Biasi slowly losing pressure, but slightly different...

[benjybobs]

Hi everyone

I registered here for some advice, as after reading many threads about slow pressure loss on combi boilers, none seem to be quite the same as what I'm experiencing and would just like to help narrow things down before I call an engineer out.

I have a Biasi M90D.24s Its fairly new I believe but was in the house when I moved in 18months ago.

Ominously the filling loop was in place when we moved in, which implies a known problem. The boiler pressure was at zero, so I filled it and everything worked fine. no problems with hot water or heating for all this time.

however the pressure does seem to fall slowly over time, I need to keep a closer eye on what conditions this happens, but it lost pressure last summer without any use of the heating.

Now the obvious thing would be a leak, its difficult to check the whole house as some pipes are under boards which are also under stone tiles, but I'm fairly sure I've no leaks on the heating circuit.

This seems backed up by the fact that despite having recharged the heating maybe 4-5 times in the last 6 months (not full loss of pressure, just a top up) I never find any air in the rad's when I bleed them.

Now the curious bit, the hotwater tap in the bathroom is a cheap one and is probably getting to the point where I need to change the washer as its sometimes tricky to turn off without a drip. Only when the heating is on I can hear air passing through this tap, and the hot water arrives at it suspiciously quickly after the heating has been on recently, followed by a cold period and then hot again.

Can anyone explain this, and is the pressure loss related, or is it more likely I have two different issues?

I wondered if the bypass valve was somehow letting water circulate the DHW circuit just a little when the heating is on?

Could there be a leak in the hotwater heat exchanger allowing CHW to escape into the DHW?

Can the diverter valves allow a little to go each way?

Any guidance would be appreciated.

Thanks

Ben

 

[Agile]

You do seeem to have been overthinking and come up with many wrong conclusions!

That model should not have been installed after April 2005! They were not available to buy for long after that. Or perhaps you think seven years old is nearly new? The last four digits on the serial number is the make date! What is yours?

The filling loop is left in place on 99% of installations and means nothing.

Have you even read the FAQ on this site? And checked the PRV vent? And isolated the boiler overnight?

Its normal for the pressure to need topping up every 2-4 months.

Your description of something odd at the hot tap does not seem to make any sense based on normal situations.

Tony

 

[benjybobs]

Tony et al

Thanks for getting back to me, I'm not sure I've come to any conclusions yet (at least regarding the boiler anyway) as that is why I've joined a forum to hopefully seek some advice on things I have no experience of.

I have been thinking a lot though, and worrying a little too. I think an inquisitive frame of mind is often useful before handing blank cheques to service engineers and trying to learn a little about a problem first often results in better understanding of what is needed to move things along.

I'm not sure if you're trying to be a little condescending or if I have started out here on the wrong foot however I realise the wrong tone can be inferred easily on forums like this so I'll happily give you the benefit of the doubt as I've read some helpful posts from you on other peoples threads. Anyway apologies again, and for this lengthy reply that hopefully helps focus some assistance for me.

I cant comment on when the boiler was fitted, I stated I moved in 18months ago and the previous owner told me it was new. I have no documentation for it, or safety certificates etc. I don't even know the model number but the one I listed was written on the front of the manual that was the "closest match" on the Biasi website. The only difference on mine seems to be a less advanced programming function.

Having rented probably 15 different houses over the years with combi boilers, this is the first house that the filling loop was ready in place. I mentioned this as anecdotal evidence that perhaps the previous owner had cause to use it often. Also a 7 year old boiler is indeed in my experience relatively new compared to some of the ones I've had the misfortune of paying the gas bills on !

I haven't leak checked the heating system / boiler by locking it off over night. This is a good idea though so will try this once he missus is away and doesn't demand it be left on !

I've just been and checked the PRV and found someone has rammed a twig up it. its not sealed it but odd all the same.

Anyway, I decided to see if I could provoke it so I ran the heating on full to see what pressure the boiler ran at.

From a cold pressure of 1.3 bar, it ran at full power and the pressure was around 2.5 bar. I then turned on a kitchen hot tap, and noticed no change. When I turned on my suspect bathroom sink tap, the pressure rose to nearly 3 bar, the pipes to the sink clanged and vibrated and indeed I saw some drips from the PRV pipe out the back. I immediately turned the tap off and the pressure slowly dropped down again.

Now at risk of thinking too much again, I wonder if its possible that the bathroom sink hot tap is more restricted somehow than any other hot tap in the house. This reduced flow results in increased water temps and thus pressure temporarily in the system, especially exaggerated when the boiler has residual heat in it from running the heating before the tap was turned on. This then clearly blows the excess out of the PRV and thus once cooled down results in a lower system pressure than before.

This coupled with my leaky tap that I can audibly hear pressure escaping from might be introducing some air into that same hot water pipe resulting in hot pockets of air which makes the pipe clang and rattle as its hotter than it should be. (the water from this tap is hotter than any other in the house and does spit and bubble a bit if not fully open).

So when conditions are perfect, i.e. the missus has whacked the heating up full and then run the sink in the bathroom it could be venting the PRV quite regularly.

Does this sound sensible or fantasy?

What could be the best way to try and improve things? simply run the boiler at lower pressure maybe just less than 1 bar cold, or maybe the expansion tank needs some attention?

I feel I've made some progress with this now, and that is thanks to this forum, and your reply Tony so Thanks.

One odd thing still remains though, I was sitting on the loo reading my Viz, and listening to the subtle but audible "hiss" of the leaky hot tap in the sink, and thinking that I should sort it out soon when it suddenly stopped, and this coincided with the timer on the heating switching to off. I cant think why pressure in that hot tap should suddenly reduce the second the heating turned off when the two loops should never meet!

Ben

 

[petertheplumber12]

The symptom you describe with air coming out of the hot water tap is a new one on me. As long as you are getting at least 3 litres /min from hot tap the secondary heat exchanger is ok. The only possibility is air is being sucked into hot water line by a ventury action. Something is causing it and it may be worth looking at the divertor valve. But since the boiler is probably near enough 8-10 years old it may be worth preparing yourself for a new boiler sooner rather than later.

 

[Agile]

Modern boilers are hoped to give a service llife of at least 10 years if you are lucky. So yours is getting close to that and not new in anyone else's vocabulary.

The pressure should only go up by about 0.3 Bar when heating is hot. Look at the FAQ about repressurising the EXV. It may be broken and when you first push the air valve then water will come out all the time. If its stops flowing then there is a good chance its retrievable.

The plate HE may well be blocked. Run a hot tap and the temp shown should not go much higher than 60 C.

Tony

 

[benjybobs]

Thanks for the help, the pressure in the EV will be checked next.

The system could do with s drain down anyway as I'd like to fit some thermoststic valves to the rads and make one or two changes to rad sizes ones too big for the room and another is too small.

From what I've gleamed from the previous owner, I think he had a "plumber" friend fit this system. The boiler might be an old model, but I dont know if it's been fitted as long ago as 7 years. He's a little sketchy and enjoys a drink so not always the most forthcoming on detail.

Anyway i plan to have it checked out by a gas safe pro, I'm not expecting him to sign it off and give it a certificate but at least he can ensure there are no glaring oversights with it. in the meantime its easy to work around the problem without provoking it and save up for the modifications it needs.

Thanks for the input