Gas -best use of boiler ??

anuvvasaver

I have no need for hot water from app 10am til 4 pm. Is it actually cheaper to turn off the boiler and reheat from cold later or to leave it running all day ?

urbanmac

How big is your tank?

I think its better to keep the water warm rather than heat up a whole tank up.

samtheman1k

It depends.

Ben84

urbanmac wrote: »

How big is your tank?

I think its better to keep the water warm rather than heat up a whole tank up.

Adding more heat energy to something doesn't make it's original heat energy which has been invested in it stick around longer. That's actually the compete opposite of what happens. In fact you wouldn't be able to burn gas and transfer heat from it to your water while it was burning if this was true. If heat causes heat to collect, how would you ever pry it away from the hot things to heat cooler things?!

Heat loss through the tank's insulation actually increases proportionally as the water temperature increases. The greater the temperature difference between two items the quicker heat flows from hotter to cooler and it will only stop when the two are the same temperature.

Every time you leave your boiler running to keep the hot water tank hot you are using more energy than if you turned off the heating source between uses. Your water is not in fact the original hot water you maintained at that temperature with lower energy input than reheating it by breaking a law of physics, it's the same water which you've been continuously passing heat through when you could have just reheated it once!

james2012_2

I went on holiday for a week and turned off the gas at the meter.

I normally use about 17 kWh of gas a day for hot water and cooking.

I turned my gas on at 8 pm and by 1pm the next day I had used 35 kWh of gas.

So I used 35 kWh of gas in 15 hours, whereas leaving the gas on all day I use 17 kWh.

So it depends.

Try it and see

Ben84

james2012 wrote: »

I went on holiday for a week and turned off the gas at the meter.

I normally use about 17 kWh of gas a day for hot water and cooking.

I turned my gas on at 8 pm and by 1pm the next day I had used 35 kWh of gas.

So I used 35 kWh of gas in 15 hours, whereas leaving the gas on all day I use 17 kWh.

So it depends.

Try it and see

I'm assuming your boiler used the gas by heating up the water tank from cold? Often these tanks are well insulated and unless you take a bath they don't get entirely emptied in one day, so the actual energy cost to heat its full volume of water is spread out over several days. Yours however lost all the energy due to the time period so you'd have seen the energy cost to heat it fully in one day. That doesn't mean leaving it on all the time is more efficient though, It would have used more gas in that time period if left on. The 35 kWh (maybe slightly less as you probably used gas for other things that day) that went in to heating the total volume of hot water from cold would have been lost through the insulation during that time anyway (adding heat definitely doesn't cause existing heat to stay in the same place for longer) and then extra would have been burnt to replace it. You'd also have experienced a higher rate of heat loss from the tank by keeping it hot all the time. You really did save gas by turning it off.

There's really no maybe or it depends to this. Nobody's boiler is breaking the laws of physics, to suggest it can is a huge misunderstanding of the situation. You might however have finally shown me where the idea comes from though, people turning on their heating or hot water after after it has cooled down completely and seeing the energy cost to heat it compressed in to a smaller time period than normal.

Premier_2

anuvvasaver wrote: »

I have no need for hot water from app 10am til 4 pm. Is it actually cheaper to turn off the boiler and reheat from cold later or to leave it running all day ?

Do you leave the kettle boiling all day for the odd occassion you want a cuppa? Or do you boil as you need it?

dannymccann

Premier wrote: »

Do you leave the kettle boiling all day for the odd occassion you want a cuppa? Or do you boil as you need it?

We need you and Cardew to put a sticky up at the top of this forum in bold block capitals explaining everything around this subject, as I have a feeling this question is going to be asked roughly 562 times again this year :rotfl:

james2012_2

It would have used more gas in that time period if left on.

I don't think you understood what I posted.

in 15 hours 35 kWh of gas was used to heat the tank up from cold.

Therefore if I heated up a tank of hot water then turned the gas off then used that hot water it would use an extra 18 kWh of gas to heat that tank of hot water up again.

Whereas normally in 24 hours, 17 kWh of gas is used to top up the temp of the water and allow for the water used which includes a bath and 2 showers.

I did an experiment a few years ago where I turned off the pilot light after we had had our showers and baths in the morning, therefore no water was heated up after then and used a kettle to heat water for dishwashing etc.

Then when I got up in the morning there was no hot water because it had all been used, I then turned on the pilot light and had to wait an hour before I could have a shower.

I used more gas doing that plus had to wait an hour for the water to heat up enough to have a shower then my wife had to wait 2 hours to have a bath.

I only did it for one day. It was more effort and inconvenience than leaving it on all the time.

As I and others said, it depends.

Try it and see.

I leave mine on 24/7/365

pstuart

There was a similar discussion this last cold winter about 'should I leave my heating on overnight, perhaps nudging the thermostat down 5 degrees'.

The 'scientists' on here came back quoting certain laws of physics (which I agree with) commenting that it would be like 'throwing money down the drain' etc.

It was so cold and we had to wait until midday until we could lead a normal life, so we started to just nudge the thermostat down by 5 degrees and left the heating on.

Result was a very comfortable house 24 hours a day (it had to be, we couldn't get out of our road for 6 days) and the gas bill came in at us owing £3.00 in mid February!

Leave it on, the physics are correct but the saving isn't quantifiable.

Ben84

james2012 wrote: »

I don't think you understood what I posted.

Your account of the situation made sense, but your conclusion of what is happening is wrong. I understand that your house has a heated tank of water that is insulated and you can roughly track the energy going in to it by the gas meter. The tank however won't get emptied fully every day and some of the contained energy rolls over to the next day, so the energy cost of heating its full volume of water is spread out over multiple days. When you turn it off, let it cool entirely, then heat it from cold however the energy cost of heating it is no longer spread out over multiple days. You seem to have thought that the initial gas consumption being higher than the average for one day means that stopping and starting the boiler costs more energy because you have only considered the first day's consumption from a completely cold start and have assumed it extrapolates endlessly to every time the boiler is switched off. You haven't appreciated that the trend changes over a longer time period and that turning off the boiler doesn't make the hot water speed up cooling down, in fact heat loss slows down.

Take for example an air plane travelling from London to Moscow. It takes maybe four hours and the speed and direction of the plane changes throughout the flight. Someone who just looked at the first three minutes of that flight during take off at London airport and extrapolated it for the next four hours would not predict the plane to end up in Moscow though, or anywhere close, they'd put its final destination as somewhere in outer space! You've fallen in to the same mistake, your sample measurement is too small and is taken from a small time period that is a poor representation of the total trend and not considered that it changes over time.

james2012 wrote: »

in 15 hours 35 kWh of gas was used to heat the tank up from cold.

Therefore if I heated up a tank of hot water then turned the gas off then used that hot water it would use an extra 18 kWh of gas to heat that tank of hot water up again.

Whereas normally in 24 hours, 17 kWh of gas is used to top up the temp of the water and allow for the water used which includes a bath and 2 showers.

I did an experiment a few years ago where I turned off the pilot light after we had had our showers and baths in the morning, therefore no water was heated up after then and used a kettle to heat water for dishwashing etc.

Then when I got up in the morning there was no hot water because it had all been used, I then turned on the pilot light and had to wait an hour before I could have a shower.

I used more gas doing that plus had to wait an hour for the water to heat up enough to have a shower then my wife had to wait 2 hours to have a bath.

I only did it for one day. It was more effort and inconvenience than leaving it on all the time.

Use the setting that work for you, its purpose is to heat water after all. Leaving it on all the time might be more convenient, just don't be mistaken that it's more efficient. I use mine with the timer to switch on twice a day for one hour each time so we make more hot water in the morning and in the afternoon. We have a large tank and good insulation so it nearly always lasts until we need to use it.

james2012 wrote: »

As I and others said, it depends.

Different boilers don't experience different laws of physics! It's the interpretation of their behaviour that depends.

james2012 wrote: »

Try it and see.

I leave mine on 24/7/365

I could try this test and I would see in my house exactly what happened in your house, it's just that it doesn't prove what you're saying because its your conclusion about the meaning of this observation that's wrong, not the observation itself.