Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Cold water when waterpump comes on

tesseo
tesseo Member Posts: 4
edited November 2022 in Domestic Hot Water
When water pump comes on ,water will go from hot to cold untill the pump stops.This happens at a shower 10 feet away or the kitchen sink 15 feet away.Any ideas.Kind of a pain .Big pain in the shower.Thanks Thye heater is a rinnai tankless

Comments

  • tesseo
    tesseo Member Posts: 4
    above comment is a rinnai tankless hot water heater.
  • GGross
    GGross Member Posts: 396
    There should be a sensor inside the water heater that detects flow, that is what would turn on your water heater. When you turn on a hot faucet water flows through, the sensor detects a flowrate and fires the heater based on that. If the sensor were fouled up or otherwise not functioning correctly, it could intermittently show that there is no flow through the unit thus shutting it down. This can result in the hot-cold-hot-cold water you describe. There can be other issues that cause this as well related more to the gas side of things. You should be able to look in the manual and find the location of the flow sensor, if it is a style that looks like it can be cleaned you can clean it, otherwise you may want to replace it depending on price and availability.

    This is assuming the unit has worked previously and is just now acting up. If this is a newly installed unit or has never functioned correctly, you should double check the flowrates that you are using for hot water and make sure the tankless unit can keep up with that flow
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,782
    Hi, I'll add that there could be a crossover in the plumbing, and the change in relative pressure caused by the pump would make the problem worse. Try shutting water off the the heater and open just a hot tap. Does it just run, or stop flowing in seconds?

    Yours, Larry
    GGross
  • pedmec
    pedmec Member Posts: 713
    what pump are you referring to? recirc pump?
  • tesseo
    tesseo Member Posts: 4
    main water pump .The only time it goes from hot to cold is when the water pump come om when the hot water is running and it will get hot again when the water pump is done with the cycle.the heater didn't do it when it was first installed a few years ago. Also no codes come up.
  • Jamie Hall
    Jamie Hall Member Posts: 20,460
    I;m sorry to seem stupid, but by "main water pump" do you mean the circulating pump for the house heating? A primary circuit pump for the boiler? Of... the main water pump for the house water supply (well water)?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • tesseo
    tesseo Member Posts: 4
    Main water pump
  • Jamie Hall
    Jamie Hall Member Posts: 20,460
    How much does the pressure (and hence flow) in your domestic water change when the main pump is running vs. when it isn't? And, related, does the tankless stay running, or does it actually turn off?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,782
    Hi @tesseo , Could you post a picture of the pump and nearby piping? I'm not clear on what pump it might be. It could be a hot water recirc pump, a pressure pump off a well, or a hydronic heating pump. I think the photo/s will help us to understand the problem better.

    Yours, Larry
  • David Penz
    David Penz Member Posts: 12
    I agree with GGross that most intermittent tankless water heating problems are due to a faulty flow sensor. As the sensor is failing, the heater controller gets confusing signals concerning whether or not there is sufficient flow to operate the burner. In my experience, after a few years of service the sensor becomes fouled with a black powdery substance, likely manganese dioxide or iron oxides. Remove the sensor assembly. If the turbine element can be removed from the assembly, the turbine wheel and axle can be cleaned with vigorous brushing. Don't breathe the dust! The thrust bearing may require to be reamed with an appropriate number-size drill bit. Reassemble. Test turbine movement before and after with a small magnet. Rotate the magnet around the turbine assembly, the turbine wheel should spin freely. If the turbine is not removable, replace the entire assembly.