Why not just take the simple option and turn the 'duff' trv down a bit.
In my experience trv calibration can vary from new.
The heat loss from a room is determined by its dimensions and construction, and by its surroundings - adjacent rooms etc. Just because one room has a heat loss of 2kW and another 4kW does not mean that the temperature in the 4kW room will drop twice as fast as the 2kW room.
I k that you are failing to understand that you cannot expect a thermostat in the house to effectively control a conservatory with higher heat losses.
The real fault is that building regulations require a conservatory to be equipped as a separate heating zone so that you don't have the problems that you are experiencing.
The plumber does not seem to have been helpful as he should have know about all these issues.
Or perhaps he correctly advised you and you refused to pay for the separate heating zone to be installed?
Tony
RAS-C2 Instructions
It might be worth checking that the flow control is correctly set. Also that the head is correctly fitted - make sure it is set to maximum when fitting.
If no joy, try swapping heads.
If still no luck, the rad is probably way out of balance (have you checked this?), so the pressure across the valve is too high and it is being kept open.
Apart from Building Regs requiring a separate circuit, with separate controls, if an extra radiator has been added to the system and the installer hasn't bothered to re-balance the system, no wonder you are having problem.
I can understand now why you have not checked the balance.
The conservatory will cool down faster than the rest of the property because it has a smaller heat capacity. This is one of the reasons why Building Regs require "external standard" walls and doors between the conservatory and the rest of the house and a separate circuit with separate controls.
See Approved Document L1B paras 3.15, 4.8 and 4.9.
1 it would seem that the valve is working correctly but has maybe drifted somewhat out of adjustment.
2 yes but the trv needs to be bi-directional, I think yours are but having never used that model am unsure (Danfoss not Drayton)
1. This seems to me that the valve may not be installed correctly or possibly it is not working at the right/expected temperature?2. Can a radiator work correctly with the hot water coming in from the wrong end first???
I am looking at the diagrams on the link provided earlier when I ask question 2 above.
http://heating.danfoss.com/PCMPDF/013R9355_RAS_C2_Keymark_VISXM302_.pdf
Putting your room stat in the conservatory would overcome most of your problems.
A wired stat can operate at over 100m so distance is not going to be a big problem.
Alternately you could use a wireless stat.
Or you could use a wireless programmable stat wired in parallel with the existing hall one.
Then you could set it at more appropriate times so that its not heated when heat is not required.
I never can understand why some people want to heat a conservatory all year round. I don't really believe that many people occupy them after dark when they look like fish in a goldfish bowl !
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