Condensation visible from flue, what is more efficient, a lot or barely visible?

WhoisDannie

I am sitting here at my computer and can see out my window quite a few flues "smoking" quite a lot of condensate but when I go outside and look at my flue there does not seem to be as much condensate visible.

So what boiler is more efficient, one giving out a lot of condensate or barely visible?

dunstonh

It would depend on the outside temperature.  The colder it is, the more visible the vapour will be.

WhoisDannie

dunstonh said:

It would depend on the outside temperature.  The colder it is, the more visible the vapour will be.

The outside temperature is the same for both, even the time is the same as I can see my flue and my neighbours at the same time.

Emmia

Was your boiler actually running?

mmmmikey

Looking at this from first principles rather than anything more specific I would imagine there are a number of things that effect the amount of visible condensation.

1. Modern boilers modulate the burner - i.e. they're not just on or off, the size of the flame / amount of gas being burned varies. The water vapour that condenses is a by-product of the combustion so the more gas being burned (i.e. the hotter the boiler is running) the more water vapour there will be.

2. The flue gases on an efficient condensing boiler will be hotter than the gases on a less efficient boiler and although this won't effect the amount of water vapour (other things being equal) it will have an effect on how quickly the flue gases cool down and the pattern of any condensation. I'd guess that the condensation will be more concentrated and visible closer to the flue outlet for a more efficient boiler and vice versa.

3. The flue itself will have a bearing on the temperature of the escaping gases - i.e. the type of flue, length, any bends, surrounding temperature and so on, so like above this will effect the pattern of condensation.

4. As @dunstonh points out the temperature of the surrounding air will effect the condensation pattern and this can vary quite significantly in proximity to structures even over short distances, for example due to the exact position of the sun, any shading, variable wind eddies and so on.

Putting all this together I don't think looking at the amount of condensation would tell you anything useful about the boiler efficiency because it is effected by so many different factors. 

An interesting question though, and my thoughts all caveated with words like "I guess that..." :-)

northernstar007

ive always wondered that aswell
my neighbour is banging steam/condensation a lot of the time i always think to myself what machine she has in and is this just throwing money out
but mmmmikey explained it very well

Staffordian11

I think combi boilers will tend to produce more vapour when heating hot water rather than just providing heat to radiators (unless just switched on) as they have to work harder to raise the water temperature more or less instantaneously. If someone is running a bath or having a shower powered from the combi rather than an electric one, I think the amount of water vapour produced will be more and hence be far more noticeable.

markin

Test it out, what is your flow temp? and if you rise it do you get more or less, and if you lower it...?

Eldi_Dos

WhoisDannie said:

So what boiler is more efficient, one giving out a lot of condensate or barely visible?

I would imagine the boiler putting the most condensate water down the condensate pipe will be the most efficient at recovering heat from the flue gasses.

QrizB

Eldi_Dos said:

WhoisDannie said:

So what boiler is more efficient, one giving out a lot of condensate or barely visible?

I would imagine the boiler putting the most condensate water down the condensate pipe will be the most efficient at recovering heat from the flue gasses.

I tend to agree. All other things being equal, the flue with a smaller plume will be condensing more water inside rge boiler and will be recovering more heat, I've. will be more efficient.
Of course, all other things might not be equal!

mmmmikey

Eldi_Dos said:

WhoisDannie said:

So what boiler is more efficient, one giving out a lot of condensate or barely visible?

I would imagine the boiler putting the most condensate water down the condensate pipe will be the most efficient at recovering heat from the flue gasses.

Yes, good point - what I guessed at in my earlier post (point 2) is probably wrong then because some of the water vapour in the exhaust gases is condensed inside the boiler and runs out a condensate pipe so never makes it to the outlet of the flue.