New Tankless Water Heater on Recirculation Line and Sprinkler Problems

[Gipsyking]

Hi Everyone,

We recently had a new Rinnai tankless water heater installed with a recirculation line which has a couple of check valves (see photo). The tankless water heater works very well. For some reason though, our sprinkler system has been creating a lot of loud banging noises in the pipes around the water heater every time the sprinkler system comes on or changes zone. Not sure if this is water hammer or some other issue. We have already replaced all the sprinkler zone valves to new valves which are soft open and close and that did not solve the issue, although it did reduce the intensity of the banging a bit.

For now we have shut the sprinkler system off. Really appreciate any leads here on how to troubleshoot the issue. We never had this issue with our old hot water heater which was a regular 'ol tank without a recirculation line. I also want to avoid paying $2k which is the cost quoted to install a separate water meter for the sprinkler system so it doesn't touch our mainline going into the house at all.

Thank you so much.

 

[Terry]

I would consider install a hammer arrestor on the system where the quick closing valve is. They act as a cushion.

 

[Sponsor]

 

[Gipsyking]

Thank you so much for the suggestion! The banging sound occurs with the opening and closing of every single one of my 4 sprinkler zone valves. Do you think putting just 1 hammer arrestor on the main line feeding the sprinkler system should work or do I need to put 1 hammer arrestor behind each 4 of the valves?

The other datapoint I just figure out last night is that when I turn on the bathtub faucet upstairs, leave it on, and then turn on the sprinkler system, there are no banging sounds at all. I never used to get any banging noises in my house pipes with my old water heater when the sprinkler system came on and off and not sure if this new tankless system with the circulation line is what is causing the issue?

 

[Dana]

Just a WAG: The old tank heater was probably behaving bit like a water hammer arrestor. The lower volume and higher rigidity of a tankless makes the pressure wave a bit sharper.

The location of the expansion tank shown in the picture may also have changed relative to the feed to the sprinkler system. (An expansion tank behaves very much like a water hammer arrestor, even though that is not it's primary function.)

 

[Gipsyking]

Thanks a lot for the insight. That makes a lot of sense on the tankless being lower volume and higher rigidity. Will have a plumber come out and see if we can install water hammer arrestors to provide more cushion.

I would have kept the old tank water heater except we live in Houston and they typically install them in the attic which doesn't make any sense to me having moved down from the northeast...its a potential hazard sitting up there!

 

[Valveman]

The little expansion tank may have too much air pressure. The air charge in the tank should be lower than the lowest pressure you see when the sprinklers are on.

 

[Dana]

The little expansion tank may have too much air pressure. The air charge in the tank should be lower than the lowest pressure you see when the sprinklers are on.

Isn't that backwards? If the tank's pre-charge pressure is too low the bladder is already fully expanded into the air-end of the tank, with no volume left to take up when the high pressure wave hits.

 

[Valveman]

If you have too much air charge in an expansion or pressure tank, it is acting like a pipe plug when the system pressure is lower than the tank air pressure. With air pressure in the tank lower than the system pressure, the tank already has some water in it, and it ready to accept more when a surge happens. With too much air pressure, the diaphragm in the tank is stuck over the inlet and the tank is just a plug in the pipe.

 

[hj]

How far is it from the meter to the sprinkler system. Hammer when a valve opens usually indicates that there is a long, small, pipe feeding it and the water's inertia takes a moment to start flowing so the pressure drops, often to zero, and then the "slug' of water starts moving and creates the hammer. Having a faucet already flowing will minimize the effect since the water in the main does not have to overcome the static inertia. If that is the problem, you have to install something like an expansion tank near the connection to maintain the pressure until the water starts flowing.

 

[Dana]

If you have too much air charge in an expansion or pressure tank, it is acting like a pipe plug when the system pressure is lower than the tank air pressure. With air pressure in the tank lower than the system pressure, the tank already has some water in it, and it ready to accept more when a surge happens. With too much air pressure, the diaphragm in the tank is stuck over the inlet and the tank is just a plug in the pipe.

Even when expanded to where it's covering the pipe connection isn't plug in the direction that matters, since it can and will move in response to the positive going pressure spike that occurs when the valve closes.

 

[Valveman]

Even when expanded to where it's covering the pipe connection isn't plug in the direction that matters, since it can and will move in response to the positive going pressure spike that occurs when the valve closes.

Pressure spikes happen too fast to push the tank diaphragm off the bottom. If it hasn't already moved up a bit, the pressure spike hits that bladder sealed over the inlet and bounces back like it hit a plug.

 

[Dana]

Pressure spikes happen too fast to push the tank diaphragm off the bottom. If it hasn't already moved up a bit, the pressure spike hits that bladder sealed over the inlet and bounces back like it hit a plug.

Count me as an extreme skeptic on that theory...

 

[Valveman]

Any bladder tank with too much air in it cannot catch a pressure surge. Transient pressure waves travel at 3000-8000 feet per second. Nothing is fast enough to catch or cancel them out, you have to have a place for it to go already opened up, which is why the bladder needs to be moved up away from the inlet. Now expansion is a different story, as it would have time to push the bladder up as the water slowly expanded.

Edit;
Actually transient waves travel so fast they can't even turn 90 degrees into an elbow or tee to get to the pressure tank. It is best to have a bladderless tank, like the old tank water heater, where water comes in one side of the tank and goes out the other side. That will catch a pressure surge.

 

[Valveman]

Slow opening valves cannot open slow enough to prevent water hammer. If there is a long distance between the zone valve and the sprinklers, keeping the line full of water will help. When the zone valve closes the sprinklers will drain all the water out of the lines. Then when the zone valve opens, it quickly fills the lines and causes water hammer when it hits the sprinkler. I use a check valve with a 3 pound spring before the sprinklers. This keeps the water from draining out, so the line is already full when the zone valve opens.