Banging noise when hot water is turned off - Rinnai Water Heaters

the banging is the water boiling/overheating Check the pump is working by undoing the large screw on the front of pump and gently putting a soft stick into the space behind to check that the rotor/impellor is turning. take care to catch the water which drips from the screw hole

this is when water is running, some tap is turned on.

either solenoid valve is closing or there is a large water hammer occurring

The noise you refer to is called Kettling, it is caused by the hot water in your boiler not being able to leave the boiler quick enough, if the problem developed quickly and your system is a "fully pumped " system this would generally point to your pump either has failed or is on its way out, i.e . your radiators would not heat up as the hot water is stuck in the boiler hence the banging......( if you know where the pump is you can generally feel if it is running by a small vibration ) if you have gravity hot water and pumped heating circuit then I would assume that it is noisy when just doing HW, and the banging is a problem that has gradually got worse it can indicate that the boiler is scaling up (are you in a hard water area). if this is correct there are various additives that can be put in the system to help quieten it down, I suggest you really need a heating engineer to diagnose fully.

There should be a pressure drain to purge out all the air. Sound like there is an air bubble in the system somewhere.

No charge for this one.
Educated guess but are you in an area where you have water scale. That is a lime build up especially in hot water appliances as the scale forms on hot surfaces. This would reduce the flow of water through to the hot taps and cause what we call kettling which accounts for the noise. Maybe give a local plumber a ring and ask about scale in your area. Other than that not too much info from you to suggest anything else. Please get make and model of hot water system and re-post problem.

What you're describing is "water hammer". This problem, left unchecked, can cause big problems for your home's plumbing. Water hammer is result of quickly shutting off the flow of water in your home. The faster the rate of water flowing when it is rapidly shut off can result in very loud banging. The banging is caused by the pipes shaking in the walls, ceilings and any place they aren't secured. This repeated shaking causes soldered and glued joints to weaken and eventually fail. The most common sources for water hammer are toilets and wash machines, but even quickly shutting off a faucet can cause it too.

An short article about water hammer is here. Below is a link to images of products designed to arrest water hammer and associated noise.

Please rate this reply - I hope this helps!

I suspect the noise is being caused by expanding metal groaning or 'popping' due to the expansion. We have a metal roof on our home and heating and cooling are always accompanied by similar noises that are inaudible inside the house but catches our attention if we are on the porch.

Ok the way this system works is from a pressure and flow switch. The pressure switch picks up the slight drop in pressure when the hot side water is turned on then turns on the burner. The flow switch measures the amount of water flowing and adjusts the size of the burner to the amount of water flow. So a couple different things might be happening. The flow switch might be sticking causing the water heater to throttle down the burner but not off when the water flow is off. Or the gas control valve might be by-passing gas when off causing the burner to stay on even when the demand is off. Run the hot water on high demand then turn the water off. If the burner is still on try banging on the water heater with your hand and see if the burner shuts off completely. If it does the gas control valve might be by-passing. You would also need to check and see if the pressure/flow switch is giving any voltage signal changes when the water flow is off. These tankless water heaters are more comlplicated to work on than a standard water heater and you may need a professional's help...good luck

Install a knocker in the line and bleed the air out