Why is the water in my shower not staying hot?

[Ladychips]

Looking for advice from the brilliant forum.  In the last few months the water in my shower starts off hot but cools rather quickly.  By the time I'm done, I have it turned all the way hot and the water is lukewarm. It's affecting the quality of my life!!

• My house is 12 years old.
• We have a tankless hot water heater (so I'm not running out of hot water)
• I have plenty of hot water everywhere else.
• This is a fairly recent problem

Thoughts?

[daverobev]

So if you run the shower 10 minutes til it goes lukewarm, then go to the kitchen and run the hot tap there, the kitchen water is properly hot?

I know you can get automatic mixers for hot and cold - so that you never have to adjust the hot/cold, you just turn the water on and off and in theory the temperature stays the same - I wonder if it's something like that but it has broken, so when it gets hot it reduces the hot flow??

Apparently the word I'm looking for is 'thermostatic' which makes sense. Hmm, maybe this will help? https://plumbingpoints.com/thermostatic-mixing-valve-problems/

[GilesMM]

Another vote for the mixing valve, if you have one. They are made of mostly plastic parts which wear out.  Easy to replace with nothing more than a couple tools.

[Gone Fishing]

Mineral build up in your water heater coupled with high demand?  Try running the shower at half flow and see if it improves.  If so, it may point to a problem with your water heater.

[yachi]

It's normal for your shower head to have a higher flow rate than any of your faucets, unless you have one of those fancy pot filler faucets.  So you're likely to see, say 1.2 GPM from a faucet and 1.8 GPM from a shower head.  It's possible your tankless water heater is only able to provide adequate heating for 1.2 GPM.  Since you mentioned it's only a recent occurrence, that could point to it needing serviced.  If gas, maybe the gas flow is being restricted from carbon buildup.  You can rule out the tankless water heater by opening two faucets.  If you still have adequate temperatures at two faucets at the same time, than the issue is not your tankless water heater.

[Ladychips]

Thank you for your suggestions. My husband is working thru them. We'd love to hear any more ideas.

[secondcor521]

It doesn't really fit your symptoms, but:

My tankless has a little plastic flywheel which spins as the water flows through it.  If the flywheel stops spinning, the tankless heater assumes that the water has stopped flowing and turns off the heating element.  This is a safety feature; if there isn't water flowing to absorb the heat then the tankless can overheat.

The flywheel also stops spinning if it gets encrusted with mineral deposits.

The solution is to disassemble that part of my tankless, take out the plastic parts, soak them in vinegar, brush the deposits off with a toothbrush, then reassemble.  It is a bit fiddly but not really that hard to do.

This happens to me every five to eight years or so.

[snic]

I don't have any experience with tankless water heaters, but I keep hearing that they require annual maintenance (and if you don't have it done by a pro, it voids the warranty). I suspect that descaling is the major reason for the maintenance requirement, but there may be other adjustments that are supposed to be made

[yachi]

Speaking of descaling, I've found citric acid works better than vinegar, and you can find it in powered form in the canning section of grocery stores or Walmart.  Keurig sells a descaling solution that is almost 100% citric acid, I think they added a second kind of acid so they don't have to label it citric acid.

[snic]

• I have plenty of hot water everywhere else.

That's a bit of a head scratcher. What kind of shower valve do you have? There are expensive varieties that automatically adjust the temperature. If you have one of those, maybe it's malfunctioning?

Another possibility: is there any place in your house where hot and cold water can mix? One example of this is a bathtub/shower with a valve that has 3 positions to direct the water: tub faucet, shower faucet, and "off". In the "off" position, no water flows out either faucet even if you have the hot and cold taps turned on. So cold water can mix in with the hot water. In this situation, you can get initially hot water in a shower somewhere else in the house that then turns cold after a few minutes as the cold water mixes in. I know this because this happened to me! What I don't remember is whether the problem was detected at other faucets in the house - I think maybe not because the shower uses the most water and you're really sensitive to it turning cold because you're standing under it naked. That's not true for sink faucets, the washing machine, etc.

[herbgeek]

Another vote for mixer valve.  We had the same symptoms.  Much improved after replacing the valve.

[ChickenStash]

If it does get narrowed down to a mixer valve, try calling the manufacturer and asking for a free replacement. Many (most?) of the major brands from the last decade or two have lifetime warranties that cover the cartridges and assorted parts inside the unit. They may only do it for the original owner or only do a one-time courtesy replacement for the new owner, though. Doesn't hurt to try, anyway.

[Jules Anne]

Speaking of descaling, I've found citric acid works better than vinegar, and you can find it in powered form in the canning section of grocery stores or Walmart.  Keurig sells a descaling solution that is almost 100% citric acid, I think they added a second kind of acid so they don't have to label it citric acid.

Every time I see that Keurig "descaling" in the store I want to scream. I can buy 8 oz. of the powder for $7 on Amazon and that darn Keurig "descaling" is $14 for a 14 oz. bottle that is already diluted and won't last for 14 applications.  The powder lasts for years and takes up less space.