How would you build it? I have wanted to begin building a solar hot water heater for a while but the only place I have to store the water is the 30 gallon HWH in my mobil home which we use now. I need to build a heat well to store the hot water before I actually build the collector(s). The collector will be something similar to what's here: It's just a simple copper pipe assembly in a frame. I will most likely paint this black. There will be more than one of these and each one will be about 2'x4' and preferably on a pivot to change the angle to capture more energy from direct sunlight. What I can't decide on is how to build the heat well. Since I live in a mobile home, I know that a factory made heat well is not an option. They are too expensive for us and I want to make one that holds at least 100 gallons of hot water but really want one around 150-200 gallons. 150 gallons could last us 5-7 days with a little conservation. That way, we'ld have hot water even thru several days without direct sunlight. The idea I'm considering at the moment is getting a large, verticle standing polyethylene tank, pouring a 4" thick slab with 6" or more of flat styrofoam panels on top of the slab to rest the tank on. Then, build a strong wooden stick frame around the tank and use A/B spray foam (expanding) to fill up the wooden frame. Once the foam expands fully, a long saw would be used to trim off the excess from the four flat sides. If you have ever seen the guys that come and spray foam new housing for insulation, they spray the foam between two studs and once it expands, they rest the saw (hand saw) on the two studs and run it downward to trim off the foam and then have a flat surface to be covered in drywall, wall board... etc... This how it would be done and then it could be protected by coating the whole thing in fiberglass or some other type of housing. New tanks can be found for a little more than a buck a gallon, so we'll call it $150 for a new 150 gal tank plus around $80-$100 for shipping. I would also need someone with spray foam equipement to apply it on the tank. I have no idea how much that would cost, then, the cost of whatever I covered the foam with as a housing. But let's change the equation a bit. Let's say we have a budget of around $300 for just the heat well. That pretty much rules out brand new materials and services. So, using $300 as a highend target for complete cost of a finished heat well mounted on a slab that will hold 150 gallons of hot water. How would YOU do it? Please give all ideas, even if they sound stupid at first. There's no telling what ideas will work and whose ideas can be used and whose can't. Don't be scared, give it a shot.