Underfloor heating causing Boiler to Shutoff

19 Jan 2021
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United Kingdom
I need bit of help with understanding a fault with my underfloor heating system. I have a Vaillant EcoTechPlus 637 system boiler. Hot water from the boiler is divided 3 ways using Honeywell 2 Port valves - Hot water to a cylinder, Radiators Upstairs and under flor heating on the ground floor. The underfloor heating system has 6 loops divided into 4 zones controlled by 4 HeatMiser NeoAir wireless Thermostats. The system is controlled by HeatMiser UH8 - RF wiring centre. I have noticed a problem with underfloor heating system.

When I turn on just the underfloor heating independently, the valve opens, the underfloor heating pump comes on and boiler fires for few minutes and then shuts itself off. The valve stays open and pump continues to operate. As the boiler has stopped firing, cold water circulates in the system. There is no issue when hot water cylinder or radiators call for heat. When I turn on underfloor heating while the radiators and/or the hot water circuit is already running, the boiler works fine and I can see hot water flowing through the underfloor heating system without the boiler shutting down itself. The problem arises when only underfloor heating is calling for heat and hot water and radiator are not calling for heat.

Any suggestions on what might be wrong.

Thanks in advance.

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Check the following values in diagnostic mode. Also when it shuts down but pump running what is the S state of the boiler?
Sounds like boiler is overheating too quick.
How many radiators and sizes have you got in the house.

When the boiler comes on, it shows S04 and then it goes blank with no display. I have 4 Type 22 radiators of about 1500mm width and 3 towel radiators. When the radiators demand for heat, the boiler runs fine with code S04. Since the underfloor heating won't run on its own, I am having to either turn the hot water thermostat on or the radiator thermostat on to demand heat so that the boiler keeps running. This way I am able to heat the ground floor until the issue is fixed.
When I said the pump keeps running, it is the underfloor heating pump connected to the Manifold that keep running. The boiler pump doesn't run and the display is blank. The Reset button does nothing.
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When was UFH added?
Is this new problem since a change or from point of install?
The entire system was installed 2.5 years ago. No new additions have been made since. It was all working fine until beginning of the month. One day I noticed that ground floor rooms were cold. I checked the flame symbol on the Thermostats. They were all on and the system was running with cold water. Worth mentioning here that when the underfloor heating system comes up the boiler heats up the water pretty quickly to about 75 degrees then cools down, hour glass symbol is displayed. The burner then tries to ignite again and few seconds afterwards it shuts down and display goes blank. Target temperature in the boiler is set to 65 degrees.
The only reason I was asking is that the boiler completely turning off seems to point towards someone having miswired the system and when its satisfied it was removing power from the boiler rather than just switching the CH call connection. It should be permanently on and never turn off.

I would check the wiring to the boiler and see that there is a permanent live direct to isolation switch and it stays live after the noted screenoff issue, if it goes dead then it points to wiring issue.
Otherwise there may be an issue with the PCB causing some fault.
Note I think the 637 when you remove the front cover it will expose the internals of the boiler which means you would need to be approved to work on your boiler unfortunately.
Thank you very much for your reply. I was also expecting some sort of wiring issues. I have the boiler's annual service booked for next Wednesday. I hope the engineer will pick up any wiring faults. Will keep the forum posted. Any other ideas please?

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