Drainback DX pump sucking air

3 Posts
Jan 24, 2012 12:50 pm
Drainback DX pump sucking air

I installed a DX drainback system with 10-gallon drainback tank.  I filled the small DX tank to within 1/2" of the top of the sight glass as the instructions called for.  When I energize the Taco pump, it does its job and pumps the water up to the panels (located on the roof), but the 10-gallon tank runs just about dry before I can hear the returning water drop into the tank (the sight glass remains completely empty while the pump is running).  Consequently, the Taco pump can be heard to be sucking air quit loudly (at least that is what I am assuming it is doing).  I've shut the system down since then, until I can get an idea of what to do about this.

I actually have two questions about this:
1. Common sense tells me that I should just add some more water to the 10-gallon drainback tank, but how much will be too much?

2. With the tank running nearly dry while the Taco pump is pumping water to the panels, I don't see how there is anyway the heat exchanger coil within the tank is able to receive the heat from the collector water, since there doesn't seem to be any water surrounding the coil at that point.  While again, the solution seems to be to add more water, I am afraid that the amount of water needed to submerge the coil while the circulator is running will be more than the capacity of the tank when the circulator is not running and the water has drained back to the tank.

The solar water panels (three panels total) are located with a supply run of about 31 feet and a return run of about 45 feet, using 3/4" copper pipe.  I calculate that I use almost 2 gallons of water for both the supply and return lines.  Does it seem reasonable that the panels themselves would hold the other (nearly) 7 gallons of water (assuming that there is a gallons worth of reserve left in the tank when the sight glass is filled to within a 1/2" of the top)?

Anyway, some insight from others who have installed this system before would be helpful.  I am used to Taco pumps operating quietly, so this brand new pump, making this noise has me concerned.

Sean
 
47 Posts
Jan 24, 2012 01:48 pm
Re: Drainback DX pump sucking air

Sean,

1. You do not have enough water in the system.  Rather than filling the tank to within 1/2" of the top, you should open up the relief valve in it, and fill it until water is coming out of the valve, to make sure it is completely filled. Then close the valve.

2. Each panel generally holds about a gallon of water, so you would have 3 gallons total in them.  With 76 feet of 3/4" pipe, at 0.02227 gallons per foot (assuming type K), that's 1.7 gallons in the tank.  So that's about 5 gallons between collectors and pipes.

3. (OK, you didn't have a #3, but I'm adding it).  You want to make sure the pump is never, ever dry. It should be installed well below the drainback tank, so even when the pump turns off and all of the water rushes back in, it doesn't have a chance to ever add air into the pump and potentially create an air lock.

Hope this helps.

Amy
AltE Store
Solar Thermal Queen
 
3 Posts
Jan 24, 2012 02:21 pm
Re: Drainback DX pump sucking air

Thanks for the reply.  I'll add the additional water tonight.  The pumps are indeed installed below the level of the tank.  They run fine and quiet until the tank empties then the noise starts, which is why I assumed it was sucking air and shut it off immediately.  I'll add the water tonight until it drains out of the P&T relief valve, and try again. How much water should be visible in the sight glass when the pump is running (so I know the heat exchanger coil is adequately covered)?

Sean

Sean,

1. You do not have enough water in the system.  Rather than filling the tank to within 1/2" of the top, you should open up the relief valve in it, and fill it until water is coming out of the valve, to make sure it is completely filled. Then close the valve.

2. Each panel generally holds about a gallon of water, so you would have 3 gallons total in them.  With 76 feet of 3/4" pipe, at 0.02227 gallons per foot (assuming type K), that's 1.7 gallons in the tank.  So that's about 5 gallons between collectors and pipes.

3. (OK, you didn't have a #3, but I'm adding it).  You want to make sure the pump is never, ever dry. It should be installed well below the drainback tank, so even when the pump turns off and all of the water rushes back in, it doesn't have a chance to ever add air into the pump and potentially create an air lock.

Hope this helps.

Amy
AltE Store
Solar Thermal Queen
 
462 Posts
Jan 28, 2012 12:33 pm
Re: Drainback DX pump sucking air

Sean, you can also add a coil of tubing around the same height as the drainback tank on the outlet side to increase system water volume......
 
3 Posts
Jan 31, 2012 12:22 am
Re: Drainback DX pump sucking air

UPDATE:
I filled the drainback tank until water began flowing out of the temperature and pressure relief valve at the top of the tank.  I turned the unit back on and now everything works great.  The pump runs quietly and the sight glass shows there to be about 2 inches of water above the level of the heat exchanger tubing Inlet and Outlet while the pump is running, so I assume that the heat exchanger coil is immersed in the collector water, while the pump is running.

As a real world check I physically just felt the copper lines, and the heat exchanger line going back to the 120-gallon storage tank feels to be within a few degrees of the temperature of the outlet tube from the drainback tank, ie. nice and warm, even on a sunny 30 degree day like we had while I was doing this.

Thanks for all the help.  I'm very satisfied now with how the system is operating.
 

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