Evohome problems on Danfoss RAS-C2 valves
New radiators, new problems
We have a Honeywell Evohome system, which is excellent for controlling the temperature in a house, especially when you want to save energy and avoid heating rooms until they are needed.
Our current house was just converted from LPG to a heat pump, and this required several new radiators. I'd specifically asked for Evohome compatible valves, and the installer brought some decent quality Danfoss RAS-C2 valves.
I'm told Danfoss valves are one of the most common in the industry, and Evohome controllers come with a specific adaptor for them.
Valve stroke issues - HR91
The problem was that the radiators never turn on, they are stone cold, despite the valve motor turning.
With a bit of investigation I found that the HR91 controllers I was using never open fully to the end stop. They always stop well before it. This is apparently a deliberate choice to save battery by winding the motor for a shorter time.
The trouble is that the control point of the Danfoss valves are different from our old ones. By default the valves are 'open' and hot water can flow. By pushing the valve pin down, the flow is restricted. Each type of valve has a slightly different range to push over and the point on the valve's movement where the flow begins to be restricted will differ as well. Evohome controllers attempt to 'calibrate' on the valve when they are first connected by winding it in and out fully.
On the Danfoss valves, the 'useful control range', ie: the distance over which the flow rate changes significantly begins VERY close to the fully out position. The flow is also restricted quickly as the pin is pushed in just a short distance. Because the HR-91 controllers never open the valve at all.
With the new radiators installed, every single one failed to work with the exception of a towel rail radiator in the bathroom with had a different style of valve.
HR80 not much better
- Rotate the black wheel fully anti-clockwise
- You will see where a the black and white plastic impact, preventing it from turning further
- Carefully twist the black section, pulling it away, so it clears the white peg and turns further.
- You can now fully unscrew the black wheel and remove it
- Use a metal file to reduce the length of the pin, about 0.5mm at a time or less
- Refit and test the controller again before you remove too much metal!