Like most houses in hot climates, the cold water that comes out if the tap in summer is hot for a while, until the hot water in the pipes is replaced by cold water from underground, etc. It’s obvious that the bare copper pipe that runs through the ceiling space soaks up the heat from the roof cavity.
I can’t figure out what happens with the hot water though. It’s insulated, like the vast majority of houses, to try to retain as much heat as possible in the pipe. But in my house, the water that comes out of the hot side of the tap is initially cold, then after the cold water is displaced it becomes hot.
I can’t figure out how a pipe full of hot water that’s insulated and located in a hot ceiling space can dump all its heat and become cold.

Any ideas?

Spiny Norman said:

Like most houses in hot climates, the cold water that comes out if the tap in summer is hot for a while, until the hot water in the pipes is replaced by cold water from underground, etc. It’s obvious that the bare copper pipe that runs through the ceiling space soaks up the heat from the roof cavity.
I can’t figure out what happens with the hot water though. It’s insulated, like the vast majority of houses, to try to retain as much heat as possible in the pipe. But in my house, the water that comes out of the hot side of the tap is initially cold, then after the cold water is displaced it becomes hot.
I can’t figure out how a pipe full of hot water that’s insulated and located in a hot ceiling space can dump all its heat and become cold.

Any ideas?

Heat conducted via the copper pipe to colder, less-insulated areas? It’d take time, but it’d happen.

Spiny Norman said:

Like most houses in hot climates, the cold water that comes out if the tap in summer is hot for a while, until the hot water in the pipes is replaced by cold water from underground, etc. It’s obvious that the bare copper pipe that runs through the ceiling space soaks up the heat from the roof cavity.
I can’t figure out what happens with the hot water though. It’s insulated, like the vast majority of houses, to try to retain as much heat as possible in the pipe. But in my house, the water that comes out of the hot side of the tap is initially cold, then after the cold water is displaced it becomes hot.
I can’t figure out how a pipe full of hot water that’s insulated and located in a hot ceiling space can dump all its heat and become cold.

Any ideas?

Either the insulations stops when it gets out of your view or the insulation doesn’t work.
The insulation on my hot water system only runs where it can be visible. Inside the walls it is uninsulated.

Spiny Norman said:

Like most houses in hot climates, the cold water that comes out if the tap in summer is hot for a while, until the hot water in the pipes is replaced by cold water from underground, etc. It’s obvious that the bare copper pipe that runs through the ceiling space soaks up the heat from the roof cavity.
I can’t figure out what happens with the hot water though. It’s insulated, like the vast majority of houses, to try to retain as much heat as possible in the pipe. But in my house, the water that comes out of the hot side of the tap is initially cold, then after the cold water is displaced it becomes hot.
I can’t figure out how a pipe full of hot water that’s insulated and located in a hot ceiling space can dump all its heat and become cold.

Any ideas?

Not really.

My hot water pipes are not insulated, but even so, it’s strange that the cooling hot water should be cooler than the heating cold water.

My only guess is that we notice the cold water from the hot tap in the morning, and notice the hot water from the cold tap in the late afternoon, so we aren’t comparing like with like.

The Rev Dodgson said:

Spiny Norman said:

Like most houses in hot climates, the cold water that comes out if the tap in summer is hot for a while, until the hot water in the pipes is replaced by cold water from underground, etc. It’s obvious that the bare copper pipe that runs through the ceiling space soaks up the heat from the roof cavity.
I can’t figure out what happens with the hot water though. It’s insulated, like the vast majority of houses, to try to retain as much heat as possible in the pipe. But in my house, the water that comes out of the hot side of the tap is initially cold, then after the cold water is displaced it becomes hot.
I can’t figure out how a pipe full of hot water that’s insulated and located in a hot ceiling space can dump all its heat and become cold.

Any ideas?

Not really.

My hot water pipes are not insulated, but even so, it’s strange that the cooling hot water should be cooler than the heating cold water.

My only guess is that we notice the cold water from the hot tap in the morning, and notice the hot water from the cold tap in the late afternoon, so we aren’t comparing like with like.

I dunno. I have my shower in the evenings and it still takes time for the hot water to come through hot.

Spiny Norman said:

Like most houses in hot climates, the cold water that comes out if the tap in summer is hot for a while, until the hot water in the pipes is replaced by cold water from underground, etc. It’s obvious that the bare copper pipe that runs through the ceiling space soaks up the heat from the roof cavity.
I can’t figure out what happens with the hot water though. It’s insulated, like the vast majority of houses, to try to retain as much heat as possible in the pipe. But in my house, the water that comes out of the hot side of the tap is initially cold, then after the cold water is displaced it becomes hot.
I can’t figure out how a pipe full of hot water that’s insulated and located in a hot ceiling space can dump all its heat and become cold.

Any ideas?

it’s not much water in whatever half inch copper pipe, or a long length of water with a small cross sectional area (comparatively the ratio of area it loses heat is high really), but you need give some idea of how quickly it loses its heat, and consider other possibilities, like the pressures on hot and cold are different or vary (differentials), non-return valves whether fitted or working properly, and the possibility some cold water is (from the cold circuit) venturing into the hot line

transition said:

Spiny Norman said:

Like most houses in hot climates, the cold water that comes out if the tap in summer is hot for a while, until the hot water in the pipes is replaced by cold water from underground, etc. It’s obvious that the bare copper pipe that runs through the ceiling space soaks up the heat from the roof cavity.
I can’t figure out what happens with the hot water though. It’s insulated, like the vast majority of houses, to try to retain as much heat as possible in the pipe. But in my house, the water that comes out of the hot side of the tap is initially cold, then after the cold water is displaced it becomes hot.
I can’t figure out how a pipe full of hot water that’s insulated and located in a hot ceiling space can dump all its heat and become cold.

Any ideas?

it’s not much water in whatever half inch copper pipe, or a long length of water with a small cross sectional area (comparatively the ratio of area it loses heat is high really), but you need give some idea of how quickly it loses its heat, and consider other possibilities, like the pressures on hot and cold are different or vary (differentials), non-return valves whether fitted or working properly, and the possibility some cold water is (from the cold circuit) venturing into the hot line

for example non-return valves can leak or not function (fail to close) it’s quite often a requirement to have two in series for this reason

pressure differentials though can put cold water back into hot line, like for example the kitchen taps or shower taps with both open, after the taps the mixer and outlets quite often have resistance/flow restriction, so it’s possible if a pressure differential existed between, say cold on mains water and hot on rainwater, that cold can find its way back into hot line

maybe

Spiny Norman said:

Like most houses in hot climates, the cold water that comes out if the tap in summer is hot for a while, until the hot water in the pipes is replaced by cold water from underground, etc. It’s obvious that the bare copper pipe that runs through the ceiling space soaks up the heat from the roof cavity.
I can’t figure out what happens with the hot water though. It’s insulated, like the vast majority of houses, to try to retain as much heat as possible in the pipe. But in my house, the water that comes out of the hot side of the tap is initially cold, then after the cold water is displaced it becomes hot.
I can’t figure out how a pipe full of hot water that’s insulated and located in a hot ceiling space can dump all its heat and become cold.

Any ideas?

What kind of insulation? Like foam tape?

dv said:

Spiny Norman said:

Like most houses in hot climates, the cold water that comes out if the tap in summer is hot for a while, until the hot water in the pipes is replaced by cold water from underground, etc. It’s obvious that the bare copper pipe that runs through the ceiling space soaks up the heat from the roof cavity.
I can’t figure out what happens with the hot water though. It’s insulated, like the vast majority of houses, to try to retain as much heat as possible in the pipe. But in my house, the water that comes out of the hot side of the tap is initially cold, then after the cold water is displaced it becomes hot.
I can’t figure out how a pipe full of hot water that’s insulated and located in a hot ceiling space can dump all its heat and become cold.

Any ideas?

What kind of insulation? Like foam tape?

I haven’t been up in the roof for several years but year it’s either a foam or whatever they used before foam insulation. The temperature gradient is going the wrong way, it’s perplexes me.

Cant answer your question but an old mate of mine told me many many years ago that if you want a cold drink on a hot day to drink out of the hot tap.

Could be faulty tempering valve (if fitted).

maybe the cold water is hot not from ceiling pipes sticking into sun, but from underground pipes not far under hot ground

In the absence of details about the insulation, I’ll just make some average kinds of assumptions to get us started.

I’ll assume then we have 12.7 mm copper tubes with 0.9 mm wall, which is pretty common in Australia.

I’m assuming the thin copper wall provides negligible insulation…

I’ll further assume the insulation is a layer of 13 mm of foamed nitrile rubber which is rated at R = 0.3 for a flat surface.

So because the tape is probably significantly thick compared to the pipe I’ll use the fancy radial formula which means the effective R referenced to the circumference of the pipe will be 0.17.

So a 1 metre length of the pipe for instance will suffer losses of

12.7e-3 * 2 * pi /0.17 = 0.469390902 W / K

meanwhile the water within this metre of pipe has a mass of around 0.109 kg. Water at room temperature has a specific heat of 4184 J/K/kg but this water will be a bit warmer and the specific heat will be a little lower, but only a couple of percent or so, let’s just use this number.

.109kg/((0.469390902 W / K) / (4184 J/K/kg)) = 16 minutes.

What this means is that at any given time, the amount of temperature loss that will occur in one minute will be about 4% of the temperature difference between the inside and the outside.

Like if the ambient temperature is 20 deg C and the water is at 70 deg C: then after a minute the water will be at 68 deg C.

It will decrease assymptotically (as the loss rate declines as the temperature gradient declines) but after an hour, the temperature of the water will not be significantly above ambient.

dv said:

In the absence of details about the insulation, I’ll just make some average kinds of assumptions to get us started.

I’ll assume then we have 12.7 mm copper tubes with 0.9 mm wall, which is pretty common in Australia.

I’m assuming the thin copper wall provides negligible insulation…

I’ll further assume the insulation is a layer of 13 mm of foamed nitrile rubber which is rated at R = 0.3 for a flat surface.

So because the tape is probably significantly thick compared to the pipe I’ll use the fancy radial formula which means the effective R referenced to the circumference of the pipe will be 0.17.

So a 1 metre length of the pipe for instance will suffer losses of

12.7e-3 * 2 * pi /0.17 = 0.469390902 W / K

meanwhile the water within this metre of pipe has a mass of around 0.109 kg. Water at room temperature has a specific heat of 4184 J/K/kg but this water will be a bit warmer and the specific heat will be a little lower, but only a couple of percent or so, let’s just use this number.

.109kg/((0.469390902 W / K) / (4184 J/K/kg)) = 16 minutes.

What this means is that at any given time, the amount of temperature loss that will occur in one minute will be about 4% of the temperature difference between the inside and the outside.

Like if the ambient temperature is 20 deg C and the water is at 70 deg C: then after a minute the water will be at 68 deg C.

It will decrease assymptotically (as the loss rate declines as the temperature gradient declines) but after an hour, the temperature of the water will not be significantly above ambient.

Sure, but the ambient temperature where the hot (and cold) water lives in in the mid 40’s I’d guess. The hot water should be around 55° so you’d expect it, over time, to cool down to ambient temps in the ceiling, but it ends up being closer to low 20’s. (I’d guess)

Spiny Norman said:

dv said:

In the absence of details about the insulation, I’ll just make some average kinds of assumptions to get us started.

I’ll assume then we have 12.7 mm copper tubes with 0.9 mm wall, which is pretty common in Australia.

I’m assuming the thin copper wall provides negligible insulation…

I’ll further assume the insulation is a layer of 13 mm of foamed nitrile rubber which is rated at R = 0.3 for a flat surface.

So because the tape is probably significantly thick compared to the pipe I’ll use the fancy radial formula which means the effective R referenced to the circumference of the pipe will be 0.17.

So a 1 metre length of the pipe for instance will suffer losses of

12.7e-3 * 2 * pi /0.17 = 0.469390902 W / K

meanwhile the water within this metre of pipe has a mass of around 0.109 kg. Water at room temperature has a specific heat of 4184 J/K/kg but this water will be a bit warmer and the specific heat will be a little lower, but only a couple of percent or so, let’s just use this number.

.109kg/((0.469390902 W / K) / (4184 J/K/kg)) = 16 minutes.

What this means is that at any given time, the amount of temperature loss that will occur in one minute will be about 4% of the temperature difference between the inside and the outside.

Like if the ambient temperature is 20 deg C and the water is at 70 deg C: then after a minute the water will be at 68 deg C.

It will decrease assymptotically (as the loss rate declines as the temperature gradient declines) but after an hour, the temperature of the water will not be significantly above ambient.

Sure, but the ambient temperature where the hot (and cold) water lives in in the mid 40’s I’d guess. The hot water should be around 55° so you’d expect it, over time, to cool down to ambient temps in the ceiling, but it ends up being closer to low 20’s. (I’d guess)

Is there some significant length of the pipes journey where it is not up in the hot roof cavity? For instance, when it is within the walls.

I mean I ain’t there but if I were I’d set up thermometers at say 3 metre intervals along the entire length of the pipe and record the temperatures over a 24 hour period at say 4 hour intervals and also measure the water temperature exactly but I guess another question is, how does the path of the hot water pipe differ from that of the cold? Does the cold pipe have a longer path in the hot part of the cavity etc.

dv said:

I mean I ain’t there but if I were I’d set up thermometers at say 3 metre intervals along the entire length of the pipe and record the temperatures over a 24 hour period at say 4 hour intervals and also measure the water temperature exactly but I guess another question is, how does the path of the hot water pipe differ from that of the cold? Does the cold pipe have a longer path in the hot part of the cavity etc.

Also, what kind of hot water system is it?

dv said:

Spiny Norman said:

dv said:

In the absence of details about the insulation, I’ll just make some average kinds of assumptions to get us started.

I’ll assume then we have 12.7 mm copper tubes with 0.9 mm wall, which is pretty common in Australia.

I’m assuming the thin copper wall provides negligible insulation…

I’ll further assume the insulation is a layer of 13 mm of foamed nitrile rubber which is rated at R = 0.3 for a flat surface.

So because the tape is probably significantly thick compared to the pipe I’ll use the fancy radial formula which means the effective R referenced to the circumference of the pipe will be 0.17.

So a 1 metre length of the pipe for instance will suffer losses of

12.7e-3 * 2 * pi /0.17 = 0.469390902 W / K

meanwhile the water within this metre of pipe has a mass of around 0.109 kg. Water at room temperature has a specific heat of 4184 J/K/kg but this water will be a bit warmer and the specific heat will be a little lower, but only a couple of percent or so, let’s just use this number.

.109kg/((0.469390902 W / K) / (4184 J/K/kg)) = 16 minutes.

What this means is that at any given time, the amount of temperature loss that will occur in one minute will be about 4% of the temperature difference between the inside and the outside.

Like if the ambient temperature is 20 deg C and the water is at 70 deg C: then after a minute the water will be at 68 deg C.

It will decrease assymptotically (as the loss rate declines as the temperature gradient declines) but after an hour, the temperature of the water will not be significantly above ambient.

Sure, but the ambient temperature where the hot (and cold) water lives in in the mid 40’s I’d guess. The hot water should be around 55° so you’d expect it, over time, to cool down to ambient temps in the ceiling, but it ends up being closer to low 20’s. (I’d guess)

Is there some significant length of the pipes journey where it is not up in the hot roof cavity? For instance, when it is within the walls.

I don’t know the street map of the water pipes, but I’d bet that the hot water pipes come straight out of the water heater unit into the wall, then up to the ceiling, along the ceiling (which branches off to the various taps on the way, then down inside the wall to the tap itself.
Now that I think about it, I was assuming that it was taking so long to get hot water out of the tap that it had to come from the water heater itself. The water pressure isn’t the best here, but I can’t imagine that the length of pipe in the (cool-ish) bathroom wall would have enough volume to explain why it takes so long for the hot water to arrive.
But, I can do a test!
I’ll measure the amount of water that comes out of the tap, until it becomes warm, then see if that volume is the same as what is held in the wall piping. I’m not convinced though, as the time taken for the water out of the cold tap to become cold (after the first little bit of cool water that becomes warm from the ambient temps in the ceiling) again is much shorter than the time taken for the hot water to arrive out of the hot tap.

furious said:

dv said:

I mean I ain’t there but if I were I’d set up thermometers at say 3 metre intervals along the entire length of the pipe and record the temperatures over a 24 hour period at say 4 hour intervals and also measure the water temperature exactly but I guess another question is, how does the path of the hot water pipe differ from that of the cold? Does the cold pipe have a longer path in the hot part of the cavity etc.

Also, what kind of hot water system is it?

Inside the house there’s always a swivel type of tap for hot/cold water. But good point about the cold water path …. though it still needs to go to the water heater unit early on, and there’s only the one (larger) plastic mains pipe coming into the house. AFAIK.

The water heater is a good old-fashioned electric unit. I believe you can’t buy them anymore, you have to get a solar-heated unit. It’s also got a small electrical pump that runs the water (I’m assuming on the entry pipe into the heater) through three large solar water panels on the roof.

Spiny Norman said:

Inside the house there’s always a swivel type of tap for hot/cold water. But good point about the cold water path …. though it still needs to go to the water heater unit early on, and there’s only the one (larger) plastic mains pipe coming into the house. AFAIK.

I’m now wondering if the mains feed comes into the house from the other end. There’s a tap (cold water only, on the outside for gardening) closest to the meter on the street that seems to have the most pressure. That makes me think that perhaps the water heater is fed by the cold water pipe, but at the far end of it.
I have to get up into the room in the next few weeks for unrelated work, I’ll see if I can get more info about it all then.
I hope you can withstand the enormous suspense.

Spiny Norman said:

Spiny Norman said:

Inside the house there’s always a swivel type of tap for hot/cold water. But good point about the cold water path …. though it still needs to go to the water heater unit early on, and there’s only the one (larger) plastic mains pipe coming into the house. AFAIK.

I’m now wondering if the mains feed comes into the house from the other end. There’s a tap (cold water only, on the outside for gardening) closest to the meter on the street that seems to have the most pressure. That makes me think that perhaps the water heater is fed by the cold water pipe, but at the far end of it.
I have to get up into the room in the next few weeks for unrelated work, I’ll see if I can get more info about it all then.
I hope you can withstand the enormous suspense.

Ah but what would life be without mystery?

> I can’t figure out how a pipe full of hot water that’s insulated and located in a hot ceiling space can dump all its heat and become cold.

> Any ideas?

The insulation isn’t that good. The ceiling space isn’t all that hot.
An insulated pipe loses heat in the roof space primarily by “free convection”.

In my house, the hot water pipe in the roof space loses about 1 degrees C per 5 minutes. So it drops from an initial 50 degrees C to 30 degrees C in an hour and a half

The region of insulated pipe between the roof space and the hot water service loses heat much faster, by a mixture of free and forced convection.

So in my house when showering I might get 2 minutes of warmish water from the roof space (a long time because I have a water saving nozzle), followed by a short period of freezing cold (15 degrees C) before the water heater water gets there.

Given that it takes 3 minutes overall to get the water temperature right, a 5 minute shower makes no sense at all.