Part IV: Solar Water HeatingChapter 7. Active Domestic Hot-Water Systems

Drainback Systems Step by Step

A pumped loop that empties into a reservoir when the pump stops, so freeze and boil are designed out.

Difficulty: Advanced
Cost: Several hundred dollars

published

A drainback system fills the collector only while the pump runs. When the pump stops, the water falls back into a reservoir indoors. The collector spends the night empty, so a freeze has nothing to burst. It spends a hot vacant afternoon empty too, so you are not boiling a full loop of water on the roof. Both of those protections are real only if every pipe actually drains.

If any run holds a low spot, you have built a direct system with extra steps. The choice guide says this plainly. This page is the sequence after you have already looked at the roof and decided the pipes can slope.

The fluid in the collector loop should not be your drinking water. Roof loops get debris, flux, and the odd solder flake. Put a heat exchanger between that loop and the potable tank, and read the exchanger page before you pick a coil. Glycol does not belong in a true drainback. The freeze protection is the empty pipe.

Difficulty

Level: Advanced
Time: Several days of plumbing after the collector and the tank exist
Skills: Pipe slope, pump head, and a drain test you are willing to fail

Cost estimate

Several hundred dollars

Pump, controller, and the copper to reach a collector you already have can land here. A collector, a solar tank, roof flashings, and electrical work are extra and often larger than this line. A complete system is not a few hundred dollars.

Piece	Typical cost
Pump that can make the lift	$100–300
Differential controller and sensors	$40–150
Pipe, insulation, and fittings to the roof	The rest of this range, or more if the run is long

Materials

Item	Qty	Notes
A collector that drains	1 or more	Headers sloped, no internal trap. A collector sold as drainback-capable, or a homebuilt plate you have already sloped.
Indoor reservoir or tank	1	Vented to the room so air can replace the water as the loop empties. Not a closed pressure vessel.
Pump sized for static lift plus friction	1	On startup the collector is empty. The pump lifts water the full height every time. A closed-loop circulator curve may not apply.
Pipe with continuous slope	Supply and return	About a quarter inch per foot, both ways down to the tank. No check valve. No automatic air vent that holds a column.
Heat exchanger to the potable tank	1	So the roof loop is not what comes out of the tap. Code may want a double wall.

Slope every pipe back to the tank

Walk the route with a level before you buy fittings. Both the supply and the return must fall, all the way, from the collector to the waterline in the tank. A beam that forces a jog up and then down is a trap. If you cannot avoid it, this is a closed glycol loop, not a drainback.

The collector has to empty too. Set it so the bottom header drains toward the return connection. People plumb a perfect slope in the attic and leave the collector itself tipped the wrong way. The puddle in the header is the part that freezes.

The tank is vented. When water drains down, air goes up. If you cap the tank, you have made a pressure vessel and you have also plugged the air path the drain needs. The open tank page is the warning for anyone tempted to "upgrade" a reservoir with a tight lid. A drainback reservoir can be a small dedicated tank above a heat exchanger, or the solar tank itself if that tank is allowed to be open on the collector side. The potable side stays on the other side of the exchanger, at household pressure, with its own relief valve.

Collector high, both pipes falling to a vented indoor tank, pump low, and no check valve to hold the water up.

The pump sees the height every time

In a closed pressurized loop the pump mostly stirs a loop that is already full. In a drainback the pump lifts water from the reservoir to the top of the collector against gravity, and the return falls on its own. The head is the elevation plus the friction of the pipe. Read the pump curve at that head. The pump and pipe pages cover the sizing. A pump that cannot make the lift will run hot and never fill the collector. You will see a little warmth in the tank from the pipe in the attic and think the system is working.

Fill the reservoir to its mark and no higher. The collector starts empty. The controller, covered on the next page, turns the pump on when the collector is hotter than the tank and off when it is not, or when the tank reaches a high limit. Off means drain. You should hear the water come home.

Then prove it. Shut the pump off and wait. Open a union at a low point you planned for this test, or use a sight glass if you have one, and confirm the collector is empty. If water keeps coming, or if a header is still heavy, you have a trap. Find it before winter. A trapped slug in sun can flash to steam even though the rest of the loop drained. That is loud and it is dangerous.

Reject the route if it cannot slope
Level on the real joists and the real roof. Any unavoidable low spot ends the drainback plan.
Set the collector and the vented tank
Tank indoors, below every collector pipe. Collector tipped so its headers drain. Vent open on the reservoir.
Pipe both runs downhill
About a quarter inch per foot. No check valve, no high-point vent that can hold water, no sag between supports.
Fit a pump that makes the lift
Curve in hand, elevation measured, strainer where you can clean it. Exchanger between this water and the tap.
Fill, run, and watch it drain
Reservoir at the mark. Pump on until the return is steady. Pump off. Confirm an empty collector. Do not call it commissioned until that is true. Then do the measurement checklist.

Difficulty

Cost estimate

Materials

Slope every pipe back to the tank

The pump sees the height every time

Reject the route if it cannot slope

Set the collector and the vented tank

Pipe both runs downhill

Fit a pump that makes the lift

Fill, run, and watch it drain

Further reading