Expansion tank location

MikeDurigon

Hydronic heating system in a 9 plex built in 1970. Circulation pumps on the return piping. Existing Weil McClain boilers (2) have the expansion tanks piped off the boiler vent tapping. No air scoop or spiro vent on the system. Will the expansion tank function better if a tee is placed on the return piping of each boiler. My thinking is lower temperature would be better for the bladder style expansion tanks. There is one expansion tank per boiler. This is not a primary secondary piping arrangement. 

EBEBRATT-Ed

The usual practice is to come out of the boiler with the supply then through the air separator with the expansion tank connected to the air sep and the circulators pumping away from the expansion tank on the supply.

You want hot water at the air separator to remove the air bubbles.

On your set up do you have plain steel compression tanks or bladder type expansion tanks??

You can leave the pumps where they are and add tees in the returns to connect the expansion tanks to the suction side of the pump.

But when you change it that way you are putting the relief valve on the boiler under pump discharge pressure which could be a problem depending on the pressure you need in the system and the building height.

What pressure are you running at on the pump discharge and what is the relief valve setting. Moving the expansion tanks also means moving the water make up connection. because you changing the point of no pressure change.

Also if you have two boilers are they on a common system or separate systems??

If they are on a common system the expansion tanks should be teed together.

Jamie Hall

An expansion tank will work much better if placed a few pipe diameters -- but not too many before the inlet to the main circulating pump. In your case on the return piping just before the circulator.

The logic is this: the expansion tank will be at the point of least pressure change (we often call it "no pressure change" but that isn't quite exact). If that point is just before the inlet to the pump, all the rest of the system will be at a somewhat higher pressure, minus any gravitational effects. If the expansion tank is after the pump, then all the rest of the piping will be at a somewhat lower pressure, and you may get air pulled in or even flashing to steam and loss of circulation or pump cavitation. Not good.

The connection to the expansion tank should also be the location of the connection to the pressure regulating valve on the makeup water.

Now... you really do need a spirovent or some other air separator on the system somewhere...

MikeDurigon

system runs at 14 to 16 psi. Single storey building. Contract is to replace boilers only. I did add in a tee at the high point of the return with an automatic air vent. I do understand about putting the expansion tanks at the point of least pressure change but that would involve quite a bit of piping changes. Existing system worked with the expansion tanks mounted off the boiler vent tapping. Not ideally the best location but will it function with a tee for the expansion tank on the return into the boiler. I guess if it starts popping the relief valve the answer will be nope

MikeDurigon

Relief valve setting is 35 psi

EdTheHeaterMan

MikeDurigon said:

system runs at 14 to 16 psi. Single storey building. Contract is to replace boilers only. I did add in a tee at the high point of the return with an automatic air vent. I do understand about putting the expansion tanks at the point of least pressure change but that would involve quite a bit of piping changes. Existing system worked with the expansion tanks mounted off the boiler vent tapping. Not ideally the best location but will it function with a tee for the expansion tank on the return into the boiler. I guess if it starts popping the relief valve the answer will be nope

To be clear. You don't put the expansion tank at the point of no pressure change. The expansion tank IS the point of no pressure change. That is because no matter what happens to the pressure in the rest of the system, the amount of water that is compressing the amount of air in the tank can not change unless you add or subtract something from the tank.

For example: if you place the pump on a loop and have the tank on the discharge side of the pump. Then turn on the pump. The pressure on the discharge side of the pump will become higher that the inlet side of the pump. When that happens, there us a fixed amount of water and air in the tank. Turning on the pump will not add water to the tank... where would that water come from? From the loop of water in the pipes? If you take say 5 ounces of water out of the loop and put it in the tank, what will replace that water in the pipe loop? and where will that come from?

Likewise if you place the expansion tank on the suction side of the pump. When you turn on the pump the discharge side will become higher pressure than the suction side of the pump. No water will leave the tank if the pipe loop is full of water. If you take water from the tank, where would it go? If the loop pipes are already full, there is no room for more water from the tank to get into the loop pipes. That is why it is called the point of no pressure change.

@Jamie Hall refers to the point of least pressure change because no system is absolutely void of air in the loops. So the air in the loop will expand or compress based on where the pump is located in reference to the air bubble. So there may be minute amounts of water going in and out of the tank based on that minor point. Also the water temperature change does actually change the volume of the water in the system. and one of the ways to change the pressure in the system is to increase the amount of water. so when the water heats up it actually becomes more water volume. But to understand the concept of the point of no pressure change, splitting hairs on this discussion only makes for confusion when trying to explain the concept.

First thing is to understand that the point of no pressure change IS the expansion tank. After that you can talk about the "what if's" that will bend the rule.

If you are "Replacing the Boiler(s) only", and you need to warranty the operation of the system after the job is completed for a year or more, then you want to do the stuff that will give you trouble free operation. Move the existing circ pumps and expansion tanks based on accepted industry standards. You will be glad you did,or regret you didn't.

hot_rod

It does get complicated if you have 9 separate circs on the return pumping into two boilers. Connecting the expansion tank before all 9 pumps is not going to be easy.

Are they 3 piece circs like B&G series 100? those do not add a lot of delta P to a system.

EBEBRATT-Ed

If they run 14-16 psi and the relief valve is 35 (sounds like it should be 30?) it may be ok pumping toward the relief valve. Time will tell

nekdahl

Excellent explanation by @EdTheHeaterMan I discuss it very similarly while raining new hires.