Boilers, condensation, flow/return temps

[RzB]

I am about to replace our 1960's thorn M gas boiler. I have been reading books, FAQ's, previous posts but still have one or two outstanding questions.

This is a really basic question about boilers in general...

It seems that condensation is linked to return temperature. How does this work..

Lets take a non condensing boiler first...

As I understand it we have a flame that burns, and is directed at the outside of a heat exchanger. The heat exchanger is some sort of metal contraption with a large surface area that the water flows through. The water is heated to typically 82C goes to CH/HW tank and returns at typically 71C (in non condensing boiler). So the water entering the heat exchanger is at 71C. The heat from the flame passes over this surface and is then directed to the outside world up the flue (whatever type that may be).

If the return temperature were to drop below the 55/60C then condensation forms and this can evidently shorten the life of the heat exchanger because of condensation.

That's the bit I don't understand. Where does the condensation come in and why? Does it form on the outside of the heat exchanger? or on the flue? Why is it affected by the return temperature? I just can't reconcile what's going on in the boiler with what I remember about Dew Point, Boyle's Law etc that I did at school a hundred years ago !

I have a further question about condensing boilers efficiency but perhaps it will become clear when I understand this first question...

Many thanks, Roy

[Christian McArdle]

Because the flue gases contain a lot of water. Not just atmospheric humidity, but the byproduct of burning natural gas. When you cool the flue gases down further (i.e. to 90C-, rather than the 150C+ of a conventional boiler) due to a better heat exchanger, the gases soon go over 100% relative humidity and the water condenses.

Of course, there are two efficiency effects for condensing boilers. Firstly, the lower flue temperature means that more heat is clearly being transferred to the water. The second effect is that the act of condensing on a surface causes that surface to heat up, much like evaporation of sweat cools you down.

As boiler efficiencies are published without reference to this previously "lost" energy from condensation, condensing boilers can even have official efficiencies of over 100%.

Christian.

[Aidan]

All the fossil fuels (oil, gas and coals) are mostly hydrocarbons, compounds of hydrogen and carbon. There's other elements mixed in as well, mostly sulphur. The exact compostion depends on what the fuel is.

Methane is CH4, Butane is C4H10. Paraffin is about 86.3% C, 13.6% H2 and 0.1% S, etc.

If you completely burn any hydrocarbon, you mostly get CO2 and H2O, carbon dioxide and water vapour. The H2O will appear as water vapour, if it is at less than 60degC, it condenses, forming water. The heat excahnger will absorb the latent heat of vaporizatrion which is released when the vapour condenses, so more energy is available for use if the water vapour can condense. The coolest bit of the heat exchanger will be where the cool return water comes in, so this is where the water usually condenses. You could have a boiler operating at less than

60 degC flow, so then you might get condensation forming over all the heat exchanger and up the flue. The return connection point is still coolest so that's where most condensation will form.

The problem occurs mostly with the traces of sulphur in the fuel. These have been burnt to form sulphur dioxide. This dissolves in the water to form sulphurous acid. The sulphurous acid will attack the boiler, UNLESS it is constructed of acid resistant materials, i.e., a condensing boiler. The damage to the boiler is known as back-end corrosion (the return usually goes in the back on commercial boilers). The condensate from condensing boilers is acidic.

There's other stuff involved, dissociation, nitrogen, partial combustion, etc., but that's the main bit. IANA chemist.

[John Rumm]

Sounds right so far...

Note that the water does not get to 82 in one pass - it takes several trips through the boiler to reach that temperature. The boiler will usually add about 10 - 12 degrees to it on each pass.

Once it is up to temperature and has reached equilibrium, yes.

Yup.

Not only the heat exchanger, but any metalwork in the boiler including the burner tray etc. (which historically would be situated under the exchanger)

Yes, and yes... primarily the first, but if due to the low exchanger temperature the flue gasses are cooled too much, then you will also see them condensing all along the exit path after the exchanger.

The lower the temperature of the heat exchanger, the greater the heat flow into it for a given flame temperature, and hence the lower the temperature of the flue gasses.

(there is an implication here that even when correctly configured a "normal" boiler will still condense a little when running up from cold - since it will take a while to get the system to normal operating temperature. I am not sure if that is actually a problem, or if it is a case of it being so short lived that any moisture generated will just be boiled off again in short order)

Well if you start from the premise that the flue gasses contain significant volumes of water (resulting from the oxidisation of a hydro carbon with oxygen - giving heat + co2 + water) this will remain in vapour form until the temperature of the gas falls below the dew point.

The dew point is not a fixed temperature, but will vary depending on the saturation of the gas. The more water vapour in there, the higher the dew point.

So in this case the vapour content percentage is set by the combustion process. Since this is plenty hot enough, the water stays in vapour form

- until it cools down to about 56 deg C IIUC. If the water vapour content percentage was different, the temperature of condensation would also be different.

[RzB]

Christian/Aidan/John, Many thanks for your excellent answers. I guess my mind was sot of linking condensation with cold surfaces. Anyway - I understand now.

I have a further linked question... I'll raise another post... Many thanks, Roy

[Andy Hall]

This would be true if a non condensing boiler were run in this mode. However, that is difficult to achieve because the thermostats tend to prevent it happening. A condensing boiler of modern design has materials suitable for coping with the mild carbonic and sulphurous acid mix that results.

The physics of it are that the latent heat of condensation from the gaseous phase of water (steam) to the liquid phase (water or water vapour) is released because of the state of change. It is exactly the same principle as a fridge. Inside the fridge, the refrigerant evaporates from liquid to gaseous phase and requires the latent heat to do that. Outside it is the reverse and the latent heat is released.

The condensing boiler achieves this by having surfaces at a temperature below the dew point. If that is the heat exchanger, then the latent heat is released and to a considerable extent will be added to the heat produced by combustion. Thus the return temperature is important. The lower it is, the more surface area of heat exchanger is used for condensing and the better the outcome.

If condensation happens within the flue system within the house or the boiler, then the heat is released within the building but through the boiler case and to some lesser extent, the heat exchanger.

Bear in mind that even if the flue gives a plume, this is actually water droplets, not steam, and the condensation has already taken place. In older non condensing boilers, the flue gases are actually steam on leaving the flue and condense to water droplets on cold days.

[<me9]

Not so. If boiler efficiency is worked out correctly it is related to the calorific value of the fuel, and the heat added to the comodity being heated. The fact that heat is being lost in combustion products is irrelevant.

[Christian McArdle]

Unless you believe in the law of conservation of energy. (We can assume that no nuclear processes are occuring here). When measuring efficiency, it can be as useful working out the amount of waste as the amount of useful work. Indeed, it is better to measure which of those that can be done most accurately, provided you can accurately know the amount of energy entering the system.

The reason you get more than 100% efficiency, is that when they worked out the official calorific value of the fuel, they forgot that the gaseous water byproduct had the latent heat of condensation that could be extracted. So the real calorific value of the fuel available to a room temperature and pressure boiler is actually higher. Obviously, the real efficiency is never better than 100%, just the artificial official one.

Christian.

[John Rumm]

No, the wrinkle in the maths is that he calorific value of the fuel used is wrong (i.e. too low). Some of the energy of combustion is "used" to convert the water produced as a by product into steam. The official figure assumes this energy does not exist (since historicaly it would have been lost in the flue gasses).

[<me9]

So the calorific value had previously been fiddled. I'l look into my old tome, 'The efficient use of fuel' to see how it was calculated in the middle of the last century.

[<me9]

Having looked into the above tome, fuels should have a gross CV which assumes condensing, and a net value without condensing.

It looks like the marketing hype has used the 'incorrect' figures.

[Aidan]

Great book, still useful. Oliver Lyle, I think. I believe he was the Lyle in Tate & Lyle.

It wasn't exactly fiddled. The energy in the water vapour simply wasn't recoverable (without ruining the boiler) and so it was ignored, quite reasonably. The efficiency figures were still valid for monitoring the efficiency of a combustion process or in comparing boilers. That technique became archaic when condensing boilers arrived. The total calorific value of the fuel was then considered.

The fiddle was that some manufacturers used the archaic method to claim higher, implausible efficiencies, of +100% in some cases, when everyone else was publishing the lower, total efficiency figures.

[<me9]

HMSO were the publishers. Various authors. The wartime equivalent of part L in some ways.

[Andy Hall]

It depends on where you are. In continental Europe, the net value is used and so efficiency of condensing boilers is quoted in the 106-109% range.

Also, the method of measurement is more laboratoty based than the UK seasonal method.

[Andrew Gabriel]

And we'll go through it again when fusion boilers arrive with ice as a waste product...