Just looking for a guide, not actual calcs, from someone who can give an authoratative "yes" or "no"...
Fitted a 330l pressurised cylinder (insulated) earlier this year and to date has been left switched on all the time due to high demand for hot water literally 24hrs a day. Now demand is dropping off dramatically and will only be needed Fri-Sun so...
Should the cylinder be switched off Monday - Fri am allowing the water to cool and reheat from what I asume would be almost cold on Friday pm. Or, is it more energy efficient to leave it on and let the thormostat regulate power consumption to keep it up to temp throughout the week?
Turn it off. It probably won't be almost cold - my brother has a 150l cylinder in his flat which was switched off last Monday and on Saturday the water was still warm.
As the water cools it will be losing heat more slowly than when it was warmer (if you see what I mean). Losing heat more slowly = more efficient. You'll have to add less heat to get it back up to temperature than you would have lost by maintaining it over the week.
In reality there's probably not that much difference. And bear in mind that if you are heating electrically that the cost of the energy may vary depending on when it is consumed.
Definitely off, if it is for that period. If it is just unused overnight, savings will be minimal. However, over the period of a week, you'll find it cheaper to only heat weekends.
However, if it is electric, you may find it takes an age to reheat, so will need to do some calculations to determine when the timer should come on. With gas, it should only take an hour or so from cold, assuming a modern system.
A little lateral thinking says that electricity used for heating water is very nearly 100% efficient in what it does - ie there is no waste heat, as there might be with a less direct method, ie a gas boiler.
However, no insulation can be 100% efficient, so there will be some heat loss. So switching off when not required must be the cheapest option. It might not be the most convenient, though, given the time needed to re-heat.
What cylinder is this ? Our cylinder goes cold in 2 to 3 days. It also costs about 10p/day just to keep the water warm without using any. Even tried adding a standard cylinder jacker over the spray foam but that didn't make a major difference - temperature between the jacket and the foam was about 2 to 3 deg above ambient.
As others have said, it's best to turn it off. However, if it's in a place where this is possible, you'll get a significant increase in time to cool down if you get a 5 quid roll of loft insulation and pack it (loosely, but leaving no air gaps) round the cylinder.
Coincidentally was thinking about the same question only this only this morning. Would there be any economy in switching the water heater tank/cylinder off for certain periods, especially since now only one person (me) living in this house?
Talking to myself driving to technical college, went like this. Self, I said;
"Normal practice here is to leave the hot water heater, which is typically
30 imperial gallons with upper and lower 3000 watt heaters with individual thermostats in a flip-flop arrangement, on, all the time".
BTW we went on vacation on one occasion for a couple of weeks and switched off the hot water circuit breaker; when we returned the tank water was still tepid! So have always assumed that heat loss to the ambient surroundings in our unheated average 55 to 60 degrees F. basement, is very small?
" What about temporarily hooking up an electric clock to the heaters. Then see how long the heaters came on while domestic hot water use was zero, to maintain the set temperature. If the clock ran for say ten minutes during a ten hour period. Then electric consumption would-be 10/60 * 3 = 0.5 Kilowatt hours or units per 10 idle/quiescent hours. An average of 0.05 units per hour? Electricity here now costs about 9 cents (Canadian) or roughly 4 pence per unit all charges, such as sales tax included Using these hypothetical numbers; then if unit left on for 24 hours, with no use of domestic hot water, 24 * 0.05 * 0.04 = approximately 5 pence per day?".
For a two week holiday I would in fact shut off the water pressure and switch off the hot water heater and, unless mid winter, the electric heat; otherwise for a quiescent two weeks it might cost, theoretically, about 14
5 = 70 pence! Does this make sense or have I slipped a decimal place somewhere?
Based on cooldown rates, for my admittedly well insulated cylinder, I believe it's about 80W or so. So, at 4p/unit, that's be around 8p/day.
It's within a factor of 10, so I'll not disagree.
Either measuring the on-time, or measuring the temperature drop with an accurate thermometer after 12 hours switched off would work, if you know the cylinder capacity.
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