Low Loss Headers
- Thread starter feebeebos
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There was a killer point mentioned only in passing in Mr Siegenthaler's article and ignored entirely in the Contractormag piece. A major objective of a condensing boiler setup is that the boiler should be able to condense.
IMHO (and apologies to DIA!) balanced headers and condensing boilers are more likely a bad marriage than a good one. If the boiler is circulating water round just the header (as it will be in low-load conditions), the Return temperature will be very close to the Flow temperature and there will be NO condensation possible, at all! It's physics: if the heat exchanger is hotter than 56 degrees or so (the 'dew point') there will be no condensation formed there.
I've encountered MANY, MANY condensing boilers that only spend minimal time condensing. I believe the situation is so bad that it threatens to challenge the whole logic of installing condensing boilers as per current UK regulations! Condensers without appropriate controls will NOT work properly but the regs say little about controls or what is ''appropriate'.
Lots of steam coming out of the flue usually shows that the boiler is NOT condensing. If it was, the water would be running into the condensate drain.
Of course, rules may be different in the US - but even G W Bush can't change physics.
IMHO (and apologies to DIA!) balanced headers and condensing boilers are more likely a bad marriage than a good one. If the boiler is circulating water round just the header (as it will be in low-load conditions), the Return temperature will be very close to the Flow temperature and there will be NO condensation possible, at all! It's physics: if the heat exchanger is hotter than 56 degrees or so (the 'dew point') there will be no condensation formed there.
I've encountered MANY, MANY condensing boilers that only spend minimal time condensing. I believe the situation is so bad that it threatens to challenge the whole logic of installing condensing boilers as per current UK regulations! Condensers without appropriate controls will NOT work properly but the regs say little about controls or what is ''appropriate'.
Lots of steam coming out of the flue usually shows that the boiler is NOT condensing. If it was, the water would be running into the condensate drain.
Of course, rules may be different in the US - but even G W Bush can't change physics.
Rubbish CC, next time we sit down for an Indian I will explain how it all works. 
Lots of steam coming out the flue means it's condensing, and in any case a condensing boiler is more effeicient than a old non-steamy boiler.
Have you done the energy course yet
Lots of steam coming out the flue means it's condensing, and in any case a condensing boiler is more effeicient than a old non-steamy boiler.
Have you done the energy course yet
Yes, thanks.
But it was PITIFULLY thin on this topic, especially concerning controls.
Before you try to wind me up with comments such as 'rubbish' maybe YOU should explain how water that's supposed to have condensed on the internal surface of the heat exchanger (and thereby 'given up its latent heat') can then appear as steam (or more correctly, water vapour) coming out of the flue. If it condensed and then vapourised again (!) (to form the water vapour), it will NOT have released it's latent heat to the boiler. If it left the boiler as steam and then formed water vapour inside the flue (as usually happens!), it WILL have released its latent heat but only to the flue walls. If that part of the flue is still inside the building - OK -you MIGHT achieve a slight benefit but any temperature gain won't be in the boiler water.
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That always seemed like logic to me too! It's like saying "look, krap boiler design". Really though it does mean that at least there is some condensing going on inside the boiler , otherwise some of it wouldn't get out to be visible.
What you see coming out of the flue is not steam or water vapour. It's water, or more precisely very tiny water droplets suspended in the air, otherwise known as mist (or cloud). As such, it is the result of condensation, since the water formed by gas combustion is initially in the gaseous form (steam, or water vapour when mixed with air) when it is invisible. Any visible manifestation of water is liquid or solid.croydoncorgi said:
OK - let's agree on a terminology truce.
When I mean 'warm white-ish cloud of water droplets coming out of a flue' I'll refer to it as 'visible plume'.
I'll not refer to water vapour at all.
And yes, I DID know that steam was invisible.
Doesn't alter the essential facts however.
The visible plume is not necessarily formed WITHIN the boiler (where it would have an efficiency benefit). OK - it may be - but only a small amount will get carried away in the airflow. The rest should stay as liquid on the surfaces and run to the drain. So an accurate test of effective condensing is the amount of water reaching the drain, not necessarily the size of the visible plume.
Particles of visible plume carried in the fluegas stream will be around 'condensing temperature', so if they get carried into the flue their heat will not contribute to boiler efficiency.
As the flue gas (above dewpoint and including steam) leaves the boiler, the size of the duct(s) increases, pressure falls, so adiabatic cooling will lower the fluegas temperature. This will cause further condensation of visible plume but with no efficiency gain.
When I mean 'warm white-ish cloud of water droplets coming out of a flue' I'll refer to it as 'visible plume'.
I'll not refer to water vapour at all.
And yes, I DID know that steam was invisible.
Doesn't alter the essential facts however.
The visible plume is not necessarily formed WITHIN the boiler (where it would have an efficiency benefit). OK - it may be - but only a small amount will get carried away in the airflow. The rest should stay as liquid on the surfaces and run to the drain. So an accurate test of effective condensing is the amount of water reaching the drain, not necessarily the size of the visible plume.
Particles of visible plume carried in the fluegas stream will be around 'condensing temperature', so if they get carried into the flue their heat will not contribute to boiler efficiency.
As the flue gas (above dewpoint and including steam) leaves the boiler, the size of the duct(s) increases, pressure falls, so adiabatic cooling will lower the fluegas temperature. This will cause further condensation of visible plume but with no efficiency gain.
Well done Chris ( Hutt ), you got in there just before me to explain that.
White clouds are water droplets which have already condensed and given up their latent heat of vapourisation!!!
Water vapour is colourless!
Does anyone understand what John and John are arguing about?
Tony.
White clouds are water droplets which have already condensed and given up their latent heat of vapourisation!!!
Water vapour is colourless!
Does anyone understand what John and John are arguing about?
Tony.
Lamb to the slaughter, Tony!
If you don't understand, you should!
If you don't understand, you should!
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croydoncorgi said:
So where does this later latent heat go?
May I suggest it warms the inner flue wall, which pre-heats the incoming air to some extent, except for Kestons; thus a (small) efficiency gain.
I do agree wholeheartedly with the original comment though, many HE boilers are set up so badly that they short cycle & lose many of their benefits.
Whilst not wanting to be a "lamb", although thats surely being better than "old mutton", can I suggest that a flue temperature of around 60°C will indicate to me that considerable condensing will have occurred.
Without having done any tests, I would have assumed that at 60°C much of the water vapour will already have condensed.
Remember that air at say 20°C will be able to contain a considerable quantity of ( invisible ) water vapour at below 100% RH before water droplets ( pluming ) become visible.
This discussion started on the topic of headers which are normally only used on larger boilers. Rather different criteria usually apply to these and like any system if the actual load is below 20% of the rated maximum then a significant reduction in the efficiency is only to be expected.
I like the concept of a number of smaller boilers which are sequentially brought into service as the load increases. At Alexandra Palace there were six boilers giving a ratio from 1:6 power output with the maximum boiler efficiency maintained at any time.
Tony
Without having done any tests, I would have assumed that at 60°C much of the water vapour will already have condensed.
Remember that air at say 20°C will be able to contain a considerable quantity of ( invisible ) water vapour at below 100% RH before water droplets ( pluming ) become visible.
This discussion started on the topic of headers which are normally only used on larger boilers. Rather different criteria usually apply to these and like any system if the actual load is below 20% of the rated maximum then a significant reduction in the efficiency is only to be expected.
I like the concept of a number of smaller boilers which are sequentially brought into service as the load increases. At Alexandra Palace there were six boilers giving a ratio from 1:6 power output with the maximum boiler efficiency maintained at any time.
Tony
Wrong assumption, I'm afraid, Tony.
UNLESS the temperature of at least part of the heat exchanger(s) is BELOW 56 degrees, NO condensation will take place inside the boiler. None. Nada.
The flue gas including steam will be significantly hotter than that before it hits a cooler heat exchanger surface, which means that depending on the design of the fluegas path, more or less of the fluegas will hardly be cooled at all! My comment about 'visible plume carried in the fluegas stream will be around 'condensing temperature' was actually b****cks: either its ABOVE dewpoint, with no condensation occurring, or below, in which case ALL the latent heat will have been released to somewhere. Problem is the process will be dynamic, with turbulence in the gasflow mixing cooler and hotter bits. The key point is whether the average temperature in the cool end of the HX is above or below dewpoint.
Correct that some heat will transfer from flue to air intake, so will not be lost to outside. This transfer also explains some of the cooling in the flue (along with expansion cooling), so that more 'visible plume' will form as it passes through. Once again, though, what percentage of the remaining latent heat in the fluegas actually finds its way into the intake air is a complicated question, depending on length of flue, wall thickness, flow rates, turbulence, .... .
My two main concerns about this efficiency question are, firstly, that the majority of retrofitted condensing boilers are VERY unlikely to operate 'efficiently' without extra work on the associated controls and pipework and, secondly, that very few people actually understand all physics and other issues that are relevant. (I exclude myself from the 'very few'! ) As a result, targets that the government expects to meet on the back of a transition to condensing boilers will be missed by miles.
Tony's point about a system operating at less than 20% of rated maximum is also confusing and 'old thinking'. Most (all?) SEDBUK A boilers have modulating burners which can usually operate over an output range from (say) 25% to 100% of rated output with no problem. It is well-known that if you run a condenser at or close to 100% it will not be very efficient. I can't see why 20 percent is relevant or even a realistic level: once heat demand falls below the equivalent of the minimum modulation level, the burner will cycle on and off. Whether this turns out to be 'efficient' is probably not a simple yes / no answer but I can see no particular reason why it should be especially INefficient. (A reason why it was considered Best Practice to operate large conventional boilers at 80% or higher was because of the RISK of condensation occurring and rotting the heat-exchanger. These rules no longer apply.)
UNLESS the temperature of at least part of the heat exchanger(s) is BELOW 56 degrees, NO condensation will take place inside the boiler. None. Nada.
The flue gas including steam will be significantly hotter than that before it hits a cooler heat exchanger surface, which means that depending on the design of the fluegas path, more or less of the fluegas will hardly be cooled at all! My comment about 'visible plume carried in the fluegas stream will be around 'condensing temperature' was actually b****cks: either its ABOVE dewpoint, with no condensation occurring, or below, in which case ALL the latent heat will have been released to somewhere. Problem is the process will be dynamic, with turbulence in the gasflow mixing cooler and hotter bits. The key point is whether the average temperature in the cool end of the HX is above or below dewpoint.
Correct that some heat will transfer from flue to air intake, so will not be lost to outside. This transfer also explains some of the cooling in the flue (along with expansion cooling), so that more 'visible plume' will form as it passes through. Once again, though, what percentage of the remaining latent heat in the fluegas actually finds its way into the intake air is a complicated question, depending on length of flue, wall thickness, flow rates, turbulence, .... .
My two main concerns about this efficiency question are, firstly, that the majority of retrofitted condensing boilers are VERY unlikely to operate 'efficiently' without extra work on the associated controls and pipework and, secondly, that very few people actually understand all physics and other issues that are relevant. (I exclude myself from the 'very few'! ) As a result, targets that the government expects to meet on the back of a transition to condensing boilers will be missed by miles.
Tony's point about a system operating at less than 20% of rated maximum is also confusing and 'old thinking'. Most (all?) SEDBUK A boilers have modulating burners which can usually operate over an output range from (say) 25% to 100% of rated output with no problem. It is well-known that if you run a condenser at or close to 100% it will not be very efficient. I can't see why 20 percent is relevant or even a realistic level: once heat demand falls below the equivalent of the minimum modulation level, the burner will cycle on and off. Whether this turns out to be 'efficient' is probably not a simple yes / no answer but I can see no particular reason why it should be especially INefficient. (A reason why it was considered Best Practice to operate large conventional boilers at 80% or higher was because of the RISK of condensation occurring and rotting the heat-exchanger. These rules no longer apply.)
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