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DIY Solar Book, home

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

Combined Hot-Water + Space-Heating System Overview

How one collector loop can serve a tank that feeds both taps and a heating zone, and where that gets complicated.

Difficulty
Advanced
Cost
Planning only
published

A combined system uses one solar collector loop to heat a storage tank, then spends that heat two ways: domestic hot water and space heat. The attraction is one set of collectors instead of two. The trap is that the two loads do not exist in the same season. Space heat wants a large array in January. Hot water in July is a small load under a large array, and that is when tanks boil and glycol breaks down.

This page covers whether to attempt that combination and leaves the build to other pages. The layout choice still decides freeze protection. The tank, the floor loop, and the backup pages are the pieces. If you have not built a small air heater or a simple water preheater, this is the wrong first project. The flowchart says so too.

Difficulty

Level
Advanced
Time
Planning only, then a permitted plumbing and heating project
Skills
Separating potable water from a heating loop, and being honest about summer

Cost estimate

Planning only

A combined system is collectors, a tank, pumps, heat exchangers, controls, a backup heater, and usually a permit. Parts often land in the thousands of dollars before anyone is paid to install them. Price the safety parts first.

Materials

ItemQtyNotes
A load splitHot water versus space heatFrom a year of bills, not from the size of the roof.
A freeze answer1Direct potable water in the collectors is off the list if the site freezes.
A summer plan1Cover the collectors, dump heat somewhere safe, or size the array for hot water and accept a modest space-heat share.

A layout that can be explained

Collectors heat a storage tank through a collector loop. That loop is drainback or closed-loop glycol if the climate freezes. The tank is not automatically drinking water. Potable water picks up heat in a heat exchanger, double-walled if the code requires it because the other fluid is a chemical or is not potable. Space heat leaves the tank through a different loop, often to a radiant floor, with a mixing valve so the floor does not see the hottest water the tank can make.

Set a priority. If the floor takes heat whenever it asks, a cloudy week gives you a warm slab and a cold shower. Heat the domestic water first, or give it a dedicated top of the tank it can finish with the backup heater. The backup page shows how the boiler or the electric element carries January without fighting the sun.

Design for summer

In summer the space-heat loop should be idle. The collectors still make heat whenever the sun is out. A tank sized for several cold days of space heat is far larger than a hot-water tank, and it will reach its high limit and then stagnate the collectors. Stagnation cooks glycol and stresses fittings. The workable choices are a smaller array that covers hot water and only a fraction of space heat, a heat dump you have actually built (a radiator or a use that can take summer heat), or covers you will put on. "The system will just shut off" means the collectors sit full of hot fluid. That is a plan only if the fluid and the piping are rated for it.

One tank, two loadsA collector heats a storage tank. A heat exchanger supplies domestic hot water. A separate loop supplies space heat. Summer has almost no space-heat load, so the array can overheat.CollectorTankTapsFloor loopSize for the summer load, or plan a way to dump heat.
Collectors feed a tank. Taps and a floor loop draw from that tank on separate paths. Summer still arrives.

What this is not

It is not a batch heater with a radiator added, and it is no reason to run potable water through a floor. It does not replace air sealing either: on a leaky house, a combined system is a large way to heat the attic.

  1. Write the two loads from the bills

    Hot water is closer to the summer fuel baseline. Space heat is the winter remainder.

  2. Pick freeze protection before collectors

    Drainback if the pipes can slope. Closed loop if they cannot. Direct only if it never freezes.

  3. Decide the summer rule

    Smaller array, a real heat dump, or covers. Write it on the sketch.

  4. Separate potable water

    Heat exchanger of the type the code accepts, mixing valve, relief valves. Then a backup that can finish a shower.

  5. Stop if the sketch needs a skill you do not have

    Plumbing, controls, and a floor loop are allowed to be hired work.

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.