right - I live in the US nowadays and nobody has them - but back in the UK they're all over the place - stay at someone's house and there's a good chance they have one
I've never heard of such a thing, let alone actually seen one in a bathroom. I've lived in the UK for all (bar a couple of years) of my life.
[edit] I found an academic paper that discusses the safety of these heaters - dated 1973. I'd guess that they've gradually disappeared thanks to better heating systems and improved safety regulations. The paper: http://hej.sagepub.com/content/32/4/120.abstract
The comment was referring to "gas fired bath water heaters", which appears to be a specific type of heater fitted in bathrooms for heating the bath water…hence the potential for increased CO risk in the bathroom.
My hot-water + heating system is gas-fired, but the boiler is in a utility closet vented to the outside.
We have central heating but the boiler runs with a heat pump driven by electricity.
You are of course right in pointing out that central heating often goes along with central water heating and that heating is often based on fossil fuels such as gas.
In Poland quite common 'till very recently; I still have one (just reminded me to buy a CO detctor). My two friends actually got poisoned by CO - one when taking a bath, the other when trying to rescue her - because the landlord didn't bother to maintain ventilation properly.
I've had one blow up in Poland. Ul. Winklera, Poznan. I'll never forget that. My s.o. was about to use the (common for three apartments) bath and the thing blasted its outer cover clear across the room. She ran into the hallway totally forgetting her state of dress or lack thereof. The neighbours, alerted by the explosion that something was up, all piled into the hallway and sure appreciated the view.
Fortunately she was only scared but otherwise ok. Quite the bang, I've had a healthy respect for old Junckers products that have gas lines fed into them since.
