Hot to stop banging when boiler calls for hot water

[Gibbo]

Hi
I have a worcester heatslave external oil boiler which makes a large bang every time we use hot water when the central heating is running. It seems that as the diverter valve moves towards the hot water position there is a single clunk which can be heard all around the house. The diverter valve has had new internals and the actuator was replaced recently for a different problem. Would appreciate any advice on this please.

 

[ShaunCorbs]

Is there an auto bypass fitted ?

 

[Gibbo]

No. Is this recommended. The boiler was installed as recommended by Worcester. They don't mention anything like that.

 

[ShaunCorbs]

I would recommend having one fitted should get rid of the clunk

 

[Paul Gibson]

Is this fitted across the the flow and return of the pump

 

[John.g]

Yes,

What exact make/model of circ pump do you have and see what mode and setting it at, you may be able to reduce its setting which may help to reduce the clunk.
Also check for clunk by shutting the boiler/circ pump down when in CH mode only then c/o to HW mode only, restart boiler, check for clunk.

 

[Deleted member 123817]

Should also have asked if it's a mid position valve that allows HW & CH on together?

 

[SimonG]

Should also have asked if it's a mid position valve that allows HW & CH on together?

No its priority to hot water, all the rads probably have trvs. All shutting down and then nowhere for pump to go, swaps to hot water and 'thud'.

For a quick fix till sorted permanently take trv head off on one of rads.

 

[John.g]

Does a diverter valve parallel the CH & HW momentarily while changing over?.

 

[SimonG]

Does a diverter valve parallel the CH & HW momentarily while changing over?.

Will do.

 

[Gibbo]

Hi
Pump is a danfoss ups 15-70. The setting is on 3 which Worcester recommended. There have been some good suggestions made and I will do some experimenting now.

 

[John.g]

I can't find the pump curves for that model but can't see the need for setting 3, I would try it on setting 2 and even setting 1 might be sufficient, you could try it on both these settings to see if it helps, also you might post a link to your pump as there are other settings as well and also details of the diverter valve.
You might also check the cylinder coil and see if it has a (balancing) gate valve installed on the return and see how many turns open.

 

[Gibbo]

I can't find the pump curves for that model but can't see the need for setting 3, I would try it on setting 2 and even setting 1 might be sufficient, you could try it on both these settings to see if it helps, also you might post a link to your pump as there are other settings as well and also details of the diverter valve.
You might also check the cylinder coil and see if it has a (balancing) gate valve installed on the return and see how many turns open.

Sorry it's a grundfoss 15-70

 

[John.g]

Still can't find any pump details for that model but you could still try it at the suggested settings.,

 

[SimonG]

Still can't find any pump details for that model but you could still try it at the suggested settings.,

Worcester require it on setting 3 to meet the hot water output requirements. Had this a lot of years back and this is what they came back with.

 

[John.g]

If one isn't too worried about ultimate HW cylinder reheat times etc then reduced flow rate won't impact greatly, the main tool (apart from coil area) is having a very high boiler flow temperature on HW duty, most boilers now with HW priority boost the boiler flow temperature up to 80C. In any case even if the boiler does cycle a bit when cylinder is approaching its SP, a oil fired boiler is well able for it as it spends its whole life continuously cycling for years on end.

 

[SimonG]

If one isn't too worried about ultimate HW cylinder reheat times etc then reduced flow rate won't impact greatly, the main tool (apart from coil area) is having a very high boiler flow temperature on HW duty, most boilers now with HW priority boost the boiler flow temperature up to 80C. In any case even if the boiler does cycle a bit when cylinder is approaching its SP, a oil fired boiler is well able for it as it spends its whole life continuously cycling for years on end.

It's an external oil combi, John.

 

[John.g]

Should have seen that, heatslave was the clue!!.