Search the forum,

Discuss Auto By-pass position in the Plumbing Jobs | The Job-board area at PlumbersForums.net

Status
Not open for further replies.

GH77

Gas Engineer
Messages
85
IMG_5903.JPG
Hi all, just a quick one that hopefully is relatively straightforward, i was asked to look at a noisy s-plan system yesterday, boiler is a Greenstar 30 CDi system Erp installed about 9 months ago, the customer has said that they get a loud clonking noise when the DHW turns off...

I looked at the system and concluded that the Auto Bypass is too close to the DHW zone valve, tried adjusting the setting on it, that makes the clonk louder or quieter depending how it is set, but never eliminates it... just wondered if anyone knows of a minimum distance it should be installed?

Thanks for your help in advance
 
Looks fine to me
 
I would have installed it to the right of the T before it branches off to the 2 ports myself where the X is marked on the flow. Whether it cures the water hammer is another thing. Is the valve installed the correct way round and not against the flow?
 
Yeah i put the x on there as the guy that fitted it is going to come back and change it so i marked the pipe to show where I would have put it ... its installed the right way round, doesn't clonk when operating CH only
 
Another line of investigation could be:

Imagine the DWH heating so the boiler is steaming away at 30kW and there is near-zero flow into the CH zone. When the DWH zone valve closes, if the CH valve is open (or leaking) the 30kW is diverted into the CH pipework which will heat up very fast and clonking is the pipes expanding rather suddenly. The ABV will mitigate but probably not cure this, as observed.

If you go old-school and put a balancing valve in the DWH circuit then you can increase the impedance of that loop slightly so that there is enough flow into the CH branch to keep it warm while the DWH zone valve is open.

You can test this idea by putting your finger on the pipe below the CH zone valve. If my guess is right it's going to be relatively cool while the DHW is heating and get very hot very fast when the DHW zone valve closes and the clonking will start soon after.
 
I can certainly test the theory with a temperature clamp, both zone valves are beand new Honeywell items though, so i'll be disappointed if it is passing
 
I can certainly test the theory with a temperature clamp, both zone valves are beand new Honeywell items though, so i'll be disappointed if it is passing

A couple more questions... What range is the circulation pump set on and can you say a bit more about the 'clonking'. How often, how loud, where from?

To address your original question I'm sure there is a minimum distance between boiler and ABV. (Strictly speaking it's specifiying the minimum volume of circulating water so the heat capacity is enough to keep the HX temperature from overshooting.) The figure I have in my mind is 5 m round-trip but I can't find that figure written down anywhere so I may be mistaken. The MIs show the external expansion vessel on the return as close as possible to the boiler so it's inside the loop. I suggest you call WB technical support and check with them.

Incidentally, the pipework looks well-done to me; nice swept bends and neat fillets on the soldered joints. Don't often see nice swept bends these days...
 
The pump is the smart pump inside the system boiler,

The clonk seems to be coming from the auto bypass when the DHW zone valve closes, if the CH is running then it doesn't do it,

The Boiler is in an adjacent room, i'm sure it's around 2-3 metres from the Auto By pass, there's no room where the boiler is to fit the external expansion vessel, there is the boilers own internal vessel though ...

I can't take credit for the Pipework as I didn't fit it :)
 
The pump is the smart pump inside the system boiler,

The clonk seems to be coming from the auto bypass when the DHW zone valve closes, if the CH is running then it doesn't do it,)

Okay, my previous suggestion isn't likely the cause then. Assuming the red vessel is the CH external expansion vessel, it seems to be connected to a branch off the boiler-ABV loop. I think you will find that moving the expansion vessel connection to a point on the return that is inside the loop, will probably fix the problem. This is the position that the MI's recommend. The explanation is that there is a shock wave that propagates back from the ABV when it closes and you want the expansion vessel in a position that will damp it as effectively as possible. When the CH Zone valve is open the CH zone acts as the damper.

Assuming you have the Greenstar 30i ErP (not quite the model you said but more consistent with your description) there are several settings for the pump that are described in the section 'Available pump head' in the installation guide. The default setting is 4 (constant pressure high) but this is unnecessarily high for a lot of properties. The MIs say "In order to save as much energy as possible and the minimise the possibility of water circulation noise, a low characteristic should be chosen." so if it's been left at the default setting try a lower one. Unless WB Tech say otherwise I'd stick with constant pressure (settings 1-4) and go for as low as possible comensurate with all the radiators getting warm.

If you're lucky, you may find you can fix the clunking just by reducing the pump pressure. This will reduce the amplitude of the hydraulic shock at source. Adjusting ABVs in constant P systems is a delicate process. You want to set it so that it is just not open when either of the zone valves is open but opens when they both close and the boiler is in 'over-run' mode.
 
Last edited:
The Boiler is a Worcester Bosch Greenstar 30 CDi Classic System Erp, I can't find anything in the MI's about adjusting the pump speed, but i scan read ... i know you can on the i models as i have done this on another installation where there was an issue with pump noise... the external expansion vessel is connected to the CH return, the installer is going back to the job to move the Auto By pass valve tomorrow, i will suggest to him to also move the expansion vessel so that it is tee'd into the return as close to the boiler as possible... thanks for the input Chuck, always good to bounce ideas off other people :)
 
The Boiler is a Worcester Bosch Greenstar 30 CDi Classic System Erp, I can't find anything in the MI's about adjusting the pump speed, but i scan read . i know you can on the i models as i have done this on another installation where there was an issue with pump noise... the external expansion vessel is connected to the CH return, the installer is going back to the job to move the Auto By pass valve tomorrow, i will suggest to him to also move the expansion vessel so that it is tee'd into the return as close to the boiler as possible... thanks for the input Chuck, always good to bounce ideas off other people :)

Pump has a selector on the side (like the old stnd grunds)
 
IMG_5909.PNG
Are you sure? ... i'll have a look, it's the erp one, thought they'd changed it ... had a look online using the part number from the mi's as above, but can't find a picture showing the required detail??
 
View attachment 30384 Are you sure? . i'll have a look, it's the erp one, thought they'd changed it . had a look online using the part number from the mi's as above, but can't find a picture showing the required detail??

Should be on the left side unless they have changed the cdi to digital display in the front panel?
 
Last edited:
I'll have to look further into it to be honest, but cheers for the steer Shaun
 
I think you are on the right track GH77 I would advise to do as you stated move the bypass where you put the x and make sure the vessel is tapped into the return with a free path back to the boiler cheers kop.
 
Status
Not open for further replies.

Reply to Auto By-pass position in the Plumbing Jobs | The Job-board area at PlumbersForums.net

Creating content since 2001. Untold Media.

OFFICIAL SPONSORS

OFFICIAL SPONSORS

Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock