Solar backup

Solar Photovoltaic Backups

This page deals with gas/electric backup water heaters for solar systems. It doesn't deal with photovoltaic backup systems (a set of batteries to store electricity for non-sunny periods). See: Solar Photovoltaic and Wind Backup Systems.

Solar Backup Designs (Water heating)

It's easy to integrate a solar water heating system with a boiler or a gas water heater - as long as the gas (or electric) heater system is compatible with the solar elements.

Look for models and manufacturers offering gas units specifically designed as a backup to preheated water produced by solar systems. The integration requires adequate water pressure and controls to switch on/off the backup system.

Check the issue. Some manufacturers are now offering gas (and electric) water heaters designed to work with solar...

The solar water heating solar design

There are different solar water heating designs: solar thermosyphon, solar drainback, glycol, active solar (opened loop and closed loop). They all can be used with backup systems... See: Solar designs.

The integration rule is always the same. The water is preheated in the solar system, and driven to a standard gas water tank (in the two-tank system) or to a tankless water heater, turning on the gas backup system if the temperatures of the preheated water are not warm enough...

If the temperatures of the preheated water are sufficiently high, the backup system remains off.

Two-Tank solar backup system

The two-tank solar backup design is particularly common. In this case the solar system has a tank of its own, connected to the storage tank of the gas heater.

The integration is simple, as depicted in the diagram. The gas water heater (or the boiler) should be able to monitor the water temperature and to adapt its flame with the temperature of the water coming from the solar sub-system.

The basic components of this design are simple, relatively inexpensive and with low maintenance. The capacity of the system (in gallons of hot water) is high. A disadvantage of this design is in the fact that it involves two tanks and consequently more space..

Image: One of several designs and possible combinations of a solar system with a gas system (from Southwest Gas Corporation).

Solar Backup system with a tankless water heater

Your solar backup system can be a tankless gas water heater system...

Several manufacturers are now offering tankless gas water heaters (Bosh AquaStar Model 1600PS, Rheem, etc...) specifically designed as a backup to solar preheated water.

In this system, the preheated water (coming from the solar storage tank) is directly driven to the tankless gas heater, switching it on only when the water isn't hot enough.

A third Solar Backup System: all-In-One-Tank solution

A third solar backup design involves an all-in-one-tank system, with a single unit for both solar storage and backup features.

This system has an high-efficient burner in the middle of the tank, ready to raise the water warmed by the solar system, whenever necessary...

This design is very energy efficient, and the capacity of the burner makes the system able to feed the requirements of hydronic space heating (baseboards, radiators...). It goes beyond the usual supplying of hot water to our kitchens and bathrooms.

But, on the other hand, is is more complex to install, more expensive, and limited to some few manufacturers and their know-how (the 3rd diagram in this page depicts a HTP/Phoenix system).

Manufacturers offering solar backup

Here it is a very incomplete list of manufacturers with water heaters designed to backup solar systems:

Atmos, Ariston, Bosch, Vaillant, Worcester/Bosh, Macdonald Engineers, Mts Group, Newark Copper Cylinder, Rheem, Solar Saving, HTP/Phoenix.