I wrote that relative humidity doubles every ten degrees centigrade, but in fact that's not strictly true. It's more like 9.2 degrees. That's an approximation too, and in fact it's closer to doubling every 9.24 degrees at 20 degrees centigrade, and every 9.19 degrees at freezing, so it's not purely exponential. These are pretty trivial differences I know, and as a rule of thumb to understand the workings of humidity it's enough to know that it will approximately double every ten degrees.
It's also something of a rule that if you have water moving slowly and occasionally stopping, sooner or later there will be a build up of gunk. The inevitable impurities will accumulate, particles will appear and biology will happen.
We called the builders, and they called the ventilation experts, who have regularly appeared in these lines. A maintenance engineer turned up within a couple of hours and fixed it, putting an end to my empirical research into the amount of water held in air. The first thing he did was take the cover off while the ventilation system was still running, which at least my manual recommends against. I wasn't watching him the whole time as I didn't want to breathe down his neck, and the next time I looked he had switched it off. I always find it difficult to balance my interest in what workmen are doing with their need to get on with their job in peace.
After a while the engineer produced a small plug that had grown into the top of the hose pipe that drains the humidity. He recommended that we take off the hose and clean inside every couple of months to stop this.
I wish they'd told me that a couple of years ago!
Since there is going to be an irregular flow of water through there, and at least some particles from the air will reach it, there's bound to be some biological build up sooner or later. The water is going at around half a litre per hour, so it's not exactly the Niagara Falls in there, and anything that builds up on a surface that is not perfectly smooth will just keep building up. There will probably be times in the year when the system is completely dry, but I suspect for over half the year there will be some water coming out for some of the day. It's at room temperature, so the growing conditions are ideal for some bacteria and fungi.
When we moved in, they had offered us a maintenance contract, and I said I'd get back to them later. At the time I didn't want to sign a contract with them because I didn't really trust them. This engineer seemed to know what he was doing. He told me that the pipe coming out of the system needed a loop in it otherwise we'd hear the noise of water dripping out. I don't know whether he realised that I'd told his company this, since they hadn't put a loop in when they installed it, and it we had dripping and gurgling noises all night.
Also they had said that maintenance needed to be done every three years, but in fact in the manuals it recommends cleaning the filters every year.
His advice to clean the hose is probably good, but the build up was actually not in the hose, and probably not coming from the hose, which was downstream. The problem is coming from above.
So, I need to find someone else to maintain the system, learn how to do it myself, or go back and humbly ask them. There is a more local heating engineer who may be able to do it, and I'd like to ask him, but there are probably invisible walls that would be impossible for them to cross.
