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

Low-Cost PEX Collector Options (and Their Limits)

Where plastic tubing is reasonable in a collector and where temperature will end the experiment.

Difficulty: Intermediate
Cost: $80–200

published

PEX is excellent pipe in a wall, at domestic temperatures, inside a house. A glazed solar collector with the pump off is not that place. Stagnation in a dry glazed box can run past 300 degrees F. Ordinary PEX is often rated near 180 degrees F at a stated pressure, with the pressure rating falling as the temperature rises. Above the rating, the tubing softens, sags out of its clips, and can burst or pull off a fitting. The layouts that work keep the tubing under that limit.

The experiments that survive are the ones where the plastic never gets much hotter than the water it is heating. An unglazed coil on a pool, a coil submerged in a tank, or a drainback coil that is either flowing or empty and is not baked inside a hot glazed box. A black PEX grid behind a pane of polycarbonate, full of water, pump off, is how people end a heating season in one afternoon.

If you need a glazed domestic collector, build it in copper or buy a listed one.

Difficulty

Level: Intermediate
Time: A weekend for a small coil, if the temperature limit is already decided
Skills: PEX fittings and the discipline to throw the design out when stagnation shows up

Cost estimate

$80–200

A coil of tubing, fittings, and clips for a low-temperature experiment. This is not the budget for a house hot-water system. UV-safe covering and a real drain path are part of the bill.

Piece	Typical cost
PEX and fittings for a small coil	$50–120
Clips, a rack, UV protection	$30–80

Materials

Item	Qty	Notes
PEX rated for the temperature you will actually hit	A coil, not a guess	Read the printed rating. Do not average it with a forum post.
Oxygen-barrier tubing	If any steel or cast iron is in the loop	Ordinary PEX lets oxygen in. Ferrous parts rust. A stainless exchanger is the other way to isolate them.
Fittings listed for that tubing	As required	A fitting that holds at room temperature can let go when the pipe softens. Follow the tubing maker, not a generic clamp.
UV shelter	1	Bare PEX in the sun gets brittle. Paint is not a UV rating unless the maker says so.
A drain path	1	If a freeze can reach the coil, it has to empty. A trapped low spot is a split waiting for January.

Three layouts that stay under the rating

An unglazed coil heating a pool or a storage tank stays near the water temperature as long as flow is moving and the tubing is not left dry in the sun. Pool mats you buy are usually a purpose-made elastomer, not a roll of plumbing PEX. If you use plumbing PEX outdoors, shelter it from ultraviolet light and from teeth. Livestock will chew a line that crosses a pen. Pool chemistry and some PEX do not agree. Read the tubing sheet before you fill a chlorinated loop with it.

A coil submerged in a tank, with the tank itself in the sun or in a simple glazed box, is a different machine. The plastic is surrounded by water, so it cannot run away to stagnation temperature unless the tank boils dry. Keep the water covering the coil. This is a batch-style idea and it belongs in a climate, or a season, where the tank will not freeze. It is not a roof collector.

A drainback loop can use plastic only on the indoor run, where a stop empties the hot collector and the indoor pipe never sees stagnation. Using PEX as the absorber inside the glazed collector is the mistake. Copper or a listed absorber goes in the box. Plastic, if you use it, stays where a failure dumps water on a floor you can see, and only if the rest of the design still drains.

Cool unglazed service on one side. A glazed stagnant box on the other, which is outside the rating of ordinary PEX.

What failure looks like

The tubing ovalizes, the clips let it droop onto the glazing, a fitting weeps, or a run that used to drain now holds a belly of water. Any of those means the experiment is over. Do not "see if it holds" through another sunny week. Replace the layout with copper or take the glazing off and accept a lower temperature.

Closed heating loops that include boilers or cast-iron radiators need oxygen-barrier PEX or a heat exchanger that keeps the plastic loop's oxygen away from the iron. This is the same rule as any hydronic retrofit.

Do not use PEX for a potable thermosiphon that sits full in the sun. The thermosiphon page assumes a layout that can stand the temperature and the code, and plastic that softens cannot.

Write down the hottest temperature the tube will see
Flowing pool water is one number. A glazed absorber with the pump off is another. If you cannot name the second number, do not glaze the coil.
Pick the tubing and the fitting as a pair
Rating, oxygen barrier if iron is involved, and UV protection. Clips that still hold if the pipe warms to its rating.
Give it a drain and a place to leak safely
Continuous slope where a freeze is possible. Indoors, a leak should be visible and should not land on a ceiling below.
Run it only while the temperature story stays true
Submerged coils stay submerged. Unglazed coils get flow or get drained. No "just this once" stagnation test behind glass.
Stop at the first sag or weep
The tubing has reached its rating. Move the job to copper or to a listed collector.

Difficulty

Cost estimate

Materials

Three layouts that stay under the rating

What failure looks like

Write down the hottest temperature the tube will see

Pick the tubing and the fitting as a pair

Give it a drain and a place to leak safely

Run it only while the temperature story stays true

Stop at the first sag or weep

Further reading