Expansion Tank Location(s) ??

dissek

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

Jamie Hall

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.

Tinman

As an aside, the buffer tank with a hydraulic separator is redundant. The buffer tank will provide hydraulic separation. 

dissek

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

Tinman

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.

hot_rod

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.
Some reading on thermal equilibrium.

https://www.caleffi.com/sites/default/files/file/idronics_23_na.pdf

dissek

The load side flow through separator is higher than the supply side at full load. Plan to use the separator in addition to the buffer tank back in the boiler room located in shop. Thanks for the references HotRod and thanks Steve for bringing this up.

Haven't mentioned this yet but this project also involves solar thermal with 4,000 square feet of radiant floor heating. I've got some fundamental questions regarding this that I'll post in the solar catagory.

Thanks again!
Kevin G Disse PE - retired
Detroit Lakes, MN