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Part IV: Solar Water HeatingChapter 6. Simple and Batch Water Heaters

Thermosiphon Systems: Let Hot Water Rise

Why the tank has to sit above the collector, and how tall that rise needs to be.

Difficulty
Intermediate
Cost
Varies
published

Hot water is lighter than cold water. If the storage tank sits above the collector, water heated in the collector rises into the tank and cooler tank water returns to the bottom of the collector. No pump. That loop is a thermosiphon. It runs only while the collector is hotter than the tank, and only if the pipes give the rising water an easy path up. A long horizontal run, a tank that is level with the collector, or a check valve that has stuck shut will stall it.

This is a different machine from the batch box, where the tank itself is the collector. It is also different from the pumped layouts in the choice guide. Direct thermosiphon, with drinking water in the collector, is for climates that do not freeze. A hard freeze bursts it, and there is no pump you can blame. If a hose bib has ever frozen at your site, pick another layout.

Difficulty

Level
Intermediate
Time
A weekend only if the tank support already exists. Building the support is most of the work.
Skills
Keeping pipes short and uphill, and treating a full tank as a structural load

Cost estimate

Varies

A small collector and a tank can be a few hundred dollars in fittings and glazing, or well above that for a listed tank, a stand, a mixing valve, and a relief valve. The stand and the valves are not the place to save money.

Materials

ItemQtyNotes
A collector below the tank1The bottom of the tank at or above the top of the collector. Higher is a stronger loop until friction wins.
Short pipes that rise toward the tankSupply and returnHot outlet of the collector to the top region of the tank. Cool return from the tank bottom to the collector inlet.
A listed potable tank if the loop is drinking water1With a temperature-and-pressure relief valve and a discharge that cannot scald anyone.
A mixing valve1On the way to the taps. The tank can get hotter than a shower should be.
Insulation on the pipes and the tankThe outdoor runsA bare riser gives the heat back.

Height and pipes

The driving force is the difference in density between the hot riser and the cooler return, times the height of that column. More height, more drive. More length and more fittings, more friction, and the drive is spent before any water moves. Put the tank clearly above the collector. Keep the pipes short, large rather than tiny, and sloping up toward the tank so air can leave and so there is no high pocket that stalls the start.

A common compact arrangement is a tank on a stand immediately above a collector on the ground or on a low roof, with the tank bottom no lower than the collector top. A tank in the attic and a collector on a wall below it can also work if the attic floor can carry the weight and the pipes do not wander across the house. A tank beside the collector at the same height will not run. Do not add a pump later and keep a check valve that fights the thermosiphon. If you need a pump, you have a pumped system. Build that on purpose.

At night the collector cools and the loop can reverse, cooling the tank. A heat trap, a generous height, or a very free check valve are the usual replies. A check valve that sticks open does nothing. A check valve that sticks shut stops the thermosiphon forever and can trap water in a collector that will freeze. If you add one, it has to be the right direction, light enough to open from buoyancy, and inspected. Many builders rely on height and insulation instead, and accept a small night loss. Know which compromise you chose.

Tank above the collectorThe storage tank sits higher than the collector. Hot water rises into the tank. Cooler water returns to the bottom of the collector. A mixing valve is on the way to the taps.TankCollectorHot risesCool returnsMix valveIf the tank is not above the collector, this loop does not run.
The tank is above the collector. Hot water rises. Cooler water returns. A mixing valve sits between the tank and the taps.

Climate and structure

Direct thermosiphon and freezing weather do not mix. Antifreeze in a jacketed tank above the collector is a different, still careful, machine: the glycol loop can thermosiphon, the potable water stays in the tank, and you still have stagnation and a fluid to maintain. If that is more complexity than you wanted, use a pumped drainback, or keep direct thermosiphon for a climate that does not freeze.

A full tank is hundreds of pounds. A roof stand is a structural and a wind problem. Tie it down the way a local wind load requires. Do not set it on a residential roof because the view is good. Attic tanks leak onto ceilings. Put a pan and a drain under any tank that can wet the house.

Pipe it as a preheater in series with a backup heater if you want hot water on a cloudy day. The backup page's series idea applies. The mixing valve is still required, because the sunniest days are when the backup is least likely to be the thing limiting the temperature.

  1. Reject the site if it freezes

    Direct potable thermosiphon is off the list. Choose drainback, a closed loop, or no water on the roof.

  2. Put the tank above the collector

    Bottom of the tank at or above the top of the collector, pipes short and rising.

  3. Support the weight and the wind

    A stand or a floor you have checked. A pan under the tank.

  4. Relief valve, mixing valve, backup

    Then run a sunny day and a night. The tank should heat by day and not collapse overnight.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.