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Step 24Appendix: Technical notes
Below are some optional technical notes, in case you are interested in learning more about why the backflow problem can happen, and about some more energy advantages of an enclosed shower.
What makes hot water backflow into the cold water pipe or vice versa? It makes sense that if
the hot water pressure is higher than the cold, it would go into the cold water pipe. But
why would the pressure be different?
In a closed system, like a hydronic heating system's hot water loop, increasing the temperature builds pressure as the water expands. But the hot water supplying the shower comes from the same pressure source as the cold water (city water or
a pump from a well), and both should be at the same pressure. At the water heater, they are at the same pressure, but if the shower is a floor or two above the water heater, the difference in elevation changes the water pressure, due to the weight of the water. The cold water density is higher than the hot water density, so its pressure is reduced more--the net result being that the hot water pressure is higher than the cold water pressure at the shower (if it's above the hot water heater).
The enclosed stall has many energy advantages. The simplest ones are that in a warm stall
you don't need the water as hot to stay warm, and that it's more comfortable to turn off the water flow if
the stall is warm.
The high humidity inside the stall is also an advantage: the water coming
out of the showerhead doesn't cool as fast by evaporation when
it's in such humid air, so you don't need to use as high a water temperature. And evaporative cooling doesn't just cool water--it cools wet people too, so the humidity helps keep you warm while your skin is wet.
Reducing the humidity released by the shower into the house is clearly an advantage in the summer when the air conditioning has to work hard to remove humidity. Many houses get too dry in the winter, in which case adding humdity might seem to be a good idea. But excessive dryness in the winter can be a sign that a house has many air leaks. It's better to fix those
leaks, following the excellent instructions
on the Energy Star web site]. You'll save heating energy as well as reducing dryness.
What makes hot water backflow into the cold water pipe or vice versa? It makes sense that if
the hot water pressure is higher than the cold, it would go into the cold water pipe. But
why would the pressure be different?
In a closed system, like a hydronic heating system's hot water loop, increasing the temperature builds pressure as the water expands. But the hot water supplying the shower comes from the same pressure source as the cold water (city water or
a pump from a well), and both should be at the same pressure. At the water heater, they are at the same pressure, but if the shower is a floor or two above the water heater, the difference in elevation changes the water pressure, due to the weight of the water. The cold water density is higher than the hot water density, so its pressure is reduced more--the net result being that the hot water pressure is higher than the cold water pressure at the shower (if it's above the hot water heater).
The enclosed stall has many energy advantages. The simplest ones are that in a warm stall
you don't need the water as hot to stay warm, and that it's more comfortable to turn off the water flow if
the stall is warm.
The high humidity inside the stall is also an advantage: the water coming
out of the showerhead doesn't cool as fast by evaporation when
it's in such humid air, so you don't need to use as high a water temperature. And evaporative cooling doesn't just cool water--it cools wet people too, so the humidity helps keep you warm while your skin is wet.
Reducing the humidity released by the shower into the house is clearly an advantage in the summer when the air conditioning has to work hard to remove humidity. Many houses get too dry in the winter, in which case adding humdity might seem to be a good idea. But excessive dryness in the winter can be a sign that a house has many air leaks. It's better to fix those
leaks, following the excellent instructions
on the Energy Star web site]. You'll save heating energy as well as reducing dryness.
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