Jason Dowdoe's posts

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.

What newer technology are you referring to?

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?

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.

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

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.

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.

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!