Drainback vs. pressure: A question for the ages

Pressure or drainback?

That’s probably the question I get asked more than any other: Should I choose the more popular pressure-glycol system, or go with a drainback system?

Here’s the answer: It depends.

Don’t you hate that kind of answer? Well, here’s a more specific answer: If the slope of the collectors and plumbing is such that all the fluid will drain out, then go with the easier and less expensive drainback design.

That doesn’t mean I favor one over the other; it really does depend on the building’s design.

Let’s backtrack for a moment. Here are definitions of each type of system.

  • Solar pressure system: Closed loop arrangement that generally has 20 psi or more at the panels. Besides the panels and the tank, basic components include check valves, expansion tank, air purge and air traps, a pressure relief valve, pump, solar control with sensors and nontoxic antifreeze.

Drainback system: Closed loop arrangement that generally has 10 psi. Besides the panels and the tank, basic components include drainback tank, a pressure relief valve, pump and a solar control with sensors. May or may not contain antifreeze.

The virtues of each system type:

  • Solar pressure system: Pressure systems generally contain glycol to provide freeze protection.
  • Drainback system: A properly designed closed-loop drainback system provides freeze protection without glycol as well as overheat protection due to the absence of fluid in the panels when either condition exists.

Let’s start with the more popular pressure glycol-filled system:

Pressure systems take time and patience to fill, charge and purge all the air out. If some air remains, controlling the flash point may be impossible.

A pressure system must be in perfect balance to handle the expansion of fluid when the pump isn’t pumping — such as a power failure or when the tank is satisfied. You must have a properly sized expansion tank that is kept at a consistent parallel pressure with the glycol loop pressure. Will this pressure stay exactly the same forever? Nope, every year you need to check and recharge the system.

And what happens if the closed-loop system pressure drops a few pounds below where you set the expansion tank pressure? The system will eventually blow antifreeze all over — via the pressure relief valve. The reason is that water-to-steam expansion is immense — 1,172% to be exact. When the pump shuts off during, say, a hot summer day, the fluid in the panels will go to boiling point and flash to steam if it is not in perfect balance.

Let’s say that we have a collector that holds one gallon of water. If the system pressure to expansion tank is maintained in concert and the pump shuts off under full sun, the panels will build heat slowly and the fluid will start to phase-change. That one gallon of water will start to phase-change and expand; it will push fluid out of the collector and down the pipes where it will stay condensed (as fluid). Eventually, you will have one gallon volume of steam in the collector and the gallon of water you pushed out will be in the expansion tank. If there is air in the system or the system pressure is not in harmony with the expansion tank, violent boiling will transfer down the plumbing, expanding several gallons of water by the aforementioned 1172%, out of control.

When the system is in balance, only a small amount of fluid will expand that immense amount, likely filling the collector’s one-gallon capacity.

Because of this, larger systems should use a constant pressure auto-fill device.

The closed-loop drainback system:

A drainback system is a fluid-filled closed loop with an air bubble and a small tank to isolate and capture the air when the pump comes on. When the pump is switched off, the heavier water drops from the panels and pushes the air bubble back up into the panels and exterior plumbing.  If the tank is satisfied or the power fails, the pump will be off, and no water will be in the panels to flash to steam. No fluid to expand, no need for expansion tank. It’s the same for freezing — when the temperature at the panels drop below freezing, there is no water in them to freeze. Fewer safety measures need to be considered.

However, a pressure glycol system will need to be considered if the slope for panels and plumbing cannot be achieved to drain all the panels’ fluid into the drainback tank.

Advantages of using some pressure

I would recommend you add a small amount of pressure in a closed loop drainback system, for two reasons. First, it will confirm you have integrity in your closed loop. If you have a completely sealed system, the air will go inert and turn mostly to harmless nitrogen.

Second, if you fill and close up the system when it’s warm, then at night when the panels get cold, the closed loop will go into negative pressure and start boiling the stored fluid in the drainback tank when it is at room temperature.

Pump and pump sizing rules change considerably with a drainback system. Pump size will be determined by the lift required to deliver from the drainback tank to the top of the panels. After the pump has overcome the lift to the panels, it’s important to have the variable solar pump speed control slow the pump down.

A variable-speed pump control should be used on both pressure and drainback systems to keep solar heat transfer to a maximum based on Delta T. Over-pumping a solar loop may void the panels’ warranty. Consider raising the drainback tank as high as possible in the building (within conditioned space) to allow a smaller pump. The pump should be sized to deliver the GPM needed for the solar and the lift from the drainback tank to the panels.

Drainback with tankless is less costly, more efficient

Drainback with tankless is less costly, more efficient

Today, as part of our ongoing series of basic solar designs, we look at a drainback system using an efficient solar storage tank and a tankless water heater as backup.

This design is fairly uncomplicated and is, perhaps, the least costly system for residential applications. For all its simplicity, it delivers freeze and overheat protection with water. It also features low heat loss.

The tank is the solar storage and the drainback tank combined. The hot water storage tank I’m using in the drawing is made of expanded polystyrene (EPS), which has an insulation factor of about R5 per inch. Dimensions of the tank are 60 inches in height and 30 inches outside diameter, with 4-inch walls. Usable Btu storage is 81 gallons,  which when filled with solar energy up to 160 degrees equals about 67,000 Btus of energy.

The system has very few components — basically the storage tank, pump, controller and collectors. The tank contains a 50-foot 1″ HX coil. Potable water is drawn through the coil, picking up heat from the contents of the tank. The backup water heating is located after the coil exits the  tank. A tankless gas or electric water heater works well as a backup, or this could be connected to any existing water heater and used as a preheat design.

This system is an excellent choice for the HTP Hydra Smart tankless water heater. That is the tankless water heater I’ve used in the design drawing. For more information on this advanced, modulating tankless, click here.

Like other drainback designs, all plumbing must be sloped from the panels for complete drainage.

Here’s more general information on how tankless water heaters work.