Viessman Vitodens 100-w pressure problem

[bulutug]

Hi

I have a Viessman Vitodens 100-W combi boiler WB1B

If topped up to pressure 1 bar when turned off and cool, it goes down to about 0.4 bar after 1-2 hours. when it runs, it can properly heat up the radiators and provide hot water but the pressure keeps raising and rising and can sometimes go all the way to 3.5 bars.

there are not fault messages at all.
then when left to cool it goes back to very low pressures below 1 bar.

a serviceman checked the expansion vessel pressure and it was topped up to 1.5 bar - there where barely any water inside.
what could be wrong with this boiler?? please help

 

[terryplumb]

Your expansion vessel isn't controlling the expansion . The PRV will be lifting at pressures around 3 bar ,and allowing water out of the system.

 

[bulutug]

but the expansion vessel has been checked and it was ok... what could be wrong with it?
the serviceman checked whether it could hold the pressure stable and it did. there wasn't any excess water in it either..
could it be anything other than the vessel?

 

[bulutug]

also - even with the boiler turned off -the pressure slowly goes down to below 0.5 bar
if the PRV only operates above 3 then why would the pressure keep going down. also I can't see any water coming out either.

 

[terryplumb]

Tie a plastic bag over the PRV pipework outside to see if water collects in there.
When did EV get checked by service guy ,and was pressure being lost immediately after his visit ?

 

[bulutug]

pressure was being lost after his visit yes - but at a very slow speed. around 0.05 bar an hour. now it seems to go down faster..

unfortunately can't tie anything around the PRV pipe outside as it is upstairs and outside the building. maybe a bucket underneath where I expect it to drip but in dry weather... it has been raining for days and it is expected to continue.. if there is any dripping from PRV pipe would that mean something is definitely wrong or does it mean it is functioning normally?

why would the pressure be going down constantly..
and why would the pressure go up way too high when the heating is on. are those two problems related?

 

[terryplumb]

When central heating is operating ,the water is heated and it expands,thus increasing the pressure in the sealed system. The expansion vessel isn't doing its job, hence the increase gets to 3 bar,and the PRV lifts and dumps water.
Once system is off ,and cools the pressure starts to drop.

 

[bulutug]

but what I'm saying is the pressure goes down even when the boiler is off...
if I turn it off and completely wait for it to cool down then if I top up the water all the way to 1.5 bar.. after a few hours it would be around 0.7 bar. without the boiler working at all.

 

[terryplumb]

That's a different problem to the system pressure reaching 3 bar.
Water is escaping out the sealed system if boiler is not operating ,and pressure is dropping.
That could be the PRV isnt fully seating and letting water out.
You could have a leak on the rads or their Pipework.
Or an internal leakage within the boiler.

 

[bulutug]

there is no sign of any leakage inside the boiler or leak from us to downstairs or around the radiators.. also the PRV isn't dripping anything out
we don't know what is causing the slow drop on the pressure..
i'm totally lost...

 

[terryplumb]

Water loss within the boiler can be through the heat exchanger ,and water goes out the condensate drain where it will not be noticeable.
If all the other earlier mentioned things are ruled out ,then the heat exchanger is really the only other suspect.
With all the wet weather we have had lately , I don't see how you can dismiss the PRV as not leaking water out ,given its position.

 

[bulutug]

i think changing that costs as much as a new boiler

could auto air vent have anything to do with this btw?

maybe I should try getting the vessel and prv replaced first.. i don't know where to start or who could do this without costing a huge sum... prv is so hard to reach. they would have to take the whole thing apart.

 

[bulutug]

does the pressure shown on the boiler pressure gauge also affect the pressure on radiator pipes around the house? I'm wondering if high pressure that I see on the boiler could cause an underfloor radiator pipe to burst..

 

[terryplumb]

The pressure gauge on the boiler shows the pressure of your sealed ,central heating system. So boiler, radiators ,and the pipes that join them all together are all under the pressure shown on the gauge.

 

[bulutug]

that is quite worrying.. I guess a 3-4 bar pressure could then cause a pipe to burst and leak to my neighbour downstairs?