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

Freeze Protection for Part-Time Homes

Drainback, antifreeze, and shutdown habits for a house that sits empty in January.

Difficulty
Advanced
Cost
Varies
published

A part-time house fails on the week you are not there. The power goes out, or the pump fails, or a valve sticks, and the night is cold. Any freeze plan that needs a person, a controller, or a running pump is a plan for a house you live in. For a cabin, the protection has to work with the power off and with nobody to open a drain.

The layout choice compares the layouts. A part-time house adds one more test: what still works in an empty house in January, and what still works in July when that same house is empty and the collectors stagnate. A batch heater does not pass the January test. Do not build one here and plan to remember to drain it.

Difficulty

Level
Advanced
Time
Design time, then a drain test you perform before you leave
Skills
Assuming you will not be there when it breaks

Cost estimate

Varies

A drainback loop that truly empties can be the cheaper life-cycle choice. A glycol loop adds fluid, an expansion tank, and replacement fluid after hot summers. A burst collector costs more than either.

Materials

ItemQtyNotes
A protection mode that does not need power1Drain to an indoor tank, or antifreeze that is still in the collector when the pump is dead.
A written drain testBefore the seasonYou watched the collectors empty. A slope on a drawing is not a test.
A summer stagnation plan1Covers, a dump, or fluid and piping rated for the pump-off temperature.

What fails when you are gone

Recirculation from a warm tank. Some controls, when they sense cold at the collector, run the pump to borrow heat from the tank. The collector stays above freezing only while the pump and the power and the sensor all work. A blackout freezes the collector. A cabin in a storm is exactly that case. Do not use this as the only protection.

A manual drain. A valve in the mechanical room works if you are standing in the mechanical room. It does not work in March when you arrive to a split riser. Manual drains are for maintenance, not for vacancy.

A check valve in a drainback line. It holds a column of water up in the collector after the pump stops. That column is a direct system that thinks it is drainback. Leave the path open so air can enter and water can leave.

What can survive

Drainback to a tank that cannot freeze. Collectors and outdoor pipe empty when the pump stops, including when the power stops. The reservoir sits in a part of the building that stays above freezing, or it is protected on purpose. Every pipe slopes. You have tested it. Supports keep the pipe from sagging into a new trap. This is the default for a vacant freeze-prone house if the architecture allows the slope.

A closed glycol loop. The collectors stay full, so a power cut does not freeze them, as long as the mixture is actually strong enough for your lowest temperature. Test the concentration. It weakens when you top up with plain water. Propylene glycol, not ethylene, in a house. A double-wall exchanger where code wants one. Glycol does not survive endless summer stagnation. Acidic, overheated fluid eats metal. Covers or a real heat dump are part of a vacant July too.

Nothing outdoors. The most reliable freeze protection is not having water on the roof. An air collector, used while you are there, does not burst in a January blackout. It is a serious option for a cabin.

What still works when the power failsA pump that was supposed to circulate warm water stops when the power stops. A drainback loop that slopes to an indoor tank can empty instead.Pump protectionStops in a blackoutDrainbackEmpties if it slopes
Pump-based protection stops when the power stops. A drainback loop can empty instead, if it actually slopes to an indoor tank.

Before you leave

Run the drain test or check the glycol with a meter, not with a memory of what you filled last year. Confirm the indoor reservoir or the mechanical room will stay above freezing even if the cabin thermostat is set low. A drainback tank in a room you set to 40 degrees F may itself freeze, and then you have protected nothing. Set the house so the protected volume stays safe, or do not leave water there.

Shut the domestic solar loop down in the way the design allows, so a leak while you are gone is a puddle in the pan and not a well pump running for a week. Tell whoever might visit which valves are already in the safe position.

  1. Reject any plan that needs you or the grid

    If a blackout freezes it, it is not a cabin system.

  2. Pick drainback or glycol

    Drainback only with a tested slope and an unfrozen reservoir. Glycol only with a tested mix and a summer plan.

  3. Walk the pipes

    No traps, no check valve holding a column, supports that cannot sag.

  4. Leave the house in the safe state

    Write the valve positions. Do not trust a reminder on your phone from a highway.

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.