Boiler vents at property boundary.

Are you sure? To form droplets it would have to condense in the air, this would warm the air but may not warm the heat exchanger. I think it is wasting heat condensing out after it has left the boiler. If it condensed inside the boiler it would drip out.

The fan is probably too fast for the flu.

Reply to
dennis
Loading thread data ...

Seems to me the biggest *single* factor about plumes is the weather. High humidity, cool/cold day. Visible plumes form easily - and I suspect that this will happen even when most of the water has already been condensed out within the boiler. You don't need that much water to make visible clouds.

Reply to
Rod

Your assertion is not borne out by reality. The flue gas temperature of mine is 40-45C. There can't be any steam in that.

Reply to
Andrew Gabriel

Who keeps bringing steam into this. I know steam is a gas and this is a mist (not a vapour). It condensed after the heat exchanger.

Anyway technically there is water as a gas in the air so there is steam there all the time its just that steam is normally accepted to be hot.

Reply to
dennis

This is where the confusion arises.

When you have heat gain and loss without a change of state the heat transfer is based on the temperature rise or fall.

This is why there is a fairly simple equation for heat loss through walls, ceilings and windows etc. Basically that one shows that he rate of heat loss is proportional to the temperature difference and the area involved. It's a linear relationship. The same principle applies if you have a emperature *without a change of state* between two points in a boiler.

However.....

Latent heat is an entirely different thing. It is the heat that is released as there is a change of state between gas and liquid or liquid and solid and is quite different to the mechanism of temperature drop. For example, a fridge uses latent heat in two places. Inside the fridge, the refrigerant inside the heat exchanger turns from a liquid to a gas. To do this, it needs to absorb its latent heat of evaporation. That comes from the content of the fridge. Outside, the compressor compresses the refrigerant gas until it condenses to beome a liquid. As it does so, it gives of its latent heat of condensation. In effect, the heat has been picked up inside the fridge and dumped outside. The condensing boiler works with the second of these and the heat given off is transferred to the heat exchanger and hence the water.

The point is that in the boiler, the condensation and the latent heat release happen before the combustion products reach the flue. Since that has already happened, it doesn't really matter about whether water is collected inside the boiler or outside. At that point, the latent heat has been released either way.

The issue is not to confuse what you can and can't see with the change of state where latent heat is released. That heat is much more than that as a result of the temperature drop after condensation.

Reply to
Andy Hall

Yes and this is not significant in termsw of energy

Reply to
Andy Hall

Phew, I was expecting a counter argument... Technically steam is water vapour and both are invisible.

Yes that happens inside the boiler and is used to up the effciency. It also produces the liquid condensate that needs to be drained away. The cooler (relative to a conventional boiler) flue gases still contain significant amounts of water vapour (steam) and as that cools after leaving the terminal under the right conditions a visible plume of condensation appears.

I was intrigued by the suggestion that the condensate could be ejected in the flue gases. I guess if you drained it away from where it condensed (giving up the latent heat to the boiler) and passed the still fairly hot flue gases over it there could be enough heat in them to evaporate some if not all of it but plumeing would be even worse. Even lower flue gas temperature and even more water vapour in it...

Reply to
Dave Liquorice

I actually took that to mean that the condensate is effectively aerosolised - rather than evaporated. (Evaporation would, of course, occur.) Yes - plum(e)ing would be worse. But you could imagine a system which aerosolised the condensate and blew it out gently whenever there is any to get rid of (not just when the boiler is burning). In which case the rate of disposal could be much lower and might cause less of a problem even though it raised higher total condensate emission.

Reply to
Rod

If it were evaporated (as in change of phase from liquid to gas) inside the boiler, it would require the Latent Heat of Evaporation. This would negate the Latent Heat of Condensation that had been gained in the heat exchanger.

The heat would have to come from somewhere.......

Reply to
Andy Hall

If there were enough heat left to do that a condensing boiler would be pointless as you are still throwing away the latent heat.

Reply to
dennis

An alternative but I think the aerosol would really need to be generated at or very close to the exit of the flue at the terminal other wise it might not get carried out of the flue and possibly run and collect in a place it shouldn't. Remember the condensate is mildly acidic not pure water.

This probably isn't a problem with most boilers wall mounted with the flue just going a foot or so straight through the wall but there are options for longer and more complex flues.

Reply to
Dave Liquorice

This is true but read what I wrote. You initially condense the flue gases at a point in the boiler that gives up the latent heat to the heat exchanger. The condensate then drains to a point away from the heat exchanger where you pass the still relatively hot flue gases over it which evaporates some or all of it. The energy required for this evaporation coming from those flue gases.

I don't know if this is practical. If it was and reliable *under all conditions* condensing boilers wouldn't have drains. But then drains is cheaper for the boiler maker...

Reply to
Dave Liquorice

There is nothing after the heat exchanger to absorb the heat required to condense it. The heat recovered by condensing is around 3kW in mine. If even a small portion of that was being dumped from the flue gas outside the heat exchanger, it would have dire consequences on the plastic parts carrying the flue gas from the heat exchanger.

That probably is a tiny factor. The exhaust will be at 100% humidity at 45C. When it mixes with outdoor air and cools to, say, 5C, you will get a tiny bit of extra mist due to it now being super saturated. This is just the same as breathing out on a cold day.

Reply to
Andrew Gabriel

Yes understood, but it's still a circular point because the flue gases are not that hot after they leave the heat exchanger. Typically they are going into plastic components at that stage.

Even if it were possible, the heat used to evaporate the water would have otherwise been transferred to th heat exchanger or into the room

Reply to
Andy Hall

This one was telling me how modulating burners work a few weeks back. Duh!!!

Reply to
Doctor Drivel

The Atmos Intergas.

Reply to
Doctor Drivel

I advise you to look at one. The best designed and made combi boiler in the world.

Reply to
Doctor Drivel

You should be delighted then. How many are you going to order?

Reply to
Doctor Drivel

You were wrong then so how about an answer this time? To the whole question that is.

Reply to
dennis

When flue gasses are less than 80C, condensation is released when the flue gas hits the cold surface of a heat exchanger. This generally is when the return water is less than 57C. The result is a plume from the flue as 100% humidity is achieved and water droplets occur.

Tertiary heat exchangers when between the incoming fresh air and the burners will heat up the air to approx 60C, dropping the flue gasses to approx 40 -

50C. This results in condensing at higher boiler water return temperatures.
Reply to
Doctor Drivel

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.