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Hot Water Off the Grid: A Simple Pressurised System
Hot water is the largest energy load in most homes. Off-grid, you either shrink it or heat it with wood.
Direct answer: An off-grid hot water system usually heats a storage tank with a wood stove coil, a solar thermal collector, or a small heat-pump water heater running off surplus solar. The tank must be pressurised correctly, insulated heavily, and fitted with a working pressure-relief valve.
How the system works
A coil or jacket inside or around the stove heats water by thermosiphon: hot water rises into the tank, cooler water returns. No pump is needed if the tank sits above the stove. A solar thermal panel does the same job in summer, with a controller and pump for a pressurised loop.
| Method | Best season | Cost | Notes |
|---|---|---|---|
| Wood stove coil | Winter | $300–$1,200 | Free heat when the stove is running anyway |
| Solar thermal | Summer | $2,000–$5,000 | Excellent in sunny climates, needs freeze protection |
| Heat-pump water heater | Year-round | $1,200–$2,500 | Efficient but needs power; runs well on surplus solar |
| Propane tankless | Year-round | $800–$2,000 | Not off-grid, but a common backup |
Parts and safety
- Pressure-relief valve rated to the tank, piped to a safe drain. Never cap it, never remove it.
- Expansion tank on a closed system to absorb thermal expansion.
- Check valve to prevent backflow into the cold supply.
- Temperature-and-pressure gauge visible without moving anything.
- Insulation on every pipe run — 20 mm minimum, more outdoors.
Hot water systems combine heat, pressure, and sometimes fuel. If you are not confident with plumbing and pressure vessels, have the tank and relief plumbing installed by a licensed plumber. A blocked relief valve can turn a tank into a bomb.
Limitations
A wood coil gives you hot water only when the stove runs. In summer, you need solar thermal or a backup. Storage size matters more than heating power: 40–60 gallons insulated well will carry a household through a day.
Frequently asked questions
Can I heat water with a rocket mass heater?
Yes, with a coil or a hot-water jacket designed for it. The same pressure-relief rules apply, and the coil must not create steam in a sealed loop.
How much storage do I need?
40–60 gallons for one or two people, 80 gallons for a family. Insulate to at least R-12 to keep heat overnight.
Is solar thermal worth it off-grid?
In sunny climates, yes — it produces hot water without using your electrical budget. In cloudy climates, a heat-pump heater running on surplus solar is usually more practical.
Pair this with the off-grid solar guide and battery sizing so hot water does not drain the bank.
Frequently asked questions
Can I heat water with a rocket mass heater?
Yes, with a coil or jacket designed for it. The same pressure-relief rules apply and the coil must not create steam in a sealed loop.
How much storage do I need?
40–60 gallons for one or two people, 80 gallons for a family. Insulate to at least R-12 to keep heat overnight.
Is solar thermal worth it off-grid?
In sunny climates yes — it makes hot water without using your electrical budget. In cloudy climates a heat-pump heater on surplus solar is more practical.