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

Radiant Floor Loops from Solar Heat

What a floor loop needs from a solar tank, and why water temperature limits the design.

Difficulty: Advanced
Cost: Varies

published

A radiant floor and a solar tank are a good match only after you respect the floor's temperature limit. Solar storage can sit well above 120 degrees F on a clear afternoon. Many floors are comfortable with supply water far cooler than that, and wood floors can be damaged by water that is merely "pretty hot." The loop needs a mixing valve, tubing that the rest of the system will not corrode, and a backup for the weeks the tank is cold.

This page assumes the heat is coming from the kind of tank in the storage guide or from a closed heating loop, not from drinking water run through the slab. The backup guide is how the boiler carries the cold start. The combined overview is the warning about summer, when this loop should be idle and the collectors still make heat.

Difficulty

Level: Advanced
Time: A floor is a construction project, not a weekend fitting
Skills: Following tubing limits and not overheating a floor

Cost estimate

Varies

Tubing, a manifold, a mixing valve, and a circulator are the mechanical parts. A new slab or a staple-up under an existing floor is the construction cost, and it dominates. Drilling a finished slab to snake tubes is usually the wrong plan.

Materials

Item	Qty	Notes
Oxygen-barrier PEX, if the loop meets ferrous parts	Per the layout	Non-barrier tubing in a system with steel or cast iron lets oxygen in and rusts those parts.
A mixing valve	1	Caps the temperature sent to the floor, using cooler return water.
Manifolds and a way to balance loops	1 set	Loops of similar length. Follow the tubing maker on maximum length.
Insulation under the tubes	On a slab or a staple-up	Heat that leaves downward or into a crawlspace is not room heat.
A floor-temperature limit you can name	From the flooring	Wood and some coverings have a maximum surface temperature. The water has to respect it.

Temperature

A slab with insulation under it can heat a room with relatively cool water, often well below the temperature a solar tank reaches. A staple-up under a wood subfloor has more resistance in the path, so it wants warmer water and still may not match a slab. Either way, put a mixing valve between the hot tank and the floor supply. Set it for the floor, not for the maximum the tank can do. Measure the supply water and, if the floor is wood, the surface on the first hard sunny day.

Do not use the tank's hottest outlet directly into tubing because the pump is simpler without a valve. The valve is the protection for the floor and for bare feet.

Tubing and layout

Keep loops similar in length so one room does not steal all the flow. The tubing maker publishes a maximum length and a minimum bend radius. Follow those. A loop that is too long becomes a pressure drop the small circulator cannot move. Space the tubes evenly. Insulate under them and at the edges of a slab so you are not heating the earth as the main product.

Oxygen-barrier PEX is the default when a steel boiler, a cast-iron pump, or a panel radiator shares the water. If the entire loop is non-ferrous and truly separate, the barrier matters less, and you should still not casually add a steel part later.

An existing finished slab is a bad retrofit. You will not drill a useful coil into it. Staple-up from below, or a new pour, or a different emitter, are the real options. Sleeping people and furniture go on top of this loop. Access later means opening the floor. Photograph the layout before you cover it.

Tank water that may be too hot is mixed down before it enters the floor. The floor has a limit even when the tank does not.

What solar will not cover

Mornings after a cloudy day, and the coldest week of the year, need the backup. A floor has some mass, which helps for a few hours and does not help for a week. Size the backup as if the solar tank were merely preheating, because some weeks it will be.

In summer, turn the floor loop off. A solar tank that is also trying not to overheat does not need a floor that nobody wants heated in July. The excess heat is the combined-system problem, not a reason to warm the slab.

Name the floor limit
From the flooring and the tubing instructions. Set the mixing valve from that number.
Choose a build you can actually do
New slab or staple-up with insulation. Not a drilled finished slab.
Lay equal loops and a mixing valve
Barrier tubing if any steel shares the water. Photograph the paths.
Connect backup and test with a thermometer
Supply water at the valve's setting, surface not painfully hot, backup able to run when the tank is cold.

Difficulty

Cost estimate

Materials

Temperature

Tubing and layout

What solar will not cover

Name the floor limit

Choose a build you can actually do

Lay equal loops and a mixing valve

Connect backup and test with a thermometer

Further reading