Hot Water Boost Applications

Fossil fuel is only used when the renewable thermal energy store is below its set-point, thus reducing the dependency on fossil fuel and reducing CO2 emissions. Rinnai units can modualte from 54kW to 2.27kW optimising gas usage in realtion to energy neeeded.

Rinnai can provide solar thermal systems with either flat plate or evacuated tube collectors and solar cylinders.

These products are also suitable for installation with ground source and air source heat pumps.
For more information please telephone:

01928 531870


Rinnai offers both flat plate and evacuated tube collectors, each with specific benefits to suit your application. Either option will provide years of low maintenance, low cost energy to heat your water provided the units are correctly sized and installed.


A solar collector works by transferring the energy contained in the solar irradiation into heat energy. There are two principle designs of solar collector; the Evacuated Tube Collector and the Flat-Plate Collector. Both these types of collector integrate in a solar circuit in the same way but their designs are radically different.

These are the benefits to both the domestic and commercial user:

  • Uses clean renewable energy source
  • Reduces your fossil fuel consumption and hence CO2 emissions
  • Save money on fuel that you would have otherwise needed to purchase
  • Enhance the life of your HE water heater by reducing the amount of switch-on / offs
  • Low maintenance technology
  • Integrates with existing heating or other renewable sources
  • Solar Thermal is backed by Government incentives to reward consumers who invest in green technology
  • Low cost and simple way for you or your company to contribute towards tackling climate change

The amount of money and CO2 that you save will be dependent upon your consumption, the fossil fuel type that you are replacing, and the energy efficiencies of both your building and boiler.

An Evacuated Tube Heat Pipe Collector is made up of a series of glass tubes containing heat pipes, that "plug-in" to a manifold. Solar irradiation is received by the tube and absorbed via a coating on the inside of the glass, this coating becomes hot and transfers heat energy through an aluminium foil to the copper heat pipe. Within the copper pipe there is an aqueous solution held in a vacuum that is allowed to rapidly boil and condense in the heat tip. The energy generated in the heat tip is then transferred to the solar circuit via the collector manifold.

A Flat Plate Collector has a large glass cover, an aluminium absorber plate and a copper pipe register. The higher performance and quality flat plates use anti-reflective glass to allow higher levels of irradiation to be absorbed. The absorber plate is heated and transfers energy to the copper pipe register attached to the back of it, fluid from the solar circuit is heated by circulating it through the copper pipes.


Evacuated tube collectors Flat plate collectors
Height (mm) 1980 2039
Width (mm) 2460 1139
Total area (m²) 4.87 2.32
Weight (kg) 96 44.4
Number of
heat pipes
30 -
Absorber area (m²) 2.42 2.14
>93% 95%
Emission coefficient <6% 5%
temperature (°C)
192 209
Maximum operating
pressure (bar)
10 6
Tube material Borosilicate glass -
Hail resistance >25mm hailstones -
Outer tube
diameter (mm)
58 -
Outer tube
thickness (mm)
1.5 -
Fluid content (ltr) - 1.2
Material coating Selective absorber Copper/Sunselect

The performance of a flat plate collector is close to that of an evacuated tube collector in the summer months when facing due south.

Evacuated tube collectors have better overall results in both early and late season and they are not affected by adverse weather conditions and even produce impressive temperatures on cloudy days.

The flat plate collectors are exceptionally sturdy and durable, perhaps more suited to vulnerable locations.


This information is intended as a guide only, it does not imply compliance with water or gas installation regulations.
Components will vary depending on the actual installation.
Check local regulations before installation.