Am I killing the mixing valve?

Gordan

So, in a previous thread I reported experiencing severe water hammer after installing a Honeywell AM-101 mixing valve on the outlet of my indirect. The mixer has two check valves, one on the cold inlet and one on the hot inlet, and I removed the cold-side check valve which resolved the hammer issue - now the pressure is allowed to equalize between the hot-side plumbing and the cold-side plumbing. Details (and pictures!) here: <a href="http://www.heatinghelp.com/forum-thread/134829/If-I-had-a-water-hammer">http://www.heatinghelp.com/forum-thread/134829/If-I-had-a-water-hammer</a> . Shameless request for feedback: any issues with how I've got things plumbed?



What I'm wondering is this: in a way, this now puts the mixer into the role of acting as a differential pressure bypass, where water shock may force it open. Can I expect this to cause longevity issues with the valve, either due to erosion (I'd figure this would be less likely because of low volumes involved) or the thermal element?

icesailor

Mix Valve:

Where do you live and where is this install? It wouldn't pas in Massachusetts.

Where is your heat trap for the mixer? The thermostatic mixer of the valve will burn out because of the constant circulation through the valve. The TPR valve isn't in the top 1/3 of the tank.

I've put a lot of those mixers in and never had the problem you describe. There's something about how you piped that that bothers me but I don't know what it is.

It's a nice clean heating install though.

Gordan

Thanks, ice -

The tank is a reverse indirect - the potable water is in the coil, so there's nowhere to tap in the tank for the T&P relief valve. The manufacturer shows it being installed on the top discharge, as I have it. The mixer manufacturer says to install the mixer right on top of the tank, within 8", and no heat traps - there's a check on the hot side to prevent gravity flow. There used to be a check on the cold side too, which I removed to get rid of hammer - maybe the reason you didn't experience hammer is that the mixers you installed didn't have the checks in them? For what it's worth, there's no gravity flow now. If that check ever gets any crud in it, though - as already happened - well, I guess that's something to consider, as I did have gravity flow in that situation...



Let me know if you figure out what's bugging you about that piping. :-) It's a bit convoluted, I know, but it was the best way I could figure out to work with where the boiler is, where the boiler side connections are, and where the domestic plumbing was... I'm in PA, as is the install. It's my home system.

Gordan

You know what bugs me about the install?

The fostapex from the boiler to the indirect. I mean, there's no functional problem with it - that 15-58 is on low speed and doing just fine - but aesthetically it bothers me. If I find myself bored, I may repipe it in black pipe.

icesailor

Tanks:

That tank must be like the Weil-McLain tank. A tank within a tank. There are special instruction for install in MA. They have a separate dip tube so you can drain it because there are no bottom drains. And you have to put a IPS tee at the hot outlet so the extended element goes into the top of the tank.

I use that valve all the time. The instructions call for a heat trap. Where the CW and HW outlet are at the top of the tank, you need to install the valve below the top of the tank to stop the syphonage. Then you don't need the checks.

If you have a lot of hardness in your water in the form of calcium, and they stick, take the valve out and soak it in white vinegar for a few hours. It will restore it to like new condition.

Gordan

Not trying to be a contrarian...

The instructions available at the below link are the ones that came with my valve; maybe the ones you installed were an older version? (Edit: correct link and figure reference provided.)



http://customer.honeywell.com/techlit/pdf/PackedLit/62-3075EFS.pdf



Fig. 1 shows installation at the water heater. Now, I'll agree with you that heat trapping it couldn't have hurt anything, but I don't want you to think that I didn't consult the instructions. :-)

Gordan

I agree with you overall, though...

Thermal-trapping the valve would remove the need for the remaining check valve. So why didn't I do it here when I did it on the heating side? Well... mostly because I was going for a relatively compact install and wanted to keep the piping on top of the heater where noone would bump into it, and the check valve (and the manufacturer's instructions) allowed me to do that.



That was before I encountered the problem caused by the check valve, so maybe I should reconsider.

Mark Eatherton

Not a tank within a tank design....

The Turbomax and Ergomax tanks are a reverse indirect tank. It is a tank, with a substantial amount of copper heat exchanger on the inside of it. Boiler water is run through the tank, and potable water is run through the coil.



Potable cold water enters the heat exchanger though the bottom of the tank, and exits the top. There are means of being able to drain both the potable and non potable sides of the tank.



It might make more sense to equate it to a large side arm coil, like an Everhot, except with capacities to 119 gallons. It is 99% efficient in transferring heat from the source to the load, and only requires boiler water temperatures around 10 degrees F hotter than your target DHW goal. Can't say the same about the tank within a tank design...



ME

Mark Eatherton

The strange thing I see...

Why the inverted trombone on the cold supply?



If you had directed the cold to the tanks potable inlet first, and then gone to the mixer, you'd automatically have had an anti-thermosiphon trap. Hot water can't naturally flow down. It has to be forced.



You have checks on the hot water outlet, eliminating the need for a siphon trap there.



As an experiment of the overall capabilities of the system, try lowering the DHW operating temperature below the mixing valves set point, and see how it handles the loads. You might find that the mixer is not necessary at all, especially if the majority of the loads are showering loads.



ME

Gordan

And I've got you to thank for even knowing that such beasts exist.

Seriously, for what it's worth, my system would look very very different than it is (for the worse, without a doubt) if it weren't for what I've had the privilege of learning from the likes of you, Dan, Brad and (NRT.)Rob, among many many others, and vet my ideas before it cost money to change my mind - and get valuable input from the likes of Chris on the (almost) finished product. Many thanks, guys!

Gordan

The thought there was to prevent siphoning from the tank inlet

My mind's eye saw cold water from the cold supply sinking from the cold piping (which is overhead) into the tank, and the hot water from the coil in the tank going up. There were three kinds of siphoning I was concerned with: tank to cold via the DHW cold inlet, and then two via the DHW hot outlet: up the mixer to the hot piping, and through the mixer to the cold piping. These last two were solved by the check valve(s), but the inlet-side flow I thought I'd need a trap to solve. In reality, I haven't seen thermosiphoning on the inlet side. Well, not the way I thought - but when the hot side check valve was stuck open, I saw hot water siphoning up that trap from the hot side via the mixing valve, and establishing a gravity loop between the outlet and the inlet of the tank! The water was much hotter than the mixed temp, which is how I know that it was flowing from the tank and not back from hot distribution.



The piping is more convoluted than I'd like, so you may be getting thrown for a loop (heh.) The leftmost upright pipe is the cold supply (goes up to the ceiling) and the one next to it, with the temperature strip on it, is the mixed outlet (likewise goes up to the ceiling.)



Things work well at the moment - no hammer, no gravity flow - so, if the longevity of the mixer won't be a problem due to it acting as the path for pressure equalization between the cold and the hot distribution piping, then I'd just as soon not tinker with it... The scenario I'm most worried about is where the hot water has not been flowing for awhile, making the mixing valve swing over to full hot. Then there's shock from a fixture closing on the cold side, propagating to the mixing valve from the cold side and squeezing through to mixed, and then equalizing back from mixed to cold by squeezing through again. We're not talking about large water volumes here but there's enough to push a slug of warm water back into that lower cross-pipe that goes from cold riser to mixer.

rlaggren

Great little lesson here

Thanks, Gordon



I wonder if the hot check had remained disabled whether your "trombone" would have stopped the siphon through the coil. It looks to me like it should have.



In that case the only need for the hot check would be a siphon _within_ the the single DHW supply riser. Not sure how much water would actually move in a small pipe - might not be able to maintain two directions of flow w/in the same pipe.



BUT. If you have water hammer issues, why not install a compression tank? Unless you're trying to maintain the absolute minimum of hardware...



Rufus

icesailor

Mix Valve:

The link to the valve on honeywell's web site shows the 8" drop for a heat trap.

I always use the Honeywell AMX 101 Direct Connect. I connect them direectly to the top of the water heater. I use them as "hot water extenders" with smaller electric water heaters where they may run out of hot water when set for 120 degrees. I set the tank temperatures higher and let the mixer control the outlet temperature.

The valve comes with one nylon check valve on the cold side only.

Gordan

I'm still committed to resolving the shock issue

I figure that this is being aggravated by high water pressure. I'll do what I need to do to fix the underlying issue; I'm just very relieved that I found and resolved the cause of something that was twenty times worse than anything I'd experienced. When I saw that pressure relief valve pop due to water shock, that really had me worried.



I wouldn't expect that trombone to stop a thermosiphon through the coil, and it didn't - I let it go overnight and in the morning that pipe was still hot. But it sure does stop siphon the other way, to the cold riser.