4 ms·

Out of curiosity, why would the water form layers of different temperatures?

Gibbon1 6y ago
I think because with a modern water heater especially electric models the heat loss is so low convection often stops completely. So the water in the tank doesn't mix. I think you can get situations too where the water at the level of the heating elements gets hotter and hotter and the water away from it gets cold. My water heater strongly suggests adding a mixing valve for that reason.
- jodrellblank 6y ago
  > "water at the level of the heating elements gets hotter and hotter and the water away from it gets cold." Eh? How come that temperature differential doesn't cause convection? The colder water being more dense and sinking down, the warmer water being less dense and rising up. If the heating elements aren't at the bottom, specifically for this reason, how come they aren't?
  - dmurray 6y ago
    > If the heating elements aren't at the bottom, specifically for this reason, how come they aren't? There are typically two heating elements, one near the bottom and one further up. You can use the higher one if you only need a half-tank of hot water. I'm not sure if this is the complete answer to your other question, or if sometimes conditions are such that convection occurs extremely slowly despite a substantial temperature difference.
    - Gibbon1 6y ago
      I think perversely it's not the density profile in the vertical dimension that drives convection but the horizontal dimension. A hotter thus lower density but perfectly flat homogeneous layer can't displace the colder denser layer above it because the pressure between the two layers is uniform.
      - jodrellblank 6y ago
        How can it become a perfectly flat homogeneous layer, from a heating element which looks like this[1]? It's your claim "the water at the level of the heating elements gets hotter and hotter and the water away from it gets cold" which is surprising me; water being quite a good conductor of heat, and "efficient boiler" suggesting that the tank will be insulated so not much heat is lost to the outside world, so the water away from the element should warm up by conduction from the other water faster than it gets cold from losing heat through the insulation (in my head). OK maybe I can imagine that in a fixed pressure with no room for expansion, the warm water cannot be less dense, so there can't be much convection - but then won't the heat radiate and conduct outwards in a "sphere" from the heating element including up and down, and not make any layers? And here[2] is a video of Thunderf00t putting an infra-red thermal camera on a closed water bottle with a peltier cooler attatched to the side of it, and showing that the cold water does sink, there is some convection happening with a cold "heating element" halfway up the "tank" on one side. (But there is compressible air in the top). Maybe it's that you don't want to run a boiler long enough for all the water to get to a uniform temperature before you can get warm water out of it, and if you don't then you risk getting super hot and cold water unmixed from "impatience" more than anything else? [1] https://ae01.alicdn.com/kf/HTB1_U_RSpXXXXbYapXXq6xXFXXXy/DC-600W-24V-Electric-Solar-Element-Boiler-Water-Heating-Element-with-1-1-4-BSP-Thread.jpg https://ae01.alicdn.com/kf/HTB1_U_RSpXXXXbYapXXq6xXFXXXy/DC-... [2] https://youtu.be/p9i1mhNsYXQ?t=903 https://youtu.be/p9i1mhNsYXQ?t=903
        Gibbon1 6y ago
        > but then won't the heat radiate and conduct outwards in a "sphere" from the heating element including up and down, and not make any layers? Suspicion of mine is part of the answer is that the viscosity of water drops with temperature. Rising hot water in a warm layer hits the cold layer above it and spreads out then turns down. Also water is a middle of the road non metallic solid when it comes to heat conduction. But it has high heat capacity and is a liquid. Complicating things stratification at least in a well designed water heater happens rarely. So it's easy enough to set up conditions where it doesn't happen. But of course it's a known problem. And not a totally solved one either. Suspect that simplified models probably don't display the behavior.

I think because with a modern water heater especially electric models the heat loss is so low convection often stops completely. So the water in the tank doesn't mix. I think you can get situations too where the water at the level of the heating elements gets hotter and hotter and the water away from it gets cold. My water heater strongly suggests adding a mixing valve for that reason.

jodrellblank 6y ago
> "water at the level of the heating elements gets hotter and hotter and the water away from it gets cold." Eh? How come that temperature differential doesn't cause convection? The colder water being more dense and sinking down, the warmer water being less dense and rising up. If the heating elements aren't at the bottom, specifically for this reason, how come they aren't?
- dmurray 6y ago
  > If the heating elements aren't at the bottom, specifically for this reason, how come they aren't? There are typically two heating elements, one near the bottom and one further up. You can use the higher one if you only need a half-tank of hot water. I'm not sure if this is the complete answer to your other question, or if sometimes conditions are such that convection occurs extremely slowly despite a substantial temperature difference.
  - Gibbon1 6y ago
    I think perversely it's not the density profile in the vertical dimension that drives convection but the horizontal dimension. A hotter thus lower density but perfectly flat homogeneous layer can't displace the colder denser layer above it because the pressure between the two layers is uniform.
    - jodrellblank 6y ago
      How can it become a perfectly flat homogeneous layer, from a heating element which looks like this[1]? It's your claim "the water at the level of the heating elements gets hotter and hotter and the water away from it gets cold" which is surprising me; water being quite a good conductor of heat, and "efficient boiler" suggesting that the tank will be insulated so not much heat is lost to the outside world, so the water away from the element should warm up by conduction from the other water faster than it gets cold from losing heat through the insulation (in my head). OK maybe I can imagine that in a fixed pressure with no room for expansion, the warm water cannot be less dense, so there can't be much convection - but then won't the heat radiate and conduct outwards in a "sphere" from the heating element including up and down, and not make any layers? And here[2] is a video of Thunderf00t putting an infra-red thermal camera on a closed water bottle with a peltier cooler attatched to the side of it, and showing that the cold water does sink, there is some convection happening with a cold "heating element" halfway up the "tank" on one side. (But there is compressible air in the top). Maybe it's that you don't want to run a boiler long enough for all the water to get to a uniform temperature before you can get warm water out of it, and if you don't then you risk getting super hot and cold water unmixed from "impatience" more than anything else? [1] https://ae01.alicdn.com/kf/HTB1_U_RSpXXXXbYapXXq6xXFXXXy/DC-600W-24V-Electric-Solar-Element-Boiler-Water-Heating-Element-with-1-1-4-BSP-Thread.jpg https://ae01.alicdn.com/kf/HTB1_U_RSpXXXXbYapXXq6xXFXXXy/DC-... [2] https://youtu.be/p9i1mhNsYXQ?t=903 https://youtu.be/p9i1mhNsYXQ?t=903
      - Gibbon1 6y ago
        > but then won't the heat radiate and conduct outwards in a "sphere" from the heating element including up and down, and not make any layers? Suspicion of mine is part of the answer is that the viscosity of water drops with temperature. Rising hot water in a warm layer hits the cold layer above it and spreads out then turns down. Also water is a middle of the road non metallic solid when it comes to heat conduction. But it has high heat capacity and is a liquid. Complicating things stratification at least in a well designed water heater happens rarely. So it's easy enough to set up conditions where it doesn't happen. But of course it's a known problem. And not a totally solved one either. Suspect that simplified models probably don't display the behavior.