Pluming, and condensing boiler efficiency.

Is it related? I feel that a condensing boiler that plumes a lot may be less efficient than a condensing boiler that hardly plumes at all. Is this statement true? If a proportion of the water vapour condenses outside the boiler then surely you are losing some energy? I'm not sure, what do others think? Don

Reply to
Donwill
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Pluming is a function of the external air temp and humidity. If it's cold a wet you get a plume, hot and dry and you won't.

As an indicator of efficency I suspect it's the other way round to what you say. My reasoning being the the exiting flue gases on the boiler that plumes a lot are cooler (ie the heat has been retained in the boiler) than the one that doesn't. Cooler flue gases will get below the dew point quicker than hotter ones. Thus the cooler gases stand more chance of forming a visible plume before becoming dispersed.

Reply to
Dave Liquorice

I think you will find it is actually the other way around...

The pluming is related to the temperature of the flue exhaust - the cooler it is (i.e. the more heat extracted, hence the lower the return temperature to the boiler, the greater the condensing efficency), the more pluming you are likely to see.

Remember that the condensing is designed to recover the latent heat of vaporisation - i.e. the energy given up on the state change from gas (i.e. steam) to liquid. What you see at the boiler flue is not steam but water vapour already well below steam temperatures.

Reply to
John Rumm

That's not quite correct. Water vapour *is* steam, and is invisible (except for its refractive characteristics).

What you see isn't water vapour, it's a fog of water droplets which have already condensed from the vapour into the liquid state.

Reply to
Ronald Raygun

What does this all mean in a 'real-world' situation though, and if it's not pluming 'correctly' what can you do about it? For instance, we have an Alpha CD32C that feeds 6 rads, a towel rad and a kickspace heater. The boiler has a control for heating (numbered 1 to 9 and set at 7 by the installer 3 years ago) and the normal rads have TRVs, all fully open apart from the two spare rooms where they are about 50%. The programmer/stat in the hallway is normally set to about 18C.

The house is the correct temp for us and the way we like it but the boiler doesn't plume very much at all, so is that 'wrong' and if so, what do we do about it?

Reply to
Pete Zahut

I know I have appeared as particularly thick of late but I don't see that at all. Surely extraction of latent heat requires no more of a temperature change than conversion into latent heat. (ie none). Thus what matters with the exhaust is its relative humidity at any given temperature which is determined by the amount of condensation that has occurred within the boiler. Water vapour turning to mist outside the boiler is latent heat lost to the system.

But surely still with some latent heat.

Reply to
Roger Chapman

Who says its happening *outside* the boiler? I'd have thought that if things are working properly, it'll all condense *within* the boiler, giving up its latent heat and turning to water. Presumably this is the water that has to be run off. Some of the *condensed* water stays as water droplets (i.e., mist) and that is what you see as a plume.

No, because it's not *steam* any longer. The mist is *water*. It won't give up more latent heat unless it freezes.

Reply to
Tim Streater

Are you sure, non condensing boilers plume a lot. The exhausts may be hotter but they cool very quickly in cold air.

It may well be droplets that have condensed but if they haven't condensed on the heat exchanger they probably haven't transferred the energy to the heat exchanger so its has been wasted. If the steam was condensing on the heat exchanger it wouldn't be in the air to form a plume.

Its not at all obvious which is true as the observations can be made to fit either theory, someone needs to measure the things to decide.

Reply to
dennis

I read John's explanation (the cooler it is ... the more pluming you see) as suggesting the transformation from water vapour to mist was occurring outside the flue.

I focused on the 'water vapour' not on the 'what you see'. Are you saying that water vapour, as opposed to mist, has no latent heat?

Reply to
Roger Chapman

Water vapour is invisible. Look at the "steam" coming out of a boiling kettle. Just at the spout, there's no mist - that is steam at 100C. [1] It then cools a bit, and condenses into a big cloud of "steam" that is actually water in the form of mist.

Now, the cloud I mentioned then disappears - where has it gone? Well, the water then evaporates again into water vapour (invisible) because air can hold a certain amount of water vapour. The amount it can hold diminishes with temperature, which is why fogs form at this time of year. Damp air cools and because the air can't hold the vapour, it condenses out - fog. Next morning the sun warms both it and the air, so it can evaporate again, and the process repeats.

I'm guessing that in a properly functioning condensing boiler, most of the water produced by the combustion condenses, gives up its latent heat, and runs away. A small amount of vapour will come out of the exhaust, and on a cold day (or when the air is fairly saturated with vapour already) will condense quickly and become visible as mist. As with the kettle, it then evaporates again and disappears. On a hot day, the air can accept the vapour directly so you don't see any mist, or much less, at any rate.

So I'm modifying what I said in the previous post (sorry). The short answer to your Q is that water vapour always has latent heat. I guess that means that in a hot damp climate a fridge has to work a lot harder than in a hot dry one.

[1] If you put a finger in the bit at the spout you'll get burnt so don't do that.
Reply to
Tim Streater

From what I have seen the plume is very large and thick when the boiler is on full tilt i.e producing hot water at full flow rate (37kw), on heating the plume is also large when the boiler is on full power and not yet modulated down.

Reply to
VAG_dude

As far as condensing mode is concerned ISTR that condensing doesn't occur at all unless the return temperature to the boiler is below something in the region of 53 degrees C and even lower temperatures are required to get extract anywhere near all the latent heat. In a house that was originally plumbed for a conventional boiler it may be that the radiators are too small to allow such a low return temperature for much of a cold winter.

Reply to
Roger Chapman

Well yes, though this is one of those situations where the non-technical/colloquial use of a word conflicts with the more precise definition.

Reply to
chris French

In message , "dennis@home" writes

No so much I'd say. We had a standard balanced flue boiler, as did our old neighbours. They had a condensing boiler installed. The condensing boiler produced much bigger plumes ISTM than the standard boiler.

I've always assumed that being hotter, the exhaust can disperse more before it condenses, so producing a less obvious plume

Reply to
chris French

Indeed, I was being imprecise...

What I wanted to distinguish between is the "fog" you can see - that people think of as steam, but in reality is not, as opposed to the transparent gas that steam actually is! ;-)

Reply to
John Rumm

There are no right and wrongs here. Plume visibility will vary with weather and also how hard the boiler is working.

Condensing boiler efficiency will improve with lower flow and return temperatures. You also get an increase in the rate of improvement of efficiency at about 54 degrees C. So a condensing boiler running at

50/30 will tend to perform a little better than one running at 80/60
Reply to
John Rumm

Indeed - you recover the latent heat on the state change...

The main change is with a conventional boiler the exhaust temp will be in excess of a couple of hundred degrees C. The water (manufactured during the combustion process) will be expelled as steam (in the true sense) in the flue gasses. They will condense later but are usually too dispersed by the time they have cooled enough to be particularly visible.

On the condenser, the state change will have mostly happened in the boiler - leaving liquid water in the flue gas rather than steam.

Heat yes, but not latent heat. What you see in the flue gas on a modern boiler is already liquid water. You only recover the latent heat once on the state change from gas to liquid. So a lower return temp will allow you to extract a bit more heat from the flue gasses, but its not a big gain like that from the initial condensation.

Reply to
John Rumm

Non condensers are designed not to condense (and hence plume) if possible - since condensation in the boiler was in those days a "bad thing" - it quickly caused corrosion of cast iron heat exchangers etc. You typically see condensing occurring on a conventional boiler when it first fires up and all the cold metalwork absorbs a proportion of the heat dropping the flue temperature to lower than normal. Atmospheric conditions effect matters a fair bit as well.

Some will condense directly on the HE, and some will do do in the air flow adjacent to it.

A better assessment of the condensing efficiency is to look at the rate of liquid water discharged from the condensate pipe than at the amount of plume visible (which is effected by to many external factors to be a reliable indicator)

I think you will find studies have been done....

There was a link Andy Hall used to post that contained one IIRC.

Reply to
John Rumm

Glad to see all the old experts are still here, despite the NG being swamped by " what colour shall I paint my Gran's front door" type questions. I knew that question would draw you out, I think I shall have to turn my pump down a tad so I can present a cooler temp on the return. Cheers Don

Reply to
Donwill

Roger is one of the very few who understands condensing boilers.

Reply to
Jeremy Brown

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