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Shower hot water lasts 10-15 seconds

I’m in a quandary here, so please help me if you can. It seems that the hot water in my ground floor powder room, shower and second floor shower gets hot for 10-15 seconds that gradually gets cold. I’ve changed the mixer cartridge in each shower, I’ve turned up the heat setting on my boiler and still no improvement. Does anyone now what could possibly be the culprit? The pipes on my boiler are very warm so there is adequate flow in and out, but the hot water just doesn’t last. I’m thinking it may be the valve on the boiler now, but if the out flow is hot then I’m not so sure. The pressure on the boiler is within range as well. Any help is greatly appreciated.


  • STEVEusaPA
    STEVEusaPA Member Posts: 5,318
    Pictures would help of the boiler.
    I assume a tankless coil?
    Is there a mixing valve at the boiler?
    With a k-type thermocouple (attached or clamp on) you should be able to measure the temperature of the water entering the coil, the temperature coming out of the coil, and the temperature after the mix at the boiler (if you have one).
    This time of the year, extremely cold incoming water reduces the output generated by the coil, plus you are competing with space heating.
    Other issues could be the coil liming up, both on the domestic side and the boiler side.
    'Very warm' isn't good. Too hot to touch coming out of the boiler is probably needed.
  • Jamie Hall
    Jamie Hall Member Posts: 17,121
    Is there a mixing valve on the lines coming from the hot water source (I take it the boiler? Tankless coil?) That's a likely villain...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 2,969
    edited January 2020
    Do you get hot water at the bathroom basins and the kitchen sink?
    Often wrong, never in doubt.
  • jsibbett75
    jsibbett75 Member Posts: 5
    Yes hot water at the sinks in the upstairs bath and kitchen sink. I thought it was the mixing valve at the boiler as well. But when the hot water flows, it comes from the boiler through the valve and out, all hot pipes.
  • jsibbett75
    jsibbett75 Member Posts: 5
    Ok Steve I gotcha. I should get a plumber out to have a look. I’m in over my head on this.
  • That's a tough one.

    Are they the same shower valve? Make and model?
    Often wrong, never in doubt.
  • jsibbett75
    jsibbett75 Member Posts: 5
    The same shower model yes. The temp on the boiler measures around 140F and the pressure is under 20.
  • What is the make and model of the shower valve?
    Often wrong, never in doubt.
  • jsibbett75
    jsibbett75 Member Posts: 5
    I replaced the shower valve cartridge. Positempt Moen 1222.
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 2,969
    edited January 2020
    Watch the video to make sure you did everything right. If you did, call the number at the end of the video in the morning and talk to Moen tech. support about the problem.

    Often wrong, never in doubt.
  • mattmia2
    mattmia2 Member Posts: 3,986
    what about a laundry tub or other big hot water draw?
  • rick in Alaska
    rick in Alaska Member Posts: 1,243
    I am also thinking possible cross connection, such as maybe a bad check valve in a recirculation line, or one of those toilet tempering valves people like to use. The fact that two of the fixtures work ok, makes me unsure of it though. But, not knowing the piping layout, makes it so we are somewhat just guessing.
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,291
    Hi, I agree with @rick in Alaska . Try this simple cross connection test. Turn off water to the hot water supply and then open a hot tap. Does it continue to run? If so, cold water is getting in someplace. Now you get to find it!

    Yours, Larry
    Alan (California Radiant) Forbes
  • dopey27177
    dopey27177 Member Posts: 756
    Not knowing whats happening with the boiler and the coil here is what you need to know

    To produce hot water thru a coil in a boiler the boiler water needs to no less than 180 degrees.

    In the warm weather you will produce more hot water than in winter. This is so because the domestic hot water can be as much as 30 degrees cooler than the summer. (depends where you live.

    If you have water enough in the boiler start checking the mixing valve for proper operation.

    As far as cross over check the connections at the washing machine, shower body, combination lavatory faucets and bath tub faucet.

    Oh did the restrictor in the shower body or shower head fail.

    You dump the coil pretty quickly if your shower head puts out more than 3 gallons of water per minute.

  • HomerJSmith
    HomerJSmith Member Posts: 1,372
    edited February 2020
    Is this something that has always been a problem from the installation of the shower valve or is it a recent thing and the valve functioned properly at an earlier time?

    "Behind the handle of a Moen PosiTemp tub and shower faucet are built-in stops that limit how far the handle can turn. By controlling the turning range of the handle on a Moen PosiTemp valve, these temperature limit stops restrict the water temperature range, meaning it cannot get too hot or too cold. The valve is designed to balance the incoming flow of hot or cold water, even when the pressure varies due to another faucet being turned on or a toilet flushing.

    This type of valve is also called a pressure balance valve, because it balances the pressure to eliminate large changes in water pressure. A Moen PosiTemp shower valve keeps your water temperature steady within 3 degrees Fahrenheit at all times, according to Moen, but requires both hot and cold pressure to operate properly. The temperature limit stops on a Moen PosiTemp valve can also be adjusted to make the range of your handle turn longer or shorter. This will increase or decrease how hot or cold your water temperature will get."

    I would think that the pressure drops on the hot side of the valve, I don't know.
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,318

    ...To produce hot water thru a coil in a boiler the boiler water needs to no less than 180 degrees...

    Not true at all.

  • Will we ever find out the answer?
    Often wrong, never in doubt.
  • If in fact it is a cross-connection, you can use an infrared camera to possibly find the culprit if it's in a mixing valve. I've never done this before, but it came to me yesterday in the shower.

    Let's say you don't get hot water at the kitchen faucet because of mixing somewhere else in the house and you suspect a bathroom faucet to be the culprit. It may have a bad o-ring that allows cold water to flow into the hot water line.

    Turn on the bathroom faucet; hot water only and let the faucet get as hot as possible. Turn on your infrared camera and train it on the bathroom faucet. Have someone else turn on the kitchen faucet and if the bathroom faucet is at fault, you should be able to see cold water coursing through the faucet.
    Often wrong, never in doubt.