Hole In Chimney
I am seeking some advice and am going to ask what might be very silly questions, and it's a fairly long story so please bear with me.
A couple of weeks ago our co monitor went off. It lived in the cupboard where the gas boiler (combi condensing) lives which is in one of the bedrooms, not the one we sleep in. I immediately switched off the boiler, opened all the windows, and went outside until it stopped, which it did quite quickly. We assumed something must be amiss with the boiler, so called the gas engineer - the boiler was due for service anyway. He came straight round, and couldn't find a thing wrong with it. He was there for ages running the analytics, running the heating, running the hot water, but nothing. He recommended a new alarm, as the one we had was more than 10 years old. The new one alerts at lower levels of co, and to ventilate. Since then though, I have been turning the boiler off and only using it for 15 minutes morning and night to run a shower/bath. Once the ventilate alarm went off, and stopped quickly when I did that.
The cupboard where the boiler lives was very dusty and full of junk, so we cleared it all out, and cleaned/dusted it thoroughly. When it was empty we found a hole, half brick size in the bottom of the cupboard. On looking into it, it's the chimney, because we can see the flue liner which we had attached to a gas fire many years ago. We had the liner because our chimney was known before we had the fire fitted, to not be gas safe. We have an electric fire there now.
So, what's going on. My next door neighbour, the other of a pair of semis (90 years old), has a gas fire. Is the carbon monixde coming from there? Now the silly question, maybe. Looking into this hole, inside the chimney is quite a bit of soot and black residue, although I am pretty sure we had it swept before having the liner in. Is it all all possible that this soot or whatever, if it gets hot from the fire next door (the chimneys are back to back), it could give off carbon monoxide). We can easily have the hole bricked up but whichever way you look at it we have carbon monxoide in a chimney that isn't suitable. If the soot isn't causing the carbon monoxide to be there, then there is a leak from next door, which opens up all kinds of scenarios none of them to their liking.
Suggestions, advice whatever much appreciated. For the next 3 weeks none of this is going to be an issue as they will be away next door and won't be having the fire on!!
We kept the window in that room open, along with other upstairs windows