why's my shower doing that?
January 14, 2019 10:26 AM Subscribe
I've got a multi-spray shower head with a 6 foot hose on my shower. When the flow of water is interrupted at the head for even a fraction of a second, the temperature of the water increases significantly. What's the physics behind this?
I put one of those extendable shower heads on my shower to make it easier to bathe my dog. It's got about 8 different spray modes, most of which are pretty much indistinguishable from each other, except the massage mode and the almost-off mode. The latter comes in handy when washing the dog - I can turn off the flow when I put down the nozzle to lather up the beast. But whenever I do that, the water becomes much, much hotter for a few moments after I resume the flow. I've discovered that this happens even if the flow is interrupted for a fraction of a second, like when I'm taking a shower myself and I switch the spray mode and the flow is reduced for a moment while the dial is in between positions. About 2 seconds later (transit time for water from the faucet to the head, presumably), there's a burst of painfully hot water.
This happens both when I'm taking a hot shower (ratio of hot to cold water about 5 to 2, as measured by turns of the taps) and when the dog is having a lukewarm bath (ratio of hot to cold about 1 to 3). So even if the hot-cold mix responds nonlinearly to flow rate, it's not just that the higher pressure tap wins when the flow is reduced.
What's happening and how!?
I put one of those extendable shower heads on my shower to make it easier to bathe my dog. It's got about 8 different spray modes, most of which are pretty much indistinguishable from each other, except the massage mode and the almost-off mode. The latter comes in handy when washing the dog - I can turn off the flow when I put down the nozzle to lather up the beast. But whenever I do that, the water becomes much, much hotter for a few moments after I resume the flow. I've discovered that this happens even if the flow is interrupted for a fraction of a second, like when I'm taking a shower myself and I switch the spray mode and the flow is reduced for a moment while the dial is in between positions. About 2 seconds later (transit time for water from the faucet to the head, presumably), there's a burst of painfully hot water.
This happens both when I'm taking a hot shower (ratio of hot to cold water about 5 to 2, as measured by turns of the taps) and when the dog is having a lukewarm bath (ratio of hot to cold about 1 to 3). So even if the hot-cold mix responds nonlinearly to flow rate, it's not just that the higher pressure tap wins when the flow is reduced.
What's happening and how!?
Since you're shutting off the water flow at a point downstream from the mixing valve but leaving the mixer taps open, any difference in water pressure between the hot and cold supply lines will cause water to flow between them. In this case your hot water pressure is evidently higher than your cold water pressure, so hot water pushes through the mixer and back into the cold water supply line. Just as yuwtze suggested, when you open the valve on the shower head you get a mixture of expected hot water from the hot supply line and the unexpected hot water that's in the cold supply line. The longer you leave the shower head valve closed while the mixer is open, the longer the effect will last (to an extent). The "almost-off mode" on your shower head is intended to prevent this by allowing some water to dribble through, but it seems that your shower head doesn't allow enough through to compensate for your hot/cold pressure differential.
This happens both when I'm taking a hot shower (ratio of hot to cold water about 5 to 2, as measured by turns of the taps) and when the dog is having a lukewarm bath (ratio of hot to cold about 1 to 3). So even if the hot-cold mix responds nonlinearly to flow rate, it's not just that the higher pressure tap wins when the flow is reduced.
More or fewer turns of the tap affects available volume from each side, and that in turn creates a change in pressure at the shower head when water is flowing freely, but when the flow is restricted at the shower head, then the precise setting of the taps is irrelevant; the difference between hot and cold pressure is likely to be small, so the flow rates from one side to the other are also so slow that the tap positions won't make a meaningful difference unless one or both are completely closed.
posted by jon1270 at 11:45 AM on January 14, 2019
This happens both when I'm taking a hot shower (ratio of hot to cold water about 5 to 2, as measured by turns of the taps) and when the dog is having a lukewarm bath (ratio of hot to cold about 1 to 3). So even if the hot-cold mix responds nonlinearly to flow rate, it's not just that the higher pressure tap wins when the flow is reduced.
More or fewer turns of the tap affects available volume from each side, and that in turn creates a change in pressure at the shower head when water is flowing freely, but when the flow is restricted at the shower head, then the precise setting of the taps is irrelevant; the difference between hot and cold pressure is likely to be small, so the flow rates from one side to the other are also so slow that the tap positions won't make a meaningful difference unless one or both are completely closed.
posted by jon1270 at 11:45 AM on January 14, 2019
Response by poster: the difference between hot and cold pressure is likely to be small, so the flow rates from one side to the other are also so slow that the tap positions won't make a meaningful difference unless one or both are completely closed.
That's interesting! Is there a simple way I could test or compare the inherent pressure of the hot vs cold water supplies to confirm that hot water is at a higher pressure? I would have guessed that the hot water pressure would be the same or lower because the hot water system is pressurized by the incoming cold water, like in a regular household hot water system, but that might not be the case. There is a very large boiler in the basement of this apartment building serving 10 apartments, so maybe it's pump-assisted.
(This is more of a science experiment for me than a problem to be solved, so I'm happy to try out things to find out about pressure that have absolutely no bearing on "fixing" the issue!)
posted by moonmilk at 12:06 PM on January 14, 2019 [1 favorite]
That's interesting! Is there a simple way I could test or compare the inherent pressure of the hot vs cold water supplies to confirm that hot water is at a higher pressure? I would have guessed that the hot water pressure would be the same or lower because the hot water system is pressurized by the incoming cold water, like in a regular household hot water system, but that might not be the case. There is a very large boiler in the basement of this apartment building serving 10 apartments, so maybe it's pump-assisted.
(This is more of a science experiment for me than a problem to be solved, so I'm happy to try out things to find out about pressure that have absolutely no bearing on "fixing" the issue!)
posted by moonmilk at 12:06 PM on January 14, 2019 [1 favorite]
Since you have more hot water flow through a more wide open valve, I think the Bernoulli effect will tend to suck water out of the cold pipe when the water is flowing, but when the flow is stopped and then started again, my guess is that orderly flow takes a few moments to reestablish itself, and during that time the Bernoulli effect is reduced or eliminated.
posted by jamjam at 1:03 PM on January 14, 2019
posted by jamjam at 1:03 PM on January 14, 2019
Response by poster: The trick we're trying to figure out is that the same effect occurs even when the cold valve is open wider!
posted by moonmilk at 1:16 PM on January 14, 2019
posted by moonmilk at 1:16 PM on January 14, 2019
I deleted a sentence about this from my first answer, but your observation would also be explained by a one-way valve on your hot water tank fill line, which would prevent the extra pressure generated by heating the water in the tank from communicating itself to the entire water system, but I don't know how common that is.
posted by jamjam at 1:56 PM on January 14, 2019
posted by jamjam at 1:56 PM on January 14, 2019
I believe the use of check valves ("one-way valve", above) in water heater pipe arrangements is mandated in the code.
posted by notsnot at 2:41 PM on January 14, 2019
posted by notsnot at 2:41 PM on January 14, 2019
Pressure-balancing valves are required on showers by some building codes. It sounds like you either don't have one, or have a defective one.
posted by Kirth Gerson at 4:40 AM on January 15, 2019 [1 favorite]
posted by Kirth Gerson at 4:40 AM on January 15, 2019 [1 favorite]
This thread is closed to new comments.
My parents' house has this issue (though from your description, it's nowhere as severe as yours is), so I've been looking into this. There exist pressure-balancing shower valves that are designed to prevent scalding when the cold water pressure drops (say someone flushes a toilet elsewhere in the house), but I don't know if they would prevent this issue or not. I'd also be interested if someone knows how to fix this.
posted by yuwtze at 10:57 AM on January 14, 2019