Taunton

I was left with a few questions after all that. I didn't look online for sources before, but what is out there now? What did HSE^▸ actually say (and why them)? And what is the technical background to how we got here?

Google can still find press reports of both those fatal incidents: Mrs. Sharon Minster (30) in Penwith 29/30 May 2002, and Rhianna Hardie (10 months) in Taunton 18/19 November 2006. These came from national news media, not the local ones that would have more detailed early reports - maybe they were not on line yet? Or don't keep old pages? There are also forums that dealt with the subject, one being on the IET^▸ site. All of those are largely taken up with missing the point, of course!

A number of pages include links to the HSE guidance note, which at don't work; however it is still there, just moved. I still can't find any real explanation of why it's HSE's business - they do have a couple of other notes for local authorities about heating systems, but only that few. None is about gas heating, for which HSE do have some kind of residual government regulator role. This letter covers the two notes about older hot water systems: https://www.hse.gov.uk/services/localgovernment/letter.htm

The second note is about the explosion risk of bringing an open fire back into use with an old back boiler. It will almost always have been drained, and should never have been sealed but a few have been. Heating by a fire with no water in it (or worse, just a little) gives such high temperatures that it weakens the cast iron and also pressurises it, leading to a violent explosion.

One fatal case is referred to. This turns out to be Christine Goodall of Twyning (65), on 11 November 2007. Her housing association landlords had decommissioned the back boiler in 1999, saying the fire was still usable, and it was - unusually - possible to find the plumbers involved (company and individual).

There have been similar, if not fatal, incidents since; perhaps they are now as common as the related problem of frozen pipes. That killed Katherine Bates (84) on 20th December 2010 in Mansfield. That was too late to be behind ellendune's comment, but I imagine such events were (like back boilers) more common in the past. I think a reminder to the public on that might be timely - if gas prices do keep rising, the temptation to burn whatever is to hand in an old open fire may become more common.

Back boilers are relevant to the immersion heater issue, since indirect vented systems originally had back boilers as a heat source, and electricity for the summer. You can't control an open fire with a thermostat, nor even the water flow as it has to always cool the boiler. And of course the header tank was made of galvanised steel (which quickly became rusty, from my experience).

I checked with a plumbing text book (a basic one for apprentices), and it says the overflow pipe is present for two reasons: firstly to ensure the system is at atmospheric pressure (it should say "when the feed to the hot tank is closed", but does not), and to let air escape on filling; and "more rarely in the event of the water becoming overheated, allows it to discharge over the cistern". That overheating would be very rare with indirect heating, but pretty common with direct heating - it just needs a rather big fire to be set.

Changing to big gas boilers with gravity indirect water heating, then fully pumped systems, with thermostatic control removes the routine overheating and allows a plastic cistern to be used. But then practices (and standards, regulation etc.) have to catch up. The requirement for those plastic cisterns to hold boiling water indefinitely (well, 500 hours is three weeks, isn't it?) only came in in 1991. (Mine was a lot older!)

I can only find one similar HSE guidance note outside the "safety at work" field. That is about the carbon monoxide poisoning risk in stores for wood pellets. That is not at all obvious, is it? Yet the note said in 2012 that there had been at least nine deaths in Europe since 2002.

The point is that cutting wood exposes organic substances to the air, and some of those self-oxidise, and some those can do so partially and yield CO. Sawdust is worst, small wood particles next, and pellets made by compressing those will show also show the effect. All it needs then is a large enough quantity, and a nearly sealed store, kept closed until someone needs to go inside...