Drainback Innovation?

Kevin_in_Denver_2

The #1 rule for drainback solar collection loops has always been that both the supply and return must be sloped continuously back to the tank with a minimum slope of 1/4" per foot. That means no traps.



Well...... sometimes the job just can't accommodate rule #1, and we've found ways to break it. You can run a 1/4" air line from the top of the tank (assuming closed loop drainback) to some point on the collector return line above all the water traps. This provides reliable drainback and no possibility of freezing if none of the traps are outside the heated envelope.



With an atmospheric tank, it's a bit simpler, the air relief line can just be a stand pipe about 24" long somewhere on the return line inside the heated envelope.



My question is has anyone you seen systems with traps that work fine, or have you experimented with this problem? Perhaps some design criteria can be developed.

Larry Weingarten

You might remember...

... the Grumman Solar system.  It used a small diameter air line as you describe.  They were tolerant of installer misbehavin.   The only real concern that comes to mind is sediment that can eventually form in any traps.  It can be flushed out periodically or perhaps drains can be installed at the traps.  I'd vote for the latter just to encourage installers to try diligently to find plumbing paths with no traps.



Yours,  Larry

Kevin_in_Denver_2

Grumman

"nothing new under the sun" applies here. Do you remember if they put the air line at the highest point on the system, or just somewhere on the return?

Larry Weingarten

Looking through my...

... Grumman file, all I see is freeze recirc and antifreeze style systems mentioned.  I do remember seeing them installed with the air line going to the system high point.  I also remember having to fix one where the installer plumbed the backup electric heater in line BEFORE the solar tank and recirculated it.  That was a stunning electric bill!



About design though, you just need a clear path for air to go to fill the collectors and exposed piping.



Yours,  Larry

Tom in Maine

Vents

We have done this type of drainback vent for over 20 years here in Maine.

It works well. If the vent is an open tube, there is some gurgling noise, but it is only near the tank and you know the system is working and good things are happening!



Tom in Maine

www.americansolartechnics.com

matt_sunwaysolar

How Large of a Trap Can This Method Overcome?

Just curious how big of a trap (in piping) this can overcome? It makes sense that adding the "air line" can aid in forcing some fluid to drain, but how much? It seems that if gravity is working against you, it'll only get you so far.



We have installed drainback systems with great success mounting the collectors level on rooftops with no traps in the piping (sometimes run level, but not necessarily pitched to drain continuously), using gravity and the weight of the fluid to drain.



It seems like a lot of extra work to run this extra line unless it can allow you to overcome at least slight traps in the piping. Can anyone with experience installing these "air lines" comment on this?



Also, where are you connecting the high point of the "air line"? The collector outlet? Does some fluid find its way down this "air line" as well?



Thanks.



Matt

matt_sunwaysolar

Clarification

I suppose that I should state that I'm primarily interested in this function in closed-loop drainback systems.

Kevin_in_Denver_2

Air line location rules

1. The air line must be installed on the downcomer (return line). It can be at the very highest point in the piping.



2. The connection must be above the first trap by a few feet. (How many feet is one of the design criteria I'm trying to develop)



3. All traps must be inside the heated space, including any traps in the air line.



4. The air line can be uninsulated.



The theory behind this is that water seeks its own level only if the air pressure on both ends of the tube is the same. This air line should equalize the air pressure.



It acts to "break the siphon", so the lower it is, the higher the flowrate can be. If it is at the very top, then the collector pump is operating at maximum head, which is high up on the pump curve. (Maximum head means minimum flowrate). If the air line is connected, say, 5ft. down from the top, then the pump will operate at maximum head minus 5ft., plus the pipe friction head loss.



Matt, having level headers makes sense to me, but I'm not sure it will always work with evacuated tube/heat pipe collectors. Those usually have a header pipe that is larger than the supply and return pipes. That would trap a risky amount of water.

Kevin_in_Denver_2

Better Explanation With Pictures

Since Gary Reysa at Build it Solar (BIS) is interested in drainback systems, he picked up on this idea.  I provided him some photos and schematics describing the strategy in more detail, and now it is posted on his website.  BIS is rapidly becoming the essential knowledge base for drainback solar as well as hundreds of other home energy saving topics:

http://www.builditsolar.com/Projects/WaterHeating/DBAirLine/DBAirLine.htm

Kevin_in_Denver_2

dp

Larry Weingarten

A Kevin question...

... about the "Hartford Loop", where it ties into the return from the collectors:  I see that it should go up from where it ties into the pipe, but is it necessary for it to make a 180 degree bend and fall down as it heads away?  That seems to make another trap that might get water in it, possibly reducing it's effectiveness.  Maybe it should be a Hartford Quarter Loop!



Yours,  Larry

Kevin_in_Denver_2

"Hartford Loop in the Air Relief Line"

Larry,



If I understand your question correctly, the air relief line must go down after the Hartford Loop because it must go downstairs to the top of the storage tank. Remember, the air relief line must meet the collector return line above all the traps.



The excess air in a drainback system is always stored in the top of the tank during collection. When the pump stops, the air travels up to the collector loop return line to provide air that relieves the suction, and allows the collector loop to drain.



The reason for the Hartford Loop isn't obvious without direct observation: When the collector pump starts up, water wants to enter the air relief line. When steady state flow is reached in the collector loop, it means that a siphon has been established on the return side. When that happens, suction occurs where the air relief line meets the return line. Now air is continually sucked up from the tank and entrained in the return flow.



The Hartford Loop has prevented water from entering the air relief line and possibly getting stuck in a trap somewhere. Water trapped in the air relief line should be sucked out, but if it isn't, it causes pressure differences that pull water up the collector return.



That also explains why the air relief line must be significantly smaller than the collector loop piping. If the air relief line were the same size as the collector loop, any traps in the air relief line wouldn't get sucked dry when the collector pump stops.

scott markle_2

Pictures

Kevin,



I'v seen this air line in drain back schematics before, but think it was just to assist the breaking of vacume in pressurized systems.



I don't mean to be a jerk but those photos don't exactly sell this very well. The work looks sloppy and not well anchored. But I guess if thermal is going to be deliverd at an economically competitive price this is what it's going to look like. I'v seen several clogged saddle valves over the years, probably a good idea to open and close that valve periodicly to avert a potential clog. Still the whole thing looks like if you tripped into these pipes you could end up taking a 180 degree shower.

scott markle_2

Pictures

Kevin,



I'v seen this air line in drain back schematics before, but think it was just to assist the breaking of vacume in pressurized systems.



I don't mean to be a jerk but those photos don't exactly sell this very well. The work looks sloppy and not well anchored. But I guess if thermal is going to be deliverd at an economically competitive price this is what it's going to look like. I'v seen several clogged saddle valves over the years, probably a good idea to open and close that valve periodicly to avert a potential clog. Still the whole thing looks like if you tripped into these pipes you could end up taking a 180 degree shower.