Flow when cold water turned from on to off firing water heater

styxplo

i have a HTP floor mount combi boiler. Whenever I shut the cold water off the hot water heater fires for a moment and shows a momentary flow of 0.28 gpm. This is with the hot water remaining off the entire time. It’s a relatively new install on a new construction house. There is only one bath room and a slip sink plumbed in. 

STEVEusaPA

When you shut the water off at a faucet, or when you close the cold water supply valve?

hot_rod

Do you have one of the small thermal expansion tanks installed?

styxplo

I don’t on the DHW side. Just on the boiler side

styxplo

At the faucet

mattmia2

It could just be bouncing on the air chambers in the plumbing system. my water will do this when i shut off a fixture but i don't have a water heater with a flow sensor. My old water heater had heat traps and you could hear those clicking as the water bounced on the air. I replaced the water heater about a year and a half ago and and still expect to hear clicking when something shuts off.

The only way i can think to fix it is to add a check valve and expansion tabk after the water heater to contain the flow within the system after the flow sensor.

styxplo

Ok. So if I add an expansion tank and check on the hot water outlet it should correct the problem?

mattmia2

Actually, it might not entirely, it could still flow in to the expansion tank, just not out. It will reduce it but may not eliminate it.

Thinking about it more, the water pressure drops when you turn on a cold fixture, the pressure in the air chambers pushes some hot water back in to the cold supply to equalize the pressure, then when the cold water turns off, that pressure in the hot water is lower so water flows in to the hot water to equalize it. If you stop the pressure in the compressible part of the hot water system from changing, the part with the air chambers, then there will be no flow when the pressure in the cold water drops and raises. Not sure if the flow sensor senses direction, so it might be producing a flow signal for both the flow out of and back in to the hot water piping. I'd see what others say, but I think my reasoning is solid. It would have to be a spring check that would already be closed when the pressure dropped in the cold piping.

SuperTech

I was always in the habit of installing the potable water expansion tank on the cold water line before the water heater. I've never seen one on the hot water side of the water heater. 

mattmia2

It should do the same thing before or after. Not sure if the potable tank is rated for hot water now that I think about it. In theory you could just add a check valve without a tank after the water heater since the water is no longer being heated at that point.

SuperTech

The only time I don't insist on a potable water expansion tank is when the home requires a storage tank from a well pump. I was taught that the storage tank handles the expansion and that code requires the expansion tank to be before the water heater when the home is on a public water supply. I'm sure @hot_rod or someone who is more of an actual super tech can explain all the reasons why that requirement exists.  

HVACNUT

If it's a single lever faucet, the flow switch in the boiler might close for a second. 
You can check it by temporarily bypassing it and opening and closing the faucet. What brand faucet? I'm not a plumber but maybe O-rings or something's not seated properly?

styxplo

It is well water with a pressure tank. The posts about air getting compressed and allowing the water to move when the faucets open and close got me thinking. There is a lot of trapped air in my system right now. Its new construction and there are 2 bathrooms full of rough in PEX thats capped and never been purged of air. I'm betting this is contributing to my problem. I will take care of this and see if it helps. Im thinking it can't be good for the boiler....its almost like its short cycling. Thanks everyone

hot_rod

I used this example, thanks to Ron George in my Safe Water presentation. It shows how water can "bounce" in a leg with a trapped air pocket. That could be enough to trigger a low flow switch a few times.

Hydraulic or elastic shock is another condition that can happens when flow is stopped quickly, especially with high pressure lines. Similar pressure wave up and below static pressure. Water hammer arrestors usually handle that surge, get all the air out and try it.

Caleffi does have low lead PlumbVents, float vents for potable water applications

mattmia2

So the "arrestor" has a damper as well as an air cushion so the bounce of the water on the air cushion decays more quickly and bounces more slowly?

JPL941

styxplo

I'm going to install a spring check on the hot side because it will be sometime before I will get to finish all the plumbing. I think it is worth a shot because I dont want to damage my boiler. That being said does the well tank cover the need for an expansion tank. It is pretty far away

HVACNUT

Yes the well tank is your domestic expansion tank. It should be pressurized  the same as the water pressure. It needs to be done with no pressure on the water side. There should be  isolation and drain  valves, plus a gauge near the bottom of the tank.

SuperTech

HVACNUT said:

Yes the well tank is your domestic expansion tank. It should be pressurized  the same as the water pressure. It needs to be done with no pressure on the water side. There should be  isolation and drain  valves, plus a gauge near the bottom of the tank.

Dont forget to install a pressure relief valve as well. 

styxplo

The check valve worked. Easy fix. Thanks!