Two gallons, and where they go
With the defaults: two panels at 0.35 gallons is 0.70, and 45 feet of 0.785 in bore pipe above the waterline is 1.13. That is 1.83 gallons that has to be somewhere when the pump is off. Add the 1.5 gallons that stays over the suction and the vessel is holding 3.33 gallons wet; keep 20 per cent of it as air space and the reservoir works out at 4.16 gallons.
The level swing is the part worth remembering when you are looking at a sight glass. The reservoir drops by the full 1.83 gallons every time the pump starts and comes back up by the same amount when it stops. A vessel sized with no margin looks fine standing still and runs the pump dry the first time the loop fills.
Only what is above the waterline drains
This is the input people get wrong. Pipe below the reservoir waterline stays full whatever happens, so it does not count towards the drain-down and does not need catching. Pipe above it does, both legs, all of it, including the vertical risers. If the reservoir is in a basement and the panels are on the roof, that is almost the whole run counted twice.
Fall is a separate failure mode from volume
A correctly sized reservoir on a route with a level section is still a loop that does not empty. Twenty two feet at a quarter inch per foot needs 5.5 inches of drop, and if the space between the two ends only gives eight inches you have room; if it gives four, you do not, and something has to change — a different route, a shorter horizontal, or a different point to turn down.
The flat-section field exists because the honest answer is often "about six feet of it is level, because that is where the joist runs". Six feet of 0.785 in bore holds 0.15 gallons, which is eight per cent of the drain-down. That fluid stays there. The reservoir will read correct and the loop will still have fluid sitting in a cold pipe.
The lift is a start-up problem, not a running one
Twenty four feet of static lift at 1.00 specific gravity is 10.4 psi. The pump has to produce that, plus friction, to get fluid to the top of the panels. Once the loop is full and both legs are running solid, the descending column balances the ascending one, the static term cancels, and all that is left is friction. That is why a drainback pump is chosen for a duty it only does for the first minute, and why a pump that manages the running case comfortably can still fail to fill the loop at all.
What is not settled here
Whether a given route actually drains is not arithmetic. It depends on pipe support and how much the hangers let a run sag over a few summers, on how the panels themselves are plumbed and tilted, on where air comes in and goes out, and on how the pump behaves at start and stop. This page adds up volumes and checks one run against one drop. The rest of it is the reason drainback systems are built by people who have built them before.
Questions people ask
How big should a drainback reservoir be?
Big enough for everything above the waterline plus whatever has to stay over the pump suction, with air space left over. With two panels at 0.35 gallons and 45 feet of 0.785 in bore pipe, the drain-down is 1.83 gallons; add 1.5 gallons held over the suction and keep 20 per cent air space and the vessel works out at 4.16 gallons. The reservoir level swings by the full 1.83 every time the pump starts.
Which pipe counts towards the drain-down volume?
Only what sits above the reservoir waterline, and both legs of it. Pipe below the waterline stays full whatever the pump does and never needs catching. If the vessel is in a basement and the collectors are on a roof, that means almost the entire run counted twice — the flow leg and the return leg — plus whatever the panels themselves hold.
How much fall does a drainback loop need?
That is a working figure you set, and this page states none. What it does is check the one you chose against the drop you actually have: 22 feet at a quarter inch per foot needs 5.5 inches, and if the space gives eight you have room. The more useful output is the flat-section field, which tells you how much fluid a level run keeps — six feet of 0.785 in bore holds 0.15 gallons, and it stays there.
Why does a drainback pump have to be so much bigger?
Because at start-up it lifts a column of fluid the full height from the waterline to the top of the panels with nothing coming back down to balance it. Twenty four feet is 10.4 psi of static pressure before any friction. Once both legs are running solid the descending column cancels the rising one and only friction is left, so the pump is sized for a duty it performs for the first minute of every cycle.
What happens if part of the run does not drain?
That fluid stays in the pipe, and the reservoir will read perfectly correct while it does, because the level only tells you how much came back rather than how much was supposed to. A level or backfalling section keeps its contents whatever the rest of the route does. The whole reason a loop was arranged to drain in the first place is what that trapped section is then exposed to, and this page names that rather than offering a way round it.