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Data Sheet

Radiator thermostats for one-pipe and two-pipe systems

4

VD.51.Q1.

12 © Danfoss 10/2003

DKCD

Radiator thermostats are sophisticated proportional
control.

1) The operating principles of thermostats
The function of the actuator unit is to sense the am-
bient temperature, compare it with the required tem-
perature, and correct the valve setting in accord-
ance with the difference, as illustrated in fig. 1.

European Standard EN215 requires that radiator
thermostats meet various operating criteria. Danfoss
radiator thermostats meet or exceed these require-
ments in every respect. A brief description of the key
considerations is given below.

2) Differential pressure dependence.
The closing characteristic of a radiator thermostat
depends on the pressure conditions in the valve. In
fig. 2, curve 1 applies to the actuator unit without
counterpressure from system differential pressure,
while curve 2 shows the offset when the
counterpressure increases proportionately to the
valve closure. The effect is a result of the elasticity
of the actuator unit which should, therefore, not be
too large.

3) Time constant
To give reasonably fast and stable control, a radia-
tor thermostat must not be too slow in its reaction to
temperature variations. The time constant (fig. 3)
depends on the specific heat and surface of the sen-
sor.
At a sudden change of the ambient temperature at
time 0, the thermostat will react as shown, relative
to time. The time constant is defined as the time
used by the spindle for 63% of its full movement.

4) Flow temperature dependence
Flow temperature dependence occurs largely in the
case of actuator units with built-in sensors since
heat transmission takes place from the valve body
to the sensor of the unit (Fig. 4), with the possible
effect, under full load conditions, of controlling the
room temperature at a lower temperature than re-
quired. Insulating plastic materials for thermostat
base, internal heat shields and good product design
in positioning the sensing element away from the
heat source, help reduce this dependence. Danfoss
recommend that radiator thermostat sensors are
mounted horizontally, away from the effects of heat
rising from hot valve and pipework.

5) Hysteresis
If the spindle movement of an actuator unit is
measured at,  first, increasing and then decreasing
temperatures, there will be a difference between the
point where the valve just closes and the point
where it opens again – see fig. 5. This difference is
termed hysteresis and is caused by friction in the
actuator unit and valve body. It is desirable that
hysteresis should be as small as possible.

Setting the temperature

1

2

3

4

5

Summary of Contents for RA 2000

Page 1: ...hout draining down the system The protective cap supplied with each new valve may be used for manual control during installation only It should not be used as isolation device A manual shut off device...

Page 2: ...ure is 2 C higher at de sign load condition 2 Remote sensors are supplied with all the capillary tube coiled within the sensor When installing the sensor only the length required should be uncoiled 3...

Page 3: ...ase request details The required room temperature is set by rotating the sensor handle to match the scale number with the setting register The correlation between scale Setting the temperature numbers...

Page 4: ...o temperature variations The time constant fig 3 depends on the specific heat and surface of the sen sor At a sudden change of the ambient temperature at time 0 the thermostat will react as shown rela...

Page 5: ...sible To ensure that the valve capacity of a fully opened valve is not significantly greater than that at 2 degrees C P deviation all RA FN valves have a modified valve characteristic This improved va...

Page 6: ...Capacities Use the following sizing charts to select the correct valve for the radiator in question Heat requirement 2 05 kW Temperature drop 11 C Differential pressure across the radiator 0 05 bar 5...

Page 7: ...ard beneath the radiator if free of surface pipes All remote sensors are now supplied with ultra thin capillary Simply pull out the length required 2m max and fix using clips provided Remote temperatu...

Page 8: ...perature adjuster DN D d2 RA FN 10 10 Rp 3 8 R3 8 60 85 27 52 22 47 96 22 27 RA FN 10 UK 10 Rp 3 8 R3 8 103 26 51 22 22 27 RA FN 15 15 Rp 1 2 R1 2 67 95 30 58 26 47 96 27 30 RA FN 15 UK 15 Rp 1 2 R1 2...

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