How does lowering the water temperature save money?

ripplyuk

My hot water used to be boiling hot, with steam coming off it. Last year it was turned down to 65°C and the heating engineer said this would save money. I’m not sure if it does. It’s oil heating with a tank in the hot press for hot water. 

Previously, the water was so hot that not much was needed for a bath since I was mixing a lot of cold water into it. It only took about 20-30 mins each day to heat enough water. 

Now, I have to heat the water for at least 3hrs because I need much more hot water and am barely adding any cold water. The bath also isn’t warm enough to stay in it for long so I have to be quick getting washed. 

Is this still saving money? It seems to be using more oil but that could be for other reasons. 

QrizB

ripplyuk said:

Is this still saving money? It seems to be using more oil but that could be for other reasons. 

If you have a modern (in the last 20 years) gas or oil boiler, it shoud be a "condensing" model. This recovers waste heat from the flue gases, but can only do this effectively if the return temperature from the heating system is lower than the temperature at which water in those gases will condense.

65C should still be plenty hot enough; if you get into water at that temperature it'll injure you. If you're barely having to add any cold water, it might mean there's something else wrong with your system.

david29dpo

Its a whast of money if you have to add cold

FreeBear

I heat my water to a maximum of 45°C (combi boiler). Simple maths tells me that it requires less energy than it would to heat to 60°C or hotter. Heating to a lower temperature reduces the amount of scale build up (a big issue if you are in a hard water area), and also reduces the effects of corrosion.

In terms of boiler efficiency, even with heating water to 45°C, the boiler is running outside of condensing mode, so it doesn't affect efficiency that much. At most, a percentage point or two.

matelodave

Lowering the temperature save money in several ways

1 - it uses less energy to heat water to a lower temperature and, as pointed out above, most boilers nowadays can recover heat by condensing the return water, meaning that you'll save even more if you can reduce your hot tank temp to around 55 degrees as the boiler efficiency improves dramatically

it takes 9.6kwh to heat a 150 litre tank from 10 to 65 degrees, whereas dropping it to 55 degrees uses 7.86 kwh, saving you nearly 2kwh add to that the improvement in boiler efficiency you could probably save around 2.5-3kwh every time you heat your hot water tank. If you do that every day then you could save 2.5kwh x 365 = over 900kwh a year or around £55 if oil is around 6p/kwh

2 - a hotter tank will lose more heat energy to the outside than a cooler one so dropping the tank temp to say 55 degrees (which is still far too hot to put your hand under) could possibly save you around 1khw a day in heat loss. Possibly not so important in the winter when that heat is enclosed in the house, but obviously not ideal in the summer.

fatbelly

Just because it says 65 doesn't mean it's heating to 65. If your bath doesn't feel hot, then it probably isn't

You could try drawing the water a little slower and see if that helps.

Otherwise nudge the thermostat up a bit

Beeblebr0x

FreeBear said:

I heat my water to a maximum of 45°C (combi boiler). Simple maths tells me that it requires less energy than it would to heat to 60°C or hotter. Heating to a lower temperature reduces the amount of scale build up (a big issue if you are in a hard water area), and also reduces the effects of corrosion.

In terms of boiler efficiency, even with heating water to 45°C, the boiler is running outside of condensing mode, so it doesn't affect efficiency that much. At most, a percentage point or two.

45°C wouldn't feel hot enough to do our washing up.

matelodave

Beeblebr0x said:

FreeBear said:

I heat my water to a maximum of 45°C (combi boiler). Simple maths tells me that it requires less energy than it would to heat to 60°C or hotter. Heating to a lower temperature reduces the amount of scale build up (a big issue if you are in a hard water area), and also reduces the effects of corrosion.

In terms of boiler efficiency, even with heating water to 45°C, the boiler is running outside of condensing mode, so it doesn't affect efficiency that much. At most, a percentage point or two.

45°C wouldn't feel hot enough to do our washing up.

I find that water at 45 degrees is more than hot enough for a shower and there's not much chance of scalding yourself

We use a dishwasher for most of our washing up and if I need water hotter than 45 degrees I boil up as much as I need in a kettle.
2 litres of very hot water costs a lot less that heating a whole hot water cylinder full or running lots of hot water through all the plumbing for a combi.

Exodi

QrizB said:

65C should still be plenty hot enough; if you get into water at that temperature it'll injure you. If you're barely having to add any cold water, it might mean there's something else wrong with your system.

^ this. To highlight this, the ideal temperature for a bath/shower is around 37-38 degrees (usually the default on digital showers). 65 degrees is third degree burns in less than 5 seconds.

I don't really understand the part about "The bath also isn’t warm enough to stay in it for long so I have to be quick getting washed." - presumably after mixing in cold water the bath is about the same temperature in either case so would cool down just the same. If the bath isn't staying warm, then use less cold water aka have a hotter bath as you likely were before. Lowering the boiler temperature hasn't changed the thermal properties of the water.

ripplyuk

Exodi said:

QrizB said:

65C should still be plenty hot enough; if you get into water at that temperature it'll injure you. If you're barely having to add any cold water, it might mean there's something else wrong with your system.

^ this. To highlight this, the ideal temperature for a bath/shower is around 37-38 degrees (usually the default on digital showers). 65 degrees is third degree burns in less than 5 seconds.

I don't really understand the part about "The bath also isn’t warm enough to stay in it for long so I have to be quick getting washed." - presumably after mixing in cold water the bath is about the same temperature in either case so would cool down just the same. If the bath isn't staying warm, then use less cold water aka have a hotter bath as you likely were before. Lowering the boiler temperature hasn't changed the thermal properties of the water.

It’s not as warm as before. I’d have to heat the tank all day to be able to fill the bath with only hot water so I tolerate it being a bit cooler. 

ripplyuk

Thanks for the replies. I don’t think this is a condensing boiler. Does that make any difference? I asked the engineer when they installed it and he said it’s not a condensing boiler, it’s a ‘conventional’ boiler. It was only installed last year but this is a council house so the fittings tend to be at least 20yrs behind everyone else. 

It’s in my back yard and pumps out steam constantly. I assume a condenser boiler would condense the steam instead?