What is the pressure at this point

metrosilo

Hey everyone. 

I’m looking at a system and my pressure is going about 5 psi over what i have been trying to set it at. I watched  Dan Holohan’s awesome Hydronics video and I had a question about what the pressure would be before the point of no pressure. 

See picture below. I want to know what the pressure will be where I made a mark. Basically before the tank tee. 

hot_rod

Do you have an autofill valve connected to the system? If so where is it connected? The exp tank was pre-charged to the fill pressure?

To know exactly what the pressure will read at various points in the system, you would need to define that piping circuit. How many feet of pipe, how many fittings, any valves, any heat emitters? The pressure at the tank should remain at the static fill.

If you pump at the tank, pressure would drop, possibly below sub atmospheric in the system.

Jamie Hall

The pressure at your arrow should be at about 12 psi, give or take, when the system is cold, regardless of whether the pump is running or not. As the system heats up, you should expect it to rise -- perhaps to as much as 18 psi -- again, regardless of whether the pump is running or not. Of it goes much higher when hot, you should check the expansion tank for correct precharge and size. If it is much lower at any time, the expansion tank again should be checked.

metrosilo

Thanks Jamie. Just to expand, as the water heats up, the water pressure increases to 18psi like you said. Once the 18psi water hits the expansion tank, I would assume it drops back to 12 as the tank takes the excess pressure away. 

Am I on the right path?

STEVEusaPA

metrosilo said:

Thanks Jamie. Just to expand, as the water heats up, the water pressure increases to 18psi like you said. Once the 18psi water hits the expansion tank, I would assume it drops back to 12 as the tank takes the excess pressure away. 

Am I on the right path?

Sort of...
If the pressure is building just from circulator differential, then yes, but if the pressure is building from thermal expansion (water heating up), then it's possible the expansion tank has failed and/or is not big enough for the system.

metrosilo

I think my tank is not big enough for the system. I’ve set the pressures up cold and I can see the pressure rising every time the boiler heats up. 

Thank you

Jamie Hall

metrosilo said:

Thanks Jamie. Just to expand, as the water heats up, the water pressure increases to 18psi like you said. Once the 18psi water hits the expansion tank, I would assume it drops back to 12 as the tank takes the excess pressure away. 

Am I on the right path?

Not really. The tank takes the increased volume from the heated water, but to do that the pressure in the tank increases (the air compresses to absorb the volume change). The change in pressure is strictly due to the change in temperature of the water and the volume of air in the tank. The smaller the tank, the less the pressure change will be. In general one kind of looks for the pressure when hot to stay below 20 to 24 pounds, but that's not fixed.

It would be more correct to say that the tank takes the excess volume away.

hot_rod

In a closed loop system the expansion tank connection develops the point of no pressure change. When the circ starts spinning it cannot change that pressure, To do so you would need to either add more water, or subtract water from the system to change that pressure. Since the system is sealed, closed that is not happening.

As a system heats up, thermal expansion will cause the pressure to to increase. Based on the fluid, the amount of fluid and the temperature increase. The gazzilion of molecules become slightly larger, the amount of fluid does not increase, per say.

A properly sized expansion tank would allow the pressure at the relief valve to reach a pressure about 5 psi lower than its opening pressure.

If you have a 30 psi relief, the system going up to 24- 25 psi is not a concern. 12- 18 indicates the tank is sized adequately, actually oversized

There are sizing programs at all the tank manufacturers sites.
The math is a bit involved.

https://hvac-eng.com/expansion-tank-sizing-formulas/