How to stop water hammer

This issue may be caused by several factors, including a malfunctioning or improperly sized water heater, clogged pipes, or a malfunctioning thermostatic mixing valve. Here are some steps you can take to troubleshoot the issue:

1. Check the size of your water heater: If your water heater is too small for your home's needs, it may not be able to supply enough hot water to all areas of your home. Consider upgrading to a larger water heater.
2. Check for clogged pipes: If your hot water pipes are clogged with mineral buildup or debris, water flow may be restricted, causing reduced hot water supply. Have a plumber inspect your pipes and clean them if necessary.
3. Check the thermostatic mixing valve: If your home is equipped with a thermostatic mixing valve, it may be malfunctioning and causing the issue. Have a plumber inspect and replace the valve if necessary.
4. Check for leaks: If there are any leaks in your hot water pipes, the water pressure may be reduced, causing reduced hot water supply. Have a plumber inspect your pipes and repair any leaks.
5. Adjust the temperature settings: Make sure the temperature settings on your water heater are properly set. If the temperature is set too low, you may not be getting enough hot water.

If these steps do not resolve the issue, it is recommended to contact a licensed plumber to inspect and repair your hot water system.

The hot side of the faucet is most likely plugged with something that came up thru the water system. The average thankless waterheater requires at least 1/2 to 3/4 gpm flow rate for the unit to sense flow and turn on. If there is not enough flow it will not fire and heat the water. Not sure about this particular unit but a few require at least one full gallon per minute flow. So chances are that if you fix the faucet, you fix the hot water issue at the same time

Install air chamber to cushion sudden-stop of flowing water.
Water flow coming to sudden stop is called water hammer, and will bang pipes, but also loosen plumbing joints, causing leaks.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#bang
http://waterheatertimer.org/images/Flow-rates-water-hammer-600.jpg

Air chamber has to be vertical, and about 18" long, with cap on top.
By code each Hot and Cold line at each tap will have air chamber located behind the wall. So there are two air chambers at each tap.

In your case, you can probably add air chamber to your Hot and Cold lines and solve problem for both dwellings.

Water hammer is caused because water does not compress like air.
So when long column of water suddenly stops... the whole pipe-full of water comes to sudden halt... and that water could weigh 1-1/2 pounds ... and just like a hammer head moving, the weight of water bangs the pipes to dissipate energy, causing noise.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Had a similar problem and found that the plumber who had replaced the recirculating pump had removed the plastic pipe hollders. These are spacers used to hol the pipes firmly as they pass through the wood studs. Even with only two spacers missing the hammering sound passed through the whole system. Once new spacers were installed the hammering sound stopped.

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!

Run through the basic checks

• Power lead from the heater plugged in a socket turned on.
• Gas on to house
• All valves below heater in on position.
• If they have controllers in the house turn on (key pads marked with Rinnai logo in kitchen or bathrooms usually)

There is a turbine that spins in the heater when a hot tap is run. If the heater has been sat for some time not operating the turbine might have stuck. Run the hot tap and keep it running, using a hammer give the centre pipe connection under the heater a firm hit a few times(not too hard). This might unstick the turbine.

Your check-valve has most likely failed at the switch.
The flow check valve works by opening when you turn the water tap. When it opens it closes a switch which turns on your heating elements. When this switch fails you will still have water flow, but no heat. Titan has parts for this but you will need to call their service Dept.

or call
Toll free support pesonnel hours (EST): Monday-Friday: 9:00am - 5:00pm

They also have an online support operator during normal business hours here
http://www.titanheater.com/livehelp/livehelp.php?department=2

1/overheat stat 2/fan 3/micro switch, best of luck.

If you have a tankless hot water heater, you may still need to install an expansion air chamber to stop the "water hammer"; If you have a tanked hot water heater, you will never get instant hot water like with a tankless unit.