Bosch Wocester 28Si pressure gauage keeps rising

[BigG]

Hi,

I've got a Bosch Worcester 28Si combi boiler which I have been using for the past 5 years or so without any issues.

The Norm......

I've always kept the pressure gauge just above 1.0bar when CH not on. The pressure normally rises to approx 2.0bar whilst CH is on and then when CH has been off for a while pressure drops back down to just above 1.0 bar again.

The past few days......

A couple of days ago I suddenly noticed the pressure gauge was reading 2.7 bar (whilst CH had been on a while). Whilst this was higher than usual (and 0.2bar over the recommended maximum pressure) I was not too concerned. However after the CH having been off for over half a day I noticed the gauge was still reading 2.7 bar. This now started to get my attention!!!

I understand that the pressure relief valve on this model is set to 3.0 bar so at this point I understand (I think) it did not need to release the pressure. I have checked the outlet pipe that is connected to the pressure relief valve just to make sure its was free from obstruction in case the valve opened to release the pressure sometime in the near future when the CH is on and I forget to manually check/release the pressure beforehand.

To release the pressure back down to 1.0 bar I remove the front panel and turn a small plastic nob on the side of the pressure release valve to release the water down a small plastic pipe (about 3 medium glasses full is required).

Once the pressure is back down to 1.0 bar it remains there. The only time the pressure rises again is when the CH is switched on....and then I'm back to the stage of having to manually release the pressure when we switch the CH back off.

I've been through all the other plumbing topics and I've seen another thread relating to increasing pressure on a Bosch combi but that indicated that mains water was still entering the CH system so I thought that for the benefit of clarity...."The pressure does increase/decrease whilst the CH is off, it only increases when the CH is on and then does not appear to drop back down to original pressure when CH is switched off".

I hope I've provided enough information.

I'd be ever so grateful if anybody has any ideas as to what the issue(s) could be? If only so I can have an idea of what might be causing the issue if I resort to calling out a plumber.

I'm a keen DIY'er but will always leave anything GAS related to a registered CORGI installer.


Regards,
BigG.

 

[bones001]

you would have been able to find out the problem yourself by searching in half the time it took you to write your post. expansion vessel. check/remove the filling loop as it may be passing slightly and slowly filling your system

 

[BigG]

Thanks a lot bones001.

I'll have a dig around and let you know how I get on.

 

[croydoncorgi]

There's only two places mains pressure water can get into a combi boiler: through an open filling loop or across a broken hot water heat exchanger. If you're sure the filling loop is not letting-by, then it's the HX.

 

[mmalone]

Hi,

I've got a Bosch Worcester 28Si combi boiler which I have been using for the past 5 years or so without any issues.

The Norm......

I've always kept the pressure gauge just above 1.0bar when CH not on. The pressure normally rises to approx 2.0bar whilst CH is on and then when CH has been off for a while pressure drops back down to just above 1.0 bar again.

The past few days......

A couple of days ago I suddenly noticed the pressure gauge was reading 2.7 bar (whilst CH had been on a while). Whilst this was higher than usual (and 0.2bar over the recommended maximum pressure) I was not too concerned. However after the CH having been off for over half a day I noticed the gauge was still reading 2.7 bar. This now started to get my attention!!!

I understand that the pressure relief valve on this model is set to 3.0 bar so at this point I understand (I think) it did not need to release the pressure. I have checked the outlet pipe that is connected to the pressure relief valve just to make sure its was free from obstruction in case the valve opened to release the pressure sometime in the near future when the CH is on and I forget to manually check/release the pressure beforehand.

To release the pressure back down to 1.0 bar I remove the front panel and turn a small plastic nob on the side of the pressure release valve to release the water down a small plastic pipe (about 3 medium glasses full is required).

Once the pressure is back down to 1.0 bar it remains there. The only time the pressure rises again is when the CH is switched on....and then I'm back to the stage of having to manually release the pressure when we switch the CH back off.

I've been through all the other plumbing topics and I've seen another thread relating to increasing pressure on a Bosch combi but that indicated that mains water was still entering the CH system so I thought that for the benefit of clarity...."The pressure does increase/decrease whilst the CH is off, it only increases when the CH is on and then does not appear to drop back down to original pressure when CH is switched off".

I hope I've provided enough information.

I'd be ever so grateful if anybody has any ideas as to what the issue(s) could be? If only so I can have an idea of what might be causing the issue if I resort to calling out a plumber.

I'm a keen DIY'er but will always leave anything GAS related to a registered CORGI installer.


Regards,
BigG.

you may find the expansion vessel has lost its charge therefore full of water and little or no air.

If the system pressure is only rising during operation Id be inclined to check this out, failing that you may have a blockage

 

[BigG]

Hi All,

Firstly thanks for all the replies so far.

I have performed some further analysis since I posted the original mail.

Here are the facts as I see them now......

Test 1
-------
1) Bleed the system down to 1.0bar with bleed knob on side of relief valve.
2) Leave to stand for 4 hours without CH on (or having been on recently).

Result - The pressure rises to 2.7bar This is despite my original posting stating that the pressure did not rise without the CH on - sorry !

Test 2
-------
1) Disconnect the filling loop. Leaving the one way valve attached to the CH return pipe - so no water can escape! Also leaving the in-line tap on the CW supply that feeds the filling loop.
2) Leave and observe for a few hours whilst CH is off.
3) Leave and observe for another few hours whilst CH is on.

Result - In both cases the pressure started to rise like test 1 (although with the CH on it rised faster as it normally does). At no point did I see a failure of the in-line tap letting out water from the CW supply nor did I see any failure of the one way valve - just to ensure that crap from CH is not pumping back out into CW supply.

Is my next best bet inspecting the expansion vessel or the HX? How can a HX transfer water from the domestic mains/DHW to the CH system? Surely within the HX these 2 systems are separate?

Can I eliminate a blockage by simply ensuring all rads are hot (because they all work when CH is on) or are we talking about a possible blockage to the expansion vessel?

 

[BigG]

Sorry missed out the following. Don't know how crucial the test was......

Test 3
-------

1) Bleed the system down to 1.0bar with bleed knob on side of relief valve.
2) Turn off CW mains supply to house.
3) Leave to stand for 4 hours without CH on (or having been on recently).

Result - no rise in pressure guage at all.

 

[chrishutt]

Leak from mains is via the secondary heat exchanger. You need to change this (as croydon.corgi pointed out above).

 

[BigG]

Just a quick update.

Had the further inner cover off the combi and can now see the burner and the HX. I assume that the secondary HX is simply the CW mains that runs through some form of internal container that contains the HW itself (a bit like my old hot water tank that used to be in the airing cupboard instead of the combi). i.e. heat transfer via water and not burner.

With the price of the HX being approx £200 I felt I ought to call the engineer that installed the boiler as it would be an expensive mistake to make if the HX is sound. (I do not wish to offend anyone that has already stated its the HX but as boilers are not my day job I'm still approaching this problem with caution).

I left all the details with his wife. I've been informed he might be able to pop round today sometime to take a look at the boiler. I will class this as a further lesson in understanding my combi. I'll be firing questions at him left right and centre (in a polite way of course).

I feel my wallet getting lighter all of a sudden!

Many thanks to everyone that has contributed so far and I'll update this subject with any new details that emerge.

 

[HarrogateGas]

YOur boiler does not have a secondary heat ex, it is a twin pass primary heat exchanger and it is passing main to ch. I changed on only yesterday on this very boiler for this problem.

Takes a bout 20 mins if you know what your doing.

 

[BigG]

Have spoken to my local engineer. He confirms from the symptoms that its the HX. He advised to speak to Bosch to find out how much a replacement would cost.

Bosch have informed me they only sell spares to merchants.

Have contacted the Plumb Centre and others. Typical price is around £260+ each time (excluding VAT at 17.5%).

Looked on internet. Found keeptheheaton.com Brand new (not reconditioned) with 12 months guarantee all for 198.69 (including VAT and delivery). I've ordered one and it should arrive within the next day or two.

I've contacted my engineer again and booked an appointment for this coming Friday. The way I see it the money I've saved on the price of the HX I can spend getting expert help (in case things go wrong) and at the same time get him to totally flush my CH and also re-introduce an inhibitor into the system.

N.B. Apologies about the phrase secondary heat exchanger. I thought this phrase was still used even if the DHW and CH connections were contained within the same HX unit.

 

[jiminblack]

Worcester would have fitted a new Heat ex for 245.00 vat and part inclusive.

 

[BigG]

On Friday the engineer came round. He changed the HX for me and all has been o.k. since.

At the same time some inhibitor was introduced into the system (at my request) via the filling loop and he gave the boiler an all round check and gave it a bill of health.

He was interested to know where I had got the part from considering it was delivered so fast and the price was good.

Would like to say thanks to all people who took the time to reply to my posting. Within the space of a few weeks I've gone from knowing absoloutly nothing about my combi to know what all the components are and what they do and how to go about diagnosing this type of fault.

I only hope this topic helps someone else as well in the future.

Keep up the good work !!!!!!