Blocked pipe to Pressure Vessel?

[Womble123]

I have an oil-fired Boulter Classic boiler, sealed system. The pressure of the system is going right up and then right down - exactly as would be expected for a faulty pressure vessel....except that there is nothing wrong with it, I've had it checked by an OFTEC engineer. I read in the FAQ that this problem could also be caused by a blockage in the pipe between PV and boiler:
1. Could someone step me through the procedure for checking this ( i don't want to just disconnect the pipe without knowing what I'm doing)?
2. Any other ideas of the cause of this fluctuation?
3. Could a leak in pipework cause pressure to go too high as well as too low? I am not over-filling the system when topping up.

Many, many thanks.

 

[Womble123]

Forgot to say that the PRV also checked out ok and the AAV doesn't seem to be hissing/leaking, certainly not noticeable anyway.

 

[Womble123]

Amendment - sorry to mess around but I beleive the PRV is now faulty, even if it wasn't before. Losing about 2 or 3 litres a day. This would explain the drop in pressure but I'm still at a loss as to what is causing the excessive rise in pressure on topping up and re-starting the boiler. I'm definately not over-filling!!

 

[kevplumb]

I've had it checked by an OFTEC engineer. I read in the FAQ that this problem could also be caused by a blockage in the pipe between PV and boiler:
1. Could someone step me through the procedure for checking this ( i

isolate and drain the boiler

remove the pipe

 

[Womble123]

Thanks for the help. I've now spoken directly to Boulter and they reckon the problem is a faulty Heat Exchanger. I've also seen this on other posts. Probably expensive and not sure my enginner is up to it - why couldn't he suggest that! Will keep this thread going (hopefully with an eventual solution) incase it helps others.

 

[Nixt]

I've now spoken directly to Boulter and they reckon the problem is a faulty Heat Exchanger

This diagnosis is unlikely to the extent of being almost impossible as the only thing that can go wrong with it is a leak. The expansion vessel is probably undersized as they are on most oil system boilers. You will need to calculate the system water volume and check it against the correct vessel sizing for your system. Any competent engineer should be able to fit an additional vessel for you.

 

[oilman]

I agree with Nixt. Fitting a 16 litre external should be more than enough for an average property. You can't have a vessel thats too large. When you get to the size of the planet, it is just a vented system.

 

[Womble123]

Thanks for the input - it is a large system (I've got a 7 bedroom guest house), the expansion vessel is 18 litres. I should also add that the boiler has been working perfectly well since installation some 6 years ago - I don't understand why the vessel would suddenly be inadequate.

 

[Nixt]

How has the vessel been checked? It would have to be removed after boiler drain and checked independently of the boiler. Attaching a gauge whilst in situ will only record the system and not vessel pre-charge air pressure. From what you describe 18ltr is not adequate size in any case.

 

[oilman]

Cars work fine for years then suddenly break down, or that is how it appears.

Fit a 32 litre vessel. You only need to relieve the system pressure to check the existing vessel, but only if it doesn't sit on the floor with the air valve at the bottom, which often seems to be the case. Fitted by someone competent?

 

[Womble123]

Hi - thanks for feedback. I have now checked the pipe from boiler to PV and there is no blockage, water flows out. I have also increased the pressure in the PV from 1 bar to 1.5 bar (which my boiler manual says is the max recommended for system) but this has made no difference and the boiler is still reaching 3.5 bar when switched on. I will talk to my engineer on Monday regarding replacing the PV with a bigger one as you both have suggested. Cheers for now.

 

[Womble123]

Two more questions:

1. I've just seen the price of expansion vessels - when you say (oilman), to fit a 32 litre vessel, I presume it's ok to add 14 litres (if they do 'em that size) to my existing 18 litres rather than buy 1 big vessel?

2. How do I calculate my system capacity? I have 22 rads, most are fairly large.

Cheers.

 

[simond]

right on, Mr Womble. You could just add a smaller one but to get to the exact size you need to work it out.

The size of the expansion vessel is directly related to the water volume in the system rather than the number of rads, although the quantity of rads gives a rule of thumb.

If in doubt, as Oilman says, just oversize it. No harm can done, providing the front door will still open

 

[ChrisR]

You shouldn't have to pay much above £1 per litre.

 

[Womble123]

After all that, it WAS the pipe between boiler and PV. Not completely blocked (explaining why water comes out when disconnected) but blocked enough to cause a problem. Cleaned it through and now everything fine. Sadly I had to pay £185 to a Worcester/Bosch engineer (they own Boulter now) to come out as all local guys had given up.

It made me feel better that the W/B guy even took about 2 1/2 hours to find the problem (no, he wasn't paid by the hour ) - the fact that it was only a PARTIAL blockage was what threw everyone.

Cheers all - I'm off for a hot bath.

PS - Chris R, if you read this, may I suggest that the section on boiler info, which mentions blocked pipe to PV, could be amended to mention partial blockage and that the pipe should be completely removed and inspected to see if there is a problem.