h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Specifically, where do the fumes go?
Our boiler is on the blink - the pilot light isn't working, and apparently a replacement part is on order. The other parts are still turning on however, whenever the timer tells it to, so I turned the boiler off so gas wouldn't keep running through it and hike up our gas bill.
This caused an argument with a flatmate, who happened to be nearby at the time. While he agreed that turning the boiler off was a good idea, he thought there would be an automatic cut-off on the gas whenever the boiler wasn't burning it. He argued that, if this were not the case, the gas would be leaking out and filling the room! My contention was that the gas would be contained by the boiler until it was piped out to wherever the burnt fumes normally go.
This is at the heart of the problem: my flatmate states that the fumes just go out harmlessly into the air of our house, which doesn't sound right to me, but I have no idea where the fumes would go otherwise. Both of us contend that the other's theory just doesn't sound plausible: either the house is normally constantly filling with CO2, a toxic gas*, or it disappears to some magical land down pipes which are now instead full of explosive natural gas.
Which of us is right?

* He highly disputes the toxicity of CO2, about which I will have to present him with Wikipedia evidence.

most boilers these days have a cut off, if the pilot light goes out, the gas is shut off

when was the boiler last serviced

ours went on the fritz this year due to dust building up in the workings and blocking the preasure equiliser pipe, just need an internal hoovering and it was good to go

CO2 isn't toxic. And don't quote me wikipedia, I have expertise in PhD level chemistry and beyond

Carbon *monoxide* (CO) is the highly toxic one and can build up when gas burns imperfectly. In your case it isn't burning at all so this isn't a problem anyway.

The only time CO2 is dangerous is if the level of oxygen around drops to an unbreathable level. Otherwise it's not a problem outside of a planetsize runaway greenhouse effect at least.

And there wouldn't be any CO2 anyway if the gas isn't burning.


Gas running through the boiler and going out anywhere - even the outside of the house constitutes a *major* leak and it would be highly illegal for a system to be left like this. The gas is highly explosive if allowed to build up and so constitutes a major danger to life and property. I think the dangers of a proper unchecked gas leak were highlighted when half a street somewhere in north England blew up last week.

The boiler's design *must* cut the gas flow off if it isn't lit. If there was any danger of gas leaking the emergency services (or corgi registered gas fitters) are *required* to cut the supply off to the appliance and it is a criminal offence for you to tamper with it.

In short, I'm afraid your flatmate is right.

All* boilers have a cut off, which will 'cut in' and turn it off if either the outflow fumes section isn't working, or the pilot light goes out... (well modern boilers anyhow of the last decade or so).

All boilers are vented to the outside for the fumes, this won't include gas as the gas is burnt inside; there shoudln't be any connection betwen the outflow fume section/outflow gas pipes, and the incoming/burning gas;

It varys somewaht though, depending on what type of boiler it is...

The effulent fumes escaping into the house is generally regarded as a very bad thing, and having had this happen with my boiler years back thanks to a dodgy gas engineer thinking that 'fixing' the boiler by bipassing the safety mechanisms was a good idea, ment the house filled with Co1 and co2, which doesn't make people feel very well... (well it explained teh headaches I had at the time...)

>CO2 isn't toxic. And don't quote me wikipedia, I have expertise in PhD level chemistry and beyond <

And never post S**t* like this until you've checked the sources.

OK so... checking wikipedia... the carbonic acid generated at high concentrations of CO2 is a little toxic.

But 1% CO2 in a room is a *huge* concentration (doesn't sound it but it is). CO is vastly more toxic than this, again quoting wikipedia, 667 ppm (0.7%) level is potentially fatal. CO by comparison causes a slight risk of acidosis at this level.

I've never known anyone worry about CO2 as a toxin before - whilst wikipedia may quite its toxicity, it isn't really regarded as a significant toxin.

sorry 667 ppm = 0.07%

I'm familiar with the toxicity of carbon monoxide, but my understanding is that dioxide is also toxic, albeit rather less so and for different reasons. I respect your expertise, but I'm not confusing CO and CO2 here.

Oops, simulpost. I'm glad we agree.

So the boiler generally vents out of the house? Makes sense, as long as the burning is properly efficient.

Someone asked about our boiler? I don't think it's been serviced in a while. We turned it on for the first time since Spring about 2 weeks ago. It was on for one evening, and then the pilot light went. That was a Friday, so we had to wait until Monday for an engineer to come. He tinkered, it seemed fixed, but it cut out again about a week later. This time he came round and said a part was faulty, and had to be ordered in. We've been waiting for the best part of a week now...

I can see it being a problem in confined spaces such as submersibles and such but otherwise I've never heard of it being an issue really.
I guess if you really deal with large quantities of it then it is an issue.

We as chemists are more worried about nitrogen depleting oxygen levels than CO2 for example - although that might be because we use N2 (particularly in its liquid form) more often.

One would of course assume that your house has some form of ventilation?

When our boiler goes down (around once a year, give or take), it takes a while for them to realise it's the little control panel on the front (always is). We dont turn it off in the meantime mainly because that'd give us a severe lack of hot water...

We have electric showers, and it seems the taps are linked to some kind of switch that turns on the heater briefly. This was the first tiem it was on for central heating purposes.

Now that I remember, we did use frozen CO2 as a coolant in a couple of university chemistry experiments. They were giving off dry ice clouds the whole time, but nobody was particularly worried.

Re: toxicity or otherwise of CO2.

Inhaled (i.e. fresh) air is about 0.04% carbon dioxide.

Exhaled air is about 4% carbon dioxide - i.e. about 100 times the normal air concentration.

IF YOU ARE OTHERWISE HEALTHY (i.e. not actually in the process of having a heart attack), breathing your own (or someone else's) exhaled air for a while will increase your blood CO2 level but will not kill or injure you. High C02 in blood is linked to panic attacks, however.

Basically, your friend is right and if the pilot light isn't on, there's no gas coming in. Legally required safety system, and very, very reliable.

Tiggy is right - there is a correlation between CO 2 and panic attacks, but it is in reverse. Hyperventilation makes your body lose more CO2 than usual, so that is why you get given a paper bag to rebreathe your exhaled CO2 until it gets back to normal levels.


As for gas leaking from anywhere, such as a possibly faulty pilot light, it contains an added ingredient to make it smell strongly so as to alert you to the slightest leak. Which works as long as you a) have a sense of smell and b) haven't be exposed to it for so long until you can't smell it anymore.

I'd have thought if your engineer had checked it out he would have slapped a blooming big sign on condemning it if it was slightly dangerous. And disconnected it.

I'll go home now and climb down for him.

take him some ?

As someone who's spent entirely too much time studying HVAC systems, I can assure you that boilers need flues/exhaust vents, though the air intake can be diffuse, that is, just from the room.

The open vent/closed system bit refers to the actual plumbing, which many people don't realise needs ventilation (unless it's a special high-pressure closed circuit, in the case of heating.)

Yes, there's a pipe sticking out of your roof that's open at the top, without which your toilet wouldn't flush properly.