Combi Boiler Use

JohnG57

I have recently had a new combi boiler installed. I wonder if anyone on here can resolve a difference of opinion between myself and a friend.

I suggest that, when running a bath, it is cheaper to run (as an example) 18 inches of water at the temperature you wish the bath water to be.
My friend suggests it would be cheaper to run (for example) 12 inches of very hot water and then add 6 inches of cold.
My argument is that it is a waste of energy using cold water to cool down water you have heated. My friend's argument is that the boiler would not be heating water for as long doing it their way.

I look forward to your take on this minor disagreement. Thank you.

chrisw99

With the winter months coming up, and the incoming water supply getting colder, you may find it's impossible to run the water so hot that you can then add a lot of cold to it to do what you suggest. I find due to the length of the run from my boiler to my bath, the maximum I can get the water to is perfect for my bath so I'd never be able to add cold water anyway.

I think they can only increase the water temperature by a max amount (which varies based on the rating of your boiler).

jk0

JohnG57 wrote: »

I have recently had a new combi boiler installed. I wonder if anyone on here can resolve a difference of opinion between myself and a friend.

I suggest that, when running a bath, it is cheaper to run (as an example) 18 inches of water at the temperature you wish the bath water to be.
My friend suggests it would be cheaper to run (for example) 12 inches of very hot water and then add 6 inches of cold.
My argument is that it is a waste of energy using cold water to cool down water you have heated. My friend's argument is that the boiler would not be heating water for as long doing it their way.

I look forward to your take on this minor disagreement. Thank you.

It's exactly the same amount of energy.

spadoosh

Lets do some fag packet maths.

Bath tubs typically hold about 80 litres.
Mains water is typicall about 7 degrees c.
Bath temp is typically about 40 degrees c.

Youre method require heating water straight to 40 degrees so will be usedv for longer. In total that requires 11051KJ of energy (roughly)

If you heated the water to 50 degrees. Youd only need to heat up about 61/62 litres of the water with rest being made up of 18/19 litres of 7 degree water to get roughly 40 degree water.

To heat 61/2 litres of water form 7 degree to 50 requires 10979kj and 11159 kj respectively.

Basically, it doesnt matter. Its exactly the same.

I run hot water and then add cold. The variable from the mains temperature would mean you would have to alter the out temperature throughout the year.

Mister_G

I suppose the only issue here could be the boiler efficiency depending on how long it has to run for.

We know that modern condensing boilers modulate, so the efficiency varies.

The boiler may be more efficient running continuously at the higher temperature for a shorter period of time, rather than cycling for longer at the lower temperature?

chrisw99

spadoosh wrote: »

If you heated the water to 50 degrees. Youd only need to heat up about 61/62 litres of the water with rest being made up of 18/19 litres of 7 degree water to get roughly 40 degree water.

Another concern here - if you alter your boiler to provide very hot water, then add cold, you've now made the water coming out of your basin and kitchen taps potentially dangerous and scalding. Unless you are going to change your boiler settings before and after each bath?

JohnG57

Thank you all for your quick replies. I guess my friend and I will have to call this one a draw.

spadoosh

chrisw99 wrote: »

Another concern here - if you alter your boiler to provide very hot water, then add cold, you've now made the water coming out of your basin and kitchen taps potentially dangerous and scalding. Unless you are going to change your boiler settings before and after each bath?

Ive got mixer taps, i just turn it colder.

Owain_Moneysaver

Slightly cheaper to heat the water to the required temperature. There will be some heat loss from the hot water in the pipe between the boiler and the bath tap. This loss will be proportional to the difference in temperature between the water and the ambient temperature. Therefore, hotter water will lose more energy.

However, that heat may not be wasted if it contributes to your house's warmth that would otherwise be made up by central heating.

teddysmum

If you mix the water as it leaves the tap, I think you could save, as filling with hot, then adding cold could result in the final mix being of higher volume than required (ie you add too much hot or the temperature of added water is too high, meaning more cold is needed to reach a suitable temperature).


I tend to mix at a very slightly higher temperature than needed, to allow for natural cooling, due to a large surface area of water.

The_Big_Bamboo

JohnG57 wrote: »

I have recently had a new combi boiler installed. I wonder if anyone on here can resolve a difference of opinion between myself and a friend.

I suggest that, when running a bath, it is cheaper to run (as an example) 18 inches of water at the temperature you wish the bath water to be.
My friend suggests it would be cheaper to run (for example) 12 inches of very hot water and then add 6 inches of cold.
My argument is that it is a waste of energy using cold water to cool down water you have heated. My friend's argument is that the boiler would not be heating water for as long doing it their way.

I look forward to your take on this minor disagreement. Thank you.

OP;
your method is cheaper;...the ‘heating curve’ of water alters as it approaches the ‘transition phase’ of changing from a liquid to a gas; i.e. it’s boiling point. :beer:

For one mole of water (18 g) the ‘Heat capacity of water’ is 75.3 J (K mol)-1.
For water, the ‘heat of fusion’ is 6.0 kJ/mol;

...thus, by extrapolating these two pieces of intrinsic data it’s very clear that your heating method is much cheaper than your mates method,...mainly due to the reduced inverse temperature coefficient involved.

The ‘heating curve of water’ was discovered by Archimedes whilst he was sat in his bath pondering the laws of physics and scratching his bollocks,..no doubt he was trying to settle a similar argument with his bezzie mate, Aristotle;

... and that’s when Archimedes had his famous “Eureka!” moment.