How do TRVs work?

[Doug99]

Having fitted my first TRV today, it got me thinking of how it works. I appreciate the valve has a device which expands/contracts with temp which then opens/closes the valve to control the flow of hot water.

But when the valve is cold and hot water flows unrestricted through the valve, how does it sense the 'room temperature' rather than the hot water flowing through the valve?

I would have thought the device would heat up more quickly due to the hot water running through it, rather than the heat of the room.

 

[Samuelle22]

Consider this a friendly bump, as I am very far from an expert, but since no one else has replied..

A TRV is much taller than a regular radiator valve, because the temp sensitive wax plug/spring is above the actual valve that opens and shuts, and only connected to it by a thin metal pin/rod. Even if hot water is running through the radiator, not enough heat his going to travel from the actual valve up the thin metal pin to the temp sensor top part to make a difference. (but that is just an uneducated guess)

 

[St0rmer66]

Also they can calibrate it to take the heat from the water into account before closing...

 

[D_Hailsham]

The effect of water temperature is very small. Drayton, for example, say that it is only 0.8 K for the TRV4.

TRVs are never marked with temperatures only numbers for two reasons:

1. The same head can be used for both Celsius and Fahrenheit.
2. The required setting to give the desired temperature can vary from house to house and room to room.

 

[Agile]

The actual close room temperature will vary a little according to if its mounted on the flow or return and if its vertical or horizontal.

They do actually work best when horizontal but that risks damage from people in the room.

They are also better on the cooler return but most installers consider that whilst bi-directional they are best on the flow as a result of the internal construction.

So you can fit them anyhow and just adjust for the room temp you require.

The liquid filled ones respond in about 1/3 the time of the cheap wax types.

Tony