Introduction: Turn Excess Attic Heat Into Hotwater

Picture of Turn Excess Attic Heat Into Hotwater

I was wondering if I could heat the water in my swimming pool using the heat in the house's attic and I started messing about with a large <s>heatsink</s> thermal battery. The water for my hotwater heater has to pass through it and picks up free calories which saves on the power bill quite a bit of the year. Even on winter days it adds some heat to the water and every calorie free is one I am not paying for...

Step 1: Make a Big Box.

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I was worried about condensation on the pipes and also concerned about freezing even though I live in Florida so I decided to try a large box of sand. I used about two cubic yards and mixed it with two sacks of mortor mix to prevent it from turning into a giant hourglass. In the event the house is knocked down in a hurricane, it will just crumble and that will be one less thing I have to clean up.

Step 2: Stick in Some Pipe

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I tried CPVC first because it was easy.

Step 3: CPVC Has a Surprisingly High R-Value

Picture of CPVC Has a Surprisingly High R-Value

I began to experiment and after modifying a large tank I began to circulate hot water to try and warm the box of sand. After burning everything I could find, I began to realize that for the water to run in hot and back out hot and the sand to remain cool I was not transferring any heat.

Step 4: Boyle's Law

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I forgot to keep an eye on it and add water. When the water got low enough for it to start circulating steam instead of liquid, the pressure spiked and it made a very loud noise.

The sand was still cool...

Step 5: Never Fear!

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Just make it bigger.

I added 150 feet of 3/4 copper tubing primered and painted to protect agaisnt corosion.

This doubled the original size of the experiment but it began transferring heat.

Step 6: Let It Warm Up

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There is enough mass so that the tempurature will only swing up or down a few degrees a day, so in the summer it averages about 90 to 100 degrees. In spring ,fall and most of the winter it is still above the tempurature of the well water, so it is stil benificial.

Step 7: Go Around It in the Cold

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I can open an irrigation valve and bypass the water around the box if it ever gets colder than the well water here. I'll use a sensor in the box to open the valve, otherwise it is shut by default.

Step 8: The End

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It won't supply all my hotwater, but it does contribute and there is nothing to maintain and not likely to cause a roof leak.


Timmy P (author)2008-12-21

hey some one may want to go with PEX Tubing. its very cheep, and flexible. find the right wholesaler and get 500 Ft of 3/8 for under 50 bucks. Many sizes. with easy to use quick connect fittings. I use it at work everyday. makes life easy. I can find out what website we use if any one is interested.

yoga_beyotch (author)Timmy P2011-10-07

Interested in your supplier. Need to replumb whole house and putting some solar water heating in while we're at it seems smart.

G.A. Murdock. Quality products. Been using them for 3 years now. Good luck

beernut (author)2010-11-09

Would Vermiculite work in the place of sand?

dangerdoug (author)2009-10-01

I like Tdrago's idea with the truck rads!! Do you brainiacs think that lengths of PVC piping in an attic would perform as well? Maybe strapped to the underside of the roof joists for maximum heat. Maybe not here in Canada, but for the southern states, I think it would be a lot less building and weight in the attic.

Senseless (author)dangerdoug2009-10-18

The pipes would get covered in condensation and drip into the attic but yes it would work during the day.  The idea with the sand was to both store heat for use at midnight and to avoid forming puddles in the attic.

Next I'm going to try using two or three 40 gallon well tanks buried in the sand. I'm not getting as much heat transfer as I'd like with 300 feet of pipe, the water passes through it too quickly.

I had originally planned on pipes like you mention inside say a ten inch pipe filled with sand but getting the sand in there would have been difficult.

Carrying 3 tons of sand up there in five gallon pails was work enough LOL.

Nostraquedeo (author)Senseless2010-05-02

!!!!!!!!!!!!!       DON'T use vehicle radiators!!!!!!

They use LEAD solder to build them since there not for human consumption. Please be careful!!!!!

musick7 (author)Nostraquedeo2010-10-25

I agree with Nostraquedeo. If you plan to be incontact with the Same water that travels through the Car/Truck radiators, you shouldn't, No Scratch that, Don't use car/truck radiators.

However, TDRAGO, it's a great thought but the Lead used to solder those together or lead that maybe in the material itself just isn't worth the health risk…
Still a good thought.

EmmettO (author)2010-03-12

While it's shut off in the cold the lines could freeze and pop. I know I've had plenty of burst lines because of the cold in the winter but thats usually in the basement. Maybe the attic would stay warmer while the house was heated but if you lost power and the house went unheated say, while you were away on vacation things could get ugly.

doon01 (author)2010-02-26

 terrific idea. But please make sure the temp of the sand pit stays below 89F & above 140F. Anything in between is proving to be the optimum breeding temp of that Legonairs virus. Shower warm & healthy

k_man93 (author)2009-09-23

so... how exactly does your attic support all this weight?

Senseless (author)k_man932009-10-18

I framed the house with 2 x 6's and then covered all my walls with plywood, glued and nailed every 5 inches for hurricane production.  I centered the mass of this over a 5 by 7 laundry room lined with 3/4 inch plywood and extended well past that with even more 2 x 6's on one foot centers and glued and nailed more 3/4 plywood over it to act as a floor / base, plus that part of the attic is not trussed but framed by hand.

Regular trusses would not support this weight and it would fall though the attic floor...

The house isn't built like most stick houses, I went out of my weigh (no pun intended) to make it take a very high windload and can sleep right through a tropical storm without a creak.

It's like a bunch of shipping containers tied together to form the rooms on the main floor.

I forget the load a standard truss can support but it is well below the mass I have sitting up there.

Overall I used 7 bunks of plywood, bundles, and about 50 cases of liquid nails, then covered that with sheet rock so it appears like a normal wall inside and a side effect is that inside the house is extremely quiet even though I live 500 feet from an evacuation route, US 331.

You can hear the traffic in my vids done outside but not the ones done inside.

For a stick framed house it is very very stout.

iwilltry (author)2007-08-10

For a given volume, few materials are better than water in terms of heat capacity. Water is also more easily moved (ie via a garden hose) and has better heat transfer to pipes running through it due to convective heat transfer. An alternative design would be to run your pipes through 4 or 5 plastic 50 gallon drums filled with water (and antifreeze). These can be picked up for about $15 each used.

Senseless (author)iwilltry2007-12-09

I could probably get away without antifreeze here in Florida even with consecutive freezing nights the mass never got below 55 degrees. I'm thinking of redoing it using a few 400 foot rolls of that polytubing people use for sprinkler systems because the water here is so low in Ph that it corrodes the copper big time. I think if I run 3 or 4 parallel tubes of one inch 400 feet long I think I'd get enough water flow to not effect the the flow of water to the tubs or showers, but would keep it within the heated area long enough to make up for the insulating value of the plastic tubing. It's not rated for hot water but it won't get much hotter than 100 degrees and seems to average around 90.

jsilve1 (author)2009-06-11

if some one wanted bigger they could put it in a commercial building

tdrago (author)2009-06-11

** A MUCH easier way would be to just use 1 or more old truck radiators. The bigger the better.
These are meant to contain hot water and only release water when the temperature of the cap is exceeded.
They could even be directly attached to the undersurface of the roof or elevated near the roof for maximum heat transfer. You could feed one radiator into a second or more radiators thereby further heating the already heated water... Might get TOO hot!
You could even get fancy and add a thermostatically controlled fan. There is a commercial product along these lines. See:

philip80 (author)2008-07-13

I'm sales of heat pump factory from china. I love my job as every heat pump unit will help to save the energy of the earth. Each unit will keep the earth existing 5 minutes longer. Through creative experience of many years, Chengda Eco-tech research center excogitated large, middle and small energy-saving and environmental friendly air to water Heat pump, swimming pool heat pump, water source heat pump heater, DC inverter air conditioner, duct air conditioner, ceiling cassette air conditioner, real VRV air conditioner. With our long years experience in heat-pump technology, it makes full use of the energy from the recyclable resource like air, sunshine, water and so on. It brings a revolution to the energy-saving industry.

mulder (author)philip802008-09-01

"Each unit will keep the earth existing 5 minutes longer. " Is this really a bragging point, considering the only way the Earth with cease to exist is when the Sun turns into a super-nova in a few million years? I doubt your water heater can stop that from happening.....

Rainh2o (author)mulder2009-03-06

LOL...I was thinking the same thing when I first read that...

Senseless (author)Rainh2o2009-03-07

It might be just a mistranslation and he meant save 5 minutes of earth energy. English as a second language kind of thing. I'd love to be able to speak Chinese.

Derin (author)Senseless2009-06-08

It matters from person to person,but I've noticed it the most in Chinese people.I don't consider this to be a bad thing though.(I wonder what would happen if a Chinese businessman spoke to an American businessman in Chinese!)


milliondead (author)mulder2008-11-18

Mulder stop raggin on people. Go and find Scully, you need to get laid.

Senseless (author)philip802008-07-13

Got a link?

jsummerlin (author)2009-03-17

I would consider a drain pan with a drain line if this is installed in the attic. I tell my customers, water issues never happen when I'm home on a Saturday with nothing else to do. Let me get 300 miles out of town, oh yea. Murphy lives at my house in one of the back rooms.

jaime9999 (author)2008-09-04

Another risk (other than weight) is potential for leaks and resulting water and/or mold damage. Any ideas for making this fail-safe from a leakage perspective? One thought it building the whole contraption in a box hanging off the side of the attic could eliminate that risk (any leaks would fall harmlessly to the earth). Another thought is to run seamless piping through the attic, with the elbows installed outside, on either end of parallel opposite walls, on the assumption that most leaks/failures will occur at the joints, not in the middle of a tube. Or use flexible tubing material that can be coiled seamlessly in the box, reducing actual joints to just a few or none. At the very least, a water sensor/alarm system (like used in basements) is probably a good idea.

cbergeron (author)2008-04-20

You might want to use copper if you want to increase your thermal gains. However, in the winter you'll be piping colder water into your pool (if you use it in the winter).

Senseless (author)cbergeron2008-04-20

I used copper on the addition to it for just that reason but unfortunately my well water has a very low Ph and it corrodes it and was leaving green stains so I have it bypassed for now. I've been pondering a second version but I haven't had the time to get up there and with summer getting near it won't be until next winter I'll feel like spending time with it. The copper pipe made a huge difference and really I need to fix the Ph but that's something else I have been meaning to get around to LOL...

Derin (author)Senseless2008-07-15

use cast iron pipes

COMMODORE64 (author)Senseless2008-05-04

A good news for you. there is a new technology advancement for water piping, there should be a plastic pvc pipes for you to use, and beside the have more thermal and chemical resistance, and outperform copper pipes. Give it a try, but remember, it is expensive.

stevenelson (author)COMMODORE642008-05-25

What is the name of this new pvc you're talking about? Is it more expensive than copper?

COMMODORE64 (author)stevenelson2008-05-25

It is called PEX pipes, which it is little expensive, kink easily ( If you aren't careful ), but good thing about it is they don't condense and don't corrodes.

This is the mandatory new code in Canada on the new building now. There is too many problems related to copper pipes in old and middle aged homes.

We have PEX pipes in our home, the only we have to use the metal pipes is to lead the outside water system.

Please correct me if I'm wrong, but this is all I can know of. If you wish, check out the hardwares stores.

COMMODORE64 (author)COMMODORE642008-05-25
Woeka (author)2006-03-06

Looking nice .. but maybe you would save more money if you used your time and effort in insulating your roof :)

Austringer (author)Woeka2007-08-29

Don't insulate your roof. The way an attic is supposed to work is that cool air enters the space at the eaves and hot air is vented up near the peak. If you insulate the roof your shingles tend to melt. (There are also a whole mess of moisture control issues to think about here too - being inside your house is like dumping 5 gallons of water on that floor every day between breathing, washing, cooking and having a toilet.) I redid the roof on my going on 100 year old house a while back and recommend Taunton's Insulate and Weatherize book if you want to jump into an insulation project.

stevenelson (author)Austringer2008-05-25

If you install "baffles" this isn't an issue. I think the brand name is Duravent. They're usually either foam or plastic sheets, shapped like a W. Staple them between your rafters so the air can circulate from your eaves up to a ridge vent. Then you can put insulation in your roof and still let the hot air escape.

Austringer (author)stevenelson2008-05-30

I know, I've even used them. (There are even tricks out there that will let you insulate a vaulted ceiling, but I've got zero experience with them.) The thing is, I think you could do an entire instructable on how to properly insulate a certain type of house and still have room for instructables on how to insulate any number of other types of houses (what with various configurations of dormers, eaves, vaulted ceilings and what have you). I didn't want to be the guy who gave people just enough information to encourage them to do the wrong thing and cause their roof to die prematurely or horrible mold problems.

servant74 (author)Woeka2007-06-30

We found that even though the house was 'insulated', we put in another layer of 'attic blanket' for about $200US, and it we think it did a 300%ROI the first year plus making the house more livable. We were in an 'all electric' house (resistive heat at the time), so it made a LOT of sense. Before we moved out we had done solar hot water heater, heat pump, insulated everything we could get to and replaced the windows with higher R value ones. The attic blanket still gave the best continuing ROI for our bucks. Still, I like this instructable!

Senseless (author)Woeka2006-03-06

Hah Woeka! Yeah let's not tell my wife that. I built the whole house actually and the attic will be very well insulated if I live long enough to finish it.

maffiou (author)Senseless2006-03-07

Funny you should say that... I'm exactly in the same kind of situation: Trying to keep the wife happy in the building site that our place has become since I felt inspired to do something with it (like 2 years ago, oups...)

Senseless (author)maffiou2006-03-07

What are you working on?

I've been trying to build this house myself for about 6 years now. I had an accident slow me down halfway.

JPKANTOR (author)2007-12-02

Senseless, check out this company their system works great I have owned one for over 14 years. I hope they don't find out about your little project because their pool heater/hotwater technology is protected by patents and is a joint venture of the University of Minnesota Mechanical Engineering Department. They may serve legal notice on you as they have done to others in the past. They are very aggressive when it comes to protecting their company technologies. The company name is SolarAttic, Incorporated located in Minneapolis, MN.

fieldman (author)JPKANTOR2008-04-11

ROFL. Might want to check the archives of Popular Mechanics. Solar Attic uses fans to circulate the air through a heat exchanger. The heat exchanger and storage medium may be proprietary but I doubt much else is. Good idea though. Neat and self contained. I think the error here is the storage medium. Rock (sand) is a good insulator. Doesn't give up or absorb its heat easily, so surface area is important. If you want to heat your pool, go buy a hundred feet of black plastic pipe and lay it on the lawn in the sun or put about 500 ft. in your attic. Put a valve on the outlet so that you can slow the flow so that it has time to heat and bingo... pool heater. Your local power company probably has specs on the best type of plastic tubing to use since they encourage ground source heat pumps.

TheMadScientist (author)2007-12-09

you could have your water system hooked into your computer with a bypass... just attach it to "water blocks" and use some thermal compound, water blocks circulate water and move heat away from the cpu and other things, so it should carry the heat with it, giving you more hot water!

(you don't realize how hot those small chips get...)

yeah, i've wanted to try to take one of those massive heatsinks and move the heat piping out on top of my computer to see if I could hook a skillet to them and make myself breakfast on top of my computer...

Senseless (author)2007-12-03

I found the place you're refering to JPKANTOR... I'm glad someone is out there trying to market ways to take advantage excess attic heat.

I'm not selling anything and I doubt I have released any propriatary information covered exclusively by a patent by pointing out that attic could be a source of heating water.

richt (author)2007-11-27

I remember my shop class going to a seminar on solar heating. I saw a box they were running water through and it had a small fan on one end which blew out hot water. I built one myself over a weekend with my dad, it was great for heating water and the rest of the house. Afterwards, we noticed during the winter that it did not heat as well, especially on those cloud covered winter days. We used a wood stove for heating the house during the winter. We ran copper tubing down from the water tank and wrapped it around the stove. We had to modify it a couple of times as the water was too hot, but it worked great. No cold showers.

About This Instructable




Bio: I've been attempting to build a house mostly by myself for the last five years... I finally more or less ... More »
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