Drainback Solar Water Heating

A drainback solar water heater introduces slightly more complexity to the process of heating domestic water than a simple thermosiphon system, but it offers a clear advantage for homeowners who experience a hard freeze periodically. The drainback system uses convection to separate warm water from cool water and uses thermal heating with assistance from two pumps and a controller.

The heart of the drainback system is the drainback heat exchanger tank (let’s call it “the drainback tank” for short), because that’s where everything comes together. A pump (let’s call it the drainback pump) pulls water from the bottom of the drainback tank and up to the bottom of the solar collector. This water gets warmer as it travels up the solar collector, and leaves the solar collector warmer than when it arrived. The same drainback pump pushes this freshly warmed water down to the top of the drainback tank. This constant flow of water from the drainback tank up through the solar collector and back down again slowly warms the water in the drainback tank. Note that this “drainback loop” will never make it into a lavatory, shower, or dishwasher; it’s a closed loop.

At the same time, a separate pump (let’s call it the storage tank pump) pulls water from the bottom of a storage tank up through a coil that runs through the drainback tank and back down to the storage tank. This constant flow from the storage tank up through the coil in the warm drainback tank and back down again slowly warms the water in the storage tank. Note that this loop of water (which will make it to the lavatory, the shower, and the dishwasher) never makes it to the solar collector itself. It picks up its heat in the drainback tank.

The brain of the drainback system is a controller that’s attached to various sensors in the solar collector and the storage tank. When the sensors announce that the sun is no longer shining, the controller stops the drainback pump. This allows the water in the solar collector to drain back into the drainback tank and removes all risk of a freeze. On the other end of the spectrum, if the sensors in the storage tank announce that the storage tank temperature is as hot as it should get, the controller stops both pumps. This allows the water from the solar collector to drain back into the drainback tank and also stops the flow between the storage tank and the drainback tank.

Because drainback systems are immune to freezing, they are fast becoming the system of choice wherever there is any risk of a hard freeze. Our friend and solar contractor Daryl Bergquist says he’s recommending drainback systems now for almost everyone. “They are more thermally efficient, more reliable, and can be easily sized to provide a larger total percentage of hot water demand, than antifreeze systems,” Daryl tells us. “All of the systems we have installed to date are drainback. I understand they are becoming the norm in Europe.”

A drainback solar water heater can provide most of the hot water you and your family will need, but it’s seldom economical to design it to provide all the water heating a family would want on the coldest, cloudiest days, because this would result in an over-designed system. Consider pairing a drainback system with a tankless electric or natural gas heater that turns on only to provide the last few degrees of temperature lift and only when needed. The tankless heater will gulp energy by the bucketful when it’s running but will run rarely. This pairing of the drainback system with a tankless heater will result in dramatic energy savings in almost any home.

A note about the power for the drainback pump: we went through our “PV period” when we were entranced with the elegance of powering the drainback pump with a PV (solar photovoltaic) panel. “What could be more simple?” The panel would produce power when the sun shines and stop when the sun went down. What we failed to understand, and have since come to grasp, is that when you invest in PV, you want to use every watt it produces whenever it’s produced. If the storage tank gets as hot as you want it to be (and that happens often during the hot months in a properly sized system), the pump should stop, but the PV is still sitting there with no load, and you have the cardinal sin of post-petroleum living, Idle PV! No, no, no. This must never happen.

So we have finally come to grips with the reality that the drainback pump needs to come from the same power source as all other components of the system and be controlled by the system controller. It really does make life simpler. There’s no reason that power source cannot be PV, but it doesn’t make sense to have PV dedicated to the drainback pump.