The best hot water tank to start with is a salvaged electric water-heater tank, which is already insulated and built for pressure; stainless drums and old boiler tanks follow, and a welded steel, lined wooden or lime-plastered concrete tank is a build-it-yourself option. Whatever it is made of, the walls must carry the water’s weight, the metal must survive hot-water corrosion, and the tank must be proved watertight before it is insulated.
Part of Hot Water Tanks
Tank construction is the choice of vessel and materials that hold hot water without leaking, collapsing or contaminating it.
Salvaged And Built Options
From the parent page:
- Salvaged: electric water-heater tanks (already insulated and pressure-rated), stainless steel drums or vessels, old boiler tanks.
- Built from scratch: a welded steel plate tank (not ideal, because of corrosion), a lined wooden tank (the traditional cooper-built barrel lined with pitch or clay), a concrete tank waterproofed with lime plaster.
Checks before you trust any of them:
- Salvaged heater tank: most are steel with an inner lining and a sacrificial rod; check for leaks, rust at the seams and the state of the rod. Test the tank with water, not air, at a pressure above its working pressure, and fill it open-vented (Pressure Testing).
- Steel: hot water attacks bare steel faster than cold; keep the water treated and the tank drained when idle.
- Wood: a wood tank needs a lining that does not soften at hot-water temperatures. Pitch softens when warm and can taint the water; test a sample first.
- Concrete: lime-plastered concrete is a cold-water cistern in the corpus (Ferrocement Tanks); heat cycles crack plaster, so use it only for warm, not hot, storage.
- Plastic barrels: creep and soften with prolonged heat; suitable for warm water only.
Worked Example: Hoop Load On A Wooden Tank
Water pressure at depth h is 9.81 kPa per metre. A round tank of radius r pulls its hoops apart with a force per unit height of p × r. Inputs: a tank 1 m in diameter (r = 0.5 m), 1 m deep, hoops spaced 0.2 m apart at the bottom.
- Pressure at the bottom = 9.81 kPa = 9,810 Pa.
- Hoop force at the bottom = p × r × spacing = 9,810 × 0.5 × 0.2 = 981 N, about 1 kN.
- Higher hoops carry less, in proportion to depth.
A forged iron hoop that holds 1 kN (the weight of about 100 kg) is easy; the issue is that wood shrinks if the tank is left empty. Keep it filled or wet. Water weighs 1 kg per litre: a full 500 L tank is 4.9 kN on its floor, so spread it (Storage Tanks & Cisterns).
What You Need
- A vessel from the list above
- Fittings: hot outlet at the top, cold inlet at the bottom, drain, vent
- A flange or bung for cleaning
- A level base
- No lead pipes or solder in contact with the water; copper or steel only
Method: Prove The Tank
Step 1: Rinse the tank thoroughly, especially if it held anything else.
Step 2: Fill it with water, leave it overnight, look for leaks at every seam and fitting.
Step 3: Heat gradually and check again when warm; metal grows, joints weep, wood swells.
Step 4: Only then insulate it.
Common Mistakes
- Insulating before testing. A leak under insulation is invisible until the insulation is soaked.
- A tank that held fuel or chemicals.
- Bare steel left wet and idle.
Safety
A hot-water tank is a pressure vessel if it can be shut in
Heated water expands by about 2.7% between 20°C and 80°C (densities 998.2 and 971.8 kg/m³, handbook values, recalled), so a tank that is sealed full can burst. Give every tank an open vent or a relief valve (Safety Valve). A tank that collapses releases hundreds of litres of scalding water at once; support it on a base that carries its weight, and never stand below one that is leaking.
What’s Next
- Learn next: Super-Insulation — keeping the water hot
- Combine with: Metal Pipes — connecting it