Cost of running dishwasher

[Munroast]

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.

seems a little worse than mine but certainly same ball park

it is certainly not just heating the copper that accounts for the loss
I think we are all agreed copper heats up instantly ( or at least Pretty Dam Fast)
4.6m copper pipe would weigh 1.2kg and hold 0.67 litres
0.67 litres of water at 55° would only loose 6° of temperature in heating up 1.2kg of copper at 15° (water & pipe would be 49°)

and yet it takes 3 litres to get up to heat - in an ideal world it should take less than half of that - is Arthur c Clarke about ?

 

[morqthana]

We need somebody with access to a pipe run to lag it, and see if it makes a difference

 

[JohnW2]

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.

If that is roughly true, and my estimate of my pipe lengths correct, then mine would amount to less than 1.5 L. You estimate yours as about 2 L, and then go on to write ...

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.

I've just repeated the experiment I did recently, with much the same results - again, around 4.5 L drawn before I notice any rise in temp at all, over 5 L before it gets 'quite warm' and over 6 L before it is too hot to touch. This is obviously pretty different from your experience.

I may possibly have appreciably underestimated the length of my pipe run, so I'll get my tape measure out and will report back.

However, about three-quarters of your 'contained water' is in your 18 feet of insulated 22mm pipe, whereas none of my pipe is insulated - so if (as I suspect), loss of heat to the surroundings is a pretty significant component of what's going on, that might go a good way to explaining the difference between your experiences and mine.

 

[JohnW2]

it is certainly not just heating the copper that accounts for the loss

I think we're all agreed about that. I imagine that we also all agree that there are essentially three factors determining how much water has to be drawn before water coming out of the other end of the pipe reaches its maximum('steady state') temp:

1... Displacement of the 'unheated' (potentially 'cold') water that had been sitting in the pipe.​

2... Rapid heating of the wall of the pipe in regions where the temp of water within it had started to rise​

3... Loss of heat to the surroundings due to conduction, convection and radiation from regions of the pipe where water temp had started to rise. This could also be pretty rapid in the case of unlagged/uninsulated copper pipe, since copper is an extremely good conductor.​

​

I think we are all agreed copper heats up instantly ( or at least Pretty Dam Fast)

Indeed. (but, as below, it does not take much heat to 'heat up' a relatively small mass of copper)

and yet it takes 3 litres to get up to heat - in an ideal world it should take less than half of that - is Arthur c Clarke about ?

Of the three components mentioned above, (1) is the most straightforward since, regardless of anything else (e.g.mixing within the pipe), all of the pre-existing water has to be 'replaced' with much higher temp water before the 'steady state' can be reached.

On reflection, I think that (2), per se, may well not be all that important. Although the heating of the copper pipe will be very rapid, there's not much of a mass of copper involved, so the total amount of heat required to raise its temp to that of the contained water will be pretty small, maybe even 'negligible' (in relation to gthe 'big picture') - so that (3) [loss of heat from pipe contents through the highly conductive copper] is probably far more significant than (2).

If that's the case, then the markedly different experiences of Sunray and myself might be at least partially explained by the fact that three-quarters of his 'in-pipe' water volume was in insulated pipes, whereas my pipes are all unlagged.

 

[JohnW2]

I may possibly have appreciably underestimated the length of my pipe run, so I'll get my tape measure out and will report back.

Not only did I get my tape measure out, but also my 'plans'. I installed most of the plumbing nearly 40 years ago, so my recollections are hazy and, needless to say, the plans I produced at the time (at least I produced some ) are far from perfect! However, it looks as if the path route is a lot more tortuous than I had thought, probably appreciably over 15m, rather than the 10m I previous estimated. That would obviously bring my experiences a bit close to yours. However ....

However, about three-quarters of your 'contained water' is in your 18 feet of insulated 22mm pipe, whereas none of my pipe is insulated - so if (as I suspect), loss of heat to the surroundings is a pretty significant component of what's going on, that might go a good way to explaining the difference between your experiences and mine.

As I've just written to Munroast, I'm coming to suspect that this may possibly be a very significant part of the reason why our experiences differ.

 

[JohnW2]

We need somebody with access to a pipe run to lag it, and see if it makes a difference

As I've been writing, I am coming to suspect that it might make a very significant difference, but I also suspect that, like me, most people will not have easy access to any substantial lengths of pipe runs!

 

[Harry Bloomfield]

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?

As I suggested, the copper pipe is quite thin, with little thermal capacity, yet what you get from the hot tap, after turning it on, is a lot of cold water, until the hot fairly gradually appears. The 'fairly gradually', for the most part, will be due to the turbulence and mixing of the pre-existing cold in the pipe, with the hot, rather than heat lost to the copper.

If you doubt it is due to mixing, then try the same trick where the pipes are all plastic, which are much less thermally conductive - the hot appearing at the tap, will still be quite a gradual process, not dissimilar to copper.

 

[Harry Bloomfield]

As I've been writing, I am coming to suspect that it might make a very significant difference, but I also suspect that, like me, most people will not have easy access to any substantial lengths of pipe runs!

I am sure you would be wasting your time, and money, adding insulation. The insulation would only have value, if you draw hot water, then might need to soon draw some more. If the time between is longer, then despite the insulation, the water in the pipe would cool, the insulation only delays the cooling.

 

[AndyPRK]

Having insulation can be annoying, as said.

You start drawing the water. It gets warm. So you put the plug in expecting some hot water to follow shortly after, however all you get is a sink full of luke warm water, and have to empty it, and start again.

 

[ericmark]

I had to try, 6 pints before it started to get warm, 8 pints before hot at the front kitchen sink. Not that it makes any difference as my dishwasher is cold fill.

 

[morqthana]

As I've been writing, I am coming to suspect that it might make a very significant difference, but I also suspect that, like me, most people will not have easy access to any substantial lengths of pipe runs!

Apart from a m or so where it's boxed in, I have access right now to the entire 7-8m run of 15mm from combi to kitchen tap.

Unfortunately I don't think I have any lagging, and given the 1,000,001 jobs of actual importance I have to do, I wouldn't have the time even if I did have the materials

 

[JohnW2]

I am sure you would be wasting your time, and money, adding insulation.

I agree, and I have absolutely no intention of doing it.

The discussion has been about the possible impact of insulation with 'hot fill' machines, and I have (and presumably never will have) any of them.

The only 'advantage' of lagging the DHW pipework would be to avoid 'wasting energy' when hot water was used (by taps, showers etc.) during the Summer months, and that is likely to be fairly trivial - certainly not enough to justify the time, effort, cost and disruption that lagging the pipes would involve.

 

[JohnW2]

Having insulation can be annoying, as said.

I don't know about 'annoying' but, as I've just written, not really worth doing!

You start drawing the water. It gets warm. So you put the plug in expecting some hot water to follow shortly after, however all you get is a sink full of luke warm water, and have to empty it, and start again.

After an appreciable amount of water has been drawn, the water in the pipes will start off 'hot' and then will gradually cool and the speed of that cooling will depend upon the insulation. (depending ion prior usage) every 24 hours If there is pretty good insulation, that could be fairly slow, so that the 'warm' water one gets soon after turning on a tap might be adequate for one purpose for quite a while after water was last drawn.

Insulation can be pretty effective, as in the case of my cylinder. My hot water is usually heated (by E7 electricity) for just 1-3 hours (depending on prior usage) once every 24 hours and unless (unusually) large amounts of hot water has been drawn, it remains very hot by the time it is next heated, nearly 24h later.

 

[Harry Bloomfield]

Not my area of expertise, but I thought I had heard there were some sort of regulations or recommendations, with regard to how long the pipe was from the HW source, to the outlet? The idea was to prevent both energy, and water wastage. A distant memory, from when we had the house go through a major refurb, in the mid-80's.

 

[AndyPRK]

I guess new builds at the time put the heating device/storage in the middle of the building.

I think the regulation is that pipes should be lagged for 3m from the tank. (edit: my phone says 1m)

Hotels of course have a 'ring'. Where water is constantly pumped around the ring and back to the tank. Rooms are then feed off the ring, so it doesn't take too long to get hot water.