26 Mar 2016
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United Kingdom
I have tried a number of local plumbers but unfortunately, have had no success.

I have a vented heating system and am running a glow worm 30 HXI boiler, approx 2 years old. It is a three zone system - one zone is the underfloor heating consisting of 5 loops, 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 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 and a return temperature of 50c after a few minutes of operation on the UFH.

The main pump is a grundfos UPS 15-50 and is set to speed 2. The boiler flow temperature is set to 68 and the return is typically between 10-12 degrees lower than flow. The expansion vessel is set to 1.2 bar.

My issue is, when the underfloor heating is running in isolation I.E. The cylinder and Radiator zones are not calling for heat, the boiler reaches 70 and the overheat protection kicks in and the pilot light is cut. The boiler remains off for a few minutes and then repeats the process. I have the same issue when the cylinder is the only zone running AND is close to reaching set temperature of 60c.

However, when calling for heat on any two zones i.e. Cylinder and underfloor, rads and underfloor or rads and cylinder, the boiler runs at a constant 68c and I do not face the cut out problem.

As an experiment, I manually opened the value for rads (but did not call for heat from the thermostat for the rads), the ufh thermostat was calling for heat. this seems to help a little, the boiler does not appear to cut out, but the temperature does fluctuate between 66 and 69 every couple of seconds.

Any idea what I am missing and the optimum setup?
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On what basis have your local plumbers attended?

What is the construction of the floor with the UFH ?

What is the area of the UFH in sq m ?

It is not normal to run UFH with much higher that a flow temperature of about 40 C. Why is your set so much higher?

My first guess is that the heat output of the UFH is a lot less than the minimum power output of the boiler.

Is there an auto bypass?

Why do you have a 30 kW boiler? Is the heat loss of the house really close to this? Or was it a "more powerful is better" ?

sounds like you need an automatic by-pass installed , 30 KW on a system boiler is a massive amount of heat to try and move around a single circuit, I would calculate what your system actually needs and it wont be anywhere near 30KW and there will be a trimmer on the PCB that can be set to that maximum value, a decent heating engineer not a plumber is your answer or contact Glow Worm and ask for a service by their own lads and explain what is happening and that engineer will range rate it for you hopefully
Hi Tony, thank you very much for your prompt reply! Really appreciate your help.

I have called local plumbers on the basis my boiler kept cycling off and on. They played with a series of settings including the UFH pump speed and the bypass value etc.

The floor construction is engineered hardwood.

The sq m of the UFH is 35sq meters

From what I understand, the polypipe suspended floor system flow is designed to run at 60c.

Forgive me if I'm talking rubbish here, but I have a bypass value fitted to the system (is this the same thing?) it's currently set to number 4.

Although I haven't calculated the heat loss of the house, if I'm honest, I would say it was sized as "the bigger the better". I inherited the system when I bought the house.
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Likewise Ian thank you very much. I will search for heating engineers instead! The maximum value is currently set to 12kw by me. I had a glow worm engineer service the boiler just two weeks ago. He recommended the calculation for the KW setting as 2KW per rad and 2KW for the cylinder, which would total 16KW without the UFH. He wasn't certain about the UFH. But as I typically never need to run all together, I started at 16KW setting and have been turning it down every couple of days to see if it helps. Any thoughts on if I should lower it further?
Those calculations are a rule of thumb but you are not far out at that so I think if you get an automatic by-pass fitted you should be fine if it is something that has just started happening it might be a water quality fault , new boilers dont like dirty water
Thanks again for your response. It turns out I do have a auto bypass value fitted. Can you please advise how I should find the correct setting on the bypass value? If it's still cutting out, do I need to open the valve further?

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