Drainback Solar

Mike Dunn

If the tank temp is satisfied but there is still sun to harvest could you just leave the pump run until the system stalled out. I was told by one of my solar instructors that the system would stall out at around 200 degrees and you wouldn't have to worry about stagnation. Not sure if I believe him but he does 30 years experience. If it is true would it be a bad thing to have a solar tank sitting there with that hot of water until there was a demand?

mike

Wayco Wayne_2

Does a typical

drainback solar system ever need the water in the drainback tank to be re-filled? I'm going back and forth on the closed loop vs drainback. The closed loop has the glycol but there can be times when the glycol could overheat and stagnate so there is a maintenance issue to check the acidity of the glycol periodically. However the drainback is safe but I imagine the pipes are noisier when filling and draining and if running through the walls of a house that could be a negative. Also does the water level have to be adjusted on drainbacks? My lack of experience is keeping me from jumping into the solar pond just yet.

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Bob Gagnon plumbing and heating

Jump In!

The green water is fine! With your hydronic experience you are probably better than a lot of solar installers. I went with closed loop it seemed easier and more reliable, but we also have a lot of minerals in our water that would build up on the heat exchangers and collector manifold if I used my big open tank to pump directly through the collectors. And your right, they say drainback pumps are noiser and more expensive to run than closed loop circulators. To eliminate the the stagnation issues and to protect my collectors I am going to install a redundant system. I'll add another circulator pump, 12 volt, tied into a high limit switch up at the collectors. This will protect my system if I have a power outage or if my circ pump or controls fail.

Go Solar! Thanks, Bob Gagnon

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Kevin_in_Denver_2

drainback tank refill

Yes, the water level should be checked several times a year.

Even if you have a "closed" drainback system like Siggy's the water level should be checked. I haven't seen a system with an automatic fill system, maybe Metro Dave knows more.

J.C.A._3

Yes....

The drainback systems that were used around here had Triangletube drainback tanks, and they wanted the vent on the sightglass left loose,so they vented the air during the drainback sequence.

Having that vent open will make the water evaporate and need to be topped off once in a while. T.T. used to want the tank about 1 " from the top of the sight glass when the system was not pumping.They also wrote that the tank level should be checked every month.And finally....yes, they make some noise when both pressurizing and draining. Chris.

Ron Huber_2

Drainback

This is a 28 year old system that I looked at last week, we are replacing the 20 panels that froze and burst this past winter. Homeowner went to Florida for a few months and his son came by to check the level on his 1200 gal atmospheric tank. He added water and shut off a valve to the panels by accident and the array froze up and burst. Depending on where you are will determine what to do. I installed many draindown and drainback systems here in NH in the late 70's and 80's and have been lucky so far. But alls it takes is one screw up and there goes the solar panel array. 28 years is a pretty good life span, but it probably would have worked for another 20 before the panels needed replacing. Closed loop systems of this size have their problems as well, so some serious engineering needs to be done for your specific site to deterime which way to go.

Wayco Wayne_2

Is there an idiot proof

method? (We got lots o them round here) If you went closed loop and had an automatic overheat dump of some sort, that might be the best from what I'm gathering. WW

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

radmix

Drainback

Does adding glycol in a solar system drop the btu output as it does in a boiler system. If it does that would be another plus for using a drainback system.

Wayco Wayne_2

I had a thought too

What if you use a boiler water level control to keep the water level right. There's got to be something.

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Ron Huber_2

glycol

Usually you have to run about 40% glycol here in the Northeast, which does indeed affect performance by a small percentage, but add the loss from the heat exchanger to that equation and yes the glycol system is not as efficient, but is safer in terms of freeze damage. That is one reason why active solar space heating is not always a viable alternative. Solar DHW is always a good investment, but for the dollars spent vs. the return, you usually can spend the money somewhere else in the mechanical system and get a better return by upgrading insulation or conventional heating equipment.

Ron Huber_2

Stall out

I am not sure what "Stall Out" means? Maybe that is the maximum temperature you can get at a particular time of year in your area. Some of the atmospheric tanks that I have seen have a Vynal liner that can only take about 150 degrees max. Some of the EPDM liners can take over 200 degrees. If a large system is left to run in the summer months, even here in NH you can go over 200 degrees on a good full sun day in late June or early July, so the control is what will govern that, beyond that you would have to have a control that will run at night to cool the tank down to be able to absorb more heat the following day to avoid or lessen stagnation time, Caleffi/Resol has such a control.

hot_rod

Many of the Wisconsin solar boys..

use glycol in their drainback systems. seems odd to me, but it does handle the errent rouge non drain or mis piped problem.

i've talked to installers that have seen drainbacks work fine for years then freeeze up when the roof and attached piping settled a bit and presented a small water trap area.

Overheat with closed loop glycol may not be as much of an issue as you think. It depends on the panel to storage, panel tilt, radiation at your location, daily use, etc.

The Caleffi I-solar controls have a number of overheat protection tricks for panel and evac tube systems. Pulses the pump to allow over temperature in the tank (tao a point) Reradiation to the night sky to pull temperature down. a second differential function on some models to dump to another tank, pool, or dump zone.

hr

Unknown

night time heat dump!?

I would assume that "stall out" is the point at which tank losses and collector/ambient delta prevent any further raise in temperature.

Glycol does not like to get that hot (200f), it breaks down, in-fact it starts degrading at even lower temperatures. Thats why closed loop glycol systems need to be monitored regularly. Glycol maintenance issues are in my view one of the most compelling reasons to consider drain-back. Although I understand that some drain-back systems incorporate glycol, as a precaution. In theory the glycol could be protected from high temperatures in such a system.

One issue I have questions about is the potential of pumping to a very hot (dry) collector. Seems like there would be a real potential to generate steam in these conditions. Does this mean that if a system drains down on a hot summer day it can not restart until the following day?

Also how well does a dry collector handle the very high temperatures it is likely to reach. If these conditions were encountered with any frequency wouldn't it eventually compromise glazing components etc.

Why would you want to cool the tank down at night (unless it was for a useful purpose) How would this control know that the sun would be shining tomorrow? Seems to me that you would want to enter the night with as much capacity as possible for morning bathing routines.

Solar systems that require heat dumps are not what we need right now. My mantra is: Small high efficiency systems systems on as many well exposed roofs as possible, 100% solar fraction systems have considerably lower utilization efficiencies than supplemental systems. It's my belief that this technology could have the biggest impact (energy politics wise) if broadly adopted as a supplemental input.( At least in northern latitudes.)

Unknown

HR, my last post was intended as a reply to Ron. Always nice to have you in on the discussions. I agree that glycol need not reach breakdown temperatures to be an effective solar dhw heat transfer medium.

Given the higher pumping cost,trickier pipe work and less balanced flow of harp type collectors I'm leaning towards closed loop glycol. This also fits with my opinion regarding the merits of modest solar fraction and high utilization efficiency. Such systems need never see excessively high operating temps (barring a power failure or circulator breakdown.) I see that bob is concerned about such a possibility. I think modern wet rotor bronze circulators are a safe bet for a decade or more, I'm more concerned with the power grid, especially since failures are often the direct result of hot summer weather.

Kevin_in_Denver_2

some answers

Relying on the "stall out" temperature as a high limit is wasteful. If the load is satisfied in summer, the pump is wasting electricity and most of the losses are going into the house to be fought off by the AC system. Can you say negative system efficiency?

Long-term dry-stagnation: Not a problem if your collector is copper, aluminum, glass, and the right type of fiberglass.

Turning the pump on during dry stagnation with an atmospheric drainback system can cause boiling and noise (sounds like water hammer), but no other problems. The differential control on the collector pump (Delta T on=20deg, Delta T off=5deg) minimizes this. The closed drainback systems can be pressurized with air to prevent the boiling completely.

Night time heat dump: also drives down system efficiency.

Glycol in drainback systems is a good solution for an installation where the collector loop can't drain completely. However, the heat exchanger required cuts efficiency and adds cost.

Mike Dunn

stagnation

Kevin,

I really like this post because it makes you think even more about not just the design but how it effects the variables around it.

When it comes to the problem of stagnation what kind of designs do you do to accomodate these issues?

Mike

Ted_5

drain back and glycol

is questionable to me. Talking to a contractor the other day about drain back -vs- closed loop. He was starting to use drain back over closed loop. He was putting glycol in the drain back systems,(WI boy) but not using high temp glycol! One thing I said was, if the pump turns on when the collectors are very hot, that first bit of flow as it goes thru the collector will flash over to steam. Over time, depending on how often the system cycles on very hot days, this will damage the glycol and shorten the life of the system if not watch carefully. Much less chance of this in a closed loop with 45 PSI system pressure.

Ted

Metro Man

panel replacement

Ron

I would plumb the new panels diff. Looks like the bottom and top collectors are in series and then run parallel. Could have issues w/ delta T and sightly longer drain back times.

my 2c

Metro Man

Paul Pollets

Closed loop

Wayne. the closed loop systems are far easier to deal with. Follow the fill procedure using the special antifreeze (we use Tyflocor) and vent the air on startup. Caleffi's manuals have the better procedures. I used drainback systems 20+ years ago...never again.



To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Wayco Wayne_2

Parallel

makes sense to me. The Oventrope Evac tube guide book said you can pipe up to 4 collectors in series. I questioned that. By the time you reach the last tubes I would think the water would be so hot that heat transfer would be minimized. It also gave a rule of thumb of 1 gpm per collector, so you could pipe 3/4 L copper for up to 4 panels. I wonder if it's the same for flat panels? WW

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Metro Man

panel piping

The problem with series is just that... way too hot by the time fluid gets to last collector. If sensor is on that last collector your system will short cycle a lot. It's better to control your flow w/ proper piping sizes.

EDIT. More accurately... the delta T will be too high in most cases. Normal delta T should be around 10 - 15* when running.

If I was replacing this I would run my feed line to feed lower and upper array then bring returns together at highest point for reverse return.

Flat panel gpm rule of thumb = .0234375/ sq ft coll area. 1 - 4 x 8 = .75 gpm / collector.

BTW - we would plumb a glycol system the exact same way. Lot of guys rely on bal. valves to accomplish this. But if you plumb correctly the system self balances.

We can install any type of system we want to . I will always design for drain back unless conditions are such that this is impossible.

Looks like a fun roof.... don't forget the sky hooks!

Metro Man

Ron Huber_2

Piping

I do not know why they did it this way, but the feed line comes into the bottom right hand side,fills bottom right 5 panels, continues up to top 5 panels on right side, runs over to top of left side, then flows down through top left and bottom left arrays and then back to storage. Be a good place to use an infrared camera to see what the transfer looks like.We will be eliminating a lot of this external piping.

Metro Man

Wow

Are you sure though? From the pic looks as though you have 2 returns top east and west?

If that's the way it is plumbed.... I'll bet all kind of funky things where happening.

What kind of collectos are you putting back up?

Metro Man

Drew_2

High Temp PG Antifreeze

Ted
Good to see you brought up higher temperature propylene glycol antifreeze. A couple of things I've run across are Distributors, Contractors, Installers and others that are not aware of higher temp PG formulations and asuming that all solar antifreezes pass FDA 21CFR 182.1666 and 21CFR 182.6285 GRAS .

Kevin_in_Denver_2

drainback solves many problems

If your system has more than two high efficiency flat plate collectors, drainback makes the most sense.

"Vacation Mode" solutions in a non-draining glycol system just waste energy.

Remember that Habitat house in Arvada? It was drainback with collectors that should perform at 30%-60% efficiency. However, for many reasons (none of which were system design) the system efficiency was down under 10%. Any sort of required "heat rejection" mode on that system would send its yearly efficiency negative.