Vokera Linea 28He Pressure goes really high then really low

[Strak]

Hi all,
I have a Vokera Linea 28he, approx 3 years old (servicing manual: http://cdn0.vokera.co.uk/wp-content/uploads/2009/04/linea-he-combi-boilers-installation.pdf). I have a problem which has been occurring since I bled a radiator then topped up the system pressure again a month or so ago.

- When the central heating is turned on, the pressure in the system goes really high (seems to settle at around 3.2-3.3 bar from what I can tell).
- Overnight when the system is off, the pressure drops to around 0.2 bar so in the morning we have to top up the pressure and reset the boiler. We're topping up the pressure to about 1 bar.

Symptoms are as described on this page, point 3:
//www.diynot.com/wiki/plumbing:faq:faq31

I had hoped this was an expansion vessel top up, and I just had the boiler serviced - the engineer said the expansion vessel pressure was low, topped it up but the problem remained.

The engineer says he thinks this is probably a blockage within the boiler as the system wasn't flushed properly on installation, which is consistent with what's described in the link above. Apparently the pressure release is opening at 3 bar when the system is hot, so when the system cools the pressure drops really low again because so much water has been released by the PRV. The problem is he's not sure of the most likely place for the blockage to know where to look first. Could anybody offer some advice on where he could start so I don't end up paying for days of his time to take my boiler apart bit by bit?

Many thanks,
Strak

 

[Tibbot]

When it's working, do you get hot water and the radiators heating up?

 

[Strak]

When it's working, do you get hot water and the radiators heating up?

Yep - when it's on hot water and heating operate fine. I did have something a couple of weeks ago where the shower went cold a couple of times when I was using it but that's the only time it's done it and I'd just turned the boiler back on I think.

 

[MANDATE]

The expansion vessel is provided to accomadate the extra volume of water which results when the water is heated.
Initially, before any water is connected, the vessel is pre charged with 'air' at approx 0.8bar.
This deflects the diaphragm to the water side, so the volume of air available for compression is the same volume as the vessel.
When the water is connected and pressurised to 0.8bar, the vessel is still at max volume. (full with air)
Over time this 'air' charge pressure drops and this results in the water pushing the diaphragm back.
This means the volume of water has increased before expansion takes place and the volume of 'air' has decreased.
It's this deacrease of 'air' volume that causes the ''air' pressure to rise along with the water pressure which results in the PRV operating at 3.0 bar.
So the vessel needs to be recharged to 0.8 bar, but it will not cure the problem alone. Any water already in the vessel has to be pushed back in order to get max volume of 'air'.
Also be aware too high a pressure of 'air' means a lot of resistance and if the extra water can't enter the vessel then it has nowhere to go other than through the PRV.
It is also feasible the vessel is correct, but if the flexible tube connecting it to boiler has a blockage, then again the extra water can't enter vessel.
Not only that, but it makes it more difficult if the flexible pipe has blockage , because any excess water in the vessel can't be pushed back to the boiler.

 

[Tibbot]

If heating and hot water are OK you can forget the bit about system blockage. Check that the vessel was charged correctly, with boiler drained and drain off left open

 

[Strak]

If heating and hot water are OK you can forget the bit about system blockage. Check that the vessel was charged correctly, with boiler drained and drain off left open

Ok, I'll check with the engineer to make sure he did that, but he checked a couple of times.

This link suggests that a blockage in the pipe to the PV could cause the same problem as a PV with low pressure:
//www.diynot.com/wiki/plumbing:faq:faq31

I guess that gives him one place to look first!

Cheers,
James

 

[Strak]

It is also feasible the vessel is correct, but if the flexible tube connecting it to boiler has a blockage, then again the extra water can't enter vessel.
Not only that, but it makes it more difficult if the flexible pipe has blockage , because any excess water in the vessel can't be pushed back to the boiler.

Thanks MANDATE, I'm going to ask him to look at the hose connected to the PV as I think pressure is set correctly and there was no water in it. The pressure was actually topped up to 1 bar which is specified by the manufacturer.