Water hammer with inline check valves
All,
Well, I’ve gone and taken a perfectly good plumbing installation (except for its inherent flaw… more on that in a minute) and screwed it up.
You see, I installed a Kohler K-404 1/2″ thermostatic mixing valve with volume control (their site has pics and specs) and it worked great. I installed air chamber water hammer arrestors (solid copper jobbies–pretty neat looking) and they do prevent hammer. However, the K-404 is apparently not designed, according to Kohler, for a house where the water pressure on one supply is significantly different from the water pressure on the other supply, such as when the sprinklers are running. When the house is served by a well, as ours is, it only serves to exacerbate the problem.
The problem? Cross-flow of the higher pressure supply into the lower pressure supply through the mixing valve.
Despite the check valves which are in the faucet, whose springs don’t appear to be strong enough to counter this flow, we had cross-flow from hot to cold. Not really a problem, except that it sorta’ surprises you when the cold comes out scalding hot in the other nearby bathroom.
So I thought I’d get clever and install a pair of check valves a bit upstream of the K-404. I had to do this so that I’d be able to get to these valves to service them and the only way to do that was to put them behind an air conditioning grille low in the wall. I also installed shutoff valves just to make servicing the check valves and other geegaws in the K-404 easier.
The attached picture shows what’s going on. At lower left are the check valves and ball valves. In the middle, behind the 2×6, is the K-404, and above it, behind the 2×4, is the diverter valve. The current air chambers are at the far right of the picture and at the far left behind the 2×6 and 2×4.
Problem solved! No cross flow! But if you read the post title, you’d discover that I now have a water hammer problem that (as you probably guessed) I didn’t have before. When the valve is shut off and only the hot side is flowing (because the hot water isn’t there yet, for example), there is a HUGE water hammer. When the cold finally kicks in to temper the water, the water hammer is less.
I’m guessing that I need either (a) to solve the problem by removing the check valves (rats! $40, wasted!) and somehow figure out how to make the Kohler check valves work or (b) to place another hammer arrestor somewhere else.
My first instinct is to add arrestors very close to the check valves on the shower side of the check valves because–as my electrical engineer’s gut tells me–the water is hammering because of the echo from the original hammer (arrested) in that length of pipe back against the closed check valve.
I also think I might need them on the supply side of the check valves because once the water train has come to a screeching halt, the echo coursing its way back down the supply will want to suck against the now-closed check valve and that only makes the valve shut faster and worsen the problem on the other side. Giving it a source of air to suck from might help things.
My second instinct is to follow KISS. No, not the rock band–“Keep It Simple, Stupid!” That is, there’s something else that can be done which is a lot simpler, will work better than everything else, and will last longer. But I’ll be darned if I know what it is.
Has anyone run into a situation like this one before? Do you have a clue for me?
Many thanks in advance,
Bill
Replies
My question has to do with the "basic problem".
"However, the K-404 is apparently not designed, according to Kohler, for a house where the water pressure on one supply is significantly different from the water pressure on the other supply, such as when the sprinklers are running. When the house is served by a well, as ours is, it only serves to exacerbate the problem."
Where are the pressure drops in the system?
You have a single source, single pressure. Then it splits off into cold supply and the WH supply.
If the sprinker take of if before the split it should affect both the cold and hot equally.
What can be done on that end to remove the pressure differnces and eliminate the need for the check valves.
Hey, that's an interesting question.Well, the sprinklers aren't the only cause of the problem. Anything which runs on the cold side is a problem. There are some days when no high-volume device runs (only regular faucet use occurs) and the problem still happens.In theory, the water pressure should be the same on both sides of the mixing valve. Sure. That makes sense. But yet there's a difference that causes flow from the hot to the cold. It wouldn't have to be much, would it? I guess if it wasn't much, then the K-404's check valve should work well enough, right?Why would the K-404's check valve fail?Oh, heck, this one's going to drive me nuts before I'm done thinking it through....
"In theory, the water pressure should be the same on both sides of the mixing valve. Sure. That makes sense. But yet there's a difference that causes flow from the hot to the cold. It wouldn't have to be much, would it? I guess if it wasn't much, then the K-404's check valve should work well enough, right?"I really don't know anything about them, but I think that they should be able to work with "normal" pressure differences.This is probably not practical at this point. But what happens if you have the mixer right at the output of the WH so that you only have a few inches or feet at the most difference between the 2 paths?
Well, the K-404 is a shower mixing valve, so the installation (pictured) is a two-handle installation, one for temperature, one for volume, adjustable by the showering person. So the mixing valve isn't a whole-house mixer, hence not installed near the water heater.
Kirchoff's Law applies to plumbing.
SamT
Check out a pressure equalizing valve. Install it just before your new, fancy Kohler valve. It will equalize the difference between your hot and cold lines and you won't have wasted your water hammer arrestors
Hey, who makes that beauty? Where do I get one?(And is it gonna' work?!)Thanks,
Bill
Here is one site.
http://www.plumbingpages.com/featurepages/Greens.cfm
Interesting, but it has 15mm compression connections. Is there a US version?
Well, things have improved.Since I don't have to worry about crossflow of cold into hot, I took out the hot check valve and that has cured the majority of the problem. There's still a very light hammer when the valve is slammed (and I mean _slammed_) shut, but there are no bangs from anything else in the house.My theory? The sudden stopping of both water lines, especially the slightly longer hot, made the shockwave turn around and head for the check valves, which are closed by that time and <wham!>.So, no need for a pressure balancing valve (that I know of), and I'll be happy enough with my solution.Though a call into Kohler is in order....Thanks, all! And I will read any further suggestions, BTW! I'm always willing to learn.Bill
Bill, sounds like you solved your problem. However, I ran into this a while back and while not quite the same, the problem was similar. I have a private well and septic system. I also have water lines run to several out buildings at my place. I also have an iron filter/water softener to remove the disolved iorn from the water so all my fixtures are not stained. The water to the outbuildings is primarly for hand washing, showers and toilet use. Fairly low volume, so rather than install iron filters at each building I connected the lines to the filtered water. The run to guest house is about 500 feet with a tee in the middle of the line to another building. My plumber reccomended that we install a pressure tank(s) at the outbuildings, which I did. A fair amount of discussion was held regarding installation of check valves on each tank. His reccomendation was not to install the check valves, for a variety of reasons. Per his advise I did not put any check valves on the tanks, the system works quite fine. However, here is the problem. I also water my lawn and use unfiltered water for this. The problem was with pressure tanks on either side of the water softner there would be reverse flow through the softner when the pressure is dropping in the main pressure tank ahead of the water sofner. The solution was to install one check valve on the outlet side of the softerner. No back flow and a bonus was steady water pressure in the house even when spinlkers are running. Roger