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Part III: Solar Space HeatingChapter 5. Water-Based Space Heating and Storage

Pipe Sizing for Solar Loops

How flow, pipe diameter, and pump head fit together so the loop is quiet and the pump is small.

Difficulty
Intermediate
Cost
Free
published

Pipe that is too small is noisy, wastes the pump on friction, and can erode over a long life. Pipe that is too large is expensive, slow to respond, and happy to hide an air pocket. For a closed hydronic loop, a velocity near 2 to 4 feet per second is a useful planning band. Drainback adds a rule velocity cannot replace: every outdoor pipe has to slope back to the tank, with no trap.

The pump guide explains why head matters. Pipe diameter is one of the things that changes it. Use inside diameter. A pipe called three-quarter inch is not 0.75 inch inside.

Difficulty

Level
Intermediate
Time
An hour with the sketch and the tubing's inside diameter
Skills
Using velocity as a check, then looking at slope

Cost estimate

Free

The arithmetic is free. Buying the next size up in copper is often cheaper than buying a larger pump to fight a small pipe.

Materials

ItemQtyNotes
The flow you intendGallons per minuteFrom the collector maker or from a conservative gallon or so per collector, checked against the pump curve.
Inside diameterInchesFrom the tubing chart for the type you will buy. Nominal size is the label, not the hole.
A slope diagramIf the loop must drainEvery foot of outdoor pipe marked downhill to the reservoir.

Velocity

Feet per second is about 0.408 times the gallons per minute, divided by the inside diameter in inches squared. The sketch does that. Land in the 2 to 4 foot per second band for a closed loop and you are in ordinary hydronic practice. Under about 1.5 feet per second, air can sit in a high spot and the loop goes quiet in the bad way. Over about 5 feet per second, expect noise and a friction head that may surprise the pump.

Velocity is not the whole friction loss. Length and fittings add head. A short loop of the right diameter is easier than a long loop of a generous diameter. Count elbows, since each one adds head.

Pipe velocity sketch

Feet per second is about 0.408 times gallons per minute, divided by the inside diameter in inches squared. Use the inside diameter, not the nominal size. A quiet closed loop often lands near 2 to 4 feet per second. Drainback still has to slope and drain. Velocity does not replace that.

2.6 ft/s

This sits in the usual planning band for a closed hydronic loop.

Slow pipe and fast pipeThe same flow in a large pipe moves slowly. The same flow in a small pipe moves fast, with more noise and more friction.Wide, slowNarrow, fastAim near 2 to 4 feet per second in a closed loop.Drainback pipes still have to slope, whatever the velocity.
The same flow is slow in a wide pipe and fast in a narrow one. Drainback still has to slope either way.

Drainback and materials

A drainback pipe can be in the right velocity band when it is full and still fail, because a sag holds a slug of water after the pump stops. That slug freezes. Pitch the pipe continuously. Support it so it cannot sag when it is warm and full. Do not add a check valve that holds a column up in the collector.

Outdoor insulation has to survive sun and birds, with a jacket, and it has to be thick enough that the pipe is not a radiator. Insulation does not fix a missing slope.

Copper handles high stagnation temperatures better than ordinary PEX. PEX is easier to run and has a temperature and pressure rating you must compare with the hottest the loop can get when the pump is off. A stagnating collector can exceed what ordinary PEX is listed for. Use PEX where the temperature will stay inside its rating, and do not use it as a way to ignore stagnation. Oxygen-barrier tubing matters if the water also touches steel or cast iron.

  1. Look up the inside diameter

    For the exact tube type. Put that number in the sketch, not the nominal size.

  2. Check velocity at the flow you will really have

    Adjust diameter until you are near 2 to 4 feet per second, then think about length and elbows.

  3. Draw the drain if it must drain

    Continuous slope, supports, no check valve holding water in the collector.

  4. Match the material to stagnation

    If the pump-off temperature is unknown, find it out before you choose PEX.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.