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Solar Drainback Systems

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This is an interesting thread that appeared on the Wall

NRT.Rob asks:
What are you guys using for tanks, specifically, if you are trying to do drainback solar WITHOUT pumping domestic water up to the roof? We're not seeing too many suitable options.

hot rod replies:
Do you mean a drain back tank with HX inside? Here is one choice.

Heat Transfer and a few others are releasing drain back tanks, not sure about HX.

I did see some interesting drain back concepts, systems and prototypes at the InterSolar show. One brand drains back into the coil HX.

This one has the pump below the tank. Another used variable speed circ to start
the flow then throttle back.

Grundfos actually has circs just for drain back applications over there, with "A" class ECM motors.

Exciting times to be into solar thermal.

R. Cavanagh replies:
Can I use a drainback system for domestic hot water only and use a solar tank without a heat exchange to bring the water directly from the collectors to plumbing system. avoiding a hx would increase the efficiency.

NRT.Rob replies:
Thanks HR; are you using an HX in your drainback designs? I think Maine won't allow for domestic to pump up to the panels for "certified" systems.

Bob Gagnon replies:
Would the minerals we have in the water up here in the Northeast build up on the solar collector like they do on a tankless coil? That's why I went closed loop.

hot rod replies:
Good point Bob. The plus side of water as the transfer fluid is the "glycol issues" go away. Cost, heat transfer penalty, upkeep, AHJ questions etc.

But if like you mentioned the minerals coat the HX surfaces, then performance drops. Similar to the issues with tube boilers, mod cons, etc where the thin layer really puts a hurt on the heat transfer.

Solar thermal is a fairly low grade use, hate to let any slip away.

Maybe DM water as the DB fluid, and HX separation from the DHW or heating fluid is best. But then the HX efficiency comes into the equasion.

I still have more DB questions than answers, but I am learning with every phone call or show I attend. There is a
very strong, experienced DB following. Not a mainstream crowd, but very experienced inn the art and science of DB use and design.

A drain back-able evac tube system might excite me. I hate the concept of dumping"free" heat with powered, or even un-powered devices :)

DB does make a lot of sense for large seasonal arrays. Overheat and freeze protection, combined with better heat
transfer needs to be recognized.

NRT.Rob replies:
I was thinking of using something like a turbomax, where the domestic is in the "coil" for max preheat.
The problem is we're not sure we can get a dip tube with siphon break combo for it that would make pressurized
drainback easy and we hate inventing stuff like that...

But it might not be that hard either. What do you think?

hot rod replies:
I am a drain back virgin to be honest with you. I do have 4 new panels on my roof, sloped to try out a DB system. I have some new components on the way including a DB tank and hopefully a prototype DB controller. I also want to see how well the SolarFlex works as a DB tube.

I can see the advantages of DB if all the components can come into place without a lot of customizing.

I'm looking at systems a bit different now that i am on the manufacturing side. I think a lot more thermal solar will be sold if we come up with kits or standard residentiall packages. So that is the mission I am on, these days.

I think the HX does make the most sense. I'm not convinced inside the tank is best. hard to get a lo of surface area in those small tanks. I feel a lot more bang for the bucks with an external HX???

Bob Gagnon replies:
I think it was Larry that talked about his PEX tank, then maybe drop in some tankless coils like I did in my grey water tank? I was thinking fiberglass for my next solar tank, but someone here said it wouldn't last?

hot rod replies:
Here is a real nice fiberglass solar tank:
I saw these at the ISH show years ago. Looks like they have set up an office in Altlanta. I hear Apricus has the NA distributorship sealed up?? Apricus was showing then at the San Diego solar show a few months back.

They have some You Tube video showing an install. looks like you need to be trained for the assembly. They make some small and huge tanks.

No idea on pricing.

It might be a nice tank for the wood boiler folks also.

Ron Huber replies:
Word of Caution - Leave no stone unturned. This photo is a 20 panel drainback system that I am in the process of replacing the panels, insurance job. Worked great for 28 years, alls it took was one moment of human error and "PoP" goes the weasel. Pitched piping, use of an oversized downcomer, quality controls, all this and in the north you are still rolling the dice. I even installed several draindown systems in the past, and even then they were looked on as a real risk in New England. Just about entering my 32nd year in the solar and HVAC business, I sleep better knowing my closed loop systems are never going to be subject to freeze ups. There may be one in a million chance that something can go wrong with a foolproof drainback system, but you do not get any second chances, 28 years is pretty good, but these panels would have lasted a lot longer

Royboy replies:
So Ron, what went wrong with that system? If it froze, I presume it was water-based rather than glycol.
when you say you did several draindowns systems - do you mean draindown or drainback? I did a couple draindowns around 1980 - using a power valve to drain water from the collector loop when below freezing. Talk about potential for failure! I don't think anyone much does that anymore.

Ron Huber replies:
Customer went to Florida for a month, his son (44 no kid) checked on the house once a week, part of the routine was to check the atmospheric solar tank for water level, had to be added to from time to time. He opened one valve he should not have opened, closed one he should not have closed, simple human error, but it happens. I installed Grumman systems for as long as they were in the solar business, and was a field tech fixing other installers systems for them for the full 5 year warranty after they got out of the business. They had a very good, almost fool proof draindown system that is installed, I also installed lots of drainbacks. I think that drainbacks have a place in the mix, but after looking at different configurations, some that use glycol, they all use heat exchangers and/or two pupms, with one using a fair amount of electricity, and some manufacturers will not warranty their panels used in a drainback system in northern climates, Viessmann, Heliodyne, etc.

Royboy replies:
I'm losing my drainback virginity in a big way ... as I'm just getting started on installing 24 drainback SDHWs on new houses in a tribal housing development near me.

I'm getting back into solar thermal after a roughly 25 year break from my initial involvement. In getting back I tried to get up to speed with current thinking/hardware and looking at Tom Lane's book, I got real interested in drainbacks - which I never did the first time around.

Tom's book led me to Solar Service in Niles IL, according to Tom the most experienced cold climate drainback folks he knew of. Brandon Leavitt is the main guy there. I signed on with them as a dealer, mostly to be able to access their experience with drainbacks.

Interesting that I'm finding that many of the more experienced solar thermal folk in WI (where I am) seem to prefer drainbacks - even though the state rebate organization has not been very favorable towards them (though is coming around, it seems).

Back to the topic. I'm starting out using AETs drainback tanks (stainless with a sightglass) and Rheem/Richmond's SolarAide HE storage tanks with a heat exchanger wrapped around the outside of the tank (creating a double wall). Though Solar Service has gone to using glycol for the drainback fluid (which allows more forgiving plumbing, such as level collector headers), I'm personally intrigued by the idea of doing water systems and making sure the slopes are positive for drainage. I'm starting out with glycol systems but am interested in the experience of others who are
doing water systems in cold climates.

Wayco Wayne replies:
I just met with a Solar contractor here in MD, who will be helping me through my first install. He recommends Drainback too. The only drawback I saw was the larger circulator needed for initial start up. He asked if the Wilo Stratus might be a good fit for that.

NRT.Rob replies:
tekmar 157 solar control allows for temporary running of one contact and then switching to another after 3 minutes I think; perfect for a high head primer pump and a low head circulation pump. But if there is a way to rig a stratos for this I'd love to hear that too!

hot rod replies:
A few ways to pump DB - I see most installers use a dual pump, and drop one out after a short period of time. We picked up a prototype control for that recently. Another is a variable speed circ, IF you have the control to drive it. I don't know if the Wilo Eco has a function built in to do delta T in a solar fashion?

Grundfos makes a special circulator for DB with a special design for "filling speed" there are some controls being built for that.

I did see another DB prototype that had a positive displacement pump built into the tank. It was a special pump with a Wilo motor. I believe it was a variable speed also, but not the Wilo Eco head. A plain Wilo with the brains in the controller.

So interesting stuff is going on in the DB field. we will see it soon. IF there is a market for it. All these components need to go through a long, and expensive listing process, like UL.

Royboy replies:
Is the dual speed Grundfos available in the states now? and would it work with the Tekmar control that Rob mentions? that seems like a good way to deal with the "larger pump" downside for drainbacks.

hot rod replies:
I'm not convinced glycol in DB's is the best idea. Seems like the only benefit is to cover up for a less then perfect

It seems like a catch 22 in WI. If you use glycol it is considered a heat transfer fluid and in some areas of WI they require double walled HX EVEN if it is non-toxic PG???

So now you have the double wammy of the glycol fluid penalty for heat transfer pump sizing, etc. Plus the efficiency loss, and cost of a double walled HX.

It would seem if the installer doesn't want to assure drainback, why not use a closed loop? Less expensive and better performance.

Also when cold glycol hit panels sitting at 300F, or higher? It seems the potential to breakdown the glycol on a regular basis is pretty high. And if some fluid does not drain back it gets cooked to the collector piping. Like a candy coating?

The good, and bad part of DB, depending on how you look at it, is they must be installed properly,

Sounds like a fun project you have, send pictures.

Royboy replies:
I agree about glycol being less than desirable. was surprised that the Solar Service folks use it as the default on new systems. they told me it allows them to put collector arrays square to the roof rather than mount so headers slope for drainage - and that gives them a look that they prefer to collectors being slightly askew. In WI, the state rebates up to 25% of installed cost - but requires glycol burst protection to -20. The project I'm doing now is single story slab-on-grade and the low lift dimension allows me to use a Grundfos UPS15-58, which is nice

hot rod replies:
The DB demo at Hot Water Products up Milwaukee way has a single circ. It takes it a while to start up, maybe 5 minutes, but it eventually gets the flow started. The collectors are an awing, not roof mount on the side of their building. it's all about the lift head.

The boys at Radiant Engineering have a clever "tube" that could revolutionize DB systems. It assures they drainback very quickly, and are completely silent. I hope they chose to build and market them.

The idea of leaving even a thin film of glycol in the piping then taking it to 300- 350 seems to lead to rapid breakdown and acidic conditions? Possibly short collector life and a drop in performance if they get coated. But how will the typical owner know?

I understand Tyfoco builds a very high temperature PG glycol for evac tube stagnation conditions. But to do so the inhibitor package is such it no longer passes as a non toxic fluid.

Wayco Wayne replies:
If the Wilo variable speed circ runs off head pressure then it might be a good fit since it would initially have all that lift, and then when the fluid fills the loop, the head would drop. Mr Hunt said he'll be in MD soon. I'll ask him then.

Metro Man replies:
I almost switched to use a var. speed (Wilo) on one of our drain back systems. After several conversations w/ Mark we decided that the possibility of the system freezing was too great and these pumps simply wouldn't work for our installations.

More thought from the manufacturers of ALL the var. speed pumps needs to be given before we will install them in the field.

BTW - we only install drain back solar systems and have been since 1982

I will gladly give up a drain back solar system pump's slightly higher power consumption for the superior efficiency and maintenance.

Royboy replies:
MM - what's your default SDHW system in terms of tanks, HXs, glycol, etc? Am I remembering correctly that you are in Colorado? I am real interested in not reinventing the wheel, either conceptually or in terms of best hardware ...

hot rod replies:
It's a lot like the ZV or pump question. In the end it comes down to what you are most comfortable with. Both DB and closed loop will provide about the same amount of energy harvest for the power they consume. Closed glycol typically run with one, small, even PV-powered circulator in most residential cases.

With drainback you MAY need two pumps or one high-head pump. You may need a second HX pump to move heat from the drainback tank into the DHW or heating system. So a bit more electrical consumption,
and control pieces.

When everything is correct, and stays that way the DB are great, efficient, simple. If or when they freeze it can be a very expensive repair if panels need to be removed and replaced.

I think there is a good application for both, but the installer and customer have to make the final choice.

Another solar contractor told me of a DB freeze up after 10 years of service. Seems the roof rafters
sagged a little bit (flat roof) after many years of snow loads. The solar piping was attached to these rafters and the sagging took the piping slope away, water pooled, piping and panels froze and split. complete re- do. it's the small details you need to watch, sometimes :)

Bob Gagnon replies:
A tree branch grew and blocked one corner of a set of collectors up here, it froze and he pumped his 1000 tank up through the leak. I guess a 1000 gallon icicle on his roof and down the side of his house was quite a sight. His wife made him take the panels down.

Royboy replies:
I like the analogy of ZVs & pumps - having pondered that one a bit, too. I am wondering about your assertion of: "Both DB and closed loop will provide about the same amount of energy harvest for the power they consume."

Tom Lane cites a 1982 study that had US Solar Eagle Sun drainback systems producing "10-15% more energy" than comparable closed loop systems. Those appear to have been water based systems with internal HX coils in the storage tanks.

Wondering if you have seen any more recent studies comparing drainbacks and closed-loop pressurized head to head?

Seems to me that that drainbacks would be favored by not having cool/cold heat- transfer fluid in the collectors at the the start of each collection cycle, and, if using water, would gain from water's greater heat transfer capacity. They would lose out on pump energy consumption, the amount varying with pumps used. Don't know how these factors balance out - but I'm curious if anyone else does ...

I'd consider 10-15% a significant number, if its relevant to the conditions I work with.

And must say I do very much like eliminating expansion tank, relief valve, air purging hardware, check valve, pressure guage from system.

Some of those seem to me like the weakest links in pressurized closed loops systems.

hot rod replies:
You're talking yourself into DB :)

Factor in the electrical consumption. It takes 1 usually two 80 w circs on DBs. Closed loop can operate on 1
and if it is a PV powered system no electrical consumption. So figure your KW cost for the operating hours also.

I feel glycol systems can run as low as 35% glycol, a bit less of a transfer penalty. You still get around -50F burst, although slush at 0 or around there.

Not a lot of solar harvest around here when it is below 0.

Pros and cons I think you should do one of each!

The beauty of the new solar controllers like the Caleffi i-solar is they can data log for you. So you get actual run hours and even KW gained figures. That is a nice feature both for solar nuts like you and I, but also for homeowners to brag about.



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