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Posted by John Cockerill on September 14, 2009 at 17:00:37:
In Reply to: Re: Boiler Condensation posted by HeatPro on September 11, 2009 at 12:15:15:
: : 1. I here that condensation occurs at 130F. for Oil and 137 F for natural gas.
: ---Whatever ... depends on the boiler design, as related to next question.
: : 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.
: --- The temperature of the fire, type of fire, and gas movement has something to do with that along with the design of metal and water content.
: : 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.
: --- Not so. There is a period during warmup where the metal is cool and the gases are hot. The delay of getting the heat from the gas to the metal to the water does count. The faster the warmup, the less the condensation. Thick cast iron will heat up the immediate metal, but have some delay getting to the water side, so there is a different effect of that to a thinner and different metal.
JC reply 1 As long as the surface of the metal is above 130 there should not be any condensation. The waterside will be cool as the citculation is on.
: A boiler with a large mass of water and metal would have a long warm-up phase.
: Another consideration is whether the fire has a radiant component as in oil to heat the metal surface, or a convective-conductance modality as in gas.
: Another consideration is the contact of the gases over an extended exchanger surface as in multiple passages where radiance is not a factor.
JC reply 1 Most exhaust temperatures at the stack reach 350F rather quickly.
: There is no hard and fast answer for all these conditions. An important consideration would be to know the exhaust temperatures at the breech, as temperatures lower than the dew point would condense water somewhere in part of the exchanger.
: Additional consideration is that sulphuric acid from oil can condense at higher temperatures than 250F in the boiler passages and in the stack.
JC reply 1 again stach temps are well over that usually.
: : 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?
: --- Yes. That can also happen all the time in retrofits of new boilers to large cast radiation.
: Also, houses that have been upgraded to new insulation, but leaving the radiation the same could have so much radiation the water temperature never gets over 130F during the season.
: New installations using condensing boilers would do well to have twice the radiation that of 180F boilers to get the heat out and create condensation all the time.
: : 5. Boilers set at 180F high limit have cold water coming back due to short circulation when the thermostat is satisfied and the heat is half way around the house. It would seem these high limit settings would be setting the system up for thermal shock from High delta temperature, and some cool parts of the heat exchanger?
: --- Not universally true. A system is ordinarily designed to have a 20F drop from boiler outlet to boiler inlet. A system that had a 180F outlet and 130F return would be grossly misdesigned, unless a boiler system is built to handle condensation using a condensing boiler, as in European designs.
JC reply 1. Most old systems are conversions of some kind. What about delta s in cold start?
: A System 2000 boiler control would pump the 'extra' hot water of a boiler to the indirect tank to preserve some. That would create a quick probability, yet there isn't enough water - about 3 gallons worth - to have that purge time last long enough to make considerable condensation.
JC reply 1 What??
: Multi-zone systems have water in the zone piping and radiation that comes back to the boiler when the valve opens, so many zones opening when only one has been open ordinarily brings in cold water.
: Boiler shock is a consideration for large commercial boilers with rolled tubes. Residential boilers are usually designed today not to leak even with radically differing water temperatures hitting them (often due to multi-zone openings); though it is wise not to design for that occasion to potentially create a problem.
JC reply don't go over 40F delta with a Weil McLain.
: : 6. Help me understand the principles and there relationship a bit better. As Dan H. says, we need to see the water. I need to see the water a little more clearly here. Is ther not a whole different theory about cold start. So many of these ideas are mossed in the general rules. In practice the general rule does not apply. When the boiler is at rest at High limit of 180 the residual heat gain from the fire chanber can run water temps up to 210F or more. Then the circulatior for a zone goes n from a radiant baseboard and brings back 100F water. ost is caused by overheat of the room and terrible lag for thermostat to go on while water is cooling in the pipes. How can one possibly have 140F return water with out primary/secondary piping. I can tell you most of the buildings do not have primary secondary piping. There best avoidance of boiler shock is cold start. Lower delta T ??? No?
: : Thank you.
: You are welcome.
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