Cost of running dishwasher

[JohnW2]

I am sure with a modern house the pipes are all lagged?

Is that the case? I thought that DHW and CH pipes totally within a house were still generally not lagged, on the basis that, at least during winter, heat lost from pipes was generally not 'wasted'?

 

[Munroast]

Is that the case? I thought that DHW and CH pipes totally within a house were still generally not lagged, on the basis that, at least during winter, heat lost from pipes was generally not 'wasted'?

and that argument can mostly be applied to DHW pipes too, or at least it can for 8 months of the year, any heat loss through the pipes stays within the fabric of the house, which will mean your CH will need to work a little less (a tiny unmeasurable amount less)

 

[JohnW2]

and that argument can mostly be applied to DHW pipes too, or at least it can for 8 months of the year

That's essentially what I wrote ("... DHWE and CH pipes ...") ... so I obviously agree with you!

 

[Munroast]

That's essentially what I wrote ("... DHWE and CH pipes ...") ... so I obviously agree with you!

indeed you did - at first I did not get DHW, so I thought i would mention hot Water pipes, and then I guess D was for Domestic ? so I thought I would use that too, completely missing the point as to why I was posting
and I haven’t even started drinking yet

I still think it is taking a disproportional amount of hot water to heat the copper pipes up - difficult to calculate, probably need speed of water and how efficient transfer of heat would be - very complex

 

[JohnW2]

indeed you did - at first I did not get DHW, so I thought i would mention hot Water pipes, and then I guess D was for Domestic ? so I thought I would use that too, completely missing the point as to why I was posting ... and I haven’t even started drinking yet

Fair enough

I still think it is taking a disproportional amount of hot water to heat the copper pipes up - difficult to calculate, probably need speed of water and how efficient transfer of heat would be - very complex

That's also something which I've also written above - the 4.5 litres I have to draw from my kitchen hot tap before any noticeable rise in temp is detectable is far more than the volume of water contained in the pipe run - so a lot of the delay in hot water arriving at the tap must be due to initial heating of the cold pipes. I doubt that varying the flow rate would make a lot of difference to this, since roughly the same amount of heat would be required to bring the pipes up to HW temp whether that happened slow or rapidly - although, certainly if taken to extremes, slower flow would presumably mean that more heat would be lost from the pipes (as well as used to heat up the pipes)

 

[Munroast]

Fair enough

That's also something which I've also written above - the 4.5 litres I have to draw from my kitchen hot tap before any noticeable rise in temp is detectable is far more than the volume of water contained in the pipe run - so a lot of the delay in hot water arriving at the tap must be due to initial heating of the cold pipes. I doubt that varying the flow rate would make a lot of difference to this, since roughly the same amount of heat would be required to bring the pipes up to HW temp whether that happened slow or rapidly - although, certainly if taken to extremes, slower flow would presumably mean that more heat would be lost from the pipes (as well as used to heat up the pipes)

yes but I need an equation - the heat must be lost to the copper, would just like to do the maths

 

[Harry Bloomfield]

although, certainly if taken to extremes, slower flow would presumably mean that more heat would be lost from the pipes (as well as used to heat up the pipes)

Copper is an excellent conductor of heat, and pipes are quite thin-walled - I would expect them to match the temperature of the pipe contents, in a fraction of a second.

 

[JohnW2]

yes but I need an equation - the heat must be lost to the copper, would just like to do the maths

The relevant equations are straightforward enough but, as you've said yourself, there are so many (largely unknown) factors that would need to be fed into them, making it an extremely difficult exercise.

However, I'm not sure what the calculations would prove (that we don't already know) even if they could be done reasonably accurately. During the appreciable period of time/flow (at least, for me) that there is no perceptible increase in water temp at the end of the pipe the heat that was in the water when it left the cylinder can only have 'gone' to a mixture of two places - heating the upstream parts of the pipe, and loss by radiation/convection/conduction from that heated pipe into the surroundings.

 

[JohnW2]

Copper is an excellent conductor of heat, and pipes are quite thin-walled - I would expect them to match the temperature of the pipe contents, in a fraction of a second.

Yes, certainly pretty quickly. Since it takes quite a long time (and flow) for any heated water to appear at the end of the pipe (at least, for me), it presumably must be the case that much of the heat which initially increases the temp of the most upstream parts of the pipe is fairly rapidly lost to the surroundings, such that the boundary of the heated contents and pipe (at similar temps) only 'creeps' down the length of the pipe fairly slowly?

 

[Harry Bloomfield]

Yes, certainly pretty quickly. Since it takes quite a long time (and flow) for any heated water to appear at the end of the pipe (at least, for me), it presumably must be the case that much of the heat which initially increases the temp of the most upstream parts of the pipe is fairly rapidly lost to the surroundings, such that the boundary of the heated contents and pipe (at similar temps) only 'creeps' down the length of the pipe fairly slowly?

I doubt much of the temperature of the heated water, is immediately lost to the surrounds of the pipe, whilst the water is being drawn, but obviously, all the static heated water remaining in the pipe, will eventually cool to ambient. When drawing water, the cold will not be pushed out, as a 'solid block', there will be some mixing of the fresh, with the existing, due to turbulence, more so in a relatively larger pipe, compared to the flow.

 

[JohnW2]

.

 

[JohnW2]

I doubt much of the temperature of the heated water, is immediately lost to the surrounds of the pipe, whilst the water is being drawn ...

Where are you suggesting that the heat goes, then? As I've said, in my case it takes quite a long time, and about 4.5 litres of flow, before any rise in temp at the end becomes noticeable.

When drawing water, the cold will not be pushed out, as a 'solid block', there will be some mixing of the fresh, with the existing, due to turbulence, more so in a relatively larger pipe, compared to the flow.

Theoretically (qualitatively) true, but I doubt that it will be much different from 'the cold water being pushed out as a solid block' in the case of a long run (about 10m) of 15mm pipe, will it?

 

[Munroast]

The relevant equations are straightforward enough but,

let us see it then ?

 

[SUNRAY]

Yes, I forgot to mention that. In my case it's about 12 metres of (mainly) 15mm pipe, but even that is still far less than the 4.5 litres I have to draw before its temp rises perceptibly. That presumably must mean that, in addition to the hot water that has to fill the pipe before hot comes out of other end, another (probably the major) factor is that the cold pipes have to be initially heated.

I haven't tried any calculations recently but I've always worked with 13ft of 15mm and 6ft of 22mm Cu pipe to a pint. I have roughly 12ft of 15mm (50:50 buried in wall: free air)-1 pint +10 ft of 22mm (insulated) and 8 ft of 22mm triwall PVC (insulated and enclosed in glass fibre insulation) - <3 pints. Total <4 pints or let's say >2L.
I'll say filling a 2,5L pan the water is running 'quite warm' but it requires about another 2L to run too hot to touch.

 

[morqthana]

Get some logging pipe thermostats, record temperature over time at various points, and plot some graphs.

We loves a good graph.