Homeowner and plumber disagreement

What i want to know is where the cold comes in and the hot goes out and how it is routed to figure out if it is ghost flow within the same pipe or between 2 different pipes. If it is just from gravity then a pipe loop should stop it. If the circulator for the aqua booster is pushing it between 2 different cold water pipes then it probably needs a mechanical check valve.

If it goes all the way back to the service before anything branches off, it is probably gravity ghost flow from just having essentially boiler hot water below the riser with the red handle valve and putting a deep loop in that riser would stop the ghost flow.

The hammer is only in the cold pipes because the hot pipes have free access to the expansion tank, there is no check valve to slam shut when the pressure drops back down after it briefly surges up.

I think that @HVACNUT knows way more than I do so that is the correct way do to an aquabooster, but I don't think that your problem is with the aquabooster but that where the cold connection is tapped in, it makes a good thermal syphon to bring the hot water in to the cold supply. It has very hot boiler heated water at the bottom of a tall riser to the cold supply, that is the perfect condition for gravity circulation within that pipe. 90 it down where it tees off the tankless coil, loop it down to the floor, then bring it up or loop it 360 to the floor and connect the cold there if you want to be extra cautious and the need for the check valve and expansion tank goes away.

Unless you put a tire gauge on the ThermTrol you don’t know what the pre charge is, lower the system pressure and check it yourself.

there is a specific way that a 3 way mix valve needs to be plumbed when a recirc pump is used

What ends up happening is recirc water enters the cold port of the mix valve and then the valve is unable to adjust. The cold port needs to see cold water not blended warm. You end up with temperature creep and the hot outlet from the mix valve will be the temperature of the tank or coil. .Ill attach a drawing

Also, 3 way valves do not necessarily need to have check valves on the h&c if they are plumbed properly

More about DHW systems here and a screen shot of the recirc piping This shows a building recirc, I think the loop from coil to tank would behave the same.

https://www.caleffi.com/sites/default/files/media/file/Idronics_11.pdf

IMO where this all went horribly wrong was the plumber's desire to re-engineer a system that worked fine for decades. He should have just come in, did some troubleshooting, and repaired / replaced whatever changed causing the ORIGINAL problem. Now with the newly engineered system there are worse problems that did not exist before.

With all the check valves and a NEW added pressure source (the air in the expansion tank on the Hot water side) you have issues. Normally an air cushion helps minimize water hammer. In this case it maybe helping to cause it.

Two old school things to minimize water hammer, larger piping reducing the flow velocity in the piping, using 3/4 inch pipe instead of 1/2 inch pipe and simple pipe based air cushion.

water hammer generally starts when a fast acting valve suddenly stops flow. The higher the velocity, speed of the flow, the higher the potential for hammer

So you may have two issues, high flow and a valve slamming quickly closed. Aka a swing check.

Water hammer arrestors really need to be at the point where the hsmmer is starting. An arresters 10’ or any distance has lost the ability to catch that shock wave, adequately

That is why there are so many versions of arresters for washers, dishwashers , ice makers, etc

Regardless, treat the problem not the symptom, an arrester treats the symptom. Find the cause of the hammer, plenty of systems like this work without issues. As you already know

there are a number of States that require a listed thermostatic mix valve on any DHW device. If you are in one of those States a licensed plumber would be required to install one.

The mix valve can both protect against scalding and also extend the amount of DHW available.

There is not much you can do to damage an expansion tank. Is there is any air pressure at all when you depress the tire stem on the end, the tank is fine.

DHW tanks typically go on the cold supply, but anywhere in the loop before check valves, they will work. Think of it as a spring in the system, it responded to pressure change created by expansion of the water as it is heated.

In that fig2 above, that system would not even require a thermal expansion tank unless the cold supply had a check or backflow preventer on the cold supply to the building. That is why the note shows “optional”

But it is a wise idea to install an expansion tank, as the water providers are adding BFD to most systems nowadays for liability reasons.

where "before the check valve" is depends on your perspective. the expansion tank and the water that is being heated in the tankless coil and the storage tank have to be on the same side of the check valve. in this case they

what the plumber did isn't exactly wrong but they didn't diagnose the problem correctly. I am pretty sure you were getting thermosiphoning/gravity flow out of that hot water pipe in to the cold water supply, not the circulator for the aquabooster forcing the water in to the cold water system. they solved the wrong problem using types of check valves that are noisy.

i think they own the problem now and some of the check valves they added are redundant if that mixing valve has integrated check valves which most do but what they did wasn't wrong per se. the extra check valves gives you more places for potential noise from the check valves.

i'm also not sure why the plumber didn't supply the parts, that makes responsibility more tricky, especially if it was your idea to supply the parts.

@steam-rookie

I can certainly understand why your stressed out and unhappy with the plumber. Anyone would.

The way I understand your problem as it now exists is that you have a water hammer problem. Nobody knows what is causing this but my suspicion is that it is a check valve issue.

Moving the expansion tank will not fix the water hammer IMHO I don't think it will have any effect on your system. Yes it should have been installed vertical and the air pressure in the tank must be set to the city water pressure to operate properly. And it should be on the CW line but IMHO that is not causing your issues.

Why no one besides me has mentioned this I don't know as most are aware of this. (proper expansion tank air pressure)

Not trying to add to your stress but I hate to see you do all this work and end up with the same issue.

the "before" and "after" the check valve depends on your perspective, what part of the system you are looking at it from.

@EBEBRATT-Ed i think i rementioned the precharge on the expansion tank a time or 2 in with other things.

i think what was happening was that when there was boiler temp water at the bottom of that ~3' riser in the feed to the tankless coil the stack effect from the height of the pipe was making the hot water at the bottom circulate to the cold water pipe at the top. there are less invasive ways to solve that.

Focus on just this part of the system. The tank and the tank :)

The HW tank can "communicate" with the expansion vessel through a few inches of 3/4 copper tube, an FIP adapter and the tee. That looks like a brass 3/4 X FIP tee that the tank screws into?

I don't see any check between the top of the HW tank and the tee connection for the expansion vessel? Is that true?

That being said the HW tank probably has heat trapper nipples on H&C. Red and Blue. They have a small ball or rubber flapper to prevent thermosiphoning. But they are not a 100% bubble-free seal, so expansion should push by.

So the only way the expansion vessel could not accept that expanded volume as the tank is heated, is if it did not have any air charge, was completely waterlogged? Usually you can shake the tank a little and determine if it is empty or full of water. Looks like someone wrote a pre-charge pressure on the tank? Or is that an install date? I always wrote the pre-charge that I put into the tank for the next service person to know. I have heard of new expansion tanks coming out of the box with 0 psi. The schrader valve leaked the pre-charge out.

I believe in California the tanks cannot be shipped with over 30, maybe 40 psi? @Larry Weingarten would know. So most every tank needs to be adjusted.

If for some reason incoming water pressure gets elevated, the tank pre-charge should be matched. That expansion tank has plenty of capacity for your system even if pre charge is off a few psi.

I agree with others IF the tank is pre-charged properly, regardless of the connection location, this is not the water hammer gremlin.

My money is on the swing checks, get them out and I'll bet the problem goes away.

@hot_rod I think the thermosyphoning was happening in this riser when the pipe at the bottom is filled with ~180f water from the tankless coil and the cold water pipe at the top is ~50f

I may be mistaken because it is hard to see in the older picture, but I think the galvanized cold water piping was replaced with copper relatively recently.

If i'm following right the numerous check valves did fix the hot water in the cold problem, it just caused a new problem with the check valve slamming shut.

Some fixtures have anti-scald features. If that has a leak hot to cold the circulator may cause a parasitic flow through the leak.

This is the type of check to look for. It has a conical flow friendly shape, rubber sealing surface, high Cv and low pop pressure. This is the check that you find in circulators now, most mix valves and many other plumbing and heating applications.

A serviceable check is a good option, they do get crud in them, from time to time. Or you may need to replace the check assembly some day.

Hi @hot_rod , I may be overly cautious about this. I've had the experience of degrading rubber from these tanks getting into the hot tap water as tiny black bits. That tank really should have a stop and waste on it. Perhaps creating a pipe heat trap also will help it to live a long and prosperous life. 🖖

Yours, Larry