DX-R-10 Drainback system efficiency questions

Me too.  I'm still planning to do with a couple of 8'x4' HW collectors and agonizing over wether to go the drainback route or closed loop/antifreeze route.  I live in Los Angeles, fairly close to the ocean so we generally have a lot of sunshine but occasional light frost.

Where are you located?  Maybe we can share some thoughts on the various design trade-offs.

I can provide more information, as my system has been up and running for about a week, during which time we have had mostly sunny days.  I have to say the system works beautifully.  It is producing all the hot water my wife and I care to use, with no assistance at all from electricity.  Now granted, it is summer here, and the performance may fall off in the fall and spring because of shading issues, but I'll deal with that as it arises. Right now the system is performing perfectly.

Here is more detail.  The single 10x40 collector is mounted horizontally on a south-facing first-floor roof.  It is slightly elevated at the top to give it a proper angle for best winter efficiency.  The roof faces about 15 degrees east of south, and I haven't chosen to compensate for this slight off-axis.

Inside the house I have two Whirlpool 12-year 50 gallon electric water heaters.  One is electrified, the other is simply for additional storage.  The drainback tank sits on top of the electric tank.

I have chosen to use only water in the drainback tank, which is what is pumped up to the collector, because I was able to properly slope all the pipes for proper drainage.  We get very hard freezes here in Central North Carolina, but it should be no problem with proper drainage.

Here's the plumbing layout.  The heat exchanger loop draws water from the bottom of the electrified tank through the original boiler drain tap, takes it up to the dranback tank, circulates it through the heat exchanger, then returns it to the standard cold water supply inlet on the storage tank.  This allows the heated water from the heat exchanger to enter the coldest part of the system, the bottom of the storage tank.  The supply of cold water also enters the bottom of the storage tank through the original boiler drain tap.  Hot water transfers from the storage tank to the electrified tank by traveling out the hot outlet of the storage tank and into the cold inlet of the electrified tank.  Hot water leaves the system for use through the standard hot supply out on the electrified tank.

The electric back-up consists of a standard hook-up of 220v for the electrified tank.  The only difference here is that I have put that 220v on a timer, which leaves it off virtually all the time.  The only time it comes on is for one hour before my wife and I gets up in the morning to make sure there is sufficient hot water for showers.  However, given the amount of storage I have, I don't expect to use much electricity. 

I have set the high-limit for the differential controller at 160 degrees and the temp for the electric tank at 140.  That way, ideally, the water from storage tank will be hotter than what's in the electric tank, further reducing the likelihood or having the electric elements come on.

The two sensors for the differential controller are located as follows:  one is hose-clamped to the copper outlet pipe at the top of the collector, the other is inserted beneath the factory-installed insulation against the metal interior of the storage tank, accessed through the door for the lower heating element, which is not electrified.

All told, the system as configured cost right at $3200.  I installed it all myself over several days, with help from some friends to get the panel on the roof.  I expect to recover about $1600 in tax credits from fed and state.

Given that heating water is probably half of our electric bill, which runs about $90/month (we're pretty frugal as it is - modest AC in the hottest part of summer, wood heat in the winter) I expect to save maybe $40-50 month.  If it is only $40 it will take me about 40 months to recover my investment.

Did I really say "10x40 collector"!?  Now that would be huge!  It's really 10x4.  Feet.  Really.

Hi, Tom,

You touched on a number of possible issues, but I'm not really convinced they are problems.  Concerning start-up I expect that if you have your tank sensor on the right part of the tank, and have the proper temperature differential set, the start-up cycling can be minimized or avoided.  The temperature in the drainback tank should always fall to about the same temp when the system isn't running, so once you get the differential set properly it should not cycle on and off at start-up.  As for running the pump dry, if it is properly installed below the level of the drainback tank this can never happen.  Concerning freeze issues, again, if the system is properly installed and drains properly, how can it freeze?  It takes more than a trace amount of water freezing in a pipe to damage the pipe.  Concerning bacteria build-up, a little chlorine takes care of that, and it is in the panel loop anyway, which is totally separate from the potable water so there is no danger of contamination.  And since the panel loop is totally closed and sealed, how can there be loss to evaporation?  If there is, then again the system is not properly installed and has a leak in the panel loop. 

I don't mean to sound glib, and perhaps you have more experience with these systems than I do, but none of the issues you raised seem to be deal breakers.  My system is currently working perfectly and meeting all my expectations with no visible problems.

Thanks for your response.

Yes, it is definitely the efficiency question which lead me to post to this forum.  Even though my system seems to be running well and producing adequate heat, Based on a very close monitoring of it I think the heat exchanger is rather inefficient.  When the collector is in full sun and both pumps are running, one to circulate the drainback fluid to the collector and one to circulate the water from the storage tank through the heat exchanger in the drainback tank, here is what I notice.  I notice there is a substantial differential across the inlet and outlet of the heat exchanger, which is good, because it means that the water going through the heat exchanger is being heated.  However, I notice no detectable differential across the inlet and outlet of the drainback tank itself.  This says to me that there is a lot of available heat in the drainback fluid which is not being taken off by the heat exchanger, and that water nearly as hot as what just came from the panel is being returned to the panel.  And no differential means inefficient heat transfer at best.

This leads me to want to install a more powerful pump in the heat exchanger loop to move more water through the heat exchanger.  Ideally, moving more water would mean moving more heat.

What do you think?  Am I wrong th think that there should be some noticeable differential across the inlet and outlet of the drainback tank?

Jason, the reason there is no real temperature difference in the drainback tank is because of the amount of water in the tank that gets sent up to the panels to be heated. If you notice, when the pump turns on, most of the water in the drainback tank is sent up to the panels leaving very little water in the drainback tank. This indicates that the amount of water to be heated pretty much equals the amount of water that the panels can hold. Since you are heating a small amount of water it heats up quickly. The other reason the temperature stays steady is the heatexchanger area. It is this area that determines the amount of heat that can be transferred. The heat exchanger, I assume, is 1/2" tubing surrounded by 3/4 to 1" tubing. So if you know the length of the 1/2 tubing you can calculate the surface area and thus the heat exchange area. Also since the hot water from the panels is in the drainback tank, the tank itself, if not insulated, can lose heat to the surroundings.

 The newer technology that I speak of are heat exchanger tanks in which the heat exchanger coil is inside the tank. These tanks are available in 45, 80 and 120 gallons. The most common of these today are "Super Stor" tanks. If you check their specs, you will see the size of the coils and will see that the surface area is much greater than your drainback coil. And since the coil is inside the tank and exposed to and surrounded by the water you are trying to heat, the efficiency is much better.   As far as a more powerful pump goes, actually the slower the water moves the better. This give the heating fluid more time to absorb and transfer it's heat.

Jason is correct.  Tom, hit the books.

I have some questions related to this thread so here it goes...

I recently added a drainback system to my house:
1 10x4 collector
10 gallon DX-10 drainback tank
80 storage tank (Rheem with the external heat exchanger wrap)

The drainback tank is constructed much like a conventional water heater with a fitting on the top connected via dip tube to the bottom of the tank. There are two fittings on the side (upper and lower) which enter the tank interior directly (no internal tubes).

Based on the attached labels, I plumbed the tank with the return from the collector entering the top of the tank (to the dip tube), the drain to the heat exchanger connected to the bottom of the tank, and the pressure relief vent connected to the upper side fitting.

This seems good at first glance but that dip tube creates potential problems.
Consider what happens when the drain back tank is full (circulation pump is off). There is little trapped air in the tank and hopefully the collector and exposed plumbing is full of air.
When the circulation pump starts, the water is pulled from the bottom of the drain back tank and pushed up to the collector. The air in the collector and plumbing is then forced into the DB tank, to the bottom through the dip tube. This air then bubbles up to the top of the DB tank until all the air in the system is at the top of the DB tank.
When the pump shuts down, the water in the collector then tries to flow back into the DB tank. The majority of this water flows counter to the normal pumping direction and thereby tries to fill the DB tank through the bottom side fitting. As the tank fills, the air at the top has no where to go since it is trapped by the dip tube.
Eventually everything drains but it takes a while.

The way I see it, I should plumb the DB tank as follows.

Cap the inlet at the top of the tank since the internal dip tube makes this useless (except for a possible line to the heat exchanger?). Attach the return from the collector to the upper side fitting on the DB tank. Also 'T'd into this line is the pressure vent. The lower side fitting is as before (run to the heat exchanger).

It seems to me this would let the air be pushed into the collector as the DB tank fills from below when the pump is off.

Anyone see any flaws with this plan?

Thanks!

Thanks for the reply Michael.
I didn't notice the hole in the dip tube but it makes sense that it would need one  or else it would have a very difficult time draining. I think the dip tube is welded to the top fitting. It's not like a normal water heater where it can slip out (and is made of plastic). This one appears to be stainless steel, just like the tank. I had thought about the noise issue and certainly the dip tube helps reduce that.

I lied, there are actually four fittings on the side of the tank, two top and two bottom. one pair of top/bottom are dedicated to the sight tube, the other two are used for return to the exchanger (bottom) and pressure vent (top).