My proposed system, diagram up for comments

[ewdudley]

Never mind.

I think you're right that it's just a matter of insuring that the neck of the safety vent is higher than the cistern level such that it is certain that water would rather flow through storage circuit and not 'pump over' into the cistern.

If the pump might have to choose between the safety vent and a failed-closed malfunctioning zone valve you would want to design for that, but a 1.5 inch circuit over to Tankzilla and back can't have much static head equivalent even at 25 gpm.

I think all you have to do is compute the worst case head the pump could develop at the point from which you would prefer to plumb the safety vent and set the goose-neck of the safety high enough above the highest possible cistern level to overcome that and you're done.

Cheers --ewd

 

[Gooserider]

<much trimmed>

Are you worried about dissolved Oxygen in your expansion tank?

In a word, yes. I'm concerned more than worried, though.

From what I've heard it isn't a big issue if the system is well set up... O2 absorption is a fairly slow process, and works from the exposed surface down, so most of the water that has been exposed to the O2 will be staying in the pipes going up to the ET... I suppose if one wanted, it would even be possible to set up a sort of "buffer" tank lower down to minimize the amount of water from the ET that even makes it back to the system plumbing...

Similarly, there isn't supposed to be that much heat loss - most of the concern I've heard has been more about protecting the ET from freezing if one has it up in the attic... The two things mentioned to minimize the heat loss is to connect the ET feed pipe to the (cooler) return side plumbing, and to put a heat trap loop in the line so as to discourage any "ghost flows"

The best program will be to test it annually to be sure that the chemistry is stable and add whatever the tests suggest to maintain it. I believe that is part of owning a Garn.

That is my understanding of what a Garn requires, and at least some people say it should be done w/ ANY system, even if a lot of people don't bother...

I will research more on the idea of floating some oxygen impermeable oil on the surface in the tank. One of our European members says some do that in Scandinavia. He also says that he doesn't know anyone that uses water treatment chemicals.

That makes sense in terms of the oil, though I'd wonder how (or if it's even possible) to keep the oil from going down the drain if one did have an overflow event... Might be an issue from both a pollution standpoint, and not wanting to waste the oil...

I reckon I can also panic out and get a big bladder tank at any time but they are fierce money even from those I've seen on Ebay.

True...

Open-vented systems used to be common here in the states and still are in Europe. I'm cynical enough to think that the self interest of unions and manufacturers had as much to do with the changes in practice and code policy as much as any real problems. I would love to know more of the history of hydronic systems in the states but the backroom politics rarely makes it into the official versions.

Actually, I suspect that the codes may be working the other direction now, at least here in Mass. If we do a closed system the current rules are that one MUST use all ASME stamped components - no LP tanks, no Euroboilers, must pay extra for ASME version of Econoburn or Woodgun, etc... I would guess that this would at at least $2-3K to the cost of a system, if not more... OTOH, an open-vented system is pretty much non-restricted, so the codes are actually pushing in the other direction now...

Gooserider