Lots of water vapour from boiler at startup

[Pamela]

I see a lot of billowing steam vapour from the outside vent when my Ideal ISAR boiler starts up in the morning. It can last an hour of two. The neighbour's boiler vents far, far less.

Is this a sign of something wrong?

As it happens, the pressure gauge on the pipe going into the boiler shows only half a bar when cold, rising to 2.5 bars when in use. When I once increased the cold pressure to over 1 bar, water leaked out somewhere in the boiler when it got warm so I reduced it.

Could both vapour and wide range of pressure be related?

[simon mitchelmore]

Probably normal to have what looks like steam coming out of the (fan) external vent.

The water pressure sounds like there's a fault. It should be more than 0.5bar when cold. If you recharge it to say 1.5 bar cold it should run hot without leaking water from the pressure relief valve. If it does leak in this condition your boiler air cylinder is probably defective. There is a way to check this which I can explain.

The water leak is probably unconnected to excess water vapour.

[John Rumm]

Its a condensing boiler, so some visible vapour is to be expected. You may find as the system comes up to temperature its less able to reduce the flue gas temperature, and less vapour is visible.

Not necessarily.

Unless this is a huge system that seems excessive. I suspect that you have a problem with the expansion vessel - either it lacks air pressure, or the diaphragm has split.

If the pressure exceeds typically 3 to 3.5 bar, then the emergency pressure relief valve should open and vent some water. This should be directly to a pipe through the wall, discharging somewhere safe against the wall.

It might be you have a leave on this pipe, or its not installed correctly. (its easy not to notice since in normal operation its never used unless there is a fault causing overpressure)

Seems unlikely.

[harry]

The commonest cause of lots of water vapour from the vent is that your boiler is very cold, ie been left off for a while, Check if your neighbour leaves her boiler on all night.

In cold weather, the water vapour from the vent is more visible.

There could be a leak in the boiler but I'd expect to see water dripping out of the bottom. Somewhere there will be a safety valve that drips if there is over pressure.

On modern boilers there is also an expansion vessel that could be faulty and causing over pressure and a drip from the safety valve.

[Chris Bartram]

Sounds like your expansion vessel is shot. Pressure should be ~1bar ish at hold and shouldn't rise as high as you are seeing.

[John Rumm]

Not necessarily with a condenser - a leak in the primary heat exchanger would tend to flow to the condensate drain and not be visible outside of the boiler. The tell tail would continuous loss of system pressure and the need to top up regularly. You may also find the content of the condensate drain syphon would be discoloured (it should normally be clear)

A diagnostic for that is to sandwich bag or similar over the end of the pipe and see if it collects any water over the space of a day or so.

[Pamela]

The quantity of steam vapour is not just a lot, it's a huge amount of quite dense billowing vapour lasting a couple of hours (this cold morning), until finally there's no more.

It's something like 10 or 20 times the visible volume of what my neighbour's heating vapour looks like and, on the same day, theirs lasts only half an hour after startup.

[Chris Hogg]

I assume you have an unvented system, i.e. no header tank in the loft serving only the C/H system.

Do you have to keep topping up the pressure in the C/H system? If you do, then maybe the heat exchanger in the boiler has corroded and is leaking water into the flue system, where it evaporates and emerges from the flu as 'steam'. The leak could be exacerbated if the expansion tank in the boiler, or elsewhere in the system, is not working properly due to a damaged diaphragm.

[Harry Bloomfield, Esq.]

Pamela has brought this to us :

That is probably a faulty, it may be the expansion vessel needs to have more air added, or has failed. It will look like a small bomb shape, most often painted blue or red, one pipe in and a tyre pressure type connector. It could be inside the boiler, or somewhere else. Try pumping some air in using a bike tyre pump or similar, with the system cold, to 1 bar pressure.

The excess steam is probably normal and indicates a lightly loaded boiler running in condensing mode.

[The Natural Philosopher]

I had the same. Gate valve seal gone due to expansion vessel having gone prior.

Losing expansion vessels puts a high stress on the primary circuit seals I had two gate seals and some suspect ball valves - fortunately once the expansion vessel was replaced and the worst offenders fixed the rest furred up and stopped leaking.

[John Rumm]

Which may simply suggest your boiler is running more efficiently than theirs, by staying in condensing mode for longer.

Look at it logically - if you are not losing primary system water, then the only source of water in the exhaust is that which is manufactured in the combustion process. It being more visible suggests a lower exit temperatures (i.e. more of the useable heat has been extracted for the purposes of heating your building)

[Pamela]

That could be so as I run my boiler quite cool with the outflow at about 55 degrees while the return temperature is about 40.

I don't run it much hotter because the water pressure I mentioned can cause tiny leaks. I need to attend to that.

[John Rumm]

yup, that will give you lots of visible vapour (and maximum condensing efficiency).

(you would probably find that the visible vapour reduces noticeably when running in DHW mode rather than CH)

As a temporary measure, just drain some water from a rad so that there is a pocket of air at the top - that will give you some expansion space.

Often its easier to add a new expansion vessel elsewhere in the system that fuss too much replacing the one in the boiler since with some boilers the expansion vessel is right at the back behind the main Hx, and a pain to get to for service.

(The ISAR is pretty compact as well, which makes it a bit harder to work on)

[simon mitchelmore]

I just fitted a new expansion vessel under the bath (connected to the CH). Replacing the vessel behind a boiler is a proper PITB.

[John Rumm]

Yup, good idea. I don't know why some boiler makers make it so hard to get at a bit that is known for needing regular checks etc.

One of the things I like about the current Vaillant design is how easy they make it to get at things in the boiler. e.g. look at the position of the silver expansion vessel on the RHS: