Drainback Solar Water Heaters
Pros and Cons

Find out if the drainback solar water heater the right solution for your water heating needs. A review of the benefits, advantages, and disadvantages.

Drainback system is one of the popular solar water heating systems whose main advantages over other types are reliability, simplicity, and low maintenance. The drainback solar water heater is not the simplest, and it requires some knowledge, time, and money to design and build it properly. It is cheaper than the closed-loop system with the antifreeze (propylene-glycol based active type) but more expensive than the ICS system.

The risk of boiling water, high pressure, freezing, and bursting pipes is low, as long as the down-flow pipe is large enough and the fluid loop in solar collectors and pipes is sloped adequately for draining.

Drainback technology uses water that does not corrode the pipes and other metal parts or deteriorates as the heating fluid with additives (such as antifreeze). Furthermore, water is cheaper, its properties are better, and maintenance is lower.


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Types of the drainback systems

There are three types of drainback systems used in residential water heating. They utilize one or more well-insulated storage water tanks, one small drainback reservoir, solar collectors, a closed-loop with the pump, controllers, and a heat exchanger.

  • The first type is the system that uses the heat exchanger located inside the water tank and one pump for fluid circulation.
  • The second type includes the heat exchanger located below the minimum fluid level inside the drainback tank. A second pump is used to move the water from the water storage tank through the heat exchanger and back to the tank.
  • The third type of drainback solar water heating system uses a heat exchanger located outside the water storage tank.

Most of the time, the heating fluid is water, less propylene-glycol, which can deteriorate over time and needs replacement. Using water as the heating fluid also allows you to go with an open-loop system, but as long as you live in warmer regions, since they are prone to freezing.

Sizing a drainback tank

When sizing a drainback tank, take into account all the fluid that will fill the solar collectors, piping that is above the tank, and adds several gallons on top.

The tanks should always be insulated to prevent heat loss, equipped with the pressure relief valve, and vented.

They should be equipped with a sight glass so you can check the fluid level inside the tank. The best location of the tanks is close to the collectors.

Circulation pump

Drainback solar water heater might have one or two pumps. The two-pump system utilizes one pump to circulate the water from the drainback tank to solar collectors, while the other one, from the water storage tank to the drainback tank. The recommended pump types are the high-head, low-flow, with a variable speed or booster pumps.

When the system turns on, the pump, after filling the system with the fluid, circulates the fluid through the system, and when it turns off, the fluid drains back into the small tank.

The pumps must have sufficient head pressure to move the fluid up to the collectors, but the energy consumption can be minimized using the thermosyphon principle for fluid circulation. If you live in an area with high mineral deposits in the water, also known as hard water, the pipes might clog over time and prevent thermosyphoning and draining.

Solar/hydraulic loop

All the pipes must have a sufficient fall, such as 15-30 mm per meter (1/2"-1" per 3') of pipe, as the drainback depends on the gravity when the pumps stop. Sags and pipe damaging must be prevented, as they might lead to incomplete draining and frost damages. The use of the correct material is important, together with the proper mounting. The recommended pipe size should be 3/4".


One of the main disadvantages of the drainback solar water heater is a significant source of heat loss when the pump is not working, such as during the night exposure. This system requires the largest pumps of all systems.

The drainback concept is simple, but special attention should be given to its solar loop design. The most complicated thing is to design the system, so all the water from collectors runs down to the small reservoir when the pump stops. Water drains due to gravity while the air flows upwards into the solar panels and outdoor pipes. This is the trickiest part.

Since the presence of air must be considered in these systems, you should also take into account the possibility of metal corrosion.


The drainback solar water heater is recommended for all climates, even the coldest, since it drains the fluid out of the collector and exposed pipes. This is the reason why the system must be designed in a way to drain the fluid fast and thoroughly.

In addition, the tank must be installed as high as possible but low enough to allow complete draining. With that in mind, avoid horizontal pipes; make sure they all have at least a 15-degree slope; also, use 45-degree fittings instead of 90-degree ones.


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