How Solar and Gas Can Work Together for Reliable Hot Water

Solar water heating is most productive when sunshine is available. Hot-water demand, however, does not always follow the weather.

A family may need early-morning showers after several cloudy days. A lodge may experience an unexpected increase in guests. A home may simply use more hot water than usual.

One way to provide dependable backup without using a large battery bank is to combine solar water heating with an instantaneous gas water heater.

The idea is straightforward: use the available solar-heated water first, then use gas only when additional heating is required. Achieving this safely and effectively depends on the selected equipment, plumbing arrangement, temperature control and professional installation.

Two separate systems working as one strategy

Customers sometimes ask for a single appliance that combines a solar geyser and gas heater. In practice, it is often more useful to think of these as separate systems with coordinated roles.

The solar system can be:

• A PV geyser conversion using dedicated panels and a controller
• A solar thermal system using a collector and storage cylinder

The gas system is usually an instantaneous water heater that heats water as it flows through the appliance.

The combined arrangement must ensure that water is sent to the gas heater only in a manner the appliance is designed to accept. The temperature and flow of the incoming water matter.

The role of a solar-to-gas valve

A purpose-designed solar-to-gas valve can form part of the control arrangement. The Dewhot Solar to Gas Valve, for example, is listed for use with solar water-heating systems and is designed to divert water to the gas heater when required.

This is not a universal compatibility statement. The valve, gas heater, solar system and plumbing configuration must be selected as a complete arrangement. Always follow the relevant product instructions and the installer’s design.

Why not simply use the gas heater all the time?

An instantaneous gas heater can be a practical primary water-heating option, particularly in off-grid properties or where electrical supply is constrained. Combining it with solar can reduce the amount of gas required when solar energy has already heated the water sufficiently.

The value of the combination depends on:

• Solar resource and system sizing
• Household hot-water demand
• Gas cost and availability
• Frequency of backup use
• Temperature settings
• Pipe distances and heat loss
• The efficiency and suitability of the selected appliances

The goal should be a reliable system with a clear operating logic, not simply the maximum number of energy sources.

Choosing the gas water heater

Gas water heaters are commonly described by their litre-per-minute rating. This is a flow rating, not the amount of stored hot water.

The correct size depends on several factors:

• Number and type of outlets
• Whether outlets may be used simultaneously
• Required temperature rise
• Incoming-water temperature
• Available water pressure and flow
• Gas type and supply arrangement
• Indoor or outdoor installation requirements

A heater that suits one shower may not suit two simultaneous showers. Likewise, a larger litre rating does not remove the need to confirm water pressure, gas supply and installation conditions.

Sustainable’s gas water-heater range includes standard- and low-pressure options for different applications. Product selection must be based on the actual site.

Incoming solar-heated water needs special attention

Not every gas water heater is intended to accept pre-heated inlet water. Water arriving from a solar system may already be warm—or occasionally hotter than the temperature required at the tap.

The selected gas heater and control arrangement must manage this correctly. Confirm:

• Permitted inlet-water temperature
• Minimum flow and pressure requirements
• Temperature-control behaviour
• Whether the appliance modulates appropriately
• Required valves and mixing controls
• Manufacturer-approved installation arrangement

Do not assume that any gas geyser can simply be connected downstream of any solar geyser.

Indoor and outdoor installation

Gas appliances must be installed in the location and manner permitted by their product specification and applicable requirements.

An outdoor model may require suitable weather protection. An indoor model may require specific flue and ventilation arrangements. The gas cylinder or supply, regulator, pipework, isolation and combustion-air requirements must all be handled by the appropriate qualified professionals.

Sustainable supplies gas water heaters and gas water-heater accessories. The complete installation should be assessed and commissioned by a registered gas installer, with other qualified trades involved where electrical, plumbing or roof work is required.

Where solar and gas can work particularly well

A combined strategy may be attractive for:

• Homes wanting hot-water backup without heating from batteries
• Off-grid houses with limited stored electrical energy
• Farms and lodges with variable hot-water demand
• Households in areas with frequent electrical interruptions
• Properties that already have a suitable solar water-heating system
• Projects where LPG is readily available and safely accommodated

It may be less attractive where gas logistics are difficult, the appliance cannot accept the available inlet conditions, or the installation becomes unnecessarily complex.

Questions to answer before requesting a quotation

Provide the following information:

• Existing solar water-heating type and model
• Geyser capacity
• Existing or proposed gas-heater model
• Number of bathrooms and outlets
• Simultaneous-use requirement
• Water pressure and source
• Indoor or outdoor installation location
• Distance between the geyser, gas heater and outlets
• City or town
• LPG or natural-gas requirement where applicable

Photographs and a simple description of the plumbing layout can help the sales and installation teams understand the project, although they do not replace a site assessment.

Solar first, backup second

The most useful design principle is simple: let the solar system perform as much of the water heating as practical, then use the backup source deliberately.

That can improve reliability without automatically placing the geyser on a large inverter and battery system. It also gives the household a clearer understanding of where its hot water comes from and what happens during poor solar conditions.

In Part 5: How to Choose the Right Hot-Water System for Your Home, we bring the options together in a step-by-step decision guide.

Plan a solar-and-gas hot-water system

Explore Sustainable’s gas water heaters, solar water-heating kits and the Dewhot Solar to Gas Valve.

Contact our sales team with your household, water-pressure, solar and installation details before selecting equipment.