Summer water heating - attic is 120 degrees!

[Billy_Bob]

I put a remote thermometer in my attic and the temperature up there is 120 degrees F. when it is 90 degrees F. outside (I don't have any vents in my old house attic (built in 1930's).

So I think my plan to use this heat to heat my water in the summer will work.

I plan to run 2 inch CPVC back and forth under the ceiling rafters (for entire length of house) then circulate the water in these pipes into an insulated electric hot water tank.

I can get a solar powered electric pump and controller (designed for solar water heating systems) which will only circulate the water when the attic water is hotter than the water in the tank.

Then place valves to all my piping so I can drain everything in the fall before it freezes.

With the 2 inch pipe, the water should move quite slowly and have plenty of time to heat up.

I'm using CPVC because it is rated for hot water and it will not rust when I drain it for the winter.

I looked at solar water heaters and they wanted around $6,000.00 for the system. My system will be a lot less expensive. The only thing is I MUST drain it before it freezes or I will have a lot of water damage. Solar water heaters use an anti-freeze solution and a heat exchanger.

 

[habanero]

The only suggestion I'd make is to be sure your rafters are strong enough to handle the extra weight load of the water (8 pounds per gallon adds up fast). I thought of doing something similar to this in my house in Kansas, but I knew we'd be moving after a few years so I didn't go to the trouble of installing all this. Even with the roof vents, that attic would get up in the 130 degree range regularly.

 

[Billy_Bob]

Actually there will not be that much weight from the water. I used a tank gallons calculator using the diameter of the pipe instead of a tank and it took something like 100 feet of pipe or more to get near the capacity of a hot water heater tank. And a HW tank with water would be around 400 lbs. Or the weight of two people.

I know my roof can easily support 8 people up there working on the roof. And I will not run anywhere near that much pipe.

I think the only thing I need to do is be sure to balance the load between "this side and that side".

Another thing is that CPVC expands and contracts when it heats and cools. So I will need to use plastic pipe clamps (metal can damage the moving pipe) and be sure the pipe can move back and forth.

And then plumb it so I can get the air out in the spring and be able to drain all pipes in the fall. Basically route the pipes back and forth going upwards to the top. (air out the top, drain out the bottom.)

 

[BlueRidgeMark]

Use black flex pipe instead, and use more of it. You'll get more heat transfer that way, and it will be easier to install with fewer joints to leak.

 

[habanero]

...I think the only thing I need to do is be sure to balance the load between "this side and that side"....

That's more or less what I was getting at. I wasn't sure if you were going to have a tank in and of itself incorporated into the design or just were going to use the pipe as the tank. A tank centered on one rafter can be enough weight to cause a problem over time. But if the load is balanced out over the whole system, you should be good to go.

 

[Marc]

Use black flex pipe instead, and use more of it. You'll get more heat transfer that way, and it will be easier to install with fewer joints to leak.

Well if it's in the attic, it doesn't much matter what color it is, but I'd use Pex if I were going to do this project, and like you said, use more of it at a smaller diameter.

 

[BlueRidgeMark]

There's plenty of radiant heat in the attic - that's why it gets hot. It's not visible light, but color will still make a HUGE difference.

Yeah, PEX would be good. I was thinking of 1 inch or so diameter.

 

[tatra805]

great idea,

maybe only one thing to keep an eye on might be condensation on the pipes. it is a slow heating process so it might take some time to go over the dew point. not sure only something that jumps to mind

 

[gink595]

Could you not coil and overlap the PEX or equivaliant up (like I've seen in geothermal) and have it lay on top of the ceiling joists. That way if the roles you buy it in are long enough you could have minimal joints and lots of surface area, more than straight runs back and forth, You'd have to do the math on that. Also do you have an Idea on what the transfer rate is from Ambient air temp to water temp. I'm sure there has to be some temp loss but what? I'm thinking that just because you have 120 degree ambient temp will not transfer to 120 degree water temp.

 

[BlueRidgeMark]

I think you have the right idea - running them back and forth. But I'd put them on the underside of the rafters up against the roof. Way more heat up there.

Yeah, you won't get 120 water out of 120 attic temp, but as a pre-heater I think it would help.

Now, would it help enough to pay for itself? That's another question. Dunno!

 

[Marc]

There's plenty of radiant heat in the attic - that's why it gets hot. It's not visible light, but color will still make a HUGE difference.

Yeah, PEX would be good. I was thinking of 1 inch or so diameter.

Visible color only matters in the visible spectrum...

The color of a material (really just the bits of the visible spectrum a material reflects) is not necessarily related to the absorptivity/emissivity of the material in the infrared range of the spectrum, which is what nearly all of the radiation in the attic will be.

It's why you can still see through windows with low emissivity coatings despite that they reflect a very high percentage of infrared radiation back into your house.

 

[froghollar]

People in Bullhead az have hot water out of there normal cold water and air condition there garage to have cold out of there hot.?????

 

[spike60]

True story: There was this interesting character back in the 80's that I always ran into at bluegrass festivals. He was an OK guy, but he pretty much lived his whole life in the festival mode. He would never fill his oil tank in the summer. His hot water heater was simply a couple hundred feet of garden hose laying on the lawn. The sun hit that hose and the water would get so hot it would burn you. I think he figured that you could get a good shower out of 100 ft. of hose, and he knew exactly how long it took to get the water to the ideal temp. I actually tried it once when I went up to his place one weekend to catch a band that we both liked. Worked better than you would think.

 

[husky455rancher]

i found that out when i was prolly 10. my parents have a inground pool. well i think its build over an under ground glacier. its always freezing ass. what i did waaay back when was teh same hose trick. i was like 10 it made no difference whatsoever in 30,000 gallon pool or whatever it is i forget. but it made sense for me to do it then , makes me laugh now all the work for nuthin lol.

i like the idea but i dunno if you would ever recoop the cost for the savings.

 

[mga]

ummmm...wouldn't the pipes sweat in an attic that hot? that condensation is going to be dripping inside insulation and eventually onto the ceilings.

personally, i think it's a bad idea. and, you should make every attempt to cool that attic down. a cooler attic will make your shingles last longer, and make your house cooler.

just giving my opinion.

 

[gink595]

People in Bullhead az have hot water out of there normal cold water and air condition there garage to have cold out of there hot.?????

Yeah I had a sister that lived in Lake Havasu near Bullhead City that place gets HOT. I believe it might be near the hottest city in the Nation.

 

[beerman6]

ummmm...wouldn't the pipes sweat in an attic that hot? that condensation is going to be dripping inside insulation and eventually onto the ceilings.

personally, i think it's a bad idea. and, you should make every attempt to cool that attic down. a cooler attic will make your shingles last longer, and make your house cooler.

just giving my opinion.

yep.

I dont think we use enough hot water in the summer to justify doing anything like this.After working in the sun all day,I want that water as cold as I can stand it.

 

[Marc]

ummmm...wouldn't the pipes sweat in an attic that hot? that condensation is going to be dripping inside insulation and eventually onto the ceilings.

personally, i think it's a bad idea. and, you should make every attempt to cool that attic down. a cooler attic will make your shingles last longer, and make your house cooler.

just giving my opinion.

Depends on the relative humidity, of course. One could easily enough install drip trays anyway, probably a very cheap way to do it. I'm not hugely crazy about the idea either, truth be told. I'd rather go with some commercially made flat plate or vacuum tube panels. Home brew heat exchanger, plumb the pump and piping. Hell, if your yard lends to it, you could put the panels in your yard lower than your tank and run the whole thing w/o a pump.

 

[chowdozer]

Visible color only matters in the visible spectrum...

The color of a material (really just the bits of the visible spectrum a material reflects) is not necessarily related to the absorptivity/emissivity of the material in the infrared range of the spectrum, which is what nearly all of the radiation in the attic will be.

It's why you can still see through windows with low emissivity coatings despite that they reflect a very high percentage of infrared radiation back into your house.

As a general rule, without getting into proprietary coatings or materials heated to very high temperatures, black is a better emitter and absorber than all other colors. In the visible, black absorbs all light, which is why it looks black. Light is nothing more than photons that we can see. There are also photons we can't see ie: NIR, MWIR and LWIR. Black acts the same in those bands as it does in the visible. As mentioned, there are exceptions like specialty coatings, red hot metals and hydrocarbon paints.

Several years ago, I ran two pieces of aluminum square tube in an oven at 125 C. One was painted with VHT Flameproof, the other was left natural. Both were fitted with type K thermocouples and logged with a Fluke Hydra at 5 sec intervals. It was interesting to note the temperatures of the square tubes were never equal. When the oven was heating, the flat black Al was leading the natural Al in temp increase, when the oven was not heating, the flat black Al was leading in temp decrease. There was no visible light.

I don't know the process on Low E windows, but the idea is simple. I'm sure the process and materials the window manufacturers use is proprietary, but a primitive way would be to put a piece of glass in a vacuum, then sputter a metal on it to a certain thickness. The metal deposited would not be a thickness like Al foil, but small molecular droplets deposited such that the shorter visible wavelengths would pass through but the longer infrared wavelengths would be reflected. Nothing more than a filter. Very similar to a filter of red, green or blue in the visible. Window manufacturers may well use a process that layers several coatings for increased performance.

 

[windthrown]

As for heating the hot water with pipes in the attic: you would be better off using black pipes and putting them on the roof in the sun. Poor man's solar collectors. It would take a long time to get ambient and radiant heat inside the attic to heat the pipes. I doubt that it would be nearly as hot as back pipes placed above the roof in direct sunlight. If that was the case, there would be a cottage industry in attic pipe heaters, and there are none. I do not think that there is enough energy in air to heat water with, without a heat pump or something like that to extract the energy. I do not think that the surface area is big enough for low level radiant heat using that method either.

I once tried to collect the attic heat in my house in northern California and heat the house with it on sunny days in fall and spring. I built a fan and opened the attic access panel and it created good air circulation. However, even with hot air in the attic at about 90 degrees, it only heated the house up about 2 degrees. Why? There just was not enough in volume of hot air up there to heat the house much. The attic air volume was less than 1/10th the air volume of the house. A chill-chaser solar collector with one box and glazed 8x4 panel could heat the house about 10 degrees easilly. Solar is way more effective.