Performance of TRVs

Because themostats generally do not open and close at the same temperature. Even when hysteresis defeating mechanisms are employed it is rare and proabably confined to purely solid state devices. Certainly a TRV is never going to perform in this way.

It varies too much from make to make and unit to unit.

Identical TRVs here have slightly different characteristics.

On a small sheet of paper in the box they came in.

Try measuring the temperature right next to the TRV head. The TRV can't do better than keeping that constant.

It can't even do that under all conditions. A given knob setting and room temperature will result in a particular flow restriction. But the *required* flow restriction to maintain a constant room temperature will vary, depending on the heat losses (heavily influenced by outside temperature) and on pump pressure - which will depend on what all the other TRVs in the system are doing. At best, TRVs can only provide crude temperature control.

Danfoss RAS-C leaflet says;

frost * 8 I 12 II 16 III 20 IIII 24

I don't think there is a standard.

You won't get precise control throughout the room because, even if the TRV were operating perfectly, it would take some time for a change in output to be measureable at all points in the room. It's cheap & cheerful but good enough for most purposes. To get exact control of room temperature, you'd need to circulate something like 10 AC/hr through a convector heater.

Even then, people's perception of comfort involves both humidity & radiant heat, as well as air temperature. E.G., the kids complain they're cold in the morning despite room air temperature being 21, 'cos floors and walls are cold. Kids are warm in the evening at the same air temperature 'cos floors & walls have heated up & so less heat lost through radiation.

They are not made well enough and combined with other factors there is too much variation between units. Hence the relative scale using numbers, final user tweaking is needed to get the settings right.