Pressure of pipe leading to Expansion tanks

[Jasesun23]

I'm trying to figure out why the pressure gauge on the branch of pipe where my Expansion tanks connected is always 2-3 psi lower than the rest of the system. That is when the system is cold or when it is running. I've tested all 3 pressure gauges I have and they are all within .5 of each other.
I have two tanks because with only the Extol 60 at 170 degrees running my system would get up to 25 psi. This was setting off my relief valve which I need to replace anyway. With the second tank added my system only reaches about 21 psi.
So for example my expansion tanks are set to 13 psi. My system is also set to 13 psi. However when I open the shut off valve going to the expansion tanks the gauge on that section reads ~10.5. When the system ran last night it got up to 21 psi on the gauge in the boiler as well as another gauge I have on the drain valve near the boiler. However on that section of pipe where the expansion tanks are it is 18 psi with the system running. I find this odd. It is 3 psi less no matter if the system is running or cold. I switched gauges around to make sure it wasn't an issue with the gauge.

Is this an issue? I thought with the circulator not running the psi of the system should be the same everywhere?
A though I had are it might have to do with that green Thrush flow control valve No.115 that leads to the Expansion tanks.
*expansion tanks will be supported to ceiling once permanent setup is figured out.

thanks
Jay

 

[Reach4]

I'm trying to figure out why the pressure gauge on the branch of pipe where my Expansion tanks connected is always 2-3 psi lower than the rest of the system.

What is the altitude difference in the gauges?

 

[Sponsor]

 

[Jasesun23]

I have one pressure gauge at bottom of boiler drain valve. I just open the drain valve to test once in awhile , so that gaugue is on the floor in the pic. The second is at the top of the boiler unit , so that's 3 feet above the floor. And the one by the xpansion tanks is towards the ceiling, so that's 7 feet from the floor. So they are all within 6 feet of each other vertically

 

[Reach4]

6 feet of water/altitude is 2.6 psi.

 

[Jasesun23]

Well look at that. Can I assume that if I put a pressure gauge at a radiator on the second floor an additional 16 feet up it would be 7-8 psi lower than the one next to the expansion tanks?

 

[Reach4]

Well look at that. Can I assume that if I put a pressure gauge at a radiator on the second floor an additional 16 feet up it would be 7-8 psi lower than the one next to the expansion tanks?

6.94, or about 7.

 

[Jadnashua]

That pressure change with altitude is why the system needs pressure in the first place...you must keep it under pressure everywhere so there's no chance of it flashing to steam. Plus, it's harder to pump if you can't be assured it's always full of water.

The pressure in your potable water is from a combination of pumps, but mostly, it's from the height of the water towers. That's also why they tend to put them on top of hills, the extra height gives better water pressure without the pump having to run constantly.

It's also why a shower on the top floor might not appear as forceful as the same setup in say the basement...altitude with the associated gravity effects. The factor is 0.43#/foot elevation change.

Learning is fun...