DX-R-10 Drainback system efficiency questions

8 Posts
Jul 14, 2008 02:14 pm
DX-R-10 Drainback system efficiency questions

Does anyone have experience with one of these systems?  I would like to discuss this system with other folks who have similar systems.

Here is my set-up which is pretty simple: 
DX-R-10 Drainback tank
2 50 gallon electric water heaters, one electrified, one simply operating as heat storage
4x10 solar panel

I can go into more detail as needed.  I'm really trying to get a feel for the general heat-gathering and heat-transferring ability of this set-up, to determine if I have set it up optimally or not.  Any information/advice/commentary would be greatly appreciated.

Thanks!
 
4 Posts
Jul 18, 2008 05:56 pm
Re: 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.
 
Jul 18, 2008 08:24 pm
Re: DX-R-10 Drainback system efficiency questions

I just returned from a solar water seminar at Appalachian State. The consensus is drain back systems are the way to go due to fewer parts, reliability and low maintenance.
The AET systems sold here are some of the toughest tested systems on the market.
 I'm setting up my company to sell these systems exclusively, and feel confident they are the best in the industry. You can use potable water in these systems or add some glycol, I live in the northeast and we're much more prone to freezing. I will be adding glycol just to be sure if any liquid is left in the collectors. Hope this helps some.
Spencer Hamilton, Shore Green Energy
 
8 Posts
Jul 19, 2008 12:12 pm
Re: DX-R-10 Drainback system efficiency questions

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.
 
8 Posts
Jul 19, 2008 12:16 pm
Re: DX-R-10 Drainback system efficiency questions

No need to agonize.  If you are in LA an only have very slight danger of freezing, you should definitely go the drainback route.  Properly set up these systems are designed for environments with very hard freezes.  You just have to make sure the plumbing is sloped properly to drain completely.  And if you are worried that it might not drain completely you can certainly add some antifreeze to the drainback tank.  But again, you shouldn't need to.
 
8 Posts
Jul 19, 2008 12:22 pm
Re: DX-R-10 Drainback system efficiency questions

Did I really say "10x40 collector"!?  Now that would be huge!  It's really 10x4.  Feet.  Really.
 
462 Posts
Jul 20, 2008 12:00 pm
Re: DX-R-10 Drainback system efficiency questions

The main thing I don't like about drainback systems is the startup. An empty panel get hot quicker. This turns on the pump. Once the water enters the panel it cools and the pump turns off and the water drains back. This cycle can happen several times before the water gets hot enough to keep the pump running and can lead to early pump failure. When draining back, syphoning can drain the water from the pump, if not properly installed, and can lead to pump failure also. If you are in a climate with freezing weather, trace amounts of water can be left in the collector which can freeze and cause leaks, once again if not properly installed. Most drainback systems are an external heat exchanges which can lower efficiency. Warm water in the drainback tank can also lead to bacteria build up if the water is not treated or changed regularly.
Water levels from evaporation from should be checked regularly to ensure that there is enough water for the solar loop. I usually say, "Use the system until it start to fail, then change over to a closed loop heat exchanger tank when the time comes."
 
8 Posts
Jul 20, 2008 09:37 pm
Re: DX-R-10 Drainback system efficiency questions

The main thing I don't like about drainback systems is the startup. An empty panel get hot quicker. This turns on the pump. Once the water enters the panel it cools and the pump turns off and the water drains back. This cycle can happen several times before the water gets hot enough to keep the pump running and can lead to early pump failure. When draining back, syphoning can drain the water from the pump, if not properly installed, and can lead to pump failure also. If you are in a climate with freezing weather, trace amounts of water can be left in the collector which can freeze and cause leaks, once again if not properly installed. Most drainback systems are an external heat exchanges which can lower efficiency. Warm water in the drainback tank can also lead to bacteria build up if the water is not treated or changed regularly.
Water levels from evaporation from should be checked regularly to ensure that there is enough water for the solar loop. I usually say, "Use the system until it start to fail, then change over to a closed loop heat exchanger tank when the time comes."

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.
 
462 Posts
Jul 21, 2008 09:32 am
Re: DX-R-10 Drainback system efficiency questions

Jason, you are right. And as I and you said, most of these problems occur if the system is not properly installed. As far as the diff. temp., most systems operate on a 10-15 deg. differential regardless of where the sensors are located so there can be intermittent pump action. Try monitoring the system at startup to see if this is happening. The evaporation part usually stems from the drainback tank itself that has to be open to the atmosphere somewhere, usually in the form of a vent, in order for the water to drain back into the tank, otherwise the internal air pressure would not allow the water to return and refill the tank. The bleach scenario is correct too. The only drawback is if the heatexchanger coil has some rubber. The bleach will eventually eat it up. You can use antifreeze or a mix of antifreeze and water instead. This will reduce evaporation, bacteria build up and will keep your panels clean. It will also prevent corrosion on the inside of your panel piping that water can otherwise promote.
 Also I assume you have two pumps, one to circulate water to the panels from the drainback tank and one to circulate water from the storage tank to the drainback tank heat exchanger. So besides losing a little heat and efficiency, being an external heat exchanger, you are using a little more electricity running the two pumps.
 As I mentioned, these system do work and have worked for years. It's just that there is newer technology out there that increases efficiency over this older technology. Also if and when the time comes to service or replace the drainback portion of your system, parts may not be available.
 
8 Posts
Jul 21, 2008 10:05 pm
Re: DX-R-10 Drainback system efficiency questions

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?

 
8 Posts
Jul 21, 2008 10:06 pm
Re: DX-R-10 Drainback system efficiency questions

What newer technology are you referring to?
 
462 Posts
Jul 22, 2008 10:34 am
Re: DX-R-10 Drainback system efficiency questions

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.
 
8 Posts
Jul 24, 2008 11:50 am
Re: DX-R-10 Drainback system efficiency questions

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.

Tom, I'm wondering where you get your information.  Quite frankly, much that you have said here has been misleading and simply incorrect.  For example, closed-loop systems with heat exchanger coils inside have in fact been around since the '70's.  The drainback system was invented in order to overcome the difficulties with these systems and is the newer technology.

Concerning using a larger pump to move more water to take off more heat, you said that the water should move more slowly to take off more heat.  This is simply incorrect, if, as I said, the output from the heat exchanger is already very hot.  This means the water is taking off all the heat it can carry.  More water will take off more heat because it will be cooler as it passes through, creating a steeper gradient.  The steeper the gradient, the more efficient the heat transfer.  Simple physics.

And concerning the amount of fluid in the drainback tank, it only takes about 2 gallons to fill all the piping and the collector, which leaves 8 gallons in the tank, as it was designed for.  The heat exchanger coil is in the lower two thrds of the tank and is always immersed in the drainback fluid.

And concerning the need to have the panel loop vented in some way to allow drainback, this is also incorrect.  The tank is designed so that the inlet from the panel enters into the airspace in the tank.  Once the pump is shut off, air travels back up the outlet pipe as the weight of the water causes it to drain back into the drainback tank through the inlet pipe.  If you had a vent in the system, as soon as you turned on the pump all your water would exit through it.

It seems obvious to me that you don't really understand how these systems are installed or operate.  I'm going to end this conversation at this point in order to prevent the further spreading of misinformation, in case there are people reading this who want facts, not uninformed opinions.
 
3 Posts
Jul 24, 2008 09:24 pm
Re: DX-R-10 Drainback system efficiency questions

Jason is correct.  Tom, hit the books.
 
462 Posts
Jul 27, 2008 12:00 pm
Re: DX-R-10 Drainback system efficiency questions

Jason and David, I have already hit the books. I am a Mechanical Engineer and a Journeyman plumber. I have also worked on many of these systems in my years working in the solar hot water field and have had more problems with these systems than any other. Perhaps you should get yourself a thermodynamics and physics book and refer to it instead of using specualtion.
 As far as a slower moving fluid, just think of heating up a pan on the stove. Pass a volume of water over it fast and pass the same amount of water over it slow. Which volume of water will gain more heat? Also, Q = K *A *(T2-T1), the smaller the temperature difference between the two fluids, the less heat gained and transferred! The smaller the area, A, the less heat gained. So if you have a volume of water that is not changing temperature, the less heat you can gain. You cannot add 100 deg. heat to something that is 101 deg.
 And the TANK has to have a vent. If you suck water through a straw, similar to pushing it up with pump, if you block one end with your tounge, the water will not come out. It needs air pressure to remove the water from the straw. If the system was sealed as you suggest, the water would not move at all, since once you pressurize the tubing, the air in the tubing would push back from being compressed. Simple physics, action and reaction. You can pump water out of a bucket and return it to the same bucket without the water spilling out of the system, as you suggest it would, which is basically what the drainback tank is. So basically, if it were not vented, it would be a closed loop system, and therefore no need for a tank. Also when you consider efficiency, each heat exchanger lowers efficiency through heat losses to the surrounding, unless they are super insulated.
« Last Edit: Jul 28, 2008 09:46 am by Tom Mayrand »
 
4 Posts
Sep 23, 2008 04:14 pm
Re: DX-R-10 Drainback system efficiency questions

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!
 
4 Posts
Sep 23, 2008 06:42 pm
Re: DX-R-10 Drainback system efficiency questions

I think you have it right already.  If you look carefully at the dip tube, you should see a small hole near the top - On regular water heaters, this is to prevent siphoning - on drainback tanks, this is to let air back into the collectors when the bottom of the dip tube is below the water level.

You may like to make the hole a little bigger so that it doesn't slow down the drainback too much, but I doubt that this is necessary.  I've just put in a system that looks very much like yours, and I'm convinced the drainback speed is limited by the heat exchange coil wrapped around the water tank.  It still drains in about a minute, so the risk of a freeze-up is zero. 

You could remove the dip tube altogether, but the splashing noise might be annoying.

By the way, the two fittings in the side of the tank are for a sight-tube so you can tell if the solar loop has enough liquid (not empty while running, not over-full when drained).  Without the sight-glass it would be hard to tell if there were slow leaks etc. or that if over filled, water might be remaining in the collectors causing a potential freeze-burst.  Ugh.
 
4 Posts
Sep 24, 2008 09:38 am
Re: DX-R-10 Drainback system efficiency questions

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).
 
 
1 Posts
Sep 27, 2008 12:42 pm
Re: DX-R-10 Drainback system efficiency questions

"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."



Hello Jason, do the riser tubes on your collector run horizontally or vertically?  Most manufactures make their collectors with riser tubes running on the longer dimension of the collector. When mounting a collector horizontally, on a drain-back system, a consideration is to make sure the riser tubes are vertical as many years down the road, horizontal riser tubes will sag and this will create a place for water to pool and possible freeze.
 

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