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

Backup Heat Integration

Ways to let a boiler or heat pump carry the coldest weeks without fighting the solar tank.

Difficulty
Advanced
Cost
Planning only
published

Solar heat is a fair-weather contributor. The backup heater is what makes the house and the showers work on a dark week. The integration problem is keeping the two from fighting: the boiler firing to heat water the sun is about to heat, or the sun shut out because the boiler already filled the tank with fuel heat. The usual pattern is series. Solar preheats. The backup finishes only what is still short.

This sits downstream of the combined overview and the floor loop. For domestic water, the waste guide still applies: a backup that stores lukewarm water is not an efficiency win.

Difficulty

Level
Advanced
Time
Planning with the existing heater's manual
Skills
Letting one heater lead and the other follow

Cost estimate

Planning only

Using the heater you already have can cost little more than controls and a mixing valve. Replacing a boiler, or adding an electric element as a habit, is a fuel bill. Price a dark January, not a sunny October.

Materials

ItemQtyNotes
The existing heater's manual1Minimum flow, condensate, and what the control is allowed to do. Do not bypass its safeties.
A tank sensor that means something1Backup enables only when solar storage is below a useful temperature.
A mixing valve on domestic hot water1Solar can overshoot after the backup has already heated the tank.

Put the backup in series

For domestic water, cold water enters the solar tank, leaves warmer, and enters the conventional heater. The conventional heater's thermostat brings the water the rest of the way only when the solar outlet is not already hot enough. On a sunny afternoon the backup should barely run. On a cloudy day it should behave as it did before you added solar. That is the test.

For space heat, the floor or the radiators should take solar-tank heat when the tank is hot enough to be useful, and call the boiler when it is not. A boiler that short-cycles, firing for a minute at a time, wastes fuel and wears out. Give it a real run: a buffer, a control with a minimum runtime, or a thermostat setup that does not chatter. Read the boiler manual before you invent that control.

Do not pipe the boiler so it heats the entire solar storage on every call. You will pay to heat hundreds of gallons the sun might have heated tomorrow, and you will run the collectors against a tank that is already hot, which pushes them into stagnation.

Electric backup

An electric element in a tank is simple and expensive to run. It is a reasonable safety net for a shower. It is a poor way to heat a house through January unless electricity is cheap and you have done the envelope work. If you add an element, it needs the thermostat and the high-limit the tank listing requires, not a bare element on a switch. The PV guide is not a reason to add resistance heat and then try to cover it with modules.

Solar first, backup secondFlow passes through the solar tank and then through a backup heater. The backup runs when the tank is not hot enough, and it does not fight a tank the sun is already heating.Solar tankBackup heaterThe backup heats the rest. Keep the relief valve and the mixing valve.
Water meets the solar tank first and the backup heater second. The backup finishes a shortage. It does not heat the whole store on every call.

What you should notice in the first month

On a clear day, the backup runs less, or runs at a lower duty, and the outlets are still hot enough. On a dark day, the house does not cool off while the control waits for the sun. If the boiler clicks on and off every few minutes, the integration is wrong even if the rooms are warm. If the solar tank is always at the boiler's setpoint by midnight, the boiler is leading and the collectors have little work left.

  1. Put solar upstream

    Preheat, then finish. Write the temperatures at which the backup is allowed to start.

  2. Keep the backup's safeties

    High limits, pressure relief, combustion air, and the vent. None of those are optional for solar.

  3. Stop the boiler from heating the whole store

    Heat the load. Leave the bulk of the tank for the sun when the load is satisfied.

  4. Watch a sunny day and a dark day

    Less backup in the sun. Full comfort without it. No short-cycling.

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.