Expansion tank

Ted Huizinga

See pdf drawing with clouded question.

Gordy

Hmmmm

Good question. I will bet Brad White has the answer. Redundency, another air removal location. I do not see any valves between the two.

Mark Eatherton

I'd keep ET1 and can ET2...

Move its connection to the same as ET1. The sizing of expansion tanks is a fairly accurate science.



You could/should leave the air eliminator at the same location for ET2. With the size of pipes you are dealing with, you will have to bottom fill and top purge manually any way.



A basic rule is that ALL pumps MUST pump AWAY from the PONPC (expansion tanks). If you get rid of ET1 and keep ET2, the boiler pumps will be pumping TOWARDS the PONPC, which is definitely not preferable.



Your DHW pumps should also have individual check valves at the discharge of each pump. If (when) one of them fails, you are going to have some issues. The tanks are piped reverse return on the hydronic side, and assuming they are piped the same way on the potable side (reverse return), you could control both pumps off of one aquastat.



Are the DHW tanks supposed to be prioritized?



ME

Brad White

I agree with Mark

Pick one and stick with it. I could justify either location given that the heads on the outlying zone pumps are "likely" higher than the boiler circuit. But as Mark said, that single location where ET-1 is shown will be universally the best location. Technically ET-1 could connect anywhere in that 3 inch return line between the DHW return tee and the end of the run where it is now shown. 



Also as Mark said, the tank volumes should be verified against actual system volume. Given the size of the piping and knowing nothing else about the system, I bet these are not Amtrol 30s but larger tanks.



And yes on check valves for the DHW pumps. You bet.

Steve Whitbeck

tank location

The expansion tank should be in the ET-1 location but you still need the air seperator at the ET-2 location since it is the boiler that produces the air. They may be installing two tanks because two small tanks may be cheaper than one large one. If I was going to install two tanks I would move the ET -1 location to just infront of the first boiler on the return line instead of at the end of the line.

That way it may pick up some air that gets brought back down from the main because the boiler flow may be more than the system flow. I would also not install the check valves above the pumps ( unless they are integral )  If a slug of air rises out of the boiler the check valve will trap the air and cavitate the pump. The indirect pumps do need check valves. And I would have used close spaced tee's on the boiler supply line for priority domestic water heating. In Michigan we need to install a manuall reset LWCO and secondary high limit on each boiler. So if that is the case where this job is those parts are a lot more than auto reset units.

Brad White

Yup. Good Points!

I agree, move that ET-2 air separator upstream of the first boiler. Otherwise, that is a lump of metal (if indeed it was an air separator or just a handy place to connect the expansion tanks), at the end of that run.



There is more to having multiple expansion tanks.



My office was troubleshooting an apartment complex system with chronic problems (not enough expansion tank volume it turns out and loss of water during relief valve discharges which we discovered. Went on for 30 years!).



System tanks were sized for 2500 gallons system volume. Dye test revealed a system volume of 13,000 gallons, (more than Snooki uses in hair conditioner in a month).  Short much?



One of the suggested solutions was to place satellite tanks upstream of each secondary pump set serving each building. Glad we did not go that way. Because each tank defines a PONPC, each secondary circuit would get ONE charge of hot water off the high temperature loop but would never have sufficient pressure to deliver return water back to the high temperature return. It took a half hour to explain it to the Owner so we put all of the tanks in one place, a very crowded room.



One PONPC is all a system can stand.

Mark Eatherton

On a closer second look....

The location of the air separator ET1 is virtually useless as it pertains to air seperation/removal, other than a place to make the expansion tank connection.



Steve, where would you put the required check valve? I've not had your experience, unless it is on the inlet to the pump, where it can create a pressure drop going IN to the pump. On the outlet of the pump, no problemo...



Care to 'splain yourself? :-)



ME