central heating system noise - sealed system

[grobsy]

System:
I have a sealed s-plan system with boiler in loft, Grundfos 25-50 Pump + unvented cylinder in cupboard in top floor, 22mm pipework supplying 5 towel rails & about 13 traditional style rads all with TRVs.

Problem:
The system was heated with an underspec'd Worcester 18KW Ri, but this was recently changed to a 24KW Ri which was as much as the gas pipe could manage.

However since fitting this new boiler there is a very constant loud noise coming from the cupboard where the pump is ( but could also be pipework coming from boiler) once the heating has been on for about 5-6 hours.

The rads seem to heat up fine and there is only very little air at the bleed points. There is water at the pump asides a soft rushing bubbly sound on start up. So I dont think it is an air problem. Also, there seems to be no restrictions in terms of pump valves, iso valves etc...

any ideas what problem could be????

 

[GodKnows]

Could be multitude of reasons, given the limited information, could be the wiring centre not wired properly. If trv's all met their respective temps the boiler should shut down and maybe the pump over-run not set right... or pump set at wrong speed...

 

[grobsy]

The pump over-run was wired in from the boiler PCB a couple of weeks after the boiler was installed. It now runs on for a while after the heating switches off. The noise occured both before & after the pump overrun was wired in.

The pump speed is set to III (3). Is it better for circulation that the pump is set at a higher speed given that the pipework (22mm) could probably have done with being a wider diameter (ie 28mm)?

 

[Agile]

With an "S" plan you SHOULD have an auto bypass as well as when you have ALL TRVs. You probably have not or its wrongly set.

The pump should usually work fine on "2". It needs to be slow enough to achieve the correct temperature differential across the boiler of 15-20 degrees.

Unless the house is poorly insulated I would have expected 18 kW to be enough but it all depends on the heat loss.

Tony

 

[grobsy]

Thanks for your reply Agile.

The bypass (a Myson 22mm) was screwed completely shut (clockwise) by the initial installers. I have since slackened it off to about 0.45bar (this ABV ranges from 0.6 down to 0.1bar). Still bad noise.

I have since adjusted it down to 0.25bar. With the system starting up from cold I can feel the heat go through at a bit less than that. Hopefully that will do it....

- The 18KW was definitely insufficient. Was heating to 80degrees until you start to open up a few rads then temp plummets. It is an old Georgian mansion with huge windows and about 6meters vert between floors. About 33KW in total required.

Grobsy

 

[Agile]

I would expect that setting to solve the problem.

Tony

 

[grobsy]

yes, adjusting bypass down to 0.25bar has got rid of noise at cylinder cupboard / pump / boiler.

but, there is still noise at a couple of rads???

any suggestions???

 

[Agile]

Have the rads been balanced with the objective of obtaining a temperature differential of about 15-20°C.

Tony

 

[grobsy]

only roughly. just making sure heat makes it down to the floor below.

[the 2 troublsome rads are on the top floor (as is pump, zone valves), with boiler above in attic.]

The rads only make noise after a while, and the noise can be temporarily fixed with a turn of the TRVs, but tends to start again.