killing bacteria. If it were too hot for the bacteria it would be way too hot for you too. Washing in hot water can be quite bad for the skin [3].

Interestingly, the same EST survey found that the cold water inlet temperature for regular boilers was typically about 3C warmer than that for combi boilers. The reason for this, they suggest, is that the regular boilers take water from a cold water tank which has been slightly warmed up by sitting in the house (or at least in your loft, getting warmth from below). This doesn’t mean the combi boilers use more energy  though because although they have cooler water coming in they also usually have cooler water coming out: the hot water coming out of the combi boiler was on average 3C colder than that from a regular boiler. It does mean that when saving hot water by using cold instead, the combi boiler people will experience more refreshingly cold water than the regular boiler people who have a cold water tank. The average cold water temperature in January was about 9C for a combi system and 12.5C for a regular system (2).

You are now probably wondering now how much energy you are using in heating up your cold water tank. I will look at this, and other losses from our heating system, in another post.

This article was updated 5/Sep/2015 with the reference for hand washing temperature.

(1) Assuming a temperature rise of 37C, as used in the Building Research Establishment Standard Assessment Procedure (SAP).
(2) Measurement of Domestic Hot water Consumption in Dwellings EST 2008.
(3) Amanda R. Carrico, Micajah Spoden, Kenneth A. Wallston and Michael P. Vandenbergh (2013) The environmental cost of misinformation: why the recommendation to use elevated temperatures for handwashing is problematic International Journal of Consumer Studies


  1. Hi, I have been asked to compose a template letter asking shops to close doors to reduce carbon emissions. Please let me know if you tried sending such a letter OR if you trialled a welcome poster on closed doors? I suggest the best trial would be for a supermarket to trial open doors then, closed door on a set day of the week to test if essentially regular customers are deterred or not. Also trial the poster on as many shops as possible so customers get to read them.

    1. I have not campaigned on this myself but I reported on it here The close the doors campaign web site still exists.


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