Danfoss RAS-C, RAS-D Fact sheet

DOMESTIC HEATING CONTROLS ARE EASY
PART 5 - RADIATOR THERMOSTATS

2

Fig.1. RAS-C

and RAS-D2 radiator thermostats with RLV-D lockshield valve

Radiator Thermostats (TRVs), the original and straight forward room temperature
controls, today remain essential to the full efficiency of most ‘wet’ domestic central
heating systems.

Endorsing this fact, Good Practice Guide 302, the Government’s authoritative document
covering domestic heating controls, recommends radiator thermostats for all ‘wet’
systems (both combi and regular boilers), whether these are installed to defined
‘Minimum Set’ or ‘Best Practice’ standards. It specifies ‘TRVs on all radiators except in
rooms with a room thermostat’ for every one of these classifications.

TRVs were invented by Danfoss and first became available as long ago as1943.
Continuous development since then has resulted in modern versions that, although
based on original principles, are smaller, more efficient, more stylish and easier to use.

Despite the arrival of numerous competitors attracted over the years by the product’s
success, Danfoss still supplies over 50% of world demand and is firmly established as
the market leader in the UK.

Installers and specifiers seeking information and advice on radiator thermostats and their
application will find no better source than Danfoss.

FUNCTION

Radiator thermostats, fitted to individual radiators, control desired room temperatures
very accurately, allowing only essential heat to be generated in the process. Furthermore,
they enable lower temperatures to be maintained in unoccupied rooms or where
appropriate, thus reducing heating costs and enhancing comfort.

Requiring no external power to operate, TRVs sense room temperature changes in
individual rooms – including extraneous ‘free heat’ from sunlight, appliances, occupants,
etc. – and adjust the flow of heated water through the relevant radiators to maintain
desired comfort levels and reduce costs. Using conventional room thermostats alone
provides just single-zone temperature control based on one sensing point and one
temperature setting.

CONSTRUCTION

Radiator thermostats comprise two basic sub-assemblies – valve bodies and sensors
that can be purchased individually or in boxed sets.

Valve bodies. Available in many sizes and configurations – straight, vertical angle and
horizontal angle - to suit various applications, valve bodies provide the means of
regulating the flow of heated water through radiators. There are also special valves, e.g.
presetting types or high flow versions for single pipe systems.

Fig.2. Danfoss valve bodies for radiator thermostats

Sensors. Settable for desired comfort levels, these monitor room temperatures and
adjust valve operation accordingly.

These two component parts need to be fixed together correctly and positively.

OPERATION

A wax, liquid or gas charged capsule-and-bellows assembly within the sensor head
expands and contracts with changes in room temperature, operating a piston that acts on
the valve pressure pin (spindle). As the room temperature increases, the valve throttles
back the flow. A powerful spring returns the piston promptly when contraction (cooling)
occurs. Fig. 3 shows the internal components of a typical modern radiator valve.