What size pipe to expansion tank?

[DaveBP]

I'm pondering pipes from the near-boiler piping to the expansion tank.

I've poured over Siegenthaler and find no guidance there. It's common to see the little blue tanks hanging by their 1/2" pipe connectors. But you get into the spec sheets for the larger tanks like you will see used for systems with large heat storage tanks and the connectors are larger, up to 1-1/4" for Amtrol's SX160.

My puzzle is that even with a total system volume of more than 1000 gallons, if it is heating and cooling over a range of 100F from min to max over the several hours of the typical burn, the flow rate of the expanding water will only be something on the order of a small fraction of a gallon/hour. That flow rate wouldn't develop any appreciable head through 1/2" pipe, even if the expansion tank were 50ft. away.

So why do the larger tanks, which are usually installed in the same room as the boiler, have the large connections? Is it that these tanks are usually used with commercial systems and commercial guys don't use "weeny" little stuff like 1/2" pipes? Or is there a real need for larger pipes to larger tanks for some other reason than normal flow restriction?

And while everyone is so wound up about this question, what effect would you experience if the pipes to the expansion tank are too small?

The only situation I can imagine is, when the circulator kicks on, if it couldn't draw water out of the expansion tank easily it would have a harder time starting water to flow and that might cause a more noticeable pressure drop of short duration. Does that happen? Is it an issue under normal circumstances?

This is aimed primarily at the pros around here, but I welcome speculation from you all. That's half the fun on this forum, isn't it?

 

[Jeff S]

With my system of 660 gal of storage I have the SX-90v expansion tank which also has a 1.25" fitting but I'm only using a 1/2" pex line to connect to my system. Seems to work fine.Like you said the flow to and from the expansion tank is not that rapid,not sure for such large fittings.

 

[henfruit]

1.25 off the tank to 3/4 back to 1.25

 

[ewdudley]

You've got it right, for domestic wood boiler storage tanks the lines to the expansion tanks only have to be big enough to prevent significant pressure drop given the flow rate, which is tiny. The Amtrol engineering documentation spells it out in detail.

The large connections might be needed in some situations. For instance if you had a big pump on a large line that extended for appreciable distance, when the pump starts it needs to accelerate the water to its flow velocity but the momentum of the water tends to leave the water at rest. Without an expansion tank the pressure can spike and overload the motor, but with a fat pipe feeding into a tank the surge in pressure can be absorbed by tank as with a capacitor in an inductive circuit.

Cheers --ewd