Francis Tapon asked:
I'm in Croatia and I'm having a debate with my friend about water heaters.
I have a water heater in my bathroom. I take on shower per day. I set it at just the right level so that it gives me just enough hot water for a 5-minute shower, no more. I turn on the boiler about 1-hour before I want to take a shower and then turn it off right when I get into the shower. I don't turn it on until the next day, where I follow the same ritual.
My friend says I'm being inefficient and not saving any energy. He says I should leave the boiler on 24-hours a day, because it takes a minimal amount of energy to keep the water hot once it's hot.
If I turn it off, the water cools and then must be re-heated from scratch, requiring far more energy than if it had been on all along. Who's right?
We put this to Hugh Hunt, from Cambridge University's Engineering Department...
Hugh - This is a very interesting and topical question. If the hot water is stored in a perfectly insulated tank, then it makes no difference whether the water is heated one hour before it’s needed or one day before, or even one year before. There's nothing at all to be gained from heating the water up 22 hours in advance, but that requires that the insulation is perfect.
In reality, insulation is not perfect and the water heater will lose some heat in advance and this heat cannot be recovered. It’s the same as when we’re boiling water for a cup of tea. If we boil the water when we need it, that makes sense because the kettle is not that well-insulated. If you boil the water an hour in advance, the water would be cold when we needed it.
How much energy wastage are we talking about? Suppose a 5-minute shower uses 60 litres of water, which is about right; and suppose the water is delivered at about 40 degrees, which is about right; and in the winter the water is cold, somewhere around 0, which is about right. So we’re talking about 4 kilojoules/degree/litre of water.
Now suppose the tank loses 10% of its heat in 24 hours. Well then that's a loss of getting up to 1 Mega Joule of heat. That's equivalent to running an electric kettle for 6 minutes, enough to boil the water for several cups of tea. So if you're the kind of person who’s bothered not to over-fill the kettle when making a cup of tea then you should certainly follow our Croatian friend’s example and only heat the shower water when it’s needed.
It depends on how well insulated the tank that holds the hot water is. If it's really well insulated, you won't waste energy whether you leave it on all the time, or turn it off and on, so you might as well leave it on all the time if you find that more convenient.
Ironically, in some countries, they call the immersion heater the "geyser", Geezer.
standby heatloss is dependent on the temp difference between the hot tank & the environment around it. I do the same= drain/use the hot water storage & leave the tank cold so it cant lose heat while standing by= cold tank has no heat to lose. CZARCAR, Sun, 27th Mar 2011
BTW, and drifting off topic a bit, there are some really cool (well, hot really) heat pump water heaters. They extract heat from the ambient air in the home and use it to heat the water. This works really well in warm climates because the water heater is actually helping to cool the home.
The energy to heat the water to a temp is the same whether you do it immediately prior to shower or several hours prior. But it's the thermal isulation which is critical to the overall efficiency of the system. If the system has 100% effective insulation so the water stays at temp then there is no advantage to one system over the other. But if the water in the tank is able to cool before being used then more energy will be used by using central heating to keep it constantly upto temp.
@NakedScientists It depends entirely on the size of your boiler. Problem sorted :) was tweeted by @geo2ward @geo2ward, Mon, 28th Mar 2011
The answer is turn the water off when not being used.
@NakedScientists Is the answer the most common one in science, 'It depends...'? was tweeted by @bangscience @bangscience, Thu, 31st Mar 2011
When you've had a shower, the water taken out of the tank is replaced by an equal quantity of cold water - which needs to be heated up - just as in the case of only heating it when you need it. Factor this in, and only heating it when you need it is far and away the most efficient thing to do. Gwernogyss, Wed, 20th Apr 2011
Actually, he's being slightly inefficient in one way. I *think* he should turn the water heater off right before he has the shower.
Of course, there is another factor that should be taken into consideration here. The water in the tank is not all at the same temperature. It actually stratifies so that the hottest water is at the top of the tank, and the feed to the shower is taken from the top of the tank.
bingo CZARCAR, Fri, 22nd Apr 2011
Contrary to popular belief, electricity does not leak from power outlets when there isn't a plug in the socket. Remarkably, neither does much heat escape from a well insulated water tank. If the tank is well insulated, the financial penalty for heating the water before it's actually needed is negligible.
wolfekeeper, I do turn off the water heater right before I get in the shower. You're right to make the point. Naked Scientists, Hugh Hunt's answer is logical, but there's one assumption that is faulty. It assume that the heat loss is 10% per day. It's more like 50% per day with my water heater. I know this because if I heat up the standard level that I need for a 5-minute shower, then in two days, the water is cold if I don't use it. A 10% loss implies that roughly after 10 days, the water is cold/room-temperature. It reaches this state in just 2 days. Thus, the energy is not running the electric kettle for 6 minutes, but rather for 30 minutes. That's a bit more expensive and significant than originally assumed. Thank you for proving me right! Francis Tapon Francis Tapon, Fri, 6th May 2011
You should also consider what effect intermittent aperation of the water heater has on the longevity of the appliance. Thermal cycling through large temperature differences might possibly deteriorate the tank more quickly. Then you have to replace the appliance sooner. May not be a good bargain. Atomic-S, Mon, 9th May 2011
This is all good discussion and the theory is correct but one thing is missing. In many countries where hot water is heated with electricity the electrical power to the heater is controlled remotely by the power company. There are two tariffs for the cost of power – one or power consumed during the ‘peak’ hours and one for ‘off peak’. The reasons for this are another topic. Peak is usually a few hours in the morning, lunch time and the evening. So . . . if the hot water heater is switched on at 2 am it will heat up but if it is switched on at 6am it will not heat up until the morning peak is over (albeit some installations have an override switch BUT the override uses electricity at the peak tariff). So . . . to save energy 1) take a shorter shower and 2) use a solar assisted or heat pump hot water supply or a well insulated hot water heater with the smallest capacity your household can manage with off peak demand heating. Clearly option 1 is the only option for the person with the original question. jo nurd, Sun, 29th May 2011
If my water came out of the hot tap at 46°F I would regard that as perishing cold not blistering hot. syhprum, Mon, 13th Jun 2011
Most responses here have missed the point. The point is NOT whether heating in advance is economical, but whether switching on and off is economical. Lets assume there is some heat loss over a period of time. That would mean at some point the water would need to be reheated. Let's say I lived in Africa, where electricity and water were scarce and I only showered once a month. Surely it would be less expensive to turn on the geezer just before I had a shower, and as soon as the water was hot, I'd use it. Then leave it switched off for the rest of the month. Whereas keeping it on for the entire month, it would consume more electricity to keep the geezer on, so that any heat lost could be regained. And do the geezers remain on, at a low temperature? Or do they turn off completely when the water is hot enough? If the later is the case, then turning it off yourself is the same thing. Except it would never turn itself on again when the temperature dropped, so you'd being saving power. BTW, instant gas hot water systems have been around for decades. My Dad is using one that's at least 50 years old and still working perfectly well. JK, Wed, 3rd Aug 2011