Expansion Tank Connection to Air Separator Question

[ewdudley]

I would like to know what you think about the location of the expansion tank/air separator/make up water supply relative to the Termovar Loading Unit (which contains the primary circ pump) in this diagram. What confuses me is the pump is on the return rather than the supply side, but the expansion tank/air separator are on the supply side of the boiler. Is this less than optimal, acceptable, or just messed up.

There are many who go on and on about 'pumping away' like it's the third rail of hydronics system design decisions. Yes there are many circumstances where you can create big problems by teeing in the expansion tank after a pump, but the wood-boiler-to-storage loop is not one of them; assuming a small flat-curve pump, fat pipes, and short distances to storage.

You can pretty much rely on the Tarm reference designs to be nicely detailed and valid, although there are often ways they could be enhanced or simplified to suit a specific situation.

Amtrol advises taking the line to an expansion tank down and downhill from where it tees into the system so air can't migrate and accumulate under the bladder membrane.

 

[RobC]

I had asked some of these questions way back. What changes things for most of us is the need for the mixing valve AND the circulator pulling the water through the mixing valve and into boiler. This is a must and those two components location are critical. Because of that requirement, some of our other parts, tanks, separators etc. get moved to the next best location in the overall system.
Rob

 

[Clarkbug]

Just to follow up on Phizman's comment about about ITT Bell & Gossett, here is a handy dandy link.

http://www.scribd.com/doc/31188492/Air-Management

I just happened to have the hard copy sitting here on my desk when he brought it up. If that link dies, you can search for "Bulletin No. TEH-1196A".

More reading and theory than many people want, but good stuff and great calcs!

 

[dogwood]

Why would the best location be right after the boiler on the supply side than right before the circulator (loading unit) on the return side?

Mike

 

[RobC]

My recollection was that this is where the hottest water is so the air separator works the best after the boiler, given that the expansion tank ends up there although not ideal.
Here's where I asked the question and received a response....
https://www.hearth.com/talk/threads/40804/
Rob

 

[dogwood]

Thanks for the link Rob. Like HR says its a trade-off in our situation and the hottest water must beat out the point of lowest pressure for some reason. Has this arrangement worked for you?

Mike

 

[RobC]

I haven't had any problems.
If I ever changed anything I would consider changing out the tremovar for reasons HR mentioned. I have the Solo 60 and a larger circulator which increases, a bit more than usual, resistance at the tremovar, but, the system does work and the boiler doesn't cycle. From that point I'm running as efficiently as possible. The tremovar is a simple solution for inlet water protection.

 

[dogwood]

Rob, I've got the Termovar Loading Unit with the pump built right in so there's no moving things around for me. It would be intersting to do HR's suggested pioping arrangement.

Next thing I was wondering about the the PT3 diagram is, why wouldn't the expansion tank go on the return line just before the loading unit circulator, where the point of no pressure change is, rather than in front of the air separator on the supply line? Isn't the the point of no pressure change, right before a circulator, where an expansion tank is supposed to go? Maybe there is some other consideration.

Eliot, you mentioned if you have big fat pipe (mine is 1.5 inches), a small flat curve pump (a Grundfos 15-58 is on the loading unit) and a short distance to storage (I've got about 22 feet to the inlet of my 1000 gallon tank from the boiler) you should have no problems with the expansion being teed after the pump. Do the parenthesized sizes and distances fit that bill? I thought maybe the 22 foot distance might be too long.

Mike

 

[RobC]

If you could stick the tank between the circulator and the loading unit that could be an improvement. However with your all in one ( which is very nice ) that isn't a possibility. Then if you go further back on the supply pipe there is the interference of the loading unit. So, by default everything goes to the other side.
The idea with the tank in front of the circulator is, it allows the circulator to have constant pressure so it wont cavitate
Maybe someone that is qualified in the field might have other ideas ?

 

[ewdudley]

big fat pipe (mine is 1.5 inches), a small flat curve pump (a Grundfos 15-58 is on the loading unit) and a short distance to storage (I've got about 22 feet to the inlet of my 1000 gallon tank from the boiler) you should have no problems with the expansion being teed after the pump. Do the parenthesized sizes and distances fit that bill?

Sorry, I'm just trying to generalize as to what factors would guarantee that it won't be a problem to have a pump on the 'wrong' side of the expansion tank connection.

What matters is that there is enough pressure at various points around the loop so that the pump does not cavitate, that the boiler sees enough pressure not to kettle or flash, and that the high point of the loop sees enough pressure not to have gas coming out of solution.

The lowest pressure any point around the loop can see no matter where the expansion tank is connected is the expansion tank pressure minus the head generated by the pump.

So if you run the numbers to figure out the head for a 15-58 pump circulating through a mixing valve, boiler, X feet of 1.5" pipe, N elbows, and the storage tank array, you come up with what, a couple two three psi? The fatter the pipe, the shorter the distance to storage, the smaller the pump and the flatter its curve, the lower the lower the head developed by the pump.

And supposing the system pressure in the vicinity of the boiler-to-storage loop is something like 12 or 15 psig minimum, worst case minimum pressure in the loop no matter where you tie the expansion tank in would be something like 9 psig minimum, which I claim is plenty not to have to worry about pump cavitation, boiler flashing, or effervescence at the top of the tanks.

--ewd

 

[dogwood]

Do you think it would be any better to have the expansion tank connected in before the Temovar Loading Unit which is on the return side of the wood boiler, or on the wood boiler supply side before the air separator, or would it make no difference. I'm trying not to be too obtuse. Thanks again. This is slowly making sense.

Mike

 

[RobC]

The idea with the tank before the circulator is that that is the best place for it to act like a shock absorber.... It dampens or takes up the slack the quickest here when the circulator starts. But we have the loading unit which allows the water to come from 2 directions so that's why the 'ideal' placement is no longer "ideal". And, if that valve is fully closed it may be less than "ideal". So at that point you start looking for the next best place.

 

[dogwood]

Now I get it. Thanks Rob.

Mike