Hydronic pressure change

rr1122 · December 20

Hi,

I have been struggling to figure out whether I have an actual leak in my hydronic in-slab radiant water system and I was wondering if others have faced this issue?

Over the summer, the system was off and I would notice the psi would drop approximately 0.5 psi per month. The only part that is not functioning correctly it seems is the water feed regulator, which only works using the manual lever. During the heating season, I’ve seen the pressure change from around 15 psi all the way down to around 8 psi (when it’s on the non heating cycle). Whenever I see it around 8 psi, I fill it up again using the manual lever. Also note, this system has no way to drain it to do a pressure test currently….other info is 19 gallons of well water is flowing through the system and the system is around 23 years old.

Does this sound like a leak? Could outside temperature swings cause this? Temperature during the winter can change by as much as 30 degrees within a few days. Could it be the faulty pressure regulator?

hot_rod · December 21

does it ever drop to zero? The system could run on 5 psi if it stays at or above that

As long as the boiler doesn’t have a low pressure safety cut out

rr1122 · December 21

I’ve never seen it go down to zero before. The other thing I did notice is that if I open that drain elbow near the expansion tank, a few very small drops of water will come out and the pressure will drop a huge amount (around 10 psi).

Big Ed_4 · December 21

That 100# charged tank ?

rr1122 · December 21

yes the legend valve next to the 100# charged tank.

Big Ed_4 · December 21

Your pressure drop over that time frame would not be a concern of mine .

The tank pressure is not in the range needed in a residential heating system .. I am sure not correct ….12# needed in a 3 floor home …..

rr1122 · December 21

Oh, Interesting! My home is single level ~1000 sqft.

LRCCBJ · December 21

You must remove the tank, reduce its pressure to 15 psi, and reinstall the tank. At 100 psi, there, effectively, is no tank. It's just a pressurized pile of air that that has no chance of compressing when the system pressure varies slightly. You might as well have a cement block attached to the system…………same result.

rr1122 · December 21

sorry I think I misunderstood. I have very little understanding of expansion tanks. I labeled that tank based on the below image. I guess I don’t know what it is actually pressurized to without testing it correct?

Big Ed_4 · December 21

Nice tank …..

The pressure in the system in the systems needed to raise the water column of heating water in the system . 3' rise per # minimum ..

The tank is needed to take up the expansion of water heating up in a closed system . To equalize the system pressure . The tank pressure should match the system pressure in text book ….

hot_rod · December 21

with the NSF shield on the tank it is rated for potable water, so it probably had a 45- 50 psi precharge. The 100 psi is the listed DOT test pressure

Tanks that ship into California have a pressure limit, 40 psi maybe? Larry may know.

So you need to remove that tank and check the pressure on the tire stem valve. Adjust to 12 psi, refill the system to 12 psi.

GroundUp · December 21

Agreed with the above, but also, the expansion tank connection to the system is on the wrong side of the circulator so every time the circulator runs, it will increase the pressure at the same time because the tank can't do its job.

PC7060 · December 21

https://forum.heatinghelp.com/discussion/comment/1828347#Comment_1828347

Good point but I think it will create lower pressure on the intake / boiler side given the PONPC is on the discharge side. Minor distinction.

GroundUp · December 21

https://forum.heatinghelp.com/discussion/comment/1828386#Comment_1828386

If the pressure gauge were on the return side and this was a boiler, I'd agree.

hot_rod · December 21

it looks like a tankless water heater? I think you will want to pipe that as a primary secondary.

Jamie Hall · December 21

https://forum.heatinghelp.com/discussion/comment/1828386#Comment_1828386

That depends on if there is an automatic refill pressure maintaining valve from a water supply and, if there is, where it is hooked up. If that is on the intake side of the pump, it will try to maintain the set pressure in the system when the pump is running — and when the pump shuts off, the system pressure will then be high.

hot_rod · December 21

You kinda have two strikes against you, pumping at the expansion tank, and a high pressure drop heater on the suction side.

Here is what pumping at the PONPC looks like. If you knew the pressure drop through the tankless that could be figured into the pressure profile.

Screenshot 2024-12-21 at 3.32.22 PM.png

rr1122 · December 22

https://forum.heatinghelp.com/discussion/comment/1828448#Comment_1828448

Thanks for all the insight. So just to clarify, when a hydronic system is functioning correctly, you should not see the psi go from 12 psi down to 5 psi when the system goes from on to off in a span of 15 minutes. And these two strikes you mention could be the reason for these issues?

hot_rod · December 22

well, it depends on where the gauge us located in the system. If it is at 12 psi with no pumps running , and drops to 5 when you run the pump. Then bsck to 12 when it is turned off?

GroundUp · December 22

A properly installed hydronic system will remain the same pressure regardless of status

rr1122 · December 22

https://forum.heatinghelp.com/discussion/comment/1828526#Comment_1828526

When the pump is on, the gauge is at 12 psi. When the pump turns off, the psi goes to 5 psi within 10 minutes. Then when it turns back on, it goes back to 12 psi. The gauge is located at the returning pipe from zone 3, as highlighted in yellow below:

psb75 · December 22

It is also a bit odd to have TWO water sources (hot AND cold) feeding the autofill valve. Admittedly both water sources are likely to be at the same household system pressure (something above 30-40 psi.) being lowered by the autofill valve to 12-15 psi.

hot_rod · December 22

As the water in the system heats, the water expands and pressure will rise. Maybe a few psi, maybe 10 or more. It depends on the temperature change and the amount of water in the system. Amtrol has tables to calculate that pressure increase if you want that info. You need system volume and temperature hot and cold condition.

Pressure, dynamic pressure, is the increases pressure you see when a circulator runs. To cause fluid to move a circulator lowers pressure on one side, the inlet, and raises it that amount on the discharge.

We refer to that energy that is added to the system as head.

So an increase in pressure caused by warming the water is evidence of thermal energy being added to the fluid.

Head is evidence of mechanical energy being added. That energy is used to overcome the flow resistance of all the components in the circuit.

So a gauge on both sides of the circulator indicated how much mechanical energy has been added to the system.

The gauge you are reading is on the suction side of the circ, so it will drop, below the 12 psi static. Add a gauge to the discharge port of the circulator and it will increase above the static pressure.

If the pressure over the summer, with the system off, both pumps and burner off, fill valve turned off, does not drop to 0 you should not have a leak.

If it read 18 psi the day you turned it of, in a hot condition, yes the pressure will decrease as the water cools.

If you leave that Watts fill valve turned on, it can mask a leak as it tries to maintain the 12 psi. So do your leak testing with it tightly valved off.

Valve it off for a month, then turn it back on, if you hear water flowing for a period of time, then yes the system has lost some volume of water. If in fact you keep adding water month after month, it's going somewhere??

You must have a very accurate gauge and good eyesight if you observe a .5 psi change :)

Screenshot 2024-12-22 at 9.13.02 AM.png

rr1122 · December 22

https://forum.heatinghelp.com/discussion/comment/1828610#Comment_1828610

Sorry that part of diagram is not clear. The well water flows into that hot water tank and then that tank distributes that water to the taps in the house.

rr1122 · December 22

The system has two 90 degree drain elbows (see the diagram). One is located where the expansion tank is and the other is located at the zone 1 input( why would that be there?). I assume the one near the expansion tank is for bleeding air from the system? If so, any recommendations on how to know when bleeding air is necessary since these are manually operated.

thanks

hot_rod · December 22

The system really should have a high efficiency air separator. Every time any air bubbles are let out, water needs to go into that space. So if you are bleeding manually now and then, pressure will drop until you bump it back up.

Might be time to replace the defective fill valve and add a Discal. Pipe a primary secondary or hydraulic separator. During that process leave a pressure test on the distribution loop.

There are not too many things that are correct about the equipment and piping :(

If you isolate the boiler and expansion tank you could put 60 psi or more on the piping. That type of pressure tends to show tiny leaks much quicker than a 12 psi test.

Hydronic pressure change

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