Hot Water Heating Systems


Pipework located under the bench.

Pipework located under the bench.

The optimum heating system for larger or muti-zoned greenhouses is a Hot Water Heating System and is typically used on glasshouses requiring more than 8kw of heat.  It utilises medium weight finned galvanised steel pipes, a well developed control system, providing accurate temperature settings, even heat distribution and reliable components.  All this together with a quality boiler ensures, this the best way to heat your glasshouse. Our heating engineer is experienced in designing a system that will suit a customers individual requirements.  Contact us with details of your glasshouse, so we can calculate the heat load requirements and provide a quote.

Harrier GD Hot water Systems:

Steel Finned Heating Pipes

Installation of finned pipes on perimeter wall and in a heating duct for a greenhouse.

Installation of finned pipes on perimeter wall and in a heating duct for a greenhouse.

In a greenhouse, the system needs to last in a hot, humid and aggressive environment, so we use high quality galvanised steel pipes, which are ideally suited for this.  They are heavy duty, can be stood on covered in mud and water, and carry a 10 year guarantee.  The pipes are located, to distribute the heat efficiently and evenly and are finned to give the high outputs required.

Pipes are installed around the perimeter of the glasshouse which is where heat is lost, and/or in heating ducts in the floor covered by cast iron grates.  (More information on grates can be found in ‘Greenhouse Fittings’.) They are attached to the wall on galvanised steel arms to give a secure fixing.

Finned pipes located in a planter on the perimeter wall, and in a heating duct under grates.

Finned pipes located in a planter on the perimeter wall, and in a heating duct under grates.

The heating pipes typically run at a temperature of 82 Deg C giving a high heat output, but is variable unlike electric heating.  However, they can run on lower temperatures, if a heat pump is being used.  Other benefits are this method of heating does not spread bacteria like forced air, and its quieter to run.

The length and number of pipes required will be calculated by our heating engineer for each individual project.


Heating power needs to be supplied to the pipes, and can be provided by any source that gives a flow and return of hot water.  Traditionally the water has been heated by high performance boilers, oil or gas fired, to suit customers requirements and preferences.   They are reliable, well made and normally positioned in an adjacent potting shed or boiler house.

They operate at 82 Deg C.  This temperature works well in heating the air around the pipes.  Once the section of the greenhouse has reached its target temperature, the boiler will shut down, thereby saving fuel.

Occasionally an electric boiler is selected by a customer.  They are small and have no exhaust/flue requirements, and are quiet to run.  However, they do have high running costs.

More recently, customers have requested a greener solution.   We have installed Ground Source Heating, Biomass Boilers and utilised electrical power from hydroelectric turbines.  These are more fully explained in our ‘Sustainable Heating’ section.

Control Pipework – The Mixing Group

Control pipework located in a potting shed.

Control pipework and boiler located in a potting shed.

The heating for a glasshouse needs to be designed well to avoid huge swings of temperature, which would not be good for the plants or fuel economy.  In a typical domestic house heat is turned on and off as required, and no modulation is required as the power requirement is relatively low, and the thermal heat capacity is high.  Unfortunately, this is not the case in a glasshouse, so to achieve the heating demands, we modulate or change the temperature of the heating water in the finned pipes.

Mixing group for a large heating installation in USA.

Mixing group for a large heating installation in USA.

This is achieved using a giant shower valve, so the temperature of the pipes changes depending on the heat load, giving good heat distribution.  The system keeps circulating when no heat is required; this gives even cooling in the glasshouse.  Another problem with a glasshouse is that heat demand can go from full power to off very quickly, for instance when the sun comes on a glasshouse on a cold winter’s day.  For this reason we have a quick response valve and settings are tailored to each glasshouse.

The control pipework is modulated from a Harrier Heating Controller and an Aspirated Screen.  The Mixing Group also includes temperature gauges, to monitor pipe temperature, isolation valves and unions to aid maintenance, and a by pass depending on the application.

Harrier Heating Controllers

Harrier has developed two high quality controllers designed and manufactured for their greenhouse heating systems – the HH2 and the HH3

HH2 Controller

HH2 controller and Aspirated Screen

HH2 controller and Aspirated Screen

The HH2 controller has been designed & developed specifically by Harrier to accurately control the climate within a glasshouse.   Being the market leader in this field, gave us the insight into exactly what was needed!

Aspirated Screens

Aspirated Screens are typically hung where the plants are.

Aspirated Screens are typically hung where the plants are.

In order to maintain accurate temperates within a structure the controller needs accurate temperature readings from within the glasshouse, and this is the function of the Aspirated Screen.

HH3 Controller

It has been developed more recently because we found the functionality of the HH2 Controller was not needed in all cases.  A smaller greenhouse may have heating with manual vents, or no heating at all but with electric ventilation.  In these cases the HH2 is not necessary, and the HH3 would be better suited.

HH3 controller and temperature sensor

HH3 controller and temperature sensor adjacent to isolation switches.

Servicing & Maintenance

It is advisable that all Hot Water Systems should have an annual Mechanical & Electrical Service to ensure they run efficiently.  Please contact Harrier GD to organise an Annual Service Contract.