Some months ago I posted here a question as to how the hot & cold water in this house, during summer, seemed to swap over. A few qualifying questions were asked, there was a trip into the roof required to see what the water plumbing looked like, and I did that recently. Also mentioned was a quick check to measure the flow rate but I haven’t done that yet sorry.

Fair enough the cold water coming out hot for a while as the unshielded copper pipe in the ceiling soaks up the heat in the roof, then after running the cold tap for a minute it starts flowing cool again as the heated water is replaced by fresh cool water from below the ceiling.
But the hot water is still a mystery. Initially it runs cool, as you’d expect from the short length of pipe that runs between the ceiling and the tap, but it stays cool for another minute or so until the hot water from the big water heater down the far end of the house arrives.
One suggestion was that the water line into the house comes up into the ceiling from the end of the house closest to the street, but as far as I can tell it doesn’t, it enters the house close to the water heater at the far end.

Here’s a photo I took inside the of the ceiling, the end closest to the street, and as best I can tell that’s as far as the water pipes run. You can see the green-ish insulation that the hot water copper pipe has, it much be responsible for somehow dumping the heat of the hot water into the ceiling space and so creating cool water in that pipe. I haven’t formally studied thermodynamics but I have a reasonable basic understanding of heat flow and I cannot work out what’s going on here.

How does the hot water pipe dump its heat into the rather toasty ceiling space in summer, thus making cold water?

Spiny Norman said:

How does the hot water pipe dump its heat into the rather toasty ceiling space in summer, thus making cold water?

My understanding is that the hot water pipes are insulated, so once the water has cooled down in the pipes overnight, for example, they can remain cooler than the cold water pipes once the ceiling space warms up.

Speedy said:

Spiny Norman said:

How does the hot water pipe dump its heat into the rather toasty ceiling space in summer, thus making cold water?

My understanding is that the hot water pipes are insulated, so once the water has cooled down in the pipes overnight, for example, they can remain cooler than the cold water pipes once the ceiling space warms up.

Interesting …. but the hot water seems to still be cool for a while even in the afternoon, after not being used for a few hours.

Spiny Norman said:

Speedy said:

Spiny Norman said:

How does the hot water pipe dump its heat into the rather toasty ceiling space in summer, thus making cold water?

My understanding is that the hot water pipes are insulated, so once the water has cooled down in the pipes overnight, for example, they can remain cooler than the cold water pipes once the ceiling space warms up.

Interesting …. but the hot water seems to still be cool for a while even in the afternoon, after not being used for a few hours.

Do you wash clothes in hot, cold or warm water?

Spiny Norman said:

Speedy said:

Spiny Norman said:

How does the hot water pipe dump its heat into the rather toasty ceiling space in summer, thus making cold water?

My understanding is that the hot water pipes are insulated, so once the water has cooled down in the pipes overnight, for example, they can remain cooler than the cold water pipes once the ceiling space warms up.

Interesting …. but the hot water seems to still be cool for a while even in the afternoon, after not being used for a few hours.

How long is “a while”? The time it takes for the water in the section of pipe in the wall cavity to be used?

Michael V said:

Spiny Norman said:

Speedy said:

My understanding is that the hot water pipes are insulated, so once the water has cooled down in the pipes overnight, for example, they can remain cooler than the cold water pipes once the ceiling space warms up.

Interesting …. but the hot water seems to still be cool for a while even in the afternoon, after not being used for a few hours.

Do you wash clothes in hot, cold or warm water?

Cold.

Dark Orange said:

Spiny Norman said:

Speedy said:

My understanding is that the hot water pipes are insulated, so once the water has cooled down in the pipes overnight, for example, they can remain cooler than the cold water pipes once the ceiling space warms up.

Interesting …. but the hot water seems to still be cool for a while even in the afternoon, after not being used for a few hours.

How long is “a while”? The time it takes for the water in the section of pipe in the wall cavity to be used?

Well beyond that.

Spiny Norman said:

Dark Orange said:

Spiny Norman said:

Interesting …. but the hot water seems to still be cool for a while even in the afternoon, after not being used for a few hours.

How long is “a while”? The time it takes for the water in the section of pipe in the wall cavity to be used?

Well beyond that.

The water pipe has a volume per length, you should be quantifying this by volume rather than time.

Dark Orange said:

Spiny Norman said:

Dark Orange said:

How long is “a while”? The time it takes for the water in the section of pipe in the wall cavity to be used?

Well beyond that.

The water pipe has a volume per length, you should be quantifying this by volume rather than time.

Yeah I know, but it’s still far more than the volume of pipe just from the ceiling down to the taps. Same as how long it takes for the cold water to become cold, after getting rid of the hot water.

Lets get some figures, Bill,

Length of pipe from heater to tap.
Diameter of pipe.
Flow rate.
Time till hot water.

sibeen said:

Lets get some figures, Bill,

Length of pipe from heater to tap.
Diameter of pipe.
Flow rate.
Time till hot water.

Initial temperature of water.
Final temperature of water.

The pipe looks like it is lying on your roof insulation and therefore the thermal between the pipe and the insulation is going to be far lower than the thermal resistance between the pipe and the toasty air.

sibeen said:

sibeen said:

Lets get some figures, Bill,

Length of pipe from heater to tap.
Diameter of pipe.
Flow rate.
Time till hot water.

Initial temperature of water.
Final temperature of water.

The pipe looks like it is lying on your roof insulation and therefore the thermal between the pipe and the insulation is going to be far lower than the thermal resistance between the pipe and the toasty air.

Add another “resistance” into the above sentence ^

sibeen said:

sibeen said:

sibeen said:

Lets get some figures, Bill,

Length of pipe from heater to tap.
Diameter of pipe.
Flow rate.
Time till hot water.

Initial temperature of water.
Final temperature of water.

The pipe looks like it is lying on your roof insulation and therefore the thermal between the pipe and the insulation is going to be far lower than the thermal resistance between the pipe and the toasty air.

Add another “resistance” into the above sentence ^

It’s useless.

sibeen said:

sibeen said:

sibeen said:

Lets get some figures, Bill,

Length of pipe from heater to tap.
Diameter of pipe.
Flow rate.
Time till hot water.

Initial temperature of water.
Final temperature of water.

The pipe looks like it is lying on your roof insulation and therefore the thermal between the pipe and the insulation is going to be far lower than the thermal resistance between the pipe and the toasty air.

Add another “resistance” into the above sentence ^

Righto, I’ll get those numbers some time this week.

FWIW according to this page the ID of our pipes is 10.88 mm.

Spiny Norman said:

Michael V said:

Spiny Norman said:

Interesting …. but the hot water seems to still be cool for a while even in the afternoon, after not being used for a few hours.

Do you wash clothes in hot, cold or warm water?

Cold.

OK.

Is the hot tap to the washing machine turned off (or do you use the wash-cycle to do that)?

Peak Warming Man said:

sibeen said:

sibeen said:

Initial temperature of water.
Final temperature of water.

The pipe looks like it is lying on your roof insulation and therefore the thermal between the pipe and the insulation is going to be far lower than the thermal resistance between the pipe and the toasty air.

Add another “resistance” into the above sentence ^

It’s useless.

LOLOLOL

:)

And FWIW this is my house – the red circle is where the hot water unit is located, the yellow line is roughly the path taken by the plumbing, though it might be a little shorter. To be sure I’d have to go up into the roof again and it’s build for people much smaller than me so I’m not too keen on that. So it might be maybe 2 or 3 metres shorter?
The furthest point of the line is our bathroom, which is also the furthest that the plumbing has to travel.

I think that the water line from the street comes straight to the left-hand corner of the house as the water pressure in the tap at that corner is noticeably better than all the other taps. Then it must travel parallel to the edge of the house, down in the garage area. When I was working overseas and I came back after a tour, dad told me that the line had sprung a leak under the driveway and he had to get a new one fitted. There’s small holes drilled into the concrete driveway where the plumber people searched for the leak.
The water line joins the street line at around about under the ‘save’ part of the path window. The pressure overall is pretty crappy as we’re about 71 metres above sea level at the top of a hill. We did have a booster pump that’s connected to a 5,000 litre tank, but the pump died and for various reasons we’ve stuck to mains pressure ever since.
(And the water bills have been lower as well, which is a good thing so I’m kinda reluctant to fit a new pump)

Michael V said:

Spiny Norman said:

Michael V said:

Do you wash clothes in hot, cold or warm water?

Cold.

OK.

Is the hot tap to the washing machine turned off (or do you use the wash-cycle to do that)?

It’s not connected, hence off for sure.

Spiny Norman said:

Michael V said:

Spiny Norman said:

Cold.

OK.

Is the hot tap to the washing machine turned off (or do you use the wash-cycle to do that)?

It’s not connected, hence off for sure.

Well, it’s not flow-back from that fixture then.

Another question:

Has it always been like this?

If not, when did it start?

Michael V said:

Spiny Norman said:

Michael V said:

OK.

Is the hot tap to the washing machine turned off (or do you use the wash-cycle to do that)?

It’s not connected, hence off for sure.

Well, it’s not flow-back from that fixture then.

Another question:

Has it always been like this?

If not, when did it start?

From memory it’s always been like that. Well, mostly as when we fitted the solar panels to the roof we also removed the small booster water heater unit that lived near the street end of the house. That saved us a fair bit in electrical bills but the water one increased because we had to let the water run for longer to get hot. There was also another smaller heater in the ceiling above our bathroom, that was for the bidet but that’s also completely disconnected as well, both power and water.

Hmm, with the figures of 10.88 mm diameter and a length of 35 metres you’re only storing a bit over 3 litres in the pipe.

sibeen said:

Hmm, with the figures of 10.88 mm diameter and a length of 35 metres you’re only storing a bit over 3 litres in the pipe.

Sounds about right. I’ll fill up a one litre container in the bathroom and see how long it takes.

The thermal conductivity of copper is excellent, it’s great for losing heat, maybe that explains the cold hot water and the short delay in cold water when the pipers are hot.

poikilotherm said:

The thermal conductivity of copper is excellent, it’s great for losing heat, maybe that explains the cold hot water and the short delay in cold water when the pipers are hot.

Seems the most likely thing. Especially as it’s always been that way.

SN could lag all the pipes he can get to. Modern foam plastic pipe lagging is easy to apply and cheap. Like this. But in long rolls from a plumbing store. I think some varieties come pre-split.

https://www.bunnings.com.au/k-flex-9-x-13mm-x-1m-elastomeric-pipe-insulation-suits-1-2-pipe_p4920088

maybe everything is normal except the high temperature of the cold water in the cold line, which you presume is heated by the ambient temp in the ceiling space, when really you have a small leak (tens to hundreds millilitre range) on the cold line down toward the far end (furthest away from HWS), and the non-return valve on the input side of the HWS is slightly leaky, allows backflow when the HWS has added pressure from heating

it’s a bit early to be thinking, but are they solar hot water panels on your roof, right of the electric ones?

regard copper pipe, water flow to supply hot water, not only do you need replace the volume of water in the pipe, but also heat the copper pipe, so there’s a temperature transition period, while raising the pipe temperature

transition said:

maybe everything is normal except the high temperature of the cold water in the cold line, which you presume is heated by the ambient temp in the ceiling space, when really you have a small leak (tens to hundreds millilitre range) on the cold line down toward the far end (furthest away from HWS), and the non-return valve on the input side of the HWS is slightly leaky, allows backflow when the HWS has added pressure from heating

No leaks.

transition said:

it’s a bit early to be thinking, but are they solar hot water panels on your roof, right of the electric ones?

Yep.

transition said:

regard copper pipe, water flow to supply hot water, not only do you need replace the volume of water in the pipe, but also heat the copper pipe, so there’s a temperature transition period, while raising the pipe temperature

Yes, a bit.

Michael V said:

poikilotherm said:

The thermal conductivity of copper is excellent, it’s great for losing heat, maybe that explains the cold hot water and the short delay in cold water when the pipers are hot.

Seems the most likely thing. Especially as it’s always been that way.

SN could lag all the pipes he can get to. Modern foam plastic pipe lagging is easy to apply and cheap. Like this. But in long rolls from a plumbing store. I think some varieties come pre-split.

https://www.bunnings.com.au/k-flex-9-x-13mm-x-1m-elastomeric-pipe-insulation-suits-1-2-pipe_p4920088

I had thought of that as well, but I’d like to figure out what’s going on first.

unless you’ve inspected all the pipes, i’d just mention you can’t measure small leaks on your utility water meter, in the range tens of millilitres to low hundred of millilitres, the flow comes in under the Q-min flow registration threshold or whatever (it stops registering flow), and bigger the meter and/or more worn the higher the Q-min

there are ways of doing it though, with intentionally added flow above the Q-min you then fix the leak and by magic of subtraction you have the leak flow rate

you’ve got solar hot water, they pump the pressure in the system, in some States you require two series non-return valves on the HWS input, because of the possibility of one not shutting properly

I fnckin hate having to go up in the roof space.

Spiny Norman said:

And FWIW this is my house – the red circle is where the hot water unit is located, the yellow line is roughly the path taken by the plumbing, though it might be a little shorter. To be sure I’d have to go up into the roof again and it’s build for people much smaller than me so I’m not too keen on that. So it might be maybe 2 or 3 metres shorter?
The furthest point of the line is our bathroom, which is also the furthest that the plumbing has to travel.

I think that the water line from the street comes straight to the left-hand corner of the house as the water pressure in the tap at that corner is noticeably better than all the other taps. Then it must travel parallel to the edge of the house, down in the garage area. When I was working overseas and I came back after a tour, dad told me that the line had sprung a leak under the driveway and he had to get a new one fitted. There’s small holes drilled into the concrete driveway where the plumber people searched for the leak.
The water line joins the street line at around about under the ‘save’ part of the path window. The pressure overall is pretty crappy as we’re about 71 metres above sea level at the top of a hill. We did have a booster pump that’s connected to a 5,000 litre tank, but the pump died and for various reasons we’ve stuck to mains pressure ever since.
(And the water bills have been lower as well, which is a good thing so I’m kinda reluctant to fit a new pump)

I have to wait one to two minutes for hot water to warm up in the shower here, it goes cold before it goes hot, and my shower is much closer to the hot water service than yours is.

That said, I have had a hot water tank fail on me, and an underground water leak.

Speedy said:

Spiny Norman said:

How does the hot water pipe dump its heat into the rather toasty ceiling space in summer, thus making cold water?

My understanding is that the hot water pipes are insulated, so once the water has cooled down in the pipes overnight, for example, they can remain cooler than the cold water pipes once the ceiling space warms up.

It kind of reminds me of the reading I did about thermal energy for producing electricity , where the heat and cold exchange flips back and forth as long as there is a metal conductor that runs from one surface to the other. For example a plate on the roof heats up and the underneath is insulated but is connected via the conductor (heat transfer at some point the heat being cooled by the underside becomes the hot and then transfer heat energy back to the now cooler side (formerly the source of the heat energy). I don’t think I explained that well enough.

maybe the problem is in your ability to determine temperature accurately through your hands…

Arts said:

maybe the problem is in your ability to determine temperature accurately through your hands…

yeah, need some apparatus to accurately measure temperature.

Arts said:

maybe the problem is in your ability to determine temperature accurately through your hands…

That’s why midwives recommend using your elbow instead.

Bogsnorkler said:

Arts said:

maybe the problem is in your ability to determine temperature accurately through your hands…

yeah, need some apparatus to accurately measure temperature.

I think there are some about. Difficult to obtain, but nonetheless worth the effort. Thermo-Meters, IIRC.