WB Greenstar 30CDi with UFH: CH output temp never constant

14 Feb 2010
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United Kingdom

I have had a new system installed as part of renovating my home with a WB Greenstar 30CDi system boiler and (water) UFH throughout the house. I moved in last week and since then have been very chilly! I have the UFH set to be 'on' constantly and control the temperature of the rooms with thermostats for every loop of the UFH. The temperature of the rooms is way below the set temperature and never gets very high (eg living room only about 15-16).

If I watch the boiler, it will fire up, heat water to, say 70-80 (in about 30 seconds), then cut the flame and let the flow temp fall back down to 35-38 (over about 60 seconds), then kick back in again. It never maintains a constant output temperature (which ideally would be ~70). The return temperature is very tepid both during and after firing (if I didn't know better I'd say the boiler is kicking back in again when flow temp drops down to return temp at 35-40).

The way the UFH system works is that there is a mixer valve next to the manifold which mixes flow water with return so that anything >60 is cooled to 60 (some legal maximum, apparently) before going through the UFH pipework.

Anyway, seemingly because the boiler is not 'operating' constantly (ie flow round the system) then only a relatively small amount of hot water is getting through the mixing valve and into the UFH manifold - so the temperature into the manifold is generally ~40 no matter what the boiler thermistor is set to (I'm guessing because the flow temp at boiler is only above this temperature for brief amounts of time) and presumably because the pump does not run for long after the flame cuts out (?). The flow through the manifold on the top floor looks to be very low, but I suspect this is all linked in.

I had the UFH installer round this week who tweaked a few things but there is no real improvement. He suggested that the boiler was to fault because it kept coming in and out rather than modulating output.

The bind I have is that the UFH was installed by a different company to the boiler and I don't want to get into a finger-pointing mess - I just want it to work!

Does anyone have any thoughts about what might be going wrong here? I just feel like I need to have a bit of a firmer view so I know where to concentrate by 'chasing up'.

Many thanks
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Im not a plumber.

Could this be air?
I suppose that could be the reason the boiler is reaching such a high temperature.
Did this get resolved? Only we've got a new Worcester Bosch free star and its struggles to warm our house when it's 5 or below out. Having been set to 23 from about 4pm I get in from work at 6.30 and it's only 20. Old boiler was fine at getting warm. Like yours ours seems to cut in and out?

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