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Foggy Solar Collectors


Jim Katen
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I don't know much about solar collectors but it seems to me that these 3-year-old Velux units would work a lot better if there wasn't a layer of condensed water on the underside fogging up the glass and scattering the solar radiation.

Is this just something to be expected with these suckers or is it a genuine problem?

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I also know very little about them, but it seems to me that there's a very good reason the inside of those things are painted heat-absorbing black rather than reflective white! Certainly, the ones around my neighborhood don't look like that. Someone screwed the install would be my first guess, although I suppose it's possible they are defective units.

Knowing you, I'm sure you are also seeking more info from the manufacturer but, meanwhile, I doubt you would be going out on a limb to call those.

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Was that a seal failure on the IG unit or was the condensation inside at the collector? If it's the glass it's an easy fix, but if it's inside that means the collectors are leaking and fogging the glass just like a faulty heater core blows antifreeze all over your windshield.

I'd wager it's the latter, and that the collectors stagnated for an extended period. If the PRV fails, and the controls don't cycle the pumps those little collector plates pop like balloons.

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Was that a seal failure on the IG unit or was the condensation inside at the collector? If it's the glass it's an easy fix, but if it's inside that means the collectors are leaking and fogging the glass just like a faulty heater core blows antifreeze all over your windshield.

I'd wager it's the latter, and that the collectors stagnated for an extended period. If the PRV fails, and the controls don't cycle the pumps those little collector plates pop like balloons.

The condensation was on the underside of the glass - the glass that I could touch with my finger. It looked exactly like a seal failure on an IG panel. It also looked like water rather than glycol.

So what's the easy fix?

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As near as I can tell from sparse images on the Velux web site, deck mounted collectors have insulated glazing. The most difficult part of reglazing a Velux skylight is getting the glass onto and off of the roof. 'Easy fix' assumes it's a seal failure. I can't tell from your pics if that is IG or not.

If that is a single pane of glass, then you either have a glazing media failure or a collector breech. Media failures happen but they're rare, Velux has been making skylights a long time and have figured out how to keep water on the outside of the glass. Rare failures tend not to happen in pairs. That leaves a collector breech, and in order for both collectors to have failed together there would have had to be a significant over heating event, or a freeze.

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As near as I can tell from sparse images on the Velux web site, deck mounted collectors have insulated glazing. The most difficult part of reglazing a Velux skylight is getting the glass onto and off of the roof. 'Easy fix' assumes it's a seal failure. I can't tell from your pics if that is IG or not.

If that is a single pane of glass, then you either have a glazing media failure or a collector breech. Media failures happen but they're rare, Velux has been making skylights a long time and have figured out how to keep water on the outside of the glass. Rare failures tend not to happen in pairs. That leaves a collector breech, and in order for both collectors to have failed together there would have had to be a significant over heating event, or a freeze.

That makes sense. But, on the other hand, Velux hasn't been making solar collectors for very long and the technology that keeps a skylight seal intact might not work on a solar collector that sees vastly higher temperatures - sometimes upwards of 400 degrees. If they tried to use the same technology from their skylight division, I'll bet that it would fail in a wholesale manner on a solar collector.

On the third hand, they probably thought of that . . .

But on the fourth hand, they can't even get those vacuum tube solar collectors to stay intact and they're welded shut.

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But on the fourth hand, they can't even get those vacuum tube solar collectors to stay intact and they're welded shut.

I don't have experience with a large enough sample of evacuated tubes to know one way or the other. One project, 300 tubes, 900 gallons of storage. In the summer the home owner has to cover 2/3 of the tubes so the heat dump can keep up. In the winter he has to sweep the snow off them or he only has enough area to heat his DHW. We've replaced 4 tubes; 2 seal failures, 2 that he broke messing with them.

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But on the fourth hand, they can't even get those vacuum tube solar collectors to stay intact and they're welded shut.

I don't have experience with a large enough sample of evacuated tubes to know one way or the other. One project, 300 tubes, 900 gallons of storage. In the summer the home owner has to cover 2/3 of the tubes so the heat dump can keep up. In the winter he has to sweep the snow off them or he only has enough area to heat his DHW. We've replaced 4 tubes; 2 seal failures, 2 that he broke messing with them.

Ha! Well, that's more evacuated tubes than I've ever seen. But every time I see an installation about half of them have failed.

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According to Velux, it's fine.

They say that those units are not hermetically sealed. Weepholes at the bottom of the assembly allow air and moisture in and out. During wet weather, they expect that you'll get condensation in there early in the morning but as the day proceeds, it should burn off and drain out.

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That sucks. An optimally positioned flat plate collector only has an aperture of 70-80 degrees. The Velux unit by design cannot be positioned optimally. A significant amount of collectible energy will be expended defogging the collector. The low angle and shaded location in the OP is quite far from optimal, regardless of the direction they face. I hope they have an efficient back up DWH.

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That sucks. An optimally positioned flat plate collector only has an aperture of 70-80 degrees. The Velux unit by design cannot be positioned optimally. A significant amount of collectible energy will be expended defogging the collector. The low angle and shaded location in the OP is quite far from optimal, regardless of the direction they face. I hope they have an efficient back up DWH.

I sense a trend.

Back in the '70s, these things were mounted on elaborate motorized stands that allowed the collectors to track the sun across the sky. We gave up on that and remained satisfied with pointing the collectors at something close to the right altitude. Now, it seems, we ignore even that and simply put them on a south-facing roof plane, parallel to the roof, and with no concern at all for the altitude or azimuth of the sun. I predict that future collectors will be positioned on the north side of the roof, inside the attic, pointing down. Heck, future generations will probably have them in the crawlspace under the house.

Still & all, the glycol in these suckers was at 102 degrees - and that was first thing in the morning, with foggy glass, and the shade from a tree all over the collectors.

Oh, yes, backup heat is electric at 4 cents per kWh. (At that price, the solar system will never pay for itself anyway. They needn't have bothered, except they really wanted to be "green.")

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That sucks. An optimally positioned flat plate collector only has an aperture of 70-80 degrees. The Velux unit by design cannot be positioned optimally. A significant amount of collectible energy will be expended defogging the collector. The low angle and shaded location in the OP is quite far from optimal, regardless of the direction they face. I hope they have an efficient back up DWH.

I sense a trend.

Back in the '70s, these things were mounted on elaborate motorized stands that allowed the collectors to track the sun across the sky. We gave up on that and remained satisfied with pointing the collectors at something close to the right altitude. Now, it seems, we ignore even that and simply put them on a south-facing roof plane, parallel to the roof, and with no concern at all for the altitude or azimuth of the sun. I predict that future collectors will be positioned on the north side of the roof, inside the attic, pointing down. Heck, future generations will probably have them in the crawlspace under the house.

Still & all, the glycol in these suckers was at 102 degrees - and that was first thing in the morning, with foggy glass, and the shade from a tree all over the collectors.

Oh, yes, backup heat is electric at 4 cents per kWh. (At that price, the solar system will never pay for itself anyway. They needn't have bothered, except they really wanted to be "green.")

The reality is most likely that the installer got paid, the owner got money in rebates and/or tax incentives, and the bill was paid by us through our taxes ( or more accurately, owed by us)..

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