CH and UFH performance problem

Hi all,

I have recently had a UFH circuit added to my existing CH/HW system and had a question on performance when UFH and CH are operating at the same time.

My boiler is an Vailliant EcoTec Plus 630 and for a number of years this was running with a CH and HW loop only. Each loop had a 2 port value controlling the flow. All radiators (11 in total) are TRV and the boiler has a manual bypass fitted. I recently discovered that the boiler has an ABV fitted as standard so question the need for an additional bypass valve. Anyway, it has been performing well since 2008.

Recently a UFH loop was installed in the loft and due to space requirements the manifold had to be positioned there also.

The flow to the manifold is the first connection tee'd from the boiler flow pipe and is controlled by a 2 port valve. The return from the manifold is tee'd into the CH flow near the boiler.

The manifold temp is set at 41C and the boiler is set to 60C. The CH and UFH are controlled by separate room thermostats.

When both the CH and UFH are on and the valves open, the UFH will return water much more quickly to the boiler than the CH circuit does as it is a smaller loop. This results in the boiler temp increasing to max temp (60C) very rapidly and then enter into it's cooling cycle. The result is that the CH circuit does not really get up to an adequate temp and the radiators are only partially warm. If I turn off the UFH then all rads will become piping hot in around 45-60 mins.

Can anyone with experience of a similar setup advise on what can be done to even out the flow so that both circuits can operate simultaneously with the correct performance? Should I lower the speed of the UFH pump at the manifold?

Thanks

Are you happy that the manifold thermostatic mixing valve (TMV) is working correctly, it shouldn't take it long to reduce the manifold temperature from 60C to 41C, initially, the temperature of the water returning to the boiler should be quite cool. If this was as low as say 25C then boiler might cycle on high dT?, have you noted a high return temperature to the boiler shortly after starting up both together?.

You could reduce the manifold pump setting to minimum and then gradually increase it if the manifold flow/return dT is too high, what setting is the pump on now?.

The circuits sound like they need to be hydraulically separated...

The above is interesting but is showing hydraulic separation necessary under abnormal conditions IMO, if the boiler can run for the few minutes until the TMV reduces the mixing temperature to 41C (OPs case) then the boiler is only supplying 2.76LPM to the UFH (at 60C) and 7.17LPM to the rads which should not be a problem, there could and probably would be a problem supplying the UFH on its own but a by pass between the boiler flow/return could take care of this.