Welcome! Here are the website rules, as well as some tips for using this forum.

# Expansion Tank Location(s) ??

Member Posts: 3
Got a question regarding location of expansion tank in system with an ancillary branch off buffer tank.

Not sure how to make "pumping away" work for both the circulators on the left and off the ancillary branch shown on the right. Common practice says to locate expansion at one location as diagram below shows off buffer tank. Head loss in insulated buried piping between shop and house is 7.9 ft. with 9.2 gpm flowing through 1-1/4" insulated PEX.

This project involves creating a central boiler location in a detached heated garage to supply detached garage and house. New electric & condensing gas boilers allow dual heat electric rate.

Seeking comments and suggestions regarding what to do about location, sizing, and pressure for expansion tank and still adhere to "pumping away" for all circulators, including those on the right side of the diagram.

Thanks,
Kevin

• Member Posts: 17,786
Where you've shown it in your diagram is about the best location for it. As to size, assuming that that buffer tank is under system pressure and essentially full, you need to size the tank based -- as usual -- on total system volume -- including the tank.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 2,696
As an aside, the buffer tank with a hydraulic separator is redundant. The buffer tank will provide hydraulic separation.
Steve Minnich
Minnich Hydronic Consulting & Design, LLC
[email protected]
• Member Posts: 3
In my case the low loss header provides hydraulic separation to allow a higher 20 degree delta T on the supply side than the 15 degree delta T on the load side. The 20 degree delta tee provides lower pump and pumping costs than if supply side matched the 15 degree delta T on the load side.

Am I thinking wrong that a low loss header should be used when the delta T of the supply is different than the delta T of the load? The only other option would be a primary/secondary loop with closely spaced tees in place of the low loss header.

Thanks
• Member Posts: 2,696
Design delta T is mostly theoretical and the times I've seen it in real life, I took a picture because of the rarity. I've never heard of a hydraulic separator being used to aid in hitting a delta T. Maybe HotRod has? I think you're overthinking this.
Steve Minnich
Minnich Hydronic Consulting & Design, LLC
[email protected]
• Member Posts: 15,400
The main intent of a separator, same as closely spaced tees, is to hydraulically separate the two flows A side and B side, lets call it.
So it is true that you could have a different flow rate thru the boiler compared to the distribution side. one of 3 conditions will be present in a separator, shown below.
The delta T in a system is a snap shot in time when the system is operating. It is used for calculation and design, the system may never run at exactly the design delta, and it doesn't need to to get the job done successfully.
In fact allowing the delta to fluctuate a bit allows the load to be handled best.
With a cold start expect to see a wide delta, that delta continues to close as the emitters and space temperatures come up. You could in fact see a small 3- 5 delta just before the system shuts down.
The heat emitters dictate the boiler operating condition, not the opposite.

Do not get hung up on trying to maintain an exact delta.

As for efficiency on the boiler, if it is a condensing boiler the return temperature is what controls the condensing and efficiencies. regardless of the delta, the cooler the return the higher the efficiency.