Skip to Content

Flat Panels or Evacuated Tubes?

solar collectors

Here's another interesting thread that appeared on the Wall.

Wayco Wayne asks:

Where do you draw the line between the two types of solar collectors? How do you decide which is more appropriate for the job at hand?

Paul Pollets replies:
Flat panels are more appropriate when you have snow loads to consider. The evacuated tubes will produce more BTU's PSF of panel, especially on cloudy days... but have a higher cost. The evacuated tubes attach to a "collector header" which contains the heat exchanger. The flat panels have water in them, the evacuated tubes do not.

I use the evacuated tubes in Seattle. I've seen that the Chinese are exporting evacuated tubes at significant discount to the German product, but their ability to hold vacuum may be in question. We use a closed loop with manual fill,(with special glycol) and either the Caleffi or Viessmann pump module and controller. These modules and controllers make solar easy to install and use. There's lots to know and I recommend you bone up on solar requirements.

Caleffi has written an excellent solar document that gives all piping diagrams and a no-nonsense approach to the types of systems available. It can be downloaded from their website:

hot rod replies:
Temperature requirements, climate in your area, and budget.

When you run the numbers using the data from various manufactures on the SRCC website we find.. At 95F return to the panels, in a 15F ambient temperature (outdoor) the performance with 200 BTU/ square foot of solar radiation, is about equal.

So if your system can operate at radiant temperatures@ 110- 115F supply the (95F example works).

IF you require higher operating temperatures, or see frequent temperatures below 15F tubes perform better. I'm not 100% convinced they perform a lot better in cloudy conditions. The energy just isn't available in a cold cloudy winter sky, regardless of the 1/2" space between the glass (R-value) with a vacuum.

With evac tubes you are mainly buying a better r value between the absorber and outdoor conditions. They really don't generate addition BTUs magically as some ad slicks lead you to believe.

Where they're at, and what you're asking them to do, is the key question.

Now if the tubes cost 3 times as much to purchase, you need to pencil that into the formula.

Most all of the solar design software will let you play the money game for investment payback, etc. Ret screen is a free download, Maui Solar, T-sol, F- Chart and Poly Sun are a few others.

You might find you could double the flat panel square footage and still be less $$ than the tubes. possibly 1/2 as much.

A solar contractor in Montana sent me a large home solar design recently. His numbers using a well know German flat panel, compared to a well know quality evac tube showed 40 bucks per square foot for panels, $105 for the tubes. This put the payback another 7- 10 years down the road with generous fuel cost increases

The Srcc website has a good $$ comparison calculator. that calculates BTU production per $ cost. Caleffi Idronics 3 has formulas to show performance difference.

Other considerations are ease of installation, a plus for tubes. Looks?? Some like tubes, some prefer panels. Also warranty, I've seen some tubes with a mere 10 year warranty. Manufacture and their history would be another concern. I suspect many of the new un familiar imports may not be around for the long run, regardless of type of collector, flat or tube.

We've been through the snow pack issue before :) Keep the tubes 50 degrees in snow country to assure snow shedding.

Let us know what you decide on.

Dave Yates replies:
How it looks can be a deciding factor too. Flat panels may need to be angled to get the best exposure and that became an issue here at our own home. Lois put the kibosh on having a tilted rack. "Too ugly" The vac tubes gave me the option to install a flush-mounted array while angling each of the tubes the almost 20-degrees needed to be true-south. Although our home was to be constructed facing south, to take advantage of passive solar, I hadn't realized it was off by a few degrees from mag south - away from true south! - until we got serious about solar.

scott markle replies:
Dave, don't get too hung up on orientation. You can be 30 deg. off true south with negligible impact on performance, I believe 2.5% diminished for 30 deg.

hot rod replies:
And the panel tilt doesn't carry a huge performance penalty. This graph was compiled by Solar Skies.

Sometimes the "taste police" trump the small performance hit with less than ideal orientation. Adding an additional panel, or a few more tubes could make up the difference. Panels or tubes mounted on awkward angled leg assemblies look ugly and out of place to me.

Being able to rotate the evac tubes, on some brands only, does add some flex-ability.

Dave Yates replies:
Agreed, but that 2.5% would have cost me 750-gallons of hot water over the course of one year's solar harvest. If my customers are ok with that loss, then I am too & they get to make that call. But, on my own roof and in my system, I wasn't satisfied with the compromise & it would have been a daily distraction that bugged me. A bit picky no doubt, but that's me.

I'm still toying with the idea of PV tracking, but the cost to rotate that many panels automatically will probably keep me from going that route(G). Then again.....

Ron Huber replies:
This is one study done in Germany, duplex with tubes on one side and flat plates on the other. One problem is that the tubes and plates are at about 33 degrees. So snow takes longer to melt off the tubes. Also, on my own tube system I have monitored and data, logged temperatures every 5 minutes since January. The tubes produce heat with morning frost on them. They produce heat with a few inches of snow on them, as do flat plates. I sell both and right now I push the tubes when the orientation is off from south, or the customer just likes the looks of the tubes over plates and is willing to pay the extra money for them. I have more flat plate sales the last few weeks and economics are swaying the people who just started looking at solar but did not know if they were ready to take the leap. The difference in cost is about $2,000.00 more for the tubes vs. a two 4x8 panel system. At the end of the day the flat plates are not that far behind in annual performance to justify the costs, unless these other reasons come into play.

Singh replies:
Are you sure Ron that the difference for two 4x8 flat plates which would equal one vacuum tube array of roughly equal size is $2000.00 more?

I just had this discussion with my Viessman supplier, and we looked at their Flat Plates and Vacuum tubes, we compared coverage and efficiency on both types and sized the system , to get same coverage , example: one 2 square meter evac would equal in coverage to two flat panels. The difference list price was $800.00.

Once again , one man installing evac, or two men installing flat panels. And the difference in price is quickly recouped.

One thing was a bit surprising was installing just one 4x8 flat panel almost seemed not worth the effort, for so little in return.

Ron Huber replies:
Yes. I have a template that I use for my base price, so I know how much things cost. Look at the current Viessmann price list. A Vitosol 200 system with 49.5 sq.ft. (two of their 4x8, which are actually smaller than a standard 4x8) is $2,257 less than a Vitosol 300 (evacuated 30 tube system) with 32.3 sq. ft. , and putting anything less than 30 tubes with an 80 gallon tank will not be too impressive to a customer who laid out that kind of cash. I am basing those
prices on their suggested retail price, and with the mark up you get from them you have to be charging top dollar to make a descent profit, or you end up being a good guy and giving it away.

Michael replies:
I don't agree that tubes are better in cloudy weather. I have not seen any proven data on this. Flat plates top off about 180, tubes can keep heating up to 212. They both heat up the same until they reach that 180. The biggest advantage I see between the tubes and plates is durability. I have worked with both and plates seem more hardy. I see a lot tubes losing their vacuums and need to be replaced quite often. Once they lose vacuums they just become radiators and send the heat back out from which it came. I love the idea of the tubes, I just think they need more time to evolve.

Dave Yates replies:
I've thought about that issue and that was one reason why I haven't given the cheaper imports any consideration. However, I do wonder why we'd toss a vac tube simply because it lost its mojo. Why not purchase a new stopper and use our refrigeration vacuum pump to re-draw the vac and give it back its mojo? With our digital vacuum neg-pressure indicator, we could no doubt obtain the level of vacuum required???

scott markle replies:
Tubes are not new, GE, Philips, and other large manufacturing companies made them in the late 70's. This is a mature technology, But remember that nature abhors a vacuum. It's inherently difficult to maintain vacuum especial in a device that undergoes this type of thermal stress . I think this is why you will not find warranties over 10 years, Although many expect 20 years from the higher quality tubes.

Andrew Hagen replies:
I think the biggest advantage of tubes is ease of installation. I prefer the look and simplicity of flat plates, but installing them on a roof is much more difficult. Performance-wise, I'm willing to call it a wash.

Darin Cook replies:
I have an article that I received at Buderus about a study that was done in Germany. It did a comparison between flat panels and evacuated tubes. Bear in mind that in Europe Buderus sells both types of collectors so there is no bias here. The so called inferior thermal performance of a flat panel actually caused it to be able gather more btu's on average because the evac tubes did not shed snow or even a hard frost would keep the tubes from performing until the sun melted the frost. The flat panels self clear enabling them to still collect energy. Flat panels are definitely more forgiving when it comes to angle placement.

Personally I do not think one is better than the other. It really depends on what you are trying to accomplish and what temperature water you need to produce. One thing to keep in mind is that if you are not utilizing the energy collected and cannot store it, the panel or tube array will get too hot and the system will lock out. This means that until the array cools down you will not harness ANY of the solar energy during that day. Even if a load is incurred after the lockout. This is where the fluid stagnation issues arise. Do not add more panels to a system without sufficient storage. This is self-defeating and you will not get the results you are after. Remember we are hoping to have a solar thermal system that is somewhere around 35% efficient. We use these systems to offset our fossil fuel consumption and they will never be stand alone. Just one piece of the puzzle to help us in the direction of energy independence.

Singh replies:
It depends. My experience is frost is melted off pretty quick when the sun rises on evac tubes. Also, the curvature of the tubes and the rotation adjustments of the tubes allows the panels to collect sooner and more. Remember both evac and flats give off heat to outdoors. It's just that flat panels give back a little more, which not the best in severe winter climates. So in winter evac tubes do better. And the snow does melt off even at a 35* angle. In summer, that vacuum layer acting as insulation effects the performance, so flat panels do better.

I guess if you live in Florida , flat panels would be a better investment as it is not a heating dominate climate and using solar thermal mostly for DHW .

If you live in NY, and tying into a low temp radiant plus DHW, evac tubes a better investment, or flat panel with gas layer between. And over the life of the panels , they will offset the price difference pretty quickly.

hot rod replies:
I'm not sure that is true.

If the tubes cost 2 to 2-1/2 times as much for the same square footage, and only provide 5-8% more harvest your
argument may not hold up.

But the biggest unknown is the life expectancy. Very little if anything goes bad in a flat panel. A rare broken glass which is replacable.

If in fact evac tubes start failing at 10- 12 years and cost 100 bucks each plus labor???? How can you project your findings?

I have heard rumors of one tube manufacturer, fairly new in them US market, already recalling, and replacing tubes due to high failure rates.

There is plenty of actual real life data on the longevity of both types here in the US as you start to meet and talk to old time solar installers and repair people.

I believe Siggy's own flat panel drain back system in upstate NY is closing in on 30 years of performance. Same panels, same performance. It's hard to argue with that success record. 30 years from a panel that probably cost less than 400 bucks back then!

I'm not against evac tube, I just want to be sure all the factors and numbers are compared fairly, based on actual experience and data not hearsay.

Singh replies:
My costs for both FP vs. ET are no where near 2 - 2 1/2 times as much. Actual and not hearsay. And what it comes down to is that you need slightly larger surface area with panels than tubes to harvest same amount. So the smaller evac. tubes also has a factor in price.

Both should have an expectancy of 20 years, according to what I researched. Yep, I heard about that manufacturer also. At least they are standing by the product.

I don't think the evac tubes were around when Siggy put up his array. But, I remember he talked about having a drainback system. So, now we have to factor a large storage tank, larger circulator pump.

But he most likely paid next to nothing anyway for those panels, being he worked in the solar biz at the time. : )

Regardless, the energy is renewable, and abundant, the efficiency of one vs. the other is trivial.

Michael replies:
Just get them up and working well. I have installed both (tubes and plates)and I have my preference. But Honestly, I don't give a fly which one goes up as long as they are going up. And as long as they are put up well! No half but stuff. Make a system that will last. In the eighty's you had alot of fly by nights that went for the buck. That gave some of the solar a bad name. I hate to see that happen again.

Darin Cook replies:
Last monday we had a major storm smash through our area and it pelted us with nickel sized hail stones. Lots of property damage for folks. My garden looked like someone took a weed eater to it. I wonder how a evac tube and flat panel system would of faired in that side by side. I can't help but think what HR says about factoring in the replacement of tubes and the labor to change them could throw whatever is picked up in extra
efficiency out the window.

jp replies:
Darin, I believe "somewhere" I saw a picture of a guy standing on an vac tub? I doubt hail will bother this things, hail doesn't seem to effect car windows? I'll see if I can find that picture again

Dave Yates replies:
Interesting thoughts. I've been told the tubes can handle hail & might just find out today as we're supposed to have roving bands of severe weather.

As for one type vs. another - when I attended the Solar Decathlon in 2005, just a few of the college teams had flat panels and the majority had vac tubes. Seemed that vac tubes were more prevalent. This past year, I found just one team using flat panels. There may have been others that I missed, but I was striving to check them all. The crowds made that difficult.

I agree - any solar is better than no solar so long as it is done well and installed using components that will help to ensure we do not get another black eye. Personally, after watching the bit of the Today Show this AM dealing with oil prices and the ramping-up in barrels per day, I'm a bit concerned that we'll see yet another artificial reduction in prices that will, once again, lull the US consumer into a stupor. 2010 before GM can bring us an electric car? And it's only going to go 40-miles before its on-board gas generator has to fire up to recharge the batteries? Somebody needs a check-up from the neck-up.  

NRT.Rob replies:
Singh, what is the area conversion you are assuming when you convert from evac to flat plate? because our pricing seems to corroborate HR, but perhaps our comparison wasn't sophisticated enough.

Delmas Gehman replies:
Has anyone had experience with Apricus evac tube systems? I've been looking at Viessmann and Oventrop and now came across the Apricus and would like feedback. It appears to be less initial cost. I wonder about the longevity
of the tubes.

jrc2905 replies:
After attending a training session I have just order a 20 tube collector to install in my own home to meet the hot water needs for two people. It will be the first house that I know of using this type of collector. I am doing my house to make sure it works ok before I sell one as a job. I install boilers so the mechanics should not be an issue. I plan to install another DHW tank in addition to the existing DHW tank. The design, that is another issue being a first timer. I have no existing medium to be used as a heat dump. I am thinking of building a unit using fin tube baseboard and a solar powered fan. My electric rate is one of the highest in the country so I am considering a solar power circulator pump. Any suggestions

Bob Gagnon replies:
How about adding another tank to dump the heat, and use that tank to pre-heat your domestic water, before it goes into the primary solar tank? You could run the domestic through the cooler tank first, then through the primary solar tank. This would give you a longer period of time that you would be at 100% solar, and it would send cooler water back up to your collectors, making them more efficient.

jrc2905 replies:
I would like to but space might be as issue as well as cost. I do hate the idea of not using all the solar gain.

Bob replies:
Hey Bob, I am interested in your tank arrangement. What are the chances of you drawing up a diagram and putting it up here?

Bob Gagnon replies:
I think we are doing solar all wrong now. We go for the domestic water heating, then what is left over we use for space heating. Then we only heat for a short period in the spring and fall when 100% of our heating needs are available from the sun. And we use the super complicated and expensive piping arrangements. Using this simple piping arrangement will allow us to harvest solar energy to heat our homes every sunny day in the winter. Simply run a separate supplemental radiant panel that can be used at the same time as your primary heating system. Keep the thermostat up on the supplemental system and run the delta tee controller off the heating tank in the winter. By supplying space heating, you will keep the temperatures in the tank around 90 degrees, and these lower temperatures are a lot easier to collect with solar energy. You can collect a lot more BTU'S at the lower space heating temperatures than waiting around for 130 to 150 degree domestic hot water. You can also harvest energy all day long with space heating, I have a one set of collectors connected to a domestic tank that maxes out around one or two PM at about 140 degrees, then it doesn't collect anymore energy, but my low temp collectors, that I use for space heating, collects energy until about sunset. Because it only needs to heat the water to about 95 degrees, that's easy to make. This supplemental system may provide only 25% of your heating needs on a lot of days, but that means every 4th day you get free heat.

Now I know all the books you read and the solar classes you attend will tell you to go with the old system, but they just keep repeating the same stuff over and over. We haven't really changed much from the way we did solar in the 70'S. Take a look and let me know what you think, and I would appreciate anyone's input or improvements.

mac replies:
Factor in location & weather and the superior performance of the evac tubes really only comes into play in wintery, cloudy conditions when the amount of energy to harvest just isn't that big to begin with!

Summer time in New England? Flat tube more bang for the buck...but they sure are easier to install!!

Singh replies:
Bob, I think I miss read your post about an evac system costing 2-2 1/2 times more. I was thinking, you meant the entire system costing that much more. Other than the panels themselves, the mounting hardware, differential controller, piping , etc are basically the same.

Labor to replace a failed tube?? What about labor to install a flat panel. An evac basically, can be a one man job. vs. two men humping a flat panel up. How much additional labor does that factor in to the cost.

Darrin my tubes survived that same hail storm.

Josh (The Youngster) replies:
I'm gonna settle this with 2 words... "It depends."

Bob Gagnon replies:
According to the chart you posted from Solar Skies, it shows that we will harvest 50% more energy in the winter with a 60 degree collector tilt compared to a 30 degree collector angle tilt, that's huge! Am I reading that chart correctly? The difference would even be more if you went with a 70 degree pitch in the winter and a 10 degree pitch in the summer, I don't think we should ignore these increased efficiencies. Some people might consider a tilting collector with legs ugly, but some people consider all solar collectors ugly. If the tilting collector was placed on a flat roof, ground mount, wall mount, or as a roof for a carport or over a deck it would look a lot better and it would be easy to adjust. We are not collecting enough energy in the winter because we are doing it wrong. We are compromising on the collector angle and we are waiting around for high temperature domestic hot water, we have to improve these systems. What do you think?

hot rod replies:
"where they're at , and what you're asking them to do" That graph was built for a SDHW application. There is no question pointing at the sun at all times will give you the best performance. Typically if the array is mainly a heating application the steeper pitch helps, and can also help with overheating in the summer. I know some SW contractors that use vertical mounts for that reason. I think for the typical residential application the cost and complexity of engineering a tracking system to keep them pointed at the best position doesn't pencil out well enough. Heating arrays, as you know tend to be large square footage arrays.

In your case with the time and willing -ness to get up on the array and "move with the sun" sure it helps with the harvest-ability, no question.

I have seen some pole mount evac tubes on tracker systems. I think Apricus shows them at their website. I suspect the tracker to move the array was much more $$ than the array itself. Doubtful the payback is there.

I did see a clever reflector concept in Germany recently. "A" shaped reflectors that mounted between panels. both PV or thermal to increase performance. This might be a less expensive means to boost some performance.

They also had some cool mini solar cookers on display. I'll look for pictures.

Wayco Wayne replies:
Funny you should mention Tilt. I am about to get solar on my own house. I've been working on a project with a solar contractor where I'm providing the radiant floor and he's providing the solar. Of course I've been looking over his shoulder and peppering him with questions. To his credit he hasn't seemed to mind. :) Long story short, he needed a new central A/C system so we are trading services. Basically I'm getting 4 flat plates sized 4 x 10 on my South side roof, tilted at 50 degrees. I'm supplementing my radiant floor, and getting some domestic hot water help for my Polaris domestic heater. I'm at 40 degrees latitude so 50 is a higher tilt than I expected but it makes sense to me now. I'm certainly not above climbing up on the roof and changing my tilt between seasons to optimize my collection. Perhaps I'll start a new thread to get everyones input on my piping in the basement.

Bob Gagnon replies:
Way to go Wayco Wayne. You may only have to adjust the tilt 2 or 3 times a year. You could tilt it away from the sun if you went away on an extended vacation, or you could tilt it flat level if you have a big summertime load or a pool or hot tub to heat. Then you can tilt it to a steep pitch in the winter to collect that much more energy. You can use your hydronic experience to improve the solar system's performance. Solar guy's exchange information and work well together, I think everyone realizes that we have to work together to improve these systems. Teach him what you know about hydronics and he will teach you about solar. It's a win win situation. Thanks for going solar.

jp replies:
In the U.P (Mich) I notice the best chances for winter solar gains start the end of February, November to feb is real cloudy.

I would think about tilt angle for march and later, seems to be the best winter months for picking up solar here.

brendan replies:
Bob, I do not agree with all this tilting just set it to the optimum angle for winter sun and if necessary add additional collectors, set it and forget it best way to go.

hot rod replies:
How interactive do you want the system to be? I like the simplicity of solar, compared to fossil fueled devices. I'm not sure I would take the time to be a "tiltmaster" :) What connections and hardware would you use to allow movement and still assure you can handle wind loads. When you start moving panels off the roof surface you literally have a sail. Be sure you fasteners, etc can handle those loads. What about flexible piping. Perhaps Caleffi SolarFlex would be a good match for moving panels. 32 square feet of sail, (4X8 solar panel) plenty to move a wind surfer through the water.

michael replies:
I been using 75* -85* on a vertical tilt. This captures most the winter sun and avoids overheating of the summer sun (but enough to take care of DHW loads). These systems work well this like this. I also suggest using a solar pump on the solar loop (like a Laing D5 strong). Mount your PV panel at the same tilt of your thermal array. Sun shines, pump runs and collectors collect. No stagnation. Better for the life of the collectors.

Wayco Wayne replies:

Very interesting. They are closer than I would have guessed.

Metro Man replies:
Been saying this all along. Think evac tubes get better than they should #'s because of how they are tested @ the SRCC. Can't beat a good old flate plate.

Singh replies:
Interesting to see two flat panels at a slightly larger surface area about equal that of the evac tube surface area. Once again, labor, cost and pay back is .....

J.C.A. replies:
Interesting Email conversation last night with Jon Klima, from Conifer-Solar-Controls. He told me a few things I was and am aware of, but I got to hear it from "The Teacher".

Both have their places. There isn't a doubt about that part...but figuring which will work best is the goal.

Long exposures and steady sun will make flat panels a better choice....unless you need higher temperatures. This seems to be where the evac. tubes shine.

I say, as always....It Depends!


To quote Leo..."The More you know....the more you DON'T know!"

Metro Man replies:
Yea, Jon's been around a while. Great resource for repairing older controls. I think he likes to keep those old C-30's alive. Still haven't drank the cool-aid yet for evacs. Long term loss of that all important vacuum could be a major problem that may or may not play out. Used to think that space was the best asset for installing tubes but have since changed my mind after comparing performances of these systems. High temps also is kind of a misnomer. Flat plates will still get you to 180*F pretty easily. Not sure how much hotter you would need for residential or commercial applications.

But... it's all up to what you are comfortable installing and standing behind.

scott markle replies:
Misnomer? Dave, The issue of high temperature is a misnomer if it's not defined properly. it's not as much about high temperature as it is about temperature above ambient. Since ambient temperatures on a roof in the summer are already very hot even an unglazed collector can produce relatively high temperatures at a
reasonable efficiencies.

I think your skepticism is generally justified, however these collector types do have significantly different performance characteristics as return temperatures rise significantly above ambient. As to how these different characteristics average out in terms of annual performance is the important question.

As well considered as an system might be, actual and theoretical performance can be quite different, especially given the wild card of user behavior. Obviously the performance of a DHW system on a weekend house is going to be vastly inferior to an identical house that is fully occupied. This kills a great potential market, as the second home market in my region is a big deal, and many in this demographic are both financially capable and ideologically motivated to make alternative energy investments. But you would be stretching the truth to sell a solar DHW system to a weekender as a green investment. Wouldn't you agree?

Larry Weingarten replies:
That wasn't a disparaging remark about the C-30s was it? :~)

Suppose I'm a solar gray-beard. In the 70s and 80s I installed many collector types including evacuated tube. Things always happened to the tubes. They broke too easily in shipping or even just with too rapid temperature change. System controls starting up at the wrong time did real damage. All of those systems are long gone, but some of the flat plates are still working. No doubt design has improved.

michael replies:
Here is the guy that does the data log system. -
Chuck Wright
Chuck Wright Consulting, LLC
ph: (512)255-4067
cell: (512)434-9692

Mi39ke replies:
Would like to see more parameters. The other data logging comparisons wouldn't work for me and my old computer. Seems to be letting the PV shade the evac tube? Not as scientific as I would like at this point.


Leave a comment

Related Posts

Vetter Weingarten solar roof
Another Solar Myth Bites the Dust

Way back in 1978, I (Larry) installed my first solar water heating system. I continued with solar thermal, installing new domestic hot water (DHW), space, and pool heatin...

Published on 08/21/2023 3:13 PM by Larry Weingarten and Zak Vetter
Posted in Solar
max headshot sq
Frequently Asked Questions About Solar

Q: What is the difference between solar thermal and solar photovoltaic (PV)? A: Solar thermal panels heat water. Photovoltaic panels convert sunlight to electricity.

Published on 06/17/2014 1:34 PM by Max Rohr
Posted in Solar