I have a underfloor heating system installed in my apartment.

Each room has a Uponor T-37 thermostat, hooked up to C-35 controller.

The system is about four years old and recently I have noticed a regular clicking sound from the relay in the controller unit.

At thermostat side, when I turn the dial so I can heat into the room, the LED comes on, then after 5 seconds or so it turns off. The light then comes back on again after a half-second or so. This cycle repeats, and seems to happen simultaneously at each thermostat in every room/zone.

I assume this is bad for my boiler/heating system because it's receiving a command to supply heat on/off every few seconds.

Any ideas on what could be wrong?

I've tried to turn-off power to heating system and boiler for a few hours, thinking it might reset any fault.

I was worried about taking off the cover off the control unit incase there maybe a danger of a shock. I removed the cover of the thermostat and noticed that as the LED switches off, it flashes rapidly.

I'm in the process of calling in a plumber/heating technician, but just thought I try and do my own research on the matter to see if there is an easy fix. Hoping it won't be an expensive repair.

2 Answers 2


I called a number of Central Heating contractors and also Uponor, the company that makes the controller.

I was told that the clicking noise comes from the soft-fuses inside the controller trying to reset. They said box has failed and needs to be replaced by an electrician. Since the controller is the simplest model in their range, there are no diagnostic LEDs or other features that indicate a fault status, except the clicking noise of the soft-fuse.

The repair should only be about an hours labour plus cost of a new controller box.


I had the same problem several times with my controller: after a long period (6-12 months) the unit started clicking, then eventually failing. I found the built in power supply board had components failing/burnt out. This was promptly replaced (the whole controller as there are no service parts PCB available) with no question via my local City Plumbing outlet from Uponor.

Then it happened a second time and I asked the question from Uponor technical support, "should there be a maximum ambient temperature the controller should be operating in?" as the controller is mounted adjacent to the manifold. They advised not, I therefore took the decision to completely rewire the controller through the wall behind the manifold where it is situated in a cool place.

Since I have done that, I have had not a problem with my third unit at all and the internal PSU board is running a lot cooler. I should add that before jumping to this assumption I tested a spare controller, all the actuators' resistances and individual loading etc. to make sure there wasn't anything overloading.

Note: Uponor may have changed some components so it runs cooler, but I can't say for sure.

I am a working instrument/electrical professional hence why I could do this, but I sympathise that your Joe Bloggs just wanting his nice new UFH to work, may not always have the knowledge to do this and may have to call in an engineer which could cost a lot of money and not always do "qualified electricians" have this knowledge in my experience. It's also has left my wife and kids a few nights in a cold house, waiting for me to come home to fix it!

My advise is make sure you mount it in a cooler place, not alongside the manifold!

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