26 Mar 2016
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United Kingdom
I’ve had many engineers with no luck, please help!

I am running a glow worm 30 HXI boiler, approx 3 years old. It is a three zone system - one zone is the underfloor heating consisting of 5 loops (approx. 35sqm room), another zone is the rads consisting of 7 rads (6 have TRV's) and the final zone is the hot water cylinder.

The underfloor heating is a suspended floor system under engineered wood and has a polypipe manifold, mixing value and a grundfos alpha 2l pump set on CP1. With these settings, I am achieving flow rates of 1.5l with a flow temperature of 60c (as per the manifold design) and a return temperature of 50c on the UFH manifold.

The main pump is a grundfos UPS 15-50 and is set to speed 2. The boiler flow temperature is set to 63 and the return is typically only 6 degrees lower than flow so around 56. The expansion vessel is set to 1 bar.

My issue is when the underfloor heating is running in isolation I.E. The cylinder and Radiator zones are not calling for heat, after some time in operation the boiler flow eventually reaches 64, then 65.. the return reaches 57, 58 and the overheat protection kicks in and the pilot light is cut. The boiler then remains off for a few minutes and then fires up rapidly before cutting out again within a matter seconds. I have the same issue when the cylinder is the only zone running AND is close to reaching set temperature of 60c.

I have set the range rated the output on the boiler to the minimum of 10kw. I also have a Honeywell bypass value fitted. I’ve tried increasing the main pump to speed 3 and both opening and closing the bypass to see if that helps. Unfortunately, eventually, it still cuts out. I’m not sure where I’m going wrong?
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Sounds fairly normal to be honest, its not shutting down on overheat its shutting down on its internal stat, the load its running is too small for minimum heat output (5kws in your case) bit overkill but you could fit a low loss header or buffer tank .....or you could ignore it.
Thank you very much for a prompt response Picasso. My gut instinct told me it was oversized for the output. The problem with ignoring it is the house never warms up downstairs, the heating can run all night and won’t reach above 19c. Because after the boiler cuts the first time, the temperature shoots down to 40 and stays off for 5 mins, the ufh manifold gets cold, then it shoots back up to 70 within 20 seconds and shuts off for another 5 mins so the ufh circuit remains cold. I will look into the options you mentioned, thanks. Another question, any advice on what the bypass should be?
Your pump is too small for your system, the heat exchanger on your 30 hxi is taking up nearly half of the pump capacity, then you have turned it down to 2 , it does not leave much for the system resistance, I would strongly suggest you fit a bigger pump for starters.
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Ah so pump and either a low loss header or buffer tank. So what pump would you recommend? Thanks again in advance
The low loss header and buffer were tongue in cheek, a decent sized pump should sort out your problems, you need to have your pump sized for your system so not possible over the net without knowing how big your system is, length of pipe runs, pipe sizes ect , as a temp fix you could have another 15/50 plumbed in series with the pump you have now and see if things improve.

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