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bennelson 03-01-10 02:48 PM

Pressure tank as solar hot water tank?
Hey Guys,

What do you think of using a well-pump pressure tank as a solar hot water tank and heat exchanger?

First take a look at this photo.

On the left is a pressure tank I just found on the curb that one of my neighbors is throwing away. On the right is an old 50 gallon electric water heater with the heating elements and some insulation removed.

As you can see, the pressure tank looks like it's about the same size as the water heater once you take the insulation into account. So, I think the pressure tank is close to 50 gallons also.

The pressure tank has larger ports on it than the water heater, and all the ports use standard pipe thread. The heating element ports on the water heater are a straight thread - so a special adapter would be needed to use those with regular pipes anyways.

The pressure tank is the "old-fasioned" style - just a big metal tank. It doesn't have a rubber bladder inside. You do have to add pressure to the tank, maybe once a year, to make up for air that dissolves into the water over time.

I am guessing this tank was thrown away because of some other upgrade to the neighbor's water system, and a new, rubber-bladder pressure tank was installed.

I could run water in to the pressure tank through the lower side port, and out through the very top (the hottest point!) The middle port could be used for a temperature gauge for tank average temperature, or for future expansion. A coil of copper pipe could be wrapped around the outside of the pressure tank to exchange heat from the solar fluid to the water. A layer of insulation would go over the entire tank.
This would most likely be a layer of radiant barrier, then Great Stuff expanding foam or similar, and then some sort of protective cover.

The water heater in the photo has some rust on it, but it also has a lot of calcium build up in it as well.. The drain port on it is cheap plastic, and seized in there, I can't unscrew it.

I could buy a new electric water heater and modify it, but even a basic 50 gallon model runs several hundred dollars. This like all my projects, only has a budget of whatever I can scrounge together. Also, the less I spend on the project, the faster the economic ROI.

Another possibility would be a used water heater through Craigslist, Freecycle, or other sources. I might be able to find something pretty decent for $50? Maybe. Who knows.

So, what do you think would be the advantages and disadvantages of using this pressure tank for my solar hot water transfer and storage? Would the old electric tank work better? Some other possibility I am not even thinking of?

Please note that I have more or less ruled out building my own tank from scratch, I just don't have any room available, other than what would fit a standard tank anyways, so I am not going to re-invent the wheel.

Here's a couple more photos.


Daox 03-01-10 03:03 PM

I don't see any reason why the pressure tank wouldn't work. But, I do think the electric water heater tank will be easier to use since its already nicely insulated.

Joe 03-07-10 08:34 PM

I think it would make a good batch heater, or maybe something to set on a wood stove for a boiler setup, other than that I think you are better off with an electric like doax said.

Xringer 03-07-10 09:16 PM

No special adapters needed for the pressure tank?
Can you get heating elements that will screw into those standard threads?
(If you wanted to use one).

I like the looks of the pressure tank. It appears to be heavy duty hardware.
If you seal up the exterior paint job real well before insulating, that thing might last a life time.

The tank could be insulated a few difference ways, and it shouldn't be too hard to DIY and get a good low loss tank.

The exterior of the insulation doesn't really need to have a sheet metal jacket.
But, some bright metal foil in layers to reflect IR might be a good idea.

I say, Go for It! :thumbup:

bennelson 03-07-10 09:34 PM

The pressure tank appears to have all typical (tapered) pipe threads.

Electric water heater heater elements have straight threads, and seal with a flat o-ring. This means you can't just screw a pipe right into the heater element port, or a heater element into a standard pipe port.

I don't think that I would want to set up my solar tank with a supplemental electric heating element anyways. I believe it is more advantageous to have two separate tanks, as it promotes improved heat transfer from the panel to the storage tank. Also, I don't need to heat water with an electric element that may just be heated by sunlight later in the day anyways.

I DO like how the pressure tank has a big port right at the very top. Seems like that is the absolute best place to take hot water from!!!

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