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

Choosing a Solar Hot-Water System: Drainback vs. Closed-Loop vs. Direct

A comparison of the three common pumped layouts, aimed at freeze risk and maintenance.

Difficulty: Intermediate
Cost: Planning only

published

If you want more hot water than a single tank in a box, you are choosing a pumped system. The three layouts people actually maintain are direct, drainback, and closed loop. They move heat from a roof or a ground rack to a tank. They fail in different ways. The right choice is the one whose failure mode matches your climate and the amount of attention you will give it.

Use this page to decide which loop you can build before you build any collector. A direct loop in a town that freezes will burst the collector. A closed loop with no plan for stagnant, overheated glycol is a slow corrosion problem. A drainback system with a pipe that traps water will freeze in that pipe the way a direct system does.

If your nights never freeze and your household is small, the batch heater is still the simpler machine. Start there unless you already know you need more area than a tank-in-a-box can carry.

Difficulty

Level: Intermediate
Time: An evening of decisions, before any pipe is bought
Skills: Reading a roof pitch and being honest about maintenance

Cost estimate

Planning only

Parts for a small pumped system often run from about $1,500 to $4,000 before a new tank, racking, and any electrical work, and local prices move that range. Installed work costs more. The point of this page is to avoid buying the wrong loop.

Materials

Item	Qty	Notes
A year of hot-water fuel use	From the bills	So you know whether the prize is large enough to justify a pump and a roof.
Site sketch	1	From the site survey: sun, freeze climate, and a path for pipes that can slope if you choose drainback.
This comparison	1	Write down the layout you rejected and why, so a sale flyer does not restart the decision.

Direct: potable water in the collector

A direct system pumps household water up through the collector and back to the tank. There is no antifreeze and often no heat exchanger. It is the shortest parts list.

It has nowhere to hide from a freeze. Water in a flat-plate collector or in roof piping will burst the passages when it freezes. A controller that is supposed to circulate and "steal" a little heat from the tank on cold nights fails when the pump fails, the power fails, or the sensor is wrong. That is not freeze protection you can trust for a house you are away from.

Direct can be reasonable where freezing weather does not occur, the water is not extremely hard, and you still provide a mixing valve and a relief valve. Scaling is the maintenance you take on instead of freeze. Minerals plate out on the hottest surfaces, which are inside the collector.

Drainback: the collector empties itself

In a drainback system the collector loop holds ordinary water, often distilled, and is not full when the pump is off. The pump lifts water into the collector only while there is useful sun. When the pump stops, every drop drains back to a reservoir that sits in a conditioned space. Air replaces the water in the collector and in the outdoor pipe. A freeze then finds an empty collector.

The heat usually crosses into the potable tank through a heat exchanger, so the roof loop is not drinking water. That also means a leak on the roof is not a leak of chlorinated city water into the ceiling, though it is still a leak.

Drainback works only if the plumbing drains completely. Every outdoor pipe slopes back to the reservoir. There is no trap, no backward check valve that holds a column of water, and no sag that forms after the pipe heats and softens. The pump has to be sized for the lift from the reservoir to the top of the collector, not merely for a circulation around a full loop. The reservoir has to sit where a spill is annoying rather than destructive, and it must not itself freeze.

This is the layout to prefer in a freezing climate when you can see a drain path and you are willing to prove it with a hose before you call the job finished.

Closed loop: antifreeze under pressure

A closed loop fills the collector and the piping with an antifreeze solution, keeps that loop under pressure, and transfers heat through a heat exchanger to the potable tank. It does not have to drain. That is why it survives a roof that cannot be piped on a continuous slope, and why it is the usual commercial package.

The fluid is the maintenance. Propylene glycol is the type used when a leak could reach drinking water. Ethylene glycol is toxic and does not belong in a homemade domestic system. Even propylene glycol is not an excuse to skip the heat exchanger. Plumbing codes commonly want a double-wall exchanger, or another approved separation, between a chemical loop and potable water. Ask the local code official which separation they accept. Do not assume a single-wall coil in the tank is allowed.

Glycol that stagnates on a hot roof, with the pump off and no load, overheats. It turns acidic and then attacks metal. A closed loop needs an expansion tank, a pressure-relief valve with a safe discharge, a way to measure pressure and pH, and a plan for summer when the household is on vacation and the tank is already hot. Some systems dump a little heat to a radiator or cover the collectors. "The glycol can take it" is not a plan.

Direct puts potable water in the collector. Drainback empties the collector whenever the pump stops. Closed loop keeps glycol in a pressurized circuit and needs an exchanger and an expansion tank.

A short way to decide

Answer these in order.

Does the site freeze? If no, direct or a batch box can be on the list. If yes, direct is off the list.
Can every pipe from the collector fall, without a trap, to a reservoir indoors? If yes, drainback is the freezing-climate default. If no, drainback is off the list.
Will you check fluid condition a few times a year and replace it when it is spent? If yes, a closed loop can serve a roof that will not drain. If no, do not take on a closed loop.
Can you mount the collectors lower than the tank and accept thermosiphon, with no pump? That is a different layout, and it still needs freeze protection. It is not a shortcut around question 1.

If the answers remove every pumped option, the sensible project is conservation: less hot water used, a newer efficient backup heater, and pipe insulation. A collector that does not match the climate is not a project you have merely postponed.

What all three still require

A mixing valve on the potable outlet, because any of these can deliver scalding water.
Temperature-and-pressure relief on the potable tank, and pressure relief on any pressurized collector loop, each with a discharge that cannot scald someone.
A controller that does not run the pump at night and that fails toward "pump off" rather than "pump on into a frozen or stagnant collector," as far as the hardware allows.
Pipe insulation outdoors that survives sun and birds, and a roof penetration flashed as a roof, not as a hole with caulk.
A permit where the town requires one for plumbing and for any electrical circuit to the pump. Pumps are line-voltage devices. The same rule as the wall collector applies: listed parts, a proper circuit, or an electrician.

None of the three is a substitute for the backup heater. Size the solar side as a preheater that covers a share of the load on a sunny day. A system drawn to cover the coldest week of the year with no backup will stagnate all summer.

Write the freeze answer first
Use the coldest night you have actually seen, not the average. If water has frozen in a hose bib, the site freezes.
Walk the pipe path
From the collector location to a possible reservoir, mark every low spot. If you cannot keep a continuous downward slope, say so on the sketch.
Cross off a layout
Use the four questions. Leave with one layout, or with a decision to stay on conservation.
List the safety parts in the budget
Mixing valve, relief valves, expansion tank if the loop or the potable side is closed, heat exchanger of the type the code accepts, and the electrical circuit. If the budget cannot cover these parts, it will not cover the collector either.
Only then read a construction guide
Build the layout you chose. Do not blend drainback and a check valve from a closed-loop drawing.

Freeze, scald, toxic fluid, and pressure

A direct collector in a freezing climate bursts. The break may be inside a roof box where you notice it as a ceiling stain in March. Do not rely on a pump as freeze protection.

Drainback that holds a pocket of water will burst in that pocket. Prove the drain with water before the heating season. Do not add a check valve that defeats the drain.

Glycol loops overheat and the fluid goes bad. Ethylene glycol is poisonous. Use propylene glycol where any chemical loop is in a house, and still separate it from potable water the way the plumbing code requires. A relief valve on a glycol loop can discharge hot chemical. Pipe that discharge to a safe container, not onto a walkway and not into a drain that is not allowed to receive it.

All three layouts can scald. The mixing valve is part of the system, not an upgrade.

Roof work, potable plumbing, and pump wiring each have permit and licensing rules. This comparison is not a license to open a service panel or to skip an inspection.

Difficulty

Cost estimate

Materials

Direct: potable water in the collector

Drainback: the collector empties itself

Closed loop: antifreeze under pressure

A short way to decide

What all three still require

Write the freeze answer first

Walk the pipe path

Cross off a layout

List the safety parts in the budget

Only then read a construction guide

Further reading