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Discuss Short cycling Bosch Worcester 30CDi Greenstar - Bypass Valve? in the Plumbing Jobs | The Job-board area at PlumbersForums.net

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My first post, so apologies for any incorrect terminology

I had a Bosch Worcester 30CDi Greenstar Classic Regular Boiler installed in 2014.

The Boiler is used in a vented S-plan system, with two motorised valves, a manual Bypass Valve, a British Gas (Grundfos) Multi-head pump, Hot Water cylinder and 12 radiators of varying sizes (Total Output of the Radiators come to 18.5kW) Bosch have told be that the Boiler isn't over-spec'd for the current configuration.

I was more than happy with the installation, but have never seemed to achieve the savings I thought I may have done over my old Ideal Turbo 2 boiler.

On top of that, the majority of the radiators in my house never get to a temperature that I would expect, but assumed it was because of poor flow rates through the radiators due to sludge build-up etc.

I have since noticed that the boiler appears to be short-cycling. I assume this is the term as the Boiler frequently stops and starts through out the day despite there being a demand for heat from the Central Heating Timer. (I've checked the Switched Live to the boiler and it's there so heat is being called for)
The Room Stat is working as expected and when temperature is satisfied, Boiler shuts down and pump goes onto over-run.

After further investigation, although some heated water is reaching the radiators (some radiators getting very hot, some just warm & some cool) , it appears that a lot of the heated water is passing through the Bypass valve back into the return to the boiler and both 28mm pipes at the boiler are very hot. At the point where the Bypass valve is connected to the Return pipe, upstream of the connection, the pipe is luke warm, but downstream of the connection, it's very hot.

I understand that this is perhaps why the Boiler is short-cycling as not enough heat is dissipated through the radiators and most of the heated water is just circulating. Why this is, I don't know, but thought it may be because of blockages in radiators/pipework?

However, the supply pipes to each radiator in the 2-pipe system are extremely hot so hot water is being presented to the radiators.

I've had a couple of radiators off and although full of dirty water, weren't blocked. I've even replaced one radiator and TRV with new items, but still only get a luke warm radiator.

I've had the bleed screw off the top of the pump and the pump is spinning. When stopped, the shaft is easy to turn.

So, I suppose my question is:

Is the issue more than likely the Bypass valve, which is a manual gate valve type arrangement (with a lock shield) or an inefficient pump?

Any suggestions or methods for testing would be appreciated.
 
A lockshield type valve is not how it's done now things have moved on there needs to be a auto bypass valve fitted which will only open when the the system pressure increases as radiator TRV valves shut down and zone valves close . Reading between the lines it sound like the system would benifit from a through chemical clean and power flush a magnetic filter fitting if not already, as a experiment fully close off the bypass lockshield valve remove one trv head so the pump overrun has a route if needed and see if that improves the distribution of heat this is only a short term fix and as above I would address this asap . Cheers Kop
 
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Thanks for the reply.

I appreciate that an automatic bypass valve is now the norm. I contacted Bosch about the manual lockshield valve and they weren't unduly concerned that an ABV hadn't been fittted, but I also appreciate that fitting one would be advantageous.

A Magnetic filter was fitted at the time of the Boiler installation - it's been checked/cleaned on every yearly service.

I thought about having a power flush carried out, but as mentioned, I've removed a number of radiators and found nothing untoward in terms of sludge. Water flows well out of the rads/TRV/lockshields.

With all Radiator TRV's open, Manual Valves open and all lockshields open, very hot water is passing through the Bypass value.

I can try closing off the Bypass valve as suggested and opening just one of the 'cool' radiator valves to see what happens, but as the gate valve looks to be quite old, I don't rate my chances of it budging!

Can I ask what the normal procedure would be to test the pump?
I can see the shaft is rotating and there must be some flow, but how would I test for sufficient pump flow? (It's a British Gas Multihead G pump - Grundfos, set on Speed 3)

Thanks in advance.
 
As Shaun said, fit a ABV as this sounds like a contributing factor to you circulation issues. Also once the ABV is fitted get the system properly balanced. Lastly try offsetting the heating and hot water times on the programmer as this may help the circulation (could also try installing a gate valve on the cylinder return if the last suggestion solves the issue) Cheers.
 
Once again, thanks for the suggestions.

Can I ask, how do you normally check pump efficiency?
I mean, other than checking that the pump is actually running, is there a method for checking flow rate?

Is there an off-the-shelf flow meter that can be purchased to fit in-line that would prove sufficient flow rate or is a pressure gauge used?

Just wondering how flow rate would be normally checked.

Thanks.
 
I have exactly the same problem!
Is there a way of stopping the water bypassing the radiators
 

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