Drain back solar

ALH_4

The heat transfer is better with pure water than with glycol. Also, the high temperatures possible in solar panels is tough on the glycol. With solar, the marginal gains can make sense, depending on the size of the system.

Mark Hunt

OK


I have just about had it with the solar dude and this entire "solar"/"green" project.

Walked into the "pump panel" room yesterday only to see that the solar dude had re-located his drain back tank DIRECTLY in front of my pump panel, blocking access to a purge valve.
(Did I mention that he slipped a USED DB tank in on the job??? Did I mention that it looks like a "scratch and dent" special?)

Ok...here is my question for you guys with experience in the solar world. Do all DB systems make noise when they drain back? This is why he moved the tank from it's original location to it's current home DIRECTLY IN FRONT OF MY PUMP PANEL. It made too much noise when it drained back.

Why?

Mark H


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Kevin_in_Denver_2

drainback noise

They all make a gurgling noise for a few seconds. A closed drainback system is somewhat quieter.

In some cases you can get water hammer (loud) with a hot tank on a sunny day, if it's shutting off on high limit.

Tank relocation wouldn't solve that problem though.

Mark Hunt

UUUUUUGH!


Thanks Kevin.

So moving the tank solves nothing other than relocating the noise.

Why use a drain back tank?

Mark H

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Rich W

Freeze protection without using anti-freeze. See also, An American Hydronic tale...

hot_rod

I didn't see any drainbacks at ISH?

Seems most, if not all the systems in Germany were closed loop. I've paged through the offerings from Buderus,Schuco,Oventrop, Viessmann, Baxi, and many others. Little if any mention of drainbacks.

The glycol itself doesn't go bad, but the ph buffer package gets compromised if continually overheated. It can be checked and boosted.

Check out the solar specfic fluids that the companies above offer. They look to be a bit more engineered than the hydronic glycols we typically use. Some are designed to boil and steam back for self protection and to prevent stagnation issues. Or consider a silicone based fluid like Dow Slytherm for 700F operating range.

I'm thinking the intelligent circs that we will soon see on the market will work well with closed loop solar also. Extremely low power consumption for flowing a closed loop solar and possibly leveraging the variable speed component of those smart circs.

hot rod

Kevin_in_Denver_2

Drainback systems aren't appliances

The German offerings are trying to present solar as an appliance that can be installed with standardized procedures. In general, that's good.

Drainback solar for larger systems (more than just DHW) requires more of a custom install. The advantages are there, however: simpler, cheaper, more efficient, etc. and almost no downside.

As always, get the book, Lessons Learned, by Tom Lane

ALH_4

Tyfocor

As I remember, Viessmann's Tyfocor antifreeze is good into the upper 200-300°F range. It is expensive stuff though. One would want to keep the heat exchanger volumes as low as possible.

Drainback can add significant cost to the system. Apparently the Europeans have decided that modern glycols are adequate with an appropriately designed system. It certainly simplifies the installation to use glycol.

I don't have any direct experience with silicone-based fluids. Weren't there problems with leaks, and isn't disposal an issue?

I also notice that Viessmann recommends their flat plate collectors to be connected together without reverse-return piping. I'm guessing that the difference is so small that they have determined it is not necessary?

John Abbott

Drain back

Aside from freeze protection heat transfer issues the drainback prevents over heating in the summer months.Installed correctly they work well I have some that have been in 30 plus years.

Metro Man

Drain-back music

Mark, the reason the system makes noise is the nature of the beast. Water displacing air and visa versa on start up and shut down. During operation though the system should run relatively quiet. Flow rates and pipe sizing are critical to prevent this. Adjust flow rate @ the return on tank for proper flow. Typically .75 GPM/ 4' x 8' collector is recommended.

Drain back systems are the systems of choice for us. There is no other solar thermal system that is more efficient or simpler than drain backs. Someone mentioned that the cost are higher than closed loop glycol. Not sure about that info. No need for glycol, expansion tank, heat exchanger, pressure relief, pressure gauge, high limit recirc, and etc.....

It's like any heating system. Not the system's fault more likely operator error.

Larry Weingarten

One (or two) other thought/s...

... for drainback is to put a throttling valve low on the return. SLOWLY close it while the pump is running, just enough to slow the return flow to match supply flow. Then remove the handle. There is probably a vacuum breaker at the high point of the system. Move it to just above the pump outlet. That way no air gets in while the pump is running and if pipes are pitched correctly, everything drains back when the pump shuts off. You may not need the throttling valve once the vacuum breaker is in the right place ;~)

Yours, Larry

Metro Man

vacume breaker

We prefer not to use vacume breakers. As long as return is above water line system will always drain-back. Vacume breakers all seem to leak and in a small drain-back tank that can add up to more service or pump failure.

my 2c

Kevin_in_Denver_2

If they don't leak, they stick, then you freeze the whole array. I think Tom Lane goes over this. You just need a hole in the return pipe above water level.

Mark Hunt

But


he DID use glycol. About a 30% mixture.

Mark H

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Metro Man

But why?????

If the system drains back.. why use any glycol at all? Pretty simple to see if plumbed correctly. Shut off and if system drains back you're good.

We have drain back systems in some of the harshest cold weather conditions...... never a problem. ALWAYS use redundant freeze protection to kill the pump. A separtate snap switch circuit, aquastat, etc..... Don't use the delta T controller as the save all for freeze protection.

Larry Weingarten

Agreed.

> If they don't leak, they stick, then you freeze

> the whole array. I think Tom Lane goes over

> this. You just need a hole in the return pipe

> above water level.

Larry Weingarten

Agreed... valves fail.

But you also need a way for the supply to the collectors to drain. A small tube run between supply and return will allow the supply to the collectors to drain back, once the pump is off.

I prefer not to use air vents or vacuum breakers as neither holds up well to the harsh conditions found on collectors.

What can be done depends on system piping and how good it can be made.

Yours, Larry

Ed_26

drainback

Haven't we been here? Drainback systems must not have glycol - won't all drain out, high temps in collectors cause glycol to evaporate/steam, causes acid, eats collector.
Nuff said!

Metro Man

Tubeless

Whaaaaaaaaaaat! What's up da tube? Me thinks your adding problems to a simple design.

Kevin_in_Denver_2

Please don't overthink it

Larry,
Take a field trip to a drainback system and watch and listen when the pump stops. The aforementioned hole sucks air after a couple seconds, and the whole thing drains. Totally simple and fool-proof.

It does siphon when the pump is running, but the air hole breaks the siphon the instant the pump stops.

Mark Hunt

Well


you oughta' see what he used for a "vibration isolator"!

A piece of carpet!

The tank is in a small drain pan and he put a piece of carpeting under the tank. NOT outdoor carpeting. INDOOR carpeting. He must have had an old piece laying around and he sorta' cut it to fit under the DB tank. It is a lovely orange color.

Mark H

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Metro Man

r-value

Don't forget about the added insulation your getting from the shag. Plus the safety orange.

Takes all kinds in all the trades