Heating the pool with attic heat

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Jun 11, 2009
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Tampa, Florida
I've been lurking for a while as well......but this attic heating post got my attention. I've been working on a similar heater for several months now.

In short, I've arranged four radiators (new, of course) in a cube. It has a top and bottom. I have a 24" fan mounted to the bottom of this unit. Inside, I've built input and output manifolds to put cold water from the pool into the bottom inlets of the four radiators in parallel....the water rises through the four radiators, is heated by the hot attic air drawn in from the fan, and flows in parallel to the output manifold. The key here is the parallel flow......my unit can flow a ridiculous amount of water. Numbers? I've got about 1500 sq in of radiator surface; I draw hot air from all four sides; the fan does about 2700 cfm on it's lowest speed. This unit is suspended at the highest point in the attic. I intend to use a 24" flexible duct to exhaust the cold air the unit will produce into the garage - essentially a/c for the garage.......cold air out, warm air in through the soffit vents is heated as it rises.

The highest risk in this is a catastrophic failure resulting in a two-inch pipe pushing water into the attic at full pump output. A lot of gadgetry and controls will be utilized here, but even shutting off the pump will not prevent heavy water intrusion; it would definitely catch and intercept a small leak, but not the full failure of an inlet or outlet fitting. And no, a small catch pan won't be sufficient. My solution is to basically "tub" that part of the attic under this unit. There's really no other way to make this safe. I'll panel the floor of the attic and seal it up with about an 18" surround. The purpose isn't to hold that water for any significant time, it's to direct that water through a huge, perhaps 4-6" outlet (maybe a pair of them) to the outside. Overkill? My unit is essentially over my garage.......each installation is different. As an additional safety measure, I intend to shut off the main drain and draw water only from the skimmer line - this will inherently limit the amount of water at risk and will preclude possible catastrophe #2: a drained pool popping out of the ground.

Please share your ideas.....I'm still doing some fabrication on this and I welcome your input.
 
In order to pump air with a fan from the outdoors (which the attic essentially is as far as air pressure) into your garage, you'll need an air vent in the garage to allow air to exit the garage. Otherwise you are going to be in a static situation which will limit the air flow and restrict the performance of the system.

I suspect the AC effect in the garage will be best served by the cooling effect in the attic. If you blow the cooled air out of the attic, it will have the same effect on the pool water, but the cooling effect in the attic will be the same as that of a simple exhaust fan. You might be better served to simply recirculate the cool air in the attic if attic cooling and the resultant lower AC load for the house is a significant goal.

I would seriously consider a flow regulating device that would shut the flow off completely if it exceeds the normal flow through the radiators. That way if a line breaks, the water stops almost immediately. That would allow you to get by with a smaller catch basin for your drain system.
 
Agreed. There are three distinct goals one can choose from here, each of which, if maximized, is going to minimize the benefits of the other two.

1)Design and build as a "pool heater". Maximize the heat in the attic. Maximizing heat in the attic, as an objective, is going to minimize the benefits of "a cooler attic leading to lower a/c bills." Maximizing the heat in the attic means getting that air you just chilled out of the space you are trying to keep hot. I realize that that these two may be able to exist together in the sense that the hot air would rise and the cold air could be directed to the lower part of the attic......but a hot attic is a hot attic, and it's hotter with no cold air in it.

2)Design and build as a "means of lowering your a/c bill." People do this all the time, with attic fans, turbine fans, ridge vents, etc. But remember, these methods require soffit vents to create air flow. Of course, the air the soffits let in is cooler than the attic air, so there is a cooling benefit. But what if you were to take the "cold" out of that body of water in your backyard and place it into the attic? You are exchanging hot for cold, via radiators. We seal up and insulate our homes to keep cool in and hot out; you would end up doing the same with the attic to keep in the cold air you just used to condition it. The attic would get and stay cool.....not a very effective pool heater. Of course, if one were to have this as their goal, they would have to deal with condensation and the like.

3)Design and build as a "garage cooler." Using the points already made in 1) and 2), we can see that we would have a cool garage and a hot attic. Goals 2) and 3) are mutually exclusive, assuming each is pushed to maximum efficiency.

Each situation is different; my garage is essentially under this unit. If another person did not have this option, my advice would be to exhaust that chilled air outside. It sounds funny, but when you think about it, wouldn't you like to be able to go to your pool, snap your fingers and have the "cold" simply rise out of the water? We can accomplish that otherwise. My goal is a warmer pool. I'm not pursuing a cooler attic, as I believe it is contradictory to my stated goal.

Excellent idea on the flow regulating device......essentially an actuated valve on the inlet and outlet pipes tied to a pressure sensor; if a certain pressure drop is detected in the manifolds, the valves seal up the inlet and outlet pipes. I'm often accused of over-engineering, probably out of paranoia. Nonetheless, of all the hours into this, it's not going to take too long to panel up and seal about a six foot square by 18" tub under this thing.
 
Some good points.

My take on the engineering side was to take as much heat out of the garage attic as possible. Looking through some data that I found on the SolarAttic site I believe I have around 90-100 kbtu available when the sun is out. We like to keep the pool at 86-88F. I thought about building one from scratch like you have done but I'm not sure it is the best way for me. I have been looking into Modine and Sterling unit heaters but I can't seem to find that perfect combination of fan CFM and water flow rate. I'm and EE by trade but my brother is an ME and he design HVAC test loops so he has been helping me with all the calculations. Once I saw the formulas I started teasing him how easy ME stuff is compared to EE :-D .

If I can maintain a delta T of 30F in the attic I would need a 3000 cfm fan for 100kbtu. Not very sensible IMO for sound reasons and overall air flow so I decided that I will take all the btus I can with 30F DT and around 2000 cfm, ~ 66kbtu.

I have to finish getting the control wiring to the attic from my EasyTouch panel before I purchase/build a pool heater/attic cooler.

My main concerns now are the piping and water in the attic (similar to you) and the motor temperature rating.

For the piping and water concern I plan to use 2" flexible PVC and only two connections to the heater, inlet and outlet. No other joints. I am looking for a reliable coupler from the flex to the heater that will give me a nice piece of mind.

I'm finding most motors on the unit heaters are rated at a max of 105F. This may not be an issue if I find a heater that will pull air through the coil versus pushing it. That's counter to what they are designed to do so it is proving difficult. What type of radiator and motor combination have you used? Maybe building one is the best approach.

BTW, I do love this type of stuff. Learning (and brewing beer) keeps me ticking.
 
Odd.....I too have consulted regularly with an engineer friend for delta T's and whatnots =) I'm no engineer by trade, but I love tinkering and fabrication. As a result, I tend to over-engineer and think outside the box.

Two-inch flex is the way to go; I ran about 40' in my attic. The obvious benefits are no sharp turns, no joints, etc. Frankly, I had some concern about the one weak point there: the bond between the union fitting and the flex. There would be two of them, and I consider those points to be the weakest links in all of this. Out of curiosity, I made a test fitting. Looking at it, I figured it would be nothing to pull it apart; I had to put it in a bench vise and attack it with a hammer, chisel, and vise grips to tear it apart - that addressed my concern. Before making those flex pvc to pvc union fittings, I researched a recommendation for the pvc cement. I found Christy's Red Hot Blue Glue highly recommended for flex pvc applications.

The solution for the fan motor is to mount it in the cool air stream. My unit pulls air in through four radiators in a cube. The fan is mounted on the bottom blowing air downward; the fan motor is in the cool air stream - also, you should design to make the fan or fan motor easily replaceable on-site.

Fan noise was a concern of mine as well. My unit is not entirely over the living space, but close to it. I found that my fan, a 24" TPI at 1/4 hp, is indiscernible while running in the attic during the day - it's literally drowned out by the home a/c, fridge, etc. At night, with everything off, if I listen closely enough, I can hear it. I found the point to be moot - I'll be running this unit in the daytime, not at night.

To your concern about flow rate. If you find a heat exchanging coil or radiator you like, the easiest way to address its flow rate is to use multiple units in parallel. Ideally, I would like to have a stainless steel/aluminum heat exchanger capable of sufficient water flow - I suspect that would be cost prohibitive. I'm using four 2001 Honda Accord aluminum radiators. Each has a 1 1/4" inlet, my layman's calculations suggest an input/output capacity nearing 4-5". When I tested the cube poolside at 100% flow from the pump, I saw a psi penalty of no more than 4lbs (I went from 12 psi normal operation to 16 psi with the unit inline - 1hp pump). I am very aware of corrosion concerns - my solution is to maintain proper water chemistry, and to climb up there from time-to-time, pop the cap and visually inspect the radiator core. I've browsed many posts here about core failures on traditional heaters failing - due ultimately to poor management of water chemistry. I think it's a fair assumption that any interface where water meets metal must be properly managed.

Ultimately, my solution is to plan for a complete failure of all fittings at once, and then to build around that risk. I consider that to be easy (no doubt, an exercise which many would find silly). What if I told you that I want to be able to pump pool water into the attic, drain it straight into a basin/tub, then have that tub drain to the grass on the side of the house? Sounds goofy.......but done right, it addresses the risk. Put a heater between the inlet and the drain basin, and you're set.
 
Here are links to two products that might help you with this.

The first is just a water sensor switch to connect to an existing home alarm system. I have these in the pans below water heater and A/c units in my attic -- I live in fear of major leak in either of those with hardwood floors and MDF baseboards in the house -- that connects to a warning chime on my home security system.

The second link is to a unit that actually can shut off flow. This particular one is sized for a toilet tank fill line. My neighborhood has had a surprising number of toilet tanks that shattered without warning creating major damage. That happened in my home before I bought it, the owners were in the house and yet damages cost $5,000. Another similar break occured in a different house when owners were out of town and damages exceeded $40,000!

There are others on that site that are for other uses that may work for different size lines. You'd just have to investigate.


http://www.homesecuritystore.com/ezstore123/CUProductReviewList.asp?productID=608

http://www.homesecuritystore.com/ezstore123/DTProductZoom.asp?productID=1013&bViaAC=True
 
My forced hot air convection heating system is up and running. I've got a 1hp pump and a cartridge filter, two jets on a 12,000 gallon kidney shaped pool. Pre-install, the filter ran at about 12psi.

I've got 2" pipe up the side of the house (single story), and 40' of 2" flex pvc to the unit, and another 40' of 2" flex pvc back from the unit. Day one of operation, the unit is watertight. I'm seeing about 18psi on the filter gauge. I've got the same strong jet output that I had before installation. I don't think that 6psi is too much of a penalty considering all that I'm putting this water through.

I only ran it for about six hours today....noon to six. Temp gauge going to the unit shows about 83, the one coming from the unit showed about 86. I've never seen 86 in this pool......85 max, due to tree shading. Temp gauge in the pool showed 86.

Some variables: I'm putting water through the unit at full throttle. As I understand it, I would see a higher temperature out of the unit if i slowed the flow. I can do this easily at the bypass valve. I also understand that I am exposing a greater quantity of water to heat with the fast flow. Another variable: I can adjust my fan speed; it runs at about 2500/3250/4000fm. I had reservations about running it at the higher speed, for fear of exhausting heat from the attic too quickly.

In looking for a measure of how effective my project may be, I climbed under the unit while it was running. The rest of the attic was hot (running at 4000cfm), but the air stream from the fan was noticeably cool, with the slightest "a/c bite" to it. This immediately told me that the system was taking cold air off that pool water, then returning it to the pool a bit warmer.

I'd appreciate opinions on rates of water flow and fan cfms, as well as any other input.
 
If you slow the water flow, you will get higher temperature water out, but you will also reduce the total amount of heat it puts into the pool. For optimal heat transfer you want the water as cold as possible, which means high water flow rates.

Normally the air flow rate has the same goal, high flow rates. But because you are removing the air from the space, rather than just recirculating it, things could well be different.
 
Great point. I'm definitely inclined to run it at the higher volumes for water and air flow. I don't think I'm in any danger of exhausting the hot air supply in the attic like I though would happen. As long as the fan is outputting cold air, it's working. Now it's time to see how how many days I need to get a marked increase in overall temperature.
 

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It isn't really that good of an idea. The amount of heat available in the spring and fall, when the pool needs heat, is not all that large, the equipment is expensive, and running water up into the attic where a leak could cause major problems is not such a great risk.
 
Fun project, read with interest

One of the earlier posts mentioned venting the attic using traditional means such as soffit vents etc. With your system, it is still imperitive that you have enough airflow into the attic to make up for the outflow (the cool air is ducted into the garage, correct?).

Many a homeowner has installed extra ridge vents, powered or solar exhaust fans, wind turbines, etc without allowing for extra air inflow (soffit vents). Without the inflow, a slight vacuum is pulled in the attic. This exacerbates leakage of cool air from the house into the attic. Most homes have anywhere from 5-15% loss due to attic leakage: with a vacuum in the attic, the issue will be worse.

Have fun, sounds like an entertaining project. I would be interested if it 'pays off' on the end: the extra 1/4hp electric motor and extra load on the pool pump vs. lowering load on the house a/c.

Cheers
 
JasonLion said:
It isn't really that good of an idea. The amount of heat available in the spring and fall, when the pool needs heat, is not all that large, the equipment is expensive, and running water up into the attic where a leak could cause major problems is not such a great risk.
Click to expand...

I hate having water in the attic. In Florida, where the whole state floats on an aquifer and there are no basements or crawl spaces, houses are built on concrete slabs, and generally plumbed with copper under the concrete. When the copper begins to leak (pinhole formation is legion here) it is often cheaper to re-plumb with PEX in the attic than to break up the floor and chase the leaks. Many, many, many homes have water run through their attics. (It makes for some **HOT** water at the tap, too.)

I noticed in Houston that it was also common to install air conditioner evaporator units in the attics. I had a house with two of them in the attic. Nasty stuff happens when the condensate drains clog. Really nasty.

Also, I've seen some fire sprinklers run in home attics which are part of the domestic water feed.

All of these things make me worry about water damage, because as is stated above, I hate having water in the attic, but it's more and more inevitable these days.
 
I forgot to mention in order to get maximum heat I read in the install manual they tell you to close off all the soffit vents, roof vents, gable vents to trap all the heat inside! This is interesting because most people find their attic is not vented enought and add wind turbones and other things to better cool the attic.
 
civicturbo said:
I forgot to mention in order to get maximum heat I read in the install manual they tell you to close off all the soffit vents, roof vents, gable vents to trap all the heat inside! This is interesting because most people find their attic is not vented enought and add wind turbones and other things to better cool the attic.
Click to expand...

That would definitely exacerbate a negative static pressure in the attic whereby the system would draw conditioned air from your living space at an accelerated rate. This you do not want...

Frank
 
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