Boiler losing pressure but only when HW on and CH off

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Hi all,

We moved into the house July 22 and, apart from the occasional top up, have had no issues with boiler, HW or CH. Recently, however, we've started having issues with the pressure on the boiler but only when only the HW is on.

What happens is that I can check the pressure all day long when the CH and HW are on and it will vary from 1bar up to 2.2bar but if there is a time when only the HW is on then the pressure will drop to near zero and I can hear bubbling noises in/near the HW tank. If I repressurise at that point then the bubbling goes away. CH works no problem. No obvious leaks anywhere.

For background:
I had one of the isolation valves on the filling loop replaced last week as it was leaking which I thought the cause of the drop in pressure but it's still happening.
The HW is also connected to two radiators in the bathrooms (so when HW is on those rads are on regardless of whether CH is on). There is never any air in these and they aren't leaking.
Worcester boiler - no more than 5 years old, single flow and return for both CH and HW

Any ideas on what's causing this? What I'm wanting to know is whether this is a simple fix that an engineer can do or if it's one of those "could be anything" sort of problems will it be cheaper to replace boiler or tank?

Cheers
 
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If you are continually adding water/ topping up pressure (when only hot water is running and pressure drops) then you must be losing water out of the system somewhere. Is water coming out of the pressure relief pipework,which is usually routed externally in copper pipe ?
 
There is only one copper pipe that runs outside from the boiler and it comes out of the wall horizontally then loops back on itself so it's facing the wall. Would that be the right pipe to look at?
 
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HW circuit connected to rads?

What is the logic behind that?
 
It means the bathroom rads come on all year round for a short time so towels can dry even when it's too warm for full CH to kick in. I've found it quite a good feature over the past year that I've lived here.
 
Yes. Tie a plastic bag around it to catch any water.
So I did as you said and watched it for the rest of the day and here are the results:

From 1430-2200 (standard CH time, HW was on from 1430-1500 and 2000-2030):
Boiler at 1.5-2bar
No water from PR pipe

2200-2230 (0-30 mins after CH off):
Boiler at went from 1.5bar to 0bar over that 30 minute period
No water from PR pipe
I topped up to 1 bar at this point.

This morning, 0730 (CH kicks in 0700-0800, HW set to 0700-0730)
Boiler at 1.5bar
250ml from PR pipe overnight (at some point)

0900 (1 hour after CH off)
Boiler at 0bar
No extra water from PR pipe

So it looks like it's not the HW that's causing it as the pressure drop is happening once everything is off. I was checking every hour yesterday from 1430-2230 and will be doing the same today to see when/if the water from this mornings pressure drop gets expelled.

Any thoughts greatly appreciated.
 

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