My wife keeps berating me for leaving the hot water on continuous (tank heated by gas boiler) instead of switching it back to twice a day setting. I say it probably makes no difference to use of gas - who's right?
M.
If it's well insulated and the pipes around it are well insulated, very little and you avoid the inconvenience of the water not reheating if used in the middle of the day.
Figures I've seen for a 300 l cylinder insulated with 45mm foam are 2.88 kW lost over 24 hours.
However, this would be with an immersed cylinder thermostat. If the temp control is left to the boiler stat and the boiler is cycling continuously, heat loss might be more.
Owain
On Sun, 17 Sep 2006 20:26:15 GMT someone who may be marcb wrote this:-
The engineering answer is, not enough information. If the cylinder is very well insulated, the pipe runs are short and very well insulated then the difference is probably relatively small. If these factors are not all present then the answer is she is right. This assumes a reasonable draw off cycle, a modulating boiler and proper controls.
However, the real world answer is, yes dear, you are right.
The only real way to tell is take regular meter readings over an extended period of time, say two weeks, with the HW on and then again with the HW timed. If your heating is gas and now starting to kick in the gas used by space heating will dwarf that from the HW.
Heat loss from a cylinder maintained at 70C or more will be higher than that from one allowed to cool. It is the energy used to maintain the cylinder temp that is "wasted". There is little point in heating water if you aren't going to use it fairly quickly.
I know that when the time switch broke on one system I had the gas bill dropped very noticeably with the "on demand" method of working. But then I was single and living alone, so demands on HW were not high. The tank was well insulated and would stay hot, ok warm enough, for washing for a good 48hrs.
The message from marcb contains these words:
She is. When it's off you're not keeping it at the very highest temperature, and you're not cycling the boiler on and off with the losses from the pipework etc.
If you use hot water during the day but there's still some left by the end of the day when the evening cycle comes then you've saved even more because the partly hot tank will lose heat slower than a fully hot tank.
But, if it's all lagged very well then as David Hansen said - not a lot of difference given that we don't really have enough information.
It makes a difference, but my cvalcs on hot water usage sugest its very slight anyway..you ned to calculate the heatloss from the tank always hot versus when its half cold after standing unheated 8 hours.
If you fancy te maths, its not hard.
Measure temperature of where the tank is. Measure hot water temp when the boiler has finished heating it. Now switch off heating and wait an hour. measure hot water temp again. the difference in water temp times the cylinder volume is the loss in joules per hour. The joules per hour divided by the temperature difference between the tank and ambient i the joules per hour per degree C which is a constant for your tank and installation.
You can do a lot more maths from that to arrive at two figures for annual heat loss - one in which the tank is run 24x7 and the other for which its held hot for the period of the heating controller.
UI wish my wife had such concerns. She leaves the TVs on everywhere, considers a thermostat is how to make a room warm up faster, and that windows are there to cool it down when it gets too hot.
And that to 'save energy' she switches the lights off in the corridors and stairs so I trip over the rubbish she has piled on the one, and land in the puppy poo the puppy has thoughtfully deposited in the other.
The short answer to your question is she is right in theory: In practice leaving the hot tap running, and not turning it off has a far more drastic effect on the fuel bills.
I know, because I had to go to a three-time on solution to hot water, after several cold showers at 11a.m., after she had emptied a 250l tank entirely down the drain.
Whilst I see what the calculation is driving at the heat loss in joules per hour is the temperature drop over one hour multiplied by 4185 joules per litre multiplied by the number of litres in the tank.
In practice the temperature in the tank may stratify so the top will be hotter than the bottom so the calculation may not be quite as simple as this.
Turning to the original question, thew psts have discussed the heat loss implications from the hot water tank. There is also the efficiency of firing the boiler from cold, heating it, pumping the water round the heating system to the tank and so on. These are also causes of heat loss. A single long burn to put heat into the boiler should use less fuel than a series of shorter ones with the same overall heat input due to these greater losses.
If you have sufficient hot water on the timer, then that must be the way to go. If you can heat once in 24 hours (i.e. put the two periods back-to-back) and have enough water then that would be even better too.
Using two cylinder stats will give one long burn and stop cycling. Allowing a cylinder to cool right off and then reheat promotes efficiency in condensing boilers too.
Best keep it on the timer and have a programmer with a boost function. hit the button and the DHW is heated only for an hour. Many homes have the cylinder on the clock and when someone has a bath or shower the boost button is hit. If already heated up it makes no difference, if cool it will heat up the cylinder, or top it up as it should not be cold. Have the system a DHW priority system and a largish boiler and little waiting when the boost is hit. There will probably be no waiting when having a shower.
So, have two cylinder stats, A DHW priority system (usually just changing the control wiring) a programmer with a boost button and keep the DHW on the time clock.
I assume you mean 2.88 KWh...?
That seems excessive to me. Almost £100 a year to keep your tank hot BEFORE you draw any water off?
What does it take to heat a tank? say 50C rise, on 300 liters, 15 mega calories? about 60 megajoules..or 16KWh..
U value of 50mm polystyrene is .45...say the cylinder is 1.5 meters tall, it must have 0.2 sq meter cross section..so 250mm radius so 2.377 is the area of the cylinder, plus 0.4 for the ends..2.77 sq meters so at
0.45 U heat loss is 1.225 watts per degree C difference..say its in the loft at 10C thats 50C difference or 62W constant...1.488 KWh per 24 hour period.Ok. similar ballpark, but out by a factor of two.
Now heatloss of this order shows that if the heating is switched off, it loses at worst 18% of that heat over a 24 hour period. Personally I'd say 9%. So if you never draw the water off, heating it once a day and letting it cool probably saves you at best 20% of the total energy -
210KWh per year. About 20 quid? I suspect that its less. 9% of MY calcs show 48 KWh so about £5.Unless you actually use the water up, and have a cold tank throughout the day, its peanuts.
However it does show that an (electric) combi, with NO hot water storage at all, could save you maybe £100 a year worst case..and maybe £50 a year. I guess gas/oil is about half that.
You probably lose FAR more heat from hot water pipes going cold. So insulating the pipes, and or shoving a second layer of foam round the tank, or pulling the tank inside the house where that 62W loss can make an airing cupboard warm, is far more efficacious.
.
That's a pretty large one. On this basis, the more typical 100 - 120l cylinder would lose around 1kWh. Of course this would contribute to space heating for 9 months of the year, so the real heat loss is around 300Wh for the summer months when heat isn't wanted.
That would be unwise....
No. 2.88 kW lost over 24 hrs, i.e. AIUI 120W per hour would be adequate to keep the water hot.
And during the winter that warmth contributes to the general house warmth and (usually) does the airing, of course.
Not *that* far off then :-)
Or, to put it into context, no more than (approx) 10 50W halogen downlighters in the kitchen for ~5 hours a day.
Owain
80 l 1.12 kW/24hr 130 l 1.50 kw/24hr
(Source: Dimplex SCx range unvented cylinders)
Owain
I think the CHeSS 2005 "basic" best practice specifies no more than 2.3 kWh loss per 24 hours for a 117 litre cylinder and the "recommended" best practice specifies no more than 1.8 kWh per 24 hours. I am not sure how much better modern real cylinders are than these.
2 kWH per day at about 4 p/kWh (I can't recall my current tariff but that sounds about right given recent price hikes) is about GBP 29 per year.Indeed. So perhaps only GBP 10 per year is uselessly lost for keeping the cylinder up to temperature at all times. I did this check some time ago (I hope I got it right) and concluded that the convenience of having hot water available at all times was worth the extra few pounds.
We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember marcb saying something like:
Why do you want to heat hot water?
Good calculation. I wonder ifmore heat is lost through conduction along copper pipes connected to the cylinder than through the cylinder insulation itself.
German cylinders have 100-150mm of insulation.
But rather defeats the point of a storage system - large quantities of hot water instantly.
Will you please eff off, as you are an idiot.
Thanks for confirming once again that you neither think about what you post or know what you're posting about.
Perhaps someone else might explain to you that there is no point in having a hot water storage system full of cold water. It will be less efficient than a combi at producing hot water when needed. But of course to you it's only the efficiency of the boiler that matters. What a fool you are.
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