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Your task, should you choose to accept it, is to help me solve THE HOT WATER MYSTERY!

Story:

I moved into my house 1 year ago and since day 1, I noticed the bathtub in the guest bathroom had an issue with the hot water. The water would turn hot and stay hot for 5 seconds and then go cold. I called a plumber out, the plumber changed the cartridge in the valve and the problem was fixed. Hot water steady and consistent.

Cut to a week ago where I changed that same cartridge because I'd developed a slow drip. Guess what happened when I did that ... yep, the hot water started having the same issue as before.

I called the same plumber asking him to come fix it since he did it before. This time he could not. We tried 3 brand new cartridges and nothing worked.

Things to Note:

This is a MOEN T2473EP

The cartridge is 1222

The hot water heater is Tankless Electric AO Smith - 240V / 14 KW 2-CHAMBER TANKLESS ELECTRIC WATER HEATER https://www.aosmithatlowes.com/products/water-heaters/electric-tankless/r2vr-140e/

Hot water heater is only 2+ years old

Today, I had a new plumber come out at $250 an hour who determined the hot water heater is too small. Basically, the GMP in the bathtub is too much and the water heater can't keep up. However the same shower in that tub, can. Weird. if this were true, why was the issue fixed a year ago when I replaced the cartridge?

Lastly ... when I am running the bathtub that is the only faucet running. Nothing else is on including the dishwasher, washing machine or other faucets. Also, the cartridge is set to allow the max hot water.

Thoughts? The hot water is fine everywhere else and in the shower it is purely at the bathtub faucet.

PLEASE HELP ME SOLVE THIS MYSTERY. THANKS!

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    AO "Tankless water heaters can provide continuous hot water when sized appropriately." - it should say with integral flow reduction to provide continuous HW which it doesn't because it's cheap junk. - $250 an hour? Apparently I need to give myself a raise again.
    – Mazura
    Jan 28, 2022 at 0:59
  • The other bathtubs fill fine with hot water from the same water heater? Does the water go lukewarm or cold? Is the tankless hot water heater "on" while it's filling?
    – rogerdpack
    Jan 28, 2022 at 6:38
  • The other bathtubs fill fine with hot water from the same water heater? Does the water go lukewarm or cold? This is without you moving the handle it changes temp? Is the tankless hot water heater "on" while it's filling? Reverse the cartridge do anything?
    – rogerdpack
    Jan 28, 2022 at 7:01
  • Is "GMP" supposed to be "GPM"- Gallons per Minute?
    – FreeMan
    Jan 28, 2022 at 12:58
  • To be honest, the story in the question, while compelling and entertaining, seems to have nothing to do with the question in the title. You know what the problem is, you simply need to reduce the flow from the tub spout. There is no mystery to be solved, your latest, expensive, plumber has solved the mystery for you. This really boils down to a shopping question - "What product will reduce the flow from my tub spout?".
    – FreeMan
    Jan 28, 2022 at 13:00

2 Answers 2

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The issue is due to a tankless (or on-demand) hot water heater having no reservoir of hot water to keep up with high demand. The water is heated immediately with 14 kW of electricity. Below a specific flow rate, dependent on initial water temperature, 14 kW can heat the cold water to an acceptable bath temperature, perhaps 40 °C (104 °F). Any faster flow is beyond the heater's capability.

A shower likely has a restricted flow rate, less than 8 liters/minute (8 L/min = 2.1 gal/min). With the bath faucet wide open, the flow rate is far greater, perhaps 30 L/min (7 gal/min). The water flows through before it gets hot.

What can you do?

  • Don't open the bathtub faucet to full flow. Cut it back to ~8 L/min, and cut your nails, read a book, or do something else while the tub fills.
  • Add an inline flow restrictor at the outlet of the hot water heater.
  • Add a bathtub flow restrictor. Likely, that would be put inside the wall, before the bathtub faucet. On some tubs, the faucets unscrew easily, so you could do this yourself. Otherwise, find a plumber who works for less.
  • Not recommended: Turn the hot-water shut-off valve for the tub faucet to reduce flow rate. Though this works for the short term, it is likely to damage the rubber washer inside, so that when you must turn off flow completely, that shut-off valve leaks.
  • Switch to a water heater with tank storage.
  • Take showers instead of baths.
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    *fill the tub with the shower head
    – redlude97
    Jan 27, 2022 at 0:05
  • @redlude97, Regrettably, the water cools as it's sprayed out into the air -- but the idea is good -- put a hose in p;lace of the shower head!
    – DrMoishe Pippik
    Jan 27, 2022 at 21:28
  • Then turn down the amount of cold water mixed in with the hot. My feet get and stay plenty warm when I take a shower...
    – FreeMan
    Jan 28, 2022 at 12:56
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From the link given by the OP on this water heater: "Enjoy continuous hot water for up to 1 shower when sized appropriately (shower calculations are based on a 2.5 GPM showerhead in the southern zone)."

We have a gas fired tankless water heater which is rated at 117 kBTU/hr equivalent to 34 kW electric. We find it is OK for us, but most households would have one rated at 180 kBTU/hr equivalent to 53 kW. But most household electric service cannot supply even 34 kW for heating water so electric tankless water heaters are not used to heat water for a whole house.

The OP's water heater is rated at 14 kW and so it is seriously undersized for a whole house unit.

Try lowering the setpoint for temperature to 110 F or 105 F and see if this is warm enough to shower. The heater may give higher flow rate if the temperature setpoint is lowered. If you lower the output temperature, you may be able to fill the tub with the valve on a higher flow setting.

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