If I had a (water) hammer

Gordan

Recently installed a Turbomax, although I don't really think that that has a lot to do with the issue. The issue: there's water hammer. A lot of water hammer. You can hear it bouncing back and forth in the pipes. What I think is going on is that the check valves on the hot and cold inlet of the mixing valve are basically acting as a hard-stop for the backpressure created by a fixture valve closing. Shutting the isolation valves on both sides of the Turbomax (and the mixing valve) makes the water hammer go away, and shutting only the hot outlet valve and causing the water hammer actually made the T&P relief valve pop. The pipes are 3/4" L copper, well supported by split ring hangers.



Will one of those Watts 150 arrestors fix this? It seems that, due to the check valve orientation, pressure would just be allowed to build on the hot outlet side and in the hot distribution piping anyway if shock keeps happening on the cold side. Is there any kind of pressure differential bypass that people install in these circumstances to allow the shock to short-circuit around the check valve?

Brad White

I'd Hammer in the Mor-or-ning

Water hammer (from the Wagner opera, "Die Wasserhämmer"), is not just back pressure, but total pressure. It really is a kinetic equation, a mass of water at velocity squared, coming to a sudden stop near you.



In larger street systems, the sudden closing of a drop gate valve can cause a large water main to buckle right out of the street. Just an idea of the principles involved.



I like the Watts 150's vs. the Oatey straight line types. You MAY need multiples depending on your velocity and flow pressure. There is a chart on the spec sheet to assist.  My spec sheet says based on 10 fps maximum but your pipe sizing should not be above that but well below that in plumbing work anyway. When installing multiples these sometimes take an "antler" configuration but the header is often larger than the branch pipe size. You may have to put them on each side, as you suggested.



You should see the ones we put on cooling tower fill valves...

Mark Eatherton

I'd hammer in the eeevening...

Consider putting hammer arrestors at the offending appliances (dish washer and clothes washer and any appliance with a fast closing valve. Also, if you choke the supply valves to those appliances, the only thing you will cost yourself is time required to fill, and if you are standing around with a stop watch timing your appliances, you need to find a life :-)



Also, not sure how expensive they are, but there are some non slam check valves available in larger sizes. Not sure about the smaller pipe sizes tho...



Puff the magic dragon lived by the sea and frolicked in the autumn mist in a land called Hanalei...



Gawd, I'm aging myself again :-)



ME

icesailor

Water Hammer:

Where is this building and what is it used for? What kind of "mixing valve" is being used?

Gordan

So...

Yep, the water hammer is a shock wave (and eventually multiple ones) traveling back and forth until it dissipates. Now here's what gives me doubt about any bladder-based remedy: the spring checks allow the shock originating at the cold feed to propagate to the hot outlet, but do not allow the "returning" shock to propagate back, resulting each time in slightly higher pressure on the hot distribution side. I would think that many instances of this would eventually, theoretically, raise the pressure on the hot side to the shock pressure. Hmmm, but if it did, then the check valve would not open and the shock would be contained on the cold side of the check valve, where it's got more lebensraum to dissipate its world-conquering ambition, so maybe this does work out?



I was originally thinking that a diferential pressure bypass from cold to hot might short circuit the shock (the part traveling back toward the check) around the check valve, preventing hammer alltogether. One could set it to 20 psi or so, which you'd think would only occur during a shock condition... kind of a hydraulic equivalent of a varistor.



What's bugging me is that water hammer occurs with my kitchen single-lever faucet even when I'm trying to close it gingerly. Actually, it even occurs when the faucet is opened, not only when it's closed. Could that be "pent up" water between the check and the faucet "springing loose"? Is there something I'm missing due to my lack of experience?

Gordan

It's my home

3 baths, kitchen, laundry. Nothing overly sexy. The mixing valve is a Honeywell AM-101-US; it's got built-in checks on both hot and cold inlets. Its purpose is to mix down the hot water in the indirect to a safe temperature.

World Plumber

#5 expansion tank

How would a #5 expansion tank work? The ones we put on water heaters when we install the back flow presenters. They have a bladder and air cushion. Wouldn't that act like a large hammer arrestor?

Mark Eatherton

This ain't your grandmas water hammer,,,,

Water hammer, as it pertains to potable water is related to water moving at a high velocity, coming to a screeching halt. WHAM!!



Typically caused by fast acting solenoidic type valves, and Fluidmaster type ball cocks on toilets.



The potable water coil on the inside of the tank could be acting like a spring, causing a back lash, but you say it happens even when you are drawing water, which doesn't make sense in my minds eye.



Do you have a check valve on the incoming cold water to the tank, along with a potable water expansion tank?



ME

Brad White

You bring up an interesting point, Gordan

In our house, the kitchen sink (single handled Gypsy Moth flying controls), even closed gingerly, also makes this banging. What amplifies it is that the main cold feed to the kitchen follows the "shortest distance between two points" rule, then makes a hard right. (What was their hurry?) The shaking must annoy the Weston Seismologic Observatory a few miles from us...



As I picture what you have described, water enters one check valve, flows to the tank and exits on the other side of a check valve, holding a volume captive between two check valves, with this volume connected to the DWH tank?



As a post below suggests, the use of a #5 expansion tank (NSF rated for Potable Water Please!), be installed on the captive side. If you have any check valve at all, this leaves the downstream portion captive against closed fixtures and you would need expansion volume for thermal expansion reasons alone. I would think this would help the water hammer situation somewhat in the bargain.

Brad White

I think it would help

if the static pressure is low, but the tank has to be NSF rated for potable water, as you know. These tanks are rated for 150 psig minimum by code and your shock forces could be 100 psi, additive to the static pressure, so thinking out loud, you could impose 180 psi if you have 80 psi static pressure.



Any compressible fluid which can see the system pressure would absorb the shock at a rate relative to its compressibility. (Volume ratio at a pressure difference). 



The question that has me curious is, do you put this on the "end" of a line (with the main flow exiting a tee before it), or do you have the tank on a tee perpendicular to the main? The end location would take the direct hit, the tee location would absorb the static forces between shock waves as I envision it. The perpendicular tee location would see less stress and not the velocity head directly.



At first I thought this might be an mis-application, but lots of systems have potable water expansion tanks AND water hammer. They must be holding up and must help given their similarity to the Watts 150a's on a larger scale. Personally? If you have a check or backflow valve, you need the tank anyway and I would also, if you have a water hammer problem, install the Watts 150a's as directed because they are "made for the application". Belt and suspenders with each device assigned a role and probably, helping each other.



The issue with any of these spring or compressible gas devices is their ability or tendency to "push back".

Sort of like the Tokyo Subway. "Oh yeah? Yeah! Oh yeah? Yeah! And all they want to do is get to "Konichiwa".

Gordan

No check valve on the cold input

No backflow preventer, nothing along those lines. The cold side is thermal-trapped. The only check valves I've got (other than in fixtures/appliances) are the two inlet check valves right on the mixer. So, the shock originating on the cold side could make it to the hot side two ways: short-circuit through the cold inlet of the mixer, and take the long path through the Turbomax coil to the hot inlet of the mixer, both ways "leaving" out the mixer outlet. Or so my mind's eye sees it.



Once it's on the hot outlet side, there ain't no leavin'. Then it's well and truly trapped, and it just keeps bouncing around till it dissipates its kinetic energy. Of course, shock starting on the hot side stays trapped on the hot side.



I don't have a potable expansion tank because the water volume in the tank is small, there's no check on the cold side preventing expansion in that direction, and once it gets to the trapped side, there's no more expansion happening (there is contraction, I suppose, as the hot water cools to room temp, but that would just draw a little more water through the mixer.) Or so my mind's eye sees it. I remember reading something you posted before about expansion tanks actually creating more water hammer; what was the scenario again?



Anyway... I'm befuddled as to the best way to address this.

Mark Eatherton

I dunno...

Has my mind befuddled too. I'd have to be there to see and hear what's going on.



The conversation we were having about expansion tanks on potable water had to do with a surge in pressure due to the tank discharging its stored volume into the pipe being drawn down. At this point, I guess I'd have to agree with Mark (World) and Brad, and throw a hydraulic shock absorber down stream of the mixing valve. Running out of options, other than defeating the check valves on the mixer, which in my opinion are not necessary...



ME

Gordan

Did some more experimenting.

The issue definitely seems to have to do with the Turbomax's coils. If there's water moving through them that suddenly stops OR there's shock on the cold side (which is free to propagate through the coils) then things bang and rattle. If I suddenly close a valve on the hot side where the flow bypassed the Turbomax (temporarily opened the isolation valves on the old tank) then there's no bang and rattle.



I'm wondering whether I have a defective unit where the coil supports are busted, or something along those lines. It's the bang when the water flow STARTS that makes me wonder, most of all.



I'll put in shock arresters and see whether it quiets things down.

Mark Eatherton

Shouldn't make that much of a difference....

When you stop flow, the pipes in your home LURCH, but the fluid is incompressible, so there shouldn't be a lot of recoil action going on, at lest not in my minds eye.



I suspect it has more to do with the internal checks unloading on each other. Can you remove the checks and still have a functional mixer?



I still contend that they are not necessary for proper valve operation. We didn't have them for YEARS (checks on H&C sides of 3 way mixers) and never had any cross connection issues. Code officials running scared, misguided knee jerk reactions.



ME

Gordan

What's to stop thermosiphoning?

Both from hot to cold and from hot to mixed.

Mark Eatherton

Same thing that has ALWAYS happened...

If you have the mixer below the top of the tank, per manufacturers recommendations, you won't see a lot of migration. But even if you do, as soon as you start drawing hot water, the valve kicks in and starts doing what it is designed to do, which is mix to the desired outlet temperature.



The old mixers use to go into a "lock up" condition whereby you couldn't get ANYTHING to go through the valve.



If nothing else, you could drill a 1/8" hole through the checks to alleviate these conditions.



I guess the other real question is, why have a mixer in the first place? I use them commercially, because they are required, but residentially, I set the tank for 130 degrees F (good antibacterial setting) and let 'er rip. No mixer necessary. Everyone is made aware that the water will be hot, so no one gets burned. Little kids learn that at an EARLY age in my house. If you have senior citizens, or mentally challenged children, then you might want to use a mixer, but unless one of those conditions is present, why use it? With a DHW setting of 130, you will need a boiler water temperature of 140 to satisfy that goal.



ME

Charlie from wmass

I vote for World plumber's answer

If it were my job I would install that. I do wonder what your water pressure is? Are you on a well or city water main?

Gordan

Had I anticipated this issue...

I might have gotten a mixer without checks and installed it thermal-trapped. The manufacturer's instructions actually specify installing this directly on top of the tank. This time I viewed the built-in checks as an asset, not a liability. Next time I may know better. The mixer itself, I do think is an asset, because the tank capacity is 26 gallons and the boiler is 60kBTU, so raising the water temp does give me more thermal storage at the cost of slightly less efficiency. That was the calculus prior to having this problem. Now... we'll see what it takes to fix it.



To paraphrase Cervantes, you only know what you know, ay, and often less.

Gordan

I wonder what my pressure is, as well...

And I do intend to find that out this weekend. City water. (Well... suburb, anyway.)

Gordan

I wonder about the misapplication, too.

For one, the instructions for the expansion tank are to precharge it to the mains pressure. The 150A is to be precharged to 5 psi less. Since I don't really need the capacity on the expansion side (the volume of potable water in the heater is only 1.5 gals or so) I'd be tempted to precharge the expansion tank to 5 psi less than static pressure, too. That would give it the ability to make up for low pressure shock, as well as high pressure shock. Does that sound sane?

Gordan

Less is more... but more is not always better

Problem solved.



Firstly, I've high static pressure in my plumbing. I checked the cold side - 85-90 psi. Then I checked the hot side - 95 psi! So, the theory of water rushing into the hot side and coming to a screeching halt, then not being able to equalize pressure back, can be considered confirmed.



Secondly, the mixer has check valves on both hot and cold side. I looked at my piping configuration and thought, hey, I really only need a check valve on the hot side - to prevent gravity flow from the tank. So I removed the check valve on the cold side, to see whether this would cure the monster hammer. Eureka! It sure did solve it. Then I felt around to make sure that I didn't get thermal migration, but I found that there was, in fact, thermal migration directly from the hot side, up the thermal trap, then down to the tank inlet. I had set up a little gravity loop. Since the hot-side check should have prevented that from happening, I figured that something must be wrong with it, and sure enough, there was a 1/4" splinter that had gotten stuck in there - it must have been inside the tank inlet when they crated it at the factory. Now I don't have hammer and I don't have gravity flow. Occasionally, a slug of hot water winds up on the cold side, even when I haven't run hot water - evidence that water shock wants to move water around quite a bit, and that I should therefore let it do that, and not get in its way with check valves all over the place. (On the boiler side, I avoided checks everywhere I could, winding up with only two hydrotrols. I removed the checks from the circulators.)



I'm attaching some pics so your mind's eye has something to work with. Thanks for lending me your brains, everyone!