Hot water generation is the business of heating water for washing, cleaning and warmth, which is usually the largest single energy demand in a household.
It's also the easiest demand to meet without electricity, because heat can be made directly from sun or fire instead of being converted twice.
Do not make electricity to make heat
Heating water is the single largest energy use in most households, ahead of lighting, appliances and frequently ahead of space heating. It's also the one most often solved in the most expensive possible way, by generating electricity and then turning it back into heat.
Every conversion loses something. Sun to electricity to heat throws away most of the energy that arrived. Sun to heat, directly, keeps nearly all of it, which is why a solar water heater outperforms a solar panel driving an element by a very large margin for the same area of collector.
The same applies to fire. A stove already burning to cook or to warm a room is producing far more heat than the room needs, and capturing part of it into water is nearly free. That's the argument for the back boiler, and it's why they were standard for so long.
Which makes this one of the highest-value subjects in the library for anyone reducing dependence: the biggest demand, met by the simplest technology, with no supply chain.
01
Solar water heating
This is the cheapest useful energy most households can produce, and the technology is old and undemanding.
The simplest system is a dark tank in the sun. It works, it's called a batch or breadbox heater, and it loses its heat overnight because the collector and the store are the same thing. Insulating it and covering the glazing at night largely fixes that.
A flat plate collector separates the two: a dark absorber under glass with water passing through it, feeding a separately insulated tank. Glazing traps heat, insulation behind the absorber stops it escaping backwards, and the result reaches useful temperatures in modest sun.
Circulation can happen without a pump. Hot water rises, so a tank placed above the collector will draw water through it by thermosiphon: heated water leaves the top of the collector, rises to the tank, and cooler water falls back to the bottom of the collector. It needs the tank higher than the collector and generously sized pipework with a continuous rise, and it then runs forever with no power and no moving parts.
Freezing is the failure mode in cold climates, because water expands as it freezes and splits collectors and pipes. The answers are draining the collector when it's cold, or using a separate frost-protected loop that heats the water through an exchanger rather than passing through it.
02
Wood-fired and back-boiler systems
Any fire that's already burning can heat water, and doing it takes very little.
A back boiler is a water jacket or a coil in a stove or fireplace, capturing heat that would otherwise go up the flue. It costs almost nothing to run because the fire is lit anyway, and it was standard domestic practice for a long time.
The same thermosiphon principle circulates it: a tank above the stove, pipes rising continuously to it, and no pump. The pipework has to rise all the way with no dips that could trap air or steam, because a trapped bubble stops circulation and a stopped circulation next to a fire is exactly the dangerous case.
That's the whole safety issue with wood-fired water heating. An electric element has a thermostat that turns it off; a fire does not. So the system must be designed so the water cannot stop circulating and cannot boil dry, and there must be somewhere for expansion and steam to go that cannot be shut off.
A dedicated water-heating fire, instead of a stove that does both, is worth it where hot water demand is high and space heating isn't wanted, which is most of the year in a warm climate.
03
Storing heat, and keeping it
Water is an excellent store of heat, and most of what a system produces is lost in storage, not in generation.
Insulation is the highest-return work in the whole subject. A well-insulated tank holds usable heat for a day or more; a poorly insulated one loses most of it overnight, which means it's collecting the same heat again each morning instead of accumulating.
Stratification is the effect worth designing for. Hot water is less dense, so it sits at the top of a tank while cooler water stays below, and a tall narrow tank maintains that separation better than a wide shallow one. Drawing hot from the top and returning cold to the bottom gets you usable hot water long before the whole tank is heated.
Pipe runs matter more than people expect. Every metre of uninsulated hot pipe is a radiator, and the water sitting in a long run cools between uses and is drawn off cold before hot arrives. Short, insulated runs and putting the tank near the point of use save more than most collector upgrades.
04
Making it usable and safe
A system that produces very hot water is a system that needs controls at the point of use.
Solar and wood-fired systems produce water considerably hotter than a thermostatted heater, and hot enough to scald quickly. A tempering or mixing arrangement, which blends cold in to limit the outlet temperature, is what makes that safe, and it's not an optional refinement in a household with children or older people.
Expansion and relief are the non-negotiable parts, and the caution above covers why. The relief path must be continuous from the heated vessel to somewhere safe, with nothing capable of isolating it.
Backup matters for reliability instead of safety. Cloudy weeks happen and fires aren't always lit, so most working systems have a second heat source, an element, a gas heater or a second fire, arranged so it only contributes when the primary hasn't delivered.
And keep it simple. Every valve, pump and control is something that fails, and a thermosiphon system with no moving parts at all is genuinely more reliable than a pumped one with better performance on paper.
Hot water for washing is a sanitation input, and stagnant lukewarm storage is a genuine health risk.
Where this subject stands
The library doesn't cover Hot Water Generation yet. It's in planning, and what gets written next is impacted by what readers ask for.
Tell us your climate, whether you have a fire burning anyway, and how much hot water the household actually uses. Those three decide which approach fits.
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Questions
Is solar hot water better than solar panels?
For heating water, substantially, and it's not close. A solar thermal collector turns most of the sunlight falling on it into heat in the water. A photovoltaic panel converts a fraction of it into electricity, which then has to be turned back into heat. For the same collector area, direct heating wins by a wide margin, which is why it's the first thing to do instead of the afterthought.
Can hot water circulate without a pump?
Yes, by thermosiphon, and it's the most reliable arrangement there is. Hot water rises, so if the tank sits above the collector or the stove, heated water rises to it and cooler water falls back, circulating continuously with no power and no moving parts. It needs the tank genuinely higher, generously sized pipes, and a continuous rise with no dips that can trap air.
What makes a wood-fired water system dangerous?
That a fire has no thermostat. An electric element switches off; a fire keeps burning whatever the water is doing. So if circulation stops, or the vessel boils dry, or expansion has nowhere to go, pressure rises until something fails. The protections have to be designed in: a circulation path that cannot air-lock, no way to fire it dry, and a relief path that cannot be isolated.
What is the cheapest improvement to an existing system?
Insulation, on the tank and on the pipes, ahead of anything else. Most systems lose more heat in storage and distribution than they fail to collect, and insulating is cheap, immediate and permanent. After that, shortening hot pipe runs or moving the tank closer to where the water is used.
The Ark is a library of practical knowledge that was ordinary once and isn't any more, being recovered from the sources that recorded it while it was still in use. Membership is what pays for that work, and it's what decides how fast subjects like this one get written.