Skip to content
DIY Solar Book, home

Part III: Solar Space HeatingChapter 5. Water-Based Space Heating and Storage

Building an Unpressurized Storage Tank

An open tank for storing solar heat, including liners, insulation, and the weight of the water.

Difficulty
Advanced
Cost
Several hundred dollars
published

An unpressurized tank is a vented container of water. The vent keeps the tank from becoming an unrated pressure vessel the first time the water heats and expands, or the first time a pump fills it faster than an undersized opening can relieve. These tanks store heat for a drainback collector or for a space-heat loop. They are not a casual way to store drinking water.

The combined-system overview is where this tank sits in a whole design. Freeze protection is why a drainback tank lives indoors. If the water in the tank will be pumped through floors or collectors, it is heating water, separated from taps by a heat exchanger.

Difficulty

Level
Advanced
Time
Several days, plus a hard look at the floor
Skills
Building a watertight container and knowing when the structure is beyond you

Cost estimate

Several hundred dollars

A liner or a made tank, insulation, fittings, and a pan. The number climbs if the floor needs work or if you buy a listed tank instead of building a box. Do not save money by closing the vent.

Materials

ItemQtyNotes
A vented tank or a lined boxSized to the heat you can actually useEPDM liner or a tank sold for water. Not a drum that held chemicals.
An open ventLarger than an afterthoughtThe tank must breathe. A blocked vent is a pressure accident.
Overflow to a safe place1Where hot water can go without hitting a person or soaking the structure.
Insulation around the sides and topSeveral inchesStop short of the vent. Do not enclose the tank so tightly that a leak has nowhere to show itself.
A leak pan or a floor that can get wet1The first leak should be a puddle you see, not a ceiling downstairs.

Weight and place

Water weighs about 8.3 pounds per gallon. Three hundred gallons is on the order of 2,500 pounds before the tank and the fittings. A basement slab on grade is the usual place that can carry that without a new beam. A wood-framed floor needs a structural check. If you cannot show how the joists carry the load, do not fill the tank there.

Put the tank where a leak is survivable and where the water will not freeze. An unheated shed is a bad drainback reservoir, because the reservoir is the part that is supposed to stay liquid when the collectors empty. Humidity is the other placement problem. An open tank in a small room will humidify that room. A lid that still vents, and a room that can dry, belong in the sketch. Do not hide a steaming tank in a closed closet.

Fittings

Hot collector water returns high in the tank or through a dip tube aimed to keep layers. The supply to the collectors or the heating loop leaves from a level that matches the control. Cold makeup, if you have it, enters low and must not be a permanent connection to potable water without the backflow protection the code requires. If someone later tees a heating tank into a shower, non-potable water reaches a tap. Label the tank. Keep a visible air gap or the approved backflow device on any fill line.

The overflow should be able to carry the fastest rate you can fill, and it should end where you will notice it. A relief path that dumps into a wall is not an overflow.

Put the insulation on the outside of a lined wood box, and keep it dry. Wet insulation does not insulate, and it rots the box. Leave a way to see the liner and the fittings.

An open storage tankA vented tank of water with insulation around it, an overflow, and pipes entering high and low. The vent stays open so the tank cannot become a pressure vessel.Vent, always openHotReturnWater is heavy. The floor has to be designed for it.
The vent stays open. Hot water can enter high, cooler water leaves low, and the overflow has somewhere safe to go. Insulation does not cover the vent.

Size

Bigger is not automatically better. A tank you cannot heat above a useful temperature is a lukewarm pond. Match the volume to the collectors and to one or two days of the load you actually have, and then look at summer. A huge tank in July still has to reject heat or the collectors stagnate. The combined-system page covers that summer problem.

  1. Put the gallons on a floor that can hold them

    Do the weight math. Ask for help if the floor is wood.

  2. Vent and overflow before you insulate

    Both must work when the lid is on. Test the overflow with a hose.

  3. Separate this water from drinking water

    Heat exchanger for heat. An air gap or a legal backflow preventer for filling. A label on the tank.

  4. Insulate and watch the first heat-up

    Look for leaks, a wet lid, and a room that is becoming a swamp. Fix those before you add collectors.

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.