Hi Jarod,
Lots of people use the U tube and most have no trouble with it.
I've got one drainback system that uses the through the tank wall and another that uses the U tube -- both work fine.
I think I would lean a bit toward the U tube just to avoid lining penetration, but the through wall is fine if you do it carefully.
TomTank and Tom S both offer some good advice.
I have heard from a couple people that did have trouble with the U tube emptying too much when the system drains back, and the pump losing prime, but this is not common.
I'd just go ahead and try the U tube -- the odds are it will work fine -- you can work the problem it it does occur.
I don't use the vent in the return line that TomTank mentions, but I think it might reduce the chance of the U tube emptying during drain back.
Its handy to add a garden hose style valve on a T off the U tube. This allows you to prime the U tube by just hooking a garden hose to the valve, opening the valve, and turning on the garden hose to fill the U tube and get rid of all the air for the initial prime. You should only have to do this once.
Question for TomTank:
You mention the need for non-iron pumps.
I've about convinced myself that in most cases (maybe depending on the water and pH) you can use an iron pump on this kind of drain back system:
http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htmThe reasons I'm thinking iron might OK:
- The system storage tank gets filled with water once, and then no new water is introduced -- unlike many open systems where new water is introduced regularly.
- While the system is vented to atmosphere, the vent can be quite small, and (I think) little new air is introduced into the system.
I have had an iron case pump (a Grundfos) in service for 10 years that came with the house when we bought it that circulates water to the far bathroom.
It ran 24/7 for 8 years with new water being introduced all the time. The pump has very little rust and still works fine.
I've also heard from other people who have had good luck with iron case pumps.
I'm sure Grundfos and Taco would rather sell a $300 bronze of stainless pump than a $80 cast iron pump, but I'm really wondering if its necessary in these kinds of systems with very limited introduction of new oxygen?
Have you actually seen iron pumps fail by rust in drain back applications?
Some of the internet references seem to indicate that keeping the water pH high also provides some rust protection for iron.
Gary