A while ago, a neighbor had several item on the curb, with a sign marked “Free”. Most of it was typical large junk, a projection TV, an old fridge, but was WAS appealing was a steel pressure tank in good condition!
This was the “old-fashioned” style with no rubber bladder. Instead this “maintenance-style” tank has to be occasionally refilled with air, as the tank air dissolves into the house water over time. I assume that the tank was being thrown out because of being replaced with the newer style tank.
I snarfed-up the tank, not sure exactly what I was going to do with it. At a bare minimum, I would need an expansion tank for the glycol loop in my solar system, even though this one was overkill, the price was right.
I have had an old electric water heater for some time that I was originally planning on using as my solar tank. But the more I looked at it, the more I disliked it. There was still a burn mark from where the wiring shorted, there was plenty of calcium build-up on the inside of the tank, and the heating element ports would require a special adapter to run pipes into. The advantage of the water heater was that it was well sealed up, and completely insulated.
I had already removed some of the insulation from the water heater where I was planning to wrap soft copper pipe around the outside, to act as a double-wall heat exchanger. Once stripped down to the bare tank, I realized the water heater and the pressure tank looked like they were almost the same diameter. I measured both circumferences – sure enough, almost exactly the same size!
I also noticed that the expanded foam insulation on the water heater isn’t directly on the tank itself. It is really inside two layers of plastic bag material. Once the sheet metal jacket of the water heater is removed, it’s pretty easy to take off the insulation as well.
So, here’s where I had the idea. Why not use the Pressure Tank, but with the water heater’s insulation on it? The pressure tank is in great condition, and has nice big, standard-sized pipe-thread ports on it. All I would have to do is wrap the insulation around it, and BAM! – solar water tank!
First, I needed to clean up the ports. The pressure tank has several 1&1/4″ ports on it. One on the back, two on the front, and one on top. I cleaned up the threads with an awl (there was just a little rust on some of the threads.) The back port and middle port on the front both got plugged up with 1&1/4″ stoppers. There was also a little port near the top front with a 1/4″ diameter. I plugged that as well, but it might be a good point for a temperature gauge or some other use.
The very top and lowest 1&1/4″ ports will become the in and out ports for my household water supply. Since the main waterline in my house is 3/4″ copper pipe, using the same size for these ports makes sense. I bought two reducing bushings that would shrink the size of the ports directly down to 3/4″.
I prepare to slice off the outer shell of the water heater
Now it’s time for surgery on the water heater. I had already found that a battery-powered reciprocating saw with a metal-cutting blade works great to go through both the sheet metal outside AND the insulation of the tank, all in one fierce cut. I sliced down the front of the tank – through the sheet metal, and through most of the insulation. I then finished cutting the insulation with a long drywall saw, and the insulation split right open. It felt like the whole shell was ready to come right off, except for the original Temperature and Pressure Relief Port blocking the way. A quick cut of the utility knife and a little prying, and the tank slid right out of its skin.
The naked tanks, side by side.
With both de-nuded tanks side by side, the condition of the tanks was a little easier to see. although the water heater is taller, it has a bottom that curves up – concave as viewed from the outside. The pressure tank has a convex bottom. I think both tanks must have a very similar capacity. The heater tank is plainly marked 50 GALLONS, and I think the pressure tank is the same.
The ports on the pressure tank are different than those on the heater tank, so I did have to do some minor cutting of the insulation to allow for it. Then, I wrapped the heater insulation around the pressure tank, and temporarily held it in place with a strap.
The pressure tank gets a coat and belt.
The insulation doesn’t make it perfectly around. When I cut the insulation off the heater tank, it “popped” as if under pressure when attached to itself. I believe that when the water heater tanks are manufactured, the tank is placed inside the sheet-metal skin, and then expanding foam insulation is injected between the two. The insulation is sort of “pressurized” on there its whole life. because of that, I am never going to get the insulation that tight onto the new tank. There is about a half-inch gap. I think I will simply fill that in with Great Stuff or some similar expanding foam. When I do that, I will first cover the tank and ports with plastic. That way, if I ever need to access one of the unused ports, I simply need to cut away the foam and plastic – the foam will NOT be stuck directly on the metal.
I will also need to cover that very top with insulation, and let’s not forget the bottom either!
What to do next? I still need to find a good circulating pump – something that can be PV powered, and with an appropriate flow rate. I also need to buy soft copper pipe to wrap around the tank. I haven’t been able to find any great prices on that yet. The best I have come up with is 60 feet of 1/2″ copper for $90. I hope that 60 feet is enough for a good heat exchange.
I will also need to hack into my existing plumbing and add in a diversion and several shut-off valves. I am really not looking forward to that – two many chances for disaster – and most of my plumbing is nearly inaccessible – directly behind the furnace!
Well, I’ll keep chipping away at it. As near as my estimates go, I will have a $6000 solar heating system for a cost of only about $400 total when I am all done. Plus I get all the experience of figuring it out myself to boot!