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Heating is an intensive process, and is costly for tunnel owners. Something that should not increase your electricity bill, can essentially double it if you select traditional heating methods.

Here are some alternatives:

1) Gas Water Heaters

2) Heat Pump Heaters

3) Solar Water Heaters

1)  Gas Water Heaters

Gas-fired water heaters remain the most popular system for heating large volumes of water. They may not be the most energy-efficient option but are used prolifically in areas with little solar or other obvious energy sources.

Gas water heaters use either natural gas or propane. Generally used in swimming pool heating, a pump circulates the pool's water whilst the water drawn from the pool passes through a filter and then to the heater. The gas burns in the heater's combustion chamber, generating heat that transfers to the water that's returned to the pool.

They're most efficient when heating large volumes of water for short periods of time, and they're ideal for quickly heating pools but I don't think it is appropriate in cases where sustained heating is required, especially through the winter. So unless you want a fast acting, quick control system gas heaters will be a costly pseudo-sauna system. On the up side, unlike heat pumps and solar water heaters, gas pool heaters can easily maintain any desired temperature regardless of the weather or climate regardless of system design. In other words it’s a plug and play system, buy the required size and heat.

The system designer would need to visit the site to make a size recommendation, and the cost would be based on the efficiency of the heater you select. Gas pool heaters typically last five or more years.

2) Heat Pump Heaters

Heat pumps use electricity to capture heat and move it from one place to another. They don't generate heat. As the pump circulates water in the system, the water passes through the heat pump heater. The heat pump heater has a fan that draws in the outside air and directs it over the evaporator coil. Liquid refrigerant within the evaporator coil absorbs the heat from the outside air and becomes a gas. The warm gas in the coil then passes through the compressor. The compressor increases the heat, creating a very hot gas that then passes through the condenser. The condenser transfers the heat from the hot gas to the cooler water circulating through the heater. The heated water then passes through the system in a continuously heated closed system. The hot gas, as it flows through the condenser coil, returns to liquid form and back to the evaporator, where the whole process begins again.

Heat pump heaters work efficiently as long as the outside temperature remains above the 10ºC. The cooler the outside air they draw in, the more energy they use, so if you require the heating in the coldest months, it will end up increasing your operating costs and installation is costly. Heat pump heaters can last 10 or more years.

3) Solar water heaters

They're cost competitive with both gas and heat pump heater’s installation cost, and they have very low annual operating costs. Actually, solar pool heating is the most cost-effective use of solar energy in many climates.

Most solar heating systems include a solar collector and a pump. Water is pumped through the solar collector, where it is heated before it is returned to the system. Each day the water will temperature will be increased by around 10ºC (a std 9m² black piping collector will have this impact on a closed system) if mounted to the winter angle of the sun. At night temperature will be maintained by a specialised mounting system, specifically a ground mounting system. If higher temperatures are required a flat panel solar heater can be installed with tanks.

Some systems include sensors and an automatic or manual valve to divert water through the collector when the collector temperature is sufficiently greater than the water system temperature.

A solar heating system has a payback of between 1.5 and 3 years and last 10-20 years, depending on your water salinity, local fuel costs and operating hours. They also typically last longer than gas and heat pump pool heaters. Your actual cost and payback depend on many factors. Installation requires a site evaluation and system design.

Contact a consultant in your area for a site overview which will indicate the availability of resources in your area, they should also be able to assist with any issues with terrain and topography. Once your options are outlined, you will be able to commission a system design, systems can vary by designer but ultimately they should recommend a system that does not increase your operating costs on a day to day basis.

Alternatively, buy that mini-weather station you’ve always wanted an excuse to buy. Link it up to your computer and plot the solar radiation, angle, temperature, wind direction and velocity.