Interesting physics factoid
 

Ever wonder why steam radiators aren't usually black, but instead are white? Kind of counter intuitive right. The reason is radiators are meant to heat the entire room, not just around the radiator only. Because white doesn't reradiate energy nearly as well as black does the radiator gets a lot hotter than a black radiator would for the same energy input. That means a strong convective loop in the air is created.

The radiator creates a strong energy transference through conduction to the air molecules next to the superheated radiator. Those air molecules move up to the ceiling then and new air molecules rush in and are in turn heated through conduction by the superheated radiator. A convection cycle is created through this process that theoretically heats the entire room. It wouldn't work if the radiator was at the ceiling because the heated air would already be at the ceiling and it would have no where to go for other air molecules to replace it. No convection cycle then.

The color doesn't matter. Color affects how something absorbs light, not how it radiates or convects heat. Unless the radiator in a path to receive sunlight, the radiator will have the same output and the pattern of output will be the same.

Stealth Solar Heating...
 

You're talking about sun exposure from time to time. Your air will be about as mold-laden as the air you breathe when you walk around outdoors.

That is a very viable plan. I even have a lab report laying around here someplace about doing that very thing.

It's been a while since I read the report, but as I recall, the big advantagea are that your solar collector is totally hidden and protected from the elements and from complaining neighbors, if that is relevant. Also, your incremental cost is very small if you are already doing a roof.

The biggest disadvantage is that the amount heat that will be collected is much lower than from a full on flat plate collector or evacuated tubes... something in the neighborhood of 8% to 10% as opposed to 40% or slightly higher for evac tubes. But if your whole roof is your collector, it all adds up. I have no data for heated air to use as a comparison.

If you're gonna do it, put some rigid foam under the roof/tubing setup.

And don't use HDPE, it's too likely to melt. PEX would be a better bet.

And if you're gonna do it, you only need to do it to the faces of the roof that receive significant sun.

I wish I had done this when I did my roof.

BTW, my roof is dark green and yest, it gets very hot... even on a sunny winter day.

Take lots of pix.

-AC

It will have a south facing roof at 6/12 or better. 12/12 is ideal in our latitude, but a little daunting to install.
The trick is to get good contact with the metal, and still protect whatever tubing used from being abraded. The metal roof expands and contracts every day, and moves slightly. I can hear our other roofs move from time to time.
One method had the tubing in a metal hat channel that the roofing sat on, so the roofing was not touching the tube directly.

mn,
Your ignorance is quite astounding. Rather than waste my time correcting your lack of knowledge line by line I'll just say that I've treated you with respect in all my previous comments even when I disagreed. Your lack of same to me and others here is obvious.

I realize someone with your arrogance will interpret my lack of refutation as not knowing what I'm talking about. My background is physics and your armchair physics is not worth refuting. In other words, you just don't know what your talking about most of the time and especially here.

Shall we review your claim here?
You posted an off topic comment in my shop heating thread. Check.
Did I care? No, not really. This thing happens all the time and no need to get feathers ruffled. You on the other hand have the ruffled feather thing going on just fine.

Somebody else disagreed with your science, in no disrespectful manner, he simply said no. No personal references, no accusation of ignorance, nada.
You get your panties in a wad, IN MY THREAD, and go off.
Next time you feel the need to vent, make your own thread.
Next time you want to start a new topic about radiating bodies, make your own thread.
Thank you :)

It's not a "claim". It's physics and I thought people would find it interesting. I did not put it there to divert attention from your thread. If you feel that way then I'm sorry. I'm always sorry when someone with genuine knowledge about something is disrespected just because it is not conventional wisdom.

I think that is really the entire reason for the reaction to my comment. Correct but non conventional wisdom seems to often be threating to many individuals here. I'm very happy that not all commenters on this forum feel that way. I assure you my factoid comment was not meant to hijack your thread and I will refrain from further interacting here.

PEX expands much much, more than metal... about 1"/100'/10 degrees F. But as long as the pipe doesn't run over a metal edge, it shouldn't be a problem.

If you are really worried about abrasion, go and haunt some radiant floor heating blogs to see how they handle the problem, and much more likely, is there an abrasion issue at all?

I think that a more suitable problem to resolve is how are you going to accomplish the 180 degree U-turns at the top & bottom, and not expose your PEX to sun light?

-AC

I'd put an edge on the cavity that the pex will occupy. Make the turns inside that edge. Put some insulation blocking between the loops to support the roof.
Fasten the metal roof to blocks of solid wood or ply as needed.
So, you are thinking insulating board or something similar over the roof sheeting?
Like this?
3/4" Foil Faced Insulation Panel at Menards
That does pose a fastener question regarding the metal roof. I'd have to get a long enough fastener to reach through to the OSB to tie the metal down well.

And One More Bit...
 

Since factiods are being featured here, here's another one...

When the 1973 OPEC embargo happened, many countries were affected by the sudden unavailability of cheap fuel.

The United States launched a big solar initiative, including cash incentives to people who chose to go solar. Jimmie Carter even put solar collectors on the roof of the White House (some of these very solar-collectors are in a museum in China today).

Today, you can get perfectly serviceable solar collectors for free or very cheap, that are from that era. At the same time, home heat loss standards were improved only to some modest degree.

Sweden chose to go after home heat loss head on, did major studies, totally revamped the building techniques and codes and also laws that affect home heat loss. So, today in Sweden you can't build a home that is not what we call a high efficiency home (very similar to a Passive House). If you can't afford it, the government will weigh in to make sure that everyone has a high efficiency home.

Net result is that Sweden has been transformed, with regards to home heat efficiency, while in the US, solar incentives come and go with the administration, and the solar industry is constantly whiplashed by government subsidies (or lack thereof), so US solar has never actually grown into a mature industry. Which leaves the interested fringe like us who are doing interesting things as best we can.

There are very many lessons from this story, but to me, the big one is to focus your best attention on the structure of your house, insuring maximum tightness, over-generous insulation (see Daox's office project for ideas)... but to keep an eye out for solar, too.

If you can seriously reduce the rate of heat loss from your house, amazing things are possible with alternative energy techniques, including solar.

Best,

-AC