To bypass or not to bypass (UFH Central Heating)

[jks]

I have recently had wet underfloor heating and a new combi boiler installed. The flow and return go from the boiler through a thermostatic mixing valve to the 7 port manifold for the UFH. There is a pump within the combi and I also have a pump on the flow pipe between the TMV and the manifold. I only have 3 ports on the manifold being used at the moment (ground floor only).

I am experiencing loud noises (rumbling/vibration) from the boiler at various times when the UFH is on (don't get the noise when drawing domestic hot water).

The combi boiler instructions (Glow-Worm 38cxi) state that there is an internal automatic bypass valve fitted and open one half turn. However, the diagram below it shows an optional ("if required") external bypass. It also states that a minumum flow rate of 1289 l/h (20 l/min) should be maintained at all times.

Do you think the boiler noise I am experiencing is down to me requiring another bypass in the system. Or should I try adjusting the internal bypass valve?

I really hope I have made sense!

thanks in advance for your help.

J

[The Natural Philosopher]

Yes, I think you have spotted the problem. The pump needs to push water around for the BOILER thermostat to work properly. Depending on the boiler..it may have to high a temp..

The bypass loop itself may actually be a bit loud running full bore. If you have adjustable pump speed on the boiler try reducing it a bit.

[jks]

No adjustment of speed on the boiler pump available unfortunatley. Should I try adjusting the automatic bypass valve on the boiler? The noise from it can not be anything like as loud as the noise we have at the moment (it wakes us up early in the morning and we are on the other side of the house!)!!!! I think my question still stands about whether it is normal practise to add another bypass in addition to the one on the boiler??

[The Natural Philosopher]

That I do not know..I certainly don't bother - as soon as the UFH stops wanting heat the primary circuit temp rises and the boiler shuts down. Cos when I want UFH it usually is all I can get ;) the mixing valve never closes up completely. Too much pipework under the floors and too much heat transfer..