Cold water when waterpump comes on

tesseo

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

tesseo

above comment is a rinnai tankless hot water heater.

GGross

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

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

pedmec

what pump are you referring to? recirc pump?

tesseo

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

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)?

tesseo

Main water pump

Jamie Hall

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?

Larry Weingarten

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

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.