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Posted by dana on September 23, 2009 at 17:51:45:
In Reply to: Boiler Condensation posted by John Cockerill on September 11, 2009 at 00:49:05:
: 1. I here that condensation occurs at 130F. for Oil and 137 F for natural gas.
: 2. If a boiler starts to fire the temperature of the fire is much higher than that. Surely the fire side of the boiler chamber is above 130F before any considerable condensation starts to happen.
: 3. It would seem to me that the only time considerable condensation could occur is when the boiler goes off on adjusted high limit and the waterside is still below 140 degrees.
: 4. Surely it takes a boiler in a resting state to start up will be handling water well below 140F as the system heats. You know those fall and spring days when the system has been off all day and starts up in the chilly evening?
The dew point of natural gas exhaust in a typical cast iron boiler with ~15% excess combustion air at sea level is around 122F (50C), not 137F (58C). (It's higher in more tightly controlled 8-10% excess air combustion mixtures of condensing boilers, up to practical limit of around 130F.) If the temp of the gas side of the heat exchangers or any section of flue prior to the introduction of dilution air stays below that during normal operation significant condensation will occur. In a cold start there's always some condensation, but as the temps of those surfaces rise above 122F, the condensation evaporates.
Atmospheric-vented boilers typically have dilution air introduced prior to entering the flue to lower the condensation temperature of the mix. This dilution air is typically introduced via a barometric damper in the vent piping between boiler & chimney, or a flue hood right at the boiler.
The high limit temp is inconsequential- it's the return water to the boiler that sets the lower-bound for the coolest part of the heat exchangers. At return water temps of 125F and above the heat exchanger is generally "safe", but the vent piping isn't necessarily. 130F+ return water is generally safe in most instances.
Propane exhaust dew temps are around 125F, oil a bit higher than that. Return water temps for oil typically need to be above 140F to protect the heat exchangers.
The cooler the return water, the higher the combustion efficiency and conversely. Above the condensing temp it's fairly linear, losing ~1% in raw combustion efficiency for every 25F rise in temp. Below 120F is where the fun begins, as efficiencies get enhanced by retrieving the heat-of-vaporization of the water in the exhaust gases. At 115F efficiencies are into the low 90s, at 100F return water it's up to about about 93-94%, but from there it has to drop all the way to 70F to hit the 98% mark. (Which is why only systems very low temp radiation like concrete slab radiant floors can ever get past the mid-90s.)
For a graphic representation see:
Note: While combustion efficiency falls off slowly with hotter return water temps above the condensing zone, FUEL use increases much more rapidly- on the order of 3% per 10F rise. This is due to the rapid increase in jacket & plumbing losses- most old-skool boilers have at best R5 of insulation on them, and located in a 65F basement the delta-T between boiler & room is quite large with say 180F output, 160F return. Uninsulated basement boiler rooms warmer than the insulated upstairs aren't rare. Large amounts of fuel get spent needlessly every year on that unintended basement heat due to higher than necessary operating temps, and inadequate boiler/plumbing insulation.
Also, it's never been clear to me how copper water-tube-boilers (and their cousins, tankless HW heaters with copper fin-tube heat exchangers) escape condensation damage at the generally lower (or MUCH lower, in the tankless case) entering-water temps? With too much excess combustion air the efficiency would fall off a cliff (45F water can be below the AIR's dew point!), yet non-condensing forced-draft tankless HW heaters typically test out around ~80-85% raw combustion efficiency, even in heating applications with return water temps over 100F, and take 40F water in stride without damage.
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