Simple expansion tank replacement, or MORE?

hot_rod

on your first picture, what size is that air vent that is in then pipe?

if it's 1/2" remove it and connect the expansion tank and fill valve there.

Now you are pumping away from the tank.

add the air purger as you show on the recent drawing.

now you have air removal at the hottest point and the pump to tank relationship correct

SWEI

hydronical said:

Never considered (or used) pex before, but I like the idea of flexible tubing so I can connect the x-tank to the air separator and place it on a 5-gallon bucket. Are there other acceptable flexible connectors, such as corrugated copper or braided stainless steel, like you see with water heater connectors? Or is pex the best choice?

I would trust properly installed PEX over the other options for this application.

Below, I added an air separator on the supply main, and attached the cold water feed and expansion tank to it, using a flexible connector (due to space constraints). How can I improve this system overhaul (given that the circulator pump cannot be moved)?

@bmwpowere36m3 and @Shane had suggested to keep the cold water feed on the return main, and mount the expansion tank near it, before the pump.

I'm with them. Plug the bottom port on the air separator and tee off above the pump supply.

hydronical

Conceptually, I understand that the expansion tank must be installed on the SUCTION SIDE of the circulator pump. Even though the pump is installed near my boiler on the return side, isn't it essentially sucking water out of the boiler through the supply line? This would mean the entire system is on the suction side of the pump, except for the very small length of pipe between the boiler and pump.

Does this argument have any merit?

If my logic is flawed, then I will install the x-tank on the 1/2" cold water feed pipe, between the fill valve and a tee on the vertical return main. However, if my logic is sound, I'm wondering again whether it makes sense to install the x-tank at the air separator on the supply main, as in my recent diagram?

hot_rod

here is what it looks like graphically. Pumping away from the tank adds the pressure that the circ develops when it runs. this assures good air removal.

Two pressures in a hydronic system, the fill pressure that you put in via the fill valve, called static pressure. this pressure assures the system is filled to the highest point and has at least 5 psi at the highest point in the piping.

The pump adds it's "head" or pressure increase when it starts. Somewhere around 5- 8 psi depending on the circa and the piping circuit. So the second the circa starts you see a pressure increase downstream from it, when pumping away from the tank.

When you pump towards the tank, the pressure that the circ develops, subtracts from the static fill pressure, so low pressure throughout the loop.

The circulator doesn't really pull or suck fluid, it sort of splashes the fluid around the circuit, it creates a pressure difference delta P.

In these drawings notice the 12 psi static or fill pressure, and how the circ delta P is added or subtracted. If you have a large, high head circa you can actually have a negative pressure at the circ inlet when pumping towards the tank.

Drawing credit : John Siegenthaler Modern Hydronic Heating 3rd edition

hydronical

Below, I added the Caleffi Discal Air Separator with a cap on the bottom port. The expansion tank has been moved to the 1/2" cold water feed pipe, after the fill valve. I also added the Caleffi DirtMag, Caleffi Backflow Preventer and AutoFill. The pipe sizes are now labeled.

Is my latest diagram correct? What other tweaks can I add? Should I keep the float air vent near the T&P valve? Is it okay to use 6 feet of 1/2" barrier pex to connect the x-tank, which will be standing inside a 5-gallon plastic bucket?

hot_rod

Whew! You made it:) Sure you can leave that air vent on top of the boiler, it does remove an air that traps at the highpoint. Most new boilers, mod cons especially include a high point vent.

Yes the pex to the tank can be 6 feet or more.

SWEI

That works. Go for it.

hydronical

Does it make sense to add a low water cut off? How/where?

wogpa67

They should be mandatory. If you are leaving the the air vent you could just add a tee closest to the boiler then stack the vent and prv. Cheaper insurance.

wogpa67

Or actually you have the tee capped passing the air vent put it on the bull of that tee.

hydronical

Now that the weather is getting warmer, I'm going to order the parts and do the work... How should I decide between sweat vs. NPT connections for the components? I have limited experience sweating pipes, but can get the job done. Also, is different solder and flux used on hydronic heating systems as compared to domestic water systems?