r/engineering • u/NuSurfer • 18d ago
Hot bathtubs but cool showers - all thinkers welcome. [GENERAL]
I've already posted this in r/ plumbing, I'm an engineer, and this appears to be an engineering problem.
Strange problem for my landlords. The original water heater that came with their house would supply hot water to the shower/tubs (showers are on the walls above the bathtubs - standard) in all three bathrooms in the house. They replaced the water heater. Now, the bathtubs get hot water, but when switched to shower, the showers start hot but within ten seconds become cool. Every shower in the house has the same problem. The landlords replaced the water heater again, but the same problem exists. And here's the kicker - a neighbor across the street with a similarly built house came to have the same problem with the showers.
What do you folks think? To me, it says that a water heater with unknown water flow characteristics is needed in order for the showers to be able to produce hot showers.
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u/humdinger44 18d ago
Probably an issue with the mixer valve
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u/Vogonfestival 18d ago
or r/askaplumber, very helpful folks over there.
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u/NuSurfer 17d ago
Thank you for taking the time to respond. I have forwarded this comment section to my landlords.
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u/NuSurfer 17d ago
Thank you for taking the time to respond. I have forwarded this comment section to my landlords.
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u/MaximumStoke 18d ago
My tankless heater shuts off if the flow is too low. So I would get cold shower water unless I was running a hot faucet somewhere else in the house.
Not a problem with classic tanks, though.
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u/NuSurfer 17d ago
Thank you for taking the time to respond. I have forwarded this comment section to my landlords.
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u/Engibineer P.E., Mechanical 14d ago
I had a similar problem in my previous apartment. I solved it myself by replacing the showerhead with a higher flow rate model.
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u/LateralThinkerer 18d ago edited 18d ago
If it's an "on demand" water heater - turns on with water flow - it may demand a minimum flow rate to operate. Your tub tab provides level of flow for this, but a high-efficiency shower head may not.
In a previous house we installed one and had to get used to the idea that running a little extra water was a good thing if you didn't want a slug of cold water hitting you all at once.
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u/NuSurfer 17d ago
Thank you for taking the time to respond. I have forwarded this comment section to my landlords.
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u/tennismenace3 18d ago
My guess is the new water heater is tankless, and it turns off if the flow through it is too low. The tub faucet flows a lot more than the shower.
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u/NuSurfer 17d ago
Thank you for taking the time to respond. I have forwarded this comment section to my landlords.
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u/PicnicBasketPirate 18d ago
What kind of shower? What kind of water heater?
Sounds like a mixer shower with a dodgy control loop, if the shower is getting hot water but then goes cool....
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u/NuSurfer 17d ago
Thank you for taking the time to respond. I have forwarded this comment section to my landlords.
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u/mrboomx 18d ago
bad thermostatic mixing valve, mixing valve is sometimes only in the shower head piping and not the tub.
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u/NuSurfer 17d ago
Thank you for taking the time to respond. I have forwarded this comment section to my landlords.
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u/mrpokehontas 18d ago
Does it seem like the hot water could be coming out at a lower pressure? Are all the bathtubs on the same floor/level?
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u/NuSurfer 17d ago
Thank you for taking the time to respond. I have forwarded this comment section to my landlords.
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u/hhuggles31 18d ago
I am also thinking a pressure issue with the shower taps. Big question is are shower taps newer? I had temperature fluctuations at a tub/ shower after I changed the shower head to something other than the one that came with the set. Otherwise regardless of either shower or tub , it would maintain temperature even if the toilet was flushed. Yes..I tried!
So my solution would be to switch a shower head and see if there is improvement.
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u/NuSurfer 17d ago
Thank you for taking the time to respond. I have forwarded this comment section to my landlords.
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u/eperb12 18d ago
If its the diverter valve, see if setting the temperature to just barely on to see if its hot.
For our shower, that was the sign it was failing. It oddly flips before it fails completely and only does cold water.
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u/NuSurfer 17d ago
Thank you for taking the time to respond. I have forwarded this comment section to my landlords.
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u/FreddyFerdiland 18d ago
Maybe the flow to the hot water is too constructed. What happens when you set it to "hottest water", which should be "hot water only"
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u/NuSurfer 17d ago
Thank you for taking the time to respond. I have forwarded this comment section to my landlords.
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u/Acrobatic_Show8919 18d ago
this is definitely not an engineering problem, its a call a plumber and have them replace the part problem. jeez
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u/aj_redgum_woodguy 18d ago
Assuming they're using those taps that have a diverter type mechanism. some of these use a pressure mechanism to switch.
I'm guessing (and this is total guess) ... by changing the water heater, you've now got different (less or more ) pressure at the tap, causing something to not work correctly with the diverter. Try throttling the pressure down slightly in the hot water line - then the cold line see if the different pressures help the operation of that valve.