solar drainback

hot_rod

how many panels? Heating ande DHW, or just DHW?

Pros and cons to both systems. AV powered pump on a closed loop glycol takes thew power outage issue away.

Drainbacks are another option, generally they take more pump power to start up, some noise issues. I think they have a great place in large arrays with little or no summertime loads.

Depends on your big plan.

hr

Dorsey Davis

solar drainback

I'm looking for an affordable and well engineered solar drainback system. I'm trying to get the cost down and I like the drainbacks because my customers get power outages and I don't want to fry the glycol....Any suggestions?

michael_34

use a DC pump

Larry Weingarten

If you're doing...

...DHW, I think Solahart has a thermosyphon/glycol system. In my space/DHW system I use PV powered pumps with alcohol and water as the heat transfer fluid. As Hot Rod says, it depends on your plans and what you're trying to do.

Yours, Larry

Dorsey Davis

I've got lots of interest out here in the Seattle area for DHW only. So generally it's two panels and an 80 gallon tank. I have some home owners building small with the mechanical room/closet right below the roof where the panels will be. So I've got lots of slope for drainback. Easy runs. I want to gang on-demand water heaters with the tank as pre-heat for all their domestic. I want to power the pump with PV....my customers do.
I've partnered up with a PV contractor. I can add a couple of panels next to his system because once his system is sold mine is just a small percentage of the overall costs.

Paul Cooke

Solar Drainback

Check out a system made in Portland called Sol-Reliant. It's made by a company called Mr. Sun.

Drainback w/glycol, PV powered pump, no stagnation issues and very trouble free. I have installed several and so far so good.

Larry Weingarten

Solar preheating tankless...

... may be a problem. I know some tankless electrics can adjust heat input down to almost nothing as solar does more of the load, but I don't think gas tankless can modulate down that far. Perhaps someone here has the solution.

In the last solar boom, glycol was not ever used with drainback, giving a simpler to live with system. I'm curious to know why it is called for in drainback now.

Yours, Larry

Dorsey Davis

The glycol's added in drainback if there's not enough slope, or a trap somewhere...I think. Otherwise why use it because the glycol doesn't transfer the heat as well as water. There is a design that uses a sensor/control to divert the potable. If the solar tank is hot enough it allows flow through to the fixtures. When there's not enough in the solar tank then it diverts potable cold through the on-demand heater.

Royboy

glycol drainbacks

as has been mentioned here before, in WI, a state with a very active solar community and state support program, glycol is required in drainbacks. don't know the history behind that decision, but I believe that WI only came to permit drainbacks in state-rebated systems fairly recently, and then only with glycol. my sense is that they are a little worried about poor installation and its a second layer of "insurance" that the system won't freeze.

I've been working with Solar Service of Niles, IL, who have been doing cold climate drainbacks since late 70s. many water-only systems without problems. they now mostly do glycol drainbacks, my understanding that with glycol they mount the arrays level rather than pitched and don't have to worry about being absolutely positive that the headers will drain. they feel that the looks-square-aesthetics of a level array are worth it.

as far as pairing solar with on-demand units, some models advertise as being compatible with solar pre-heat, some do not. I stick with the ones that say they are suitable. none of them modulate all the way down to nothing so I'm not sure how they handle water preheated to just under the normal output temp.

I've heard of others using a 3-way zone valve to bypass the on-demand unit if the solar tank is up to usable temp. my approach is to run it through the on-demand unit but to route the power for the (gas) on-demand through the upper thermostat on the solar tank. then when the tank is up to temp, it will cut power to the on-demand and you won't get the on-demand starting up to heat the first slug of water from the pipes and then shutting down when the tank water gets there. saves some short-cycling of the on-demand and reduces the temp fluctuation at the tap.

Roy

Sunrise Energy Services

hot_rod

I'm still concerned about glycol \"cooking\"

and "coking" on the inside of those copper collectors. Seems that potential increases when the panels are mounted level and not "encouraged" to drain?

I wonder if anyone has documanted glycol performance and lifespan in a system that allows the fluid to overheat consistently?

hr

Paul Cooke

Hot glycol

One of the things I like about the system in my post above is that the food grade glycol does not sit there and stagnate if the system reaches high limit.

The system pump stops the flow and the fluid in the risers drops down into a the header that is on the back lower side of the panel.

Having the fluid contained in that way allows the small PV pump to overcome the head even on a 2 story roof.

hot_rod

that is a clever

simple drainback system. I like simple as long as a mechanism for premature failure of fluids is in place.

Without a differential control it is possible to send heat from a warm tank up to a cold collector for a period. imagine a cold winter morning, a warm tank, and the sun comes out for only an hour or so.

A simple PV solar control could prevent that condition, provide the over-heat function, data log, provide over heat and re-cooling functions if it did over charge the tank.

Also the concern is a warm satisfied tank, hot sunny day, 325F collector and absorber and the pump fires to start a cycle when the tank calls. There will be a period where that glycol is exposed to a high, maybe higher than rated temperature. How long, and how often depends on the SDHW useage.

50% or more glycol mix does take a bit out of the heat exchange transfer ability when compared to water in a drainback. I'm surprised that small Hartell can move a cold 50% solution?


I think it would be wise to check the fluid yearly just to keep an eye out for that condition, as the owner would be unaware of it degrading the fluid. I believe OG-300 requires yearly glycol inspection in the operating manual.

hr

Paul Cooke

Concerns

I agree with you that the control strategy on these systems is the weakest link in the design. I decided that the simplicity of the system outweighed the slight drop in efficiencies. I want trouble free systems and there is little that can go wrong with these.

The Hartell pump works OK. It is a little noisy but I have never worried about any hydronic system making some music when it's operating.

I understand that Laing makes a nice little DC circ pump and I have suggested that they work it into their design but the Mr. Sun people probably stay with the Hartell because it's less expensive.

Mad Dog_2

Taco has a DC circulator that can handle the job

Ask John Barba About it...I saw it last week. Mad Dog

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Mad Dog_2

Probably for the reason of everything else today>>>>>>

Cover one's arse and to compensate for sloppy installers that can't pitch things properly. Mad Dog

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Mad Dog_2

Very good ideas Roy

I enjoy reading your posts. Mad Dog

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Mad Dog_2

And of course, you too HR

always have something important to add. Mad Dog

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Wayco Wayne_2

I like the concept of drainback

I've been working on a drainback system that was installed 25 years ago. Still working fine. Simplicity is the key. WW

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hot_rod

Paul

if ever you do need to replace that pump look into the D-5 Laing. It should be the same or less $$. I see the Laing online for under 200 bucks.

I think the Laing has a lot of unique features and is noise free The soft start and anti cycling feature really does improve performance. I have one on my home system pushing 100' of 3/4 SolarFlex to put it to the test!

Let me know if you ever want to switch and compare.

hr

Royboy

ditto on the Laing

they are very quiet, appear well made and of a durable innovative design.

cheaper than Hartell & March options, which tend to be unacceptably noisy to my tastes.

Roy

Sunrise Energy Services

Royboy

which Taco

are you referring to, MD? I couldn't find any DC pumps on their site.

I did find something new to me, a line of variable-speed AC solar circulators, with integrated solar controllers. interesting.

http://www.taco-hvac.com/en/products/Variable Speed Solar Control "00" Circulator/products.html?current_category=371#

is that the one John showed you?

Roy

Sunrise Energy Services

Royboy

interesting approach

with the Sol-Reliant. nice to be able to PV pump. not so sure if I'd want my drainback tank outside the thermal envelope. low temp here the night before last was a new record for the date of -25, which is to say I'm in a cold climate, and to store a chunk of the system fluid outside the heated envelope is a downside.

Roy

Paul Cooke

Sol-Reliant

Roy

No separate drainback tank with this system. Some of the glycol fluid drops back into the heat exchanger of the Rheem Solar-Aide solar storage tank and some remains in the panel below the riser level.

Royboy

if I understand

the Sol-Reliant system correctly, the "Fat Tube reservoir" below the collector header serves the same function as a separate drainback tank would in a more "traditional" AC pumped drainback system. for my climate, I think there would be a significant heat loss from that fluid during non-operative periods (esp. in winter).

but it highlights for me the tradeoffs that come with different designs and how challenging it is to know how different elements effect overall system performance. how does the energy saved by PV pumping compare to the energy lost by the drainback reservoir being outside? I sure don't know. without side-by-side testing in particular climates, its kind of a tough judgment call as to what's "best."

Roy

Paul Cooke

DC Pumps

Hot rod

Thanks for the offer and your insight on the best way to control these PV powered systems. That is a cool looking little pump. Laings are great. I would love to get one and switch it out with the next system we put in.

The funny thing is that even though the Hartells look funky and are noisy about 2 years ago I went back to the first radiant floor job that I ever did (around 1981) and found the original Hartell still working fine.