Pressure drops to zero after heating is turned off

[mohthom]

I've got a problem with my boiler, a Ferroli Optima 701, in that it will work just fine for an hour or two, but as soon as it's turned off the pressure fairly quickly drops to zero. After this I have to feed in water using the isolator, and the pressure stays in the system until the next time the boiler is turned on (and off) again. I can (!) get hot water and heating, but only by allowing water into the system prior to any usage.

I've checked the pressure release pipe, and it doesn't seem to be leaking, dripping or squirting at all. I've bled all the radiators (when cold, after re-pressurising) with no spectacular results, and I can't find any evidence of wetness around radiators and pipes or dripping noises anywhere. The house is timber floored, but it's tricky to get underneath in a lot of places - but none of the pipes are set into concrete.

Is there anything I can try to get this fixed? It seems like a strange one to me - I guess if anything in the system were leaking it wouldn't hold pressure overnight (while the heating is off), but I'm very happy to be wrong and someone give me some pointers!

Many thanks in advance,

MohThoM

 

[mohthom]

Just a quick addition; after raising the pressure to just under 2 bar (its former normal operating range was 2-2.25 bar) to get some hot water (without a problem!) the gauge showed 2.15 (ish) after hot water delivery.

Also, likely to be completely unrelated, a cold tap in the bathroom that always had very weak pressure seems today to be full of beans - just got water all over my jeans!

Many thanks, any help is much appreciated,

MohThoM

 

[mohthom]

Bump for the more accurate thread title!

MohThoM

 

[gasmick2]

Hi it sounds like the expansion vessel has lost its air pressure due to prob a pinhole in the diaphram allowing water to go into the air side ,
try pressing the shrieder valve in and see if any air or water comes out??
if no water comes out then the boiler needs draining and the vessell needs repressurizing u can use a bike pump and a tyre pressure guage to check it it should hoid around 0.7 bar make sure you isolate the radiators with the isolation valves on the boiler then make sure the 3 bar p r v valve is in an oipen state while you are pumping it up
if water does come out then it needs replacing with a new one or an external one can be fitted as long as it big enough to do the job

mick

 

[MANDATE]

I've been looking at a similar problem and all I can say is that you must have leak!.
It could be in pipe work or radiators. It could be the automatic air vent/valve. It could be the Pressure relief valve or within the expansion vessel. I think the last two are the most likely.
In general terms he PRV is set to open about 3.5 bar. The pressure of water due to the rise in temperture increase is about 1 bar. The pressure when cold should be about 1 or 1.5 bar.
It could be that the pressure increase is well over the 1 bar and the PRV is operated. Once operated I understand they don't always seal again properly, hence a slight leak.
Oh and the reason for excessive increase in pressure is due to faulty expansion vessel.( split diaphram or it's lost its pressure on the 'air' side)
You can tie a plastic bag on the PRV outlet pipe and see what it gathers overnight in the way of water.

 

[mohthom]

I'm pretty sure it's not the pressure release valve - I've been out and checked this while the pressure has been dropping and no dice. I'll get the expansion vessel checked out. If this needs replacing, is it likely to be something I can do myself (given that it doesn't touch any gas - is it possible to make a direct replacement)? Is it worth me trying to raise the pressure in the expansion vessel before forking out for an inspection/expansion vessel?

I'll give the valve a go, see if that's what the problem is.

Many thanks,

MohThoM

 

[mohthom]

OK, so today I went for hot water and it's not playing at all.

When I try to repressurise it only goes to 1.6, whereas before I could push it all the way up to 2.5. Could someone confirm that this sounds like a problem with the expansion vessel?

Where do I find the expansion vessel? I've looked at manuals and it doesn't seem to be in there - it's listed as an 'optional' part in the manual, does this mean that it's likely to be located somewhere else in the house? Where are the usual places if so?

Many thanks,

MohThoM

 

[MANDATE]

I understand expansion vessels are always within the boiler casing. Mine was slotted into a space at the rear, right at the top. All I needed was a step ladder. Accessible from the top. Having said that, replacing it would not be so easy, due to the flue pipe.
I also believe a repalcement can be fitted in a different location if it makes it easier.
The expansion vessel can't prevent the pressure from going beyond 1.6 bar. You can only put that down to a low mains pressure or some form of blockage I would think.
All of the above is related to the heating side so it sounds as though there other problems if HW is affected.
Perhaps it's time to call in the specialist.

 

[mohthom]

Re-inflated the expansion vessel . . . it's happy for now!

MohThoM

 

[Master of None]

Yep. had this fault.
Fill cold.
Heating on.
Pressure climbs rapidly.
Release out of prv.
Heating off lost pressure.

Mine was a passing air valve on exp. vessel.
Vessel replaced. fixed.
although as you have done re-inflate and wait.

 

[ChrisR]

Leaking main heat exchangers can do it. They tend to leak only when hot, and the water evaporates. Eventually they get worse and leak cold as well.
if it were just a lack of air in the pressure vessel, the water must have been going out through the prv pipe.

 

[mohthom]

Is there a way to test for this? This sounds like it might be worth investigating . . .

Many thanks again,

MohThoM