Musings on Boiler "Net" Output BTUH and the "Pick-Up Factor"
- lsayre
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I believe I've finally figured out why my computed efficiency figures for my AHS Coal Gun always come out at about 70% when (Axeman Anderson AA-130) literature (The detailed Bureau of Mines Report) indicates it should be 80% to 81% efficient.
I've been reading about boiler Pick-Up Factors, which are required to account for the necessary heating of all of the piping mass in the system before it can radiate useful heat.
Modern boiler manufacturer literature indicates that they have unified upon 1.15 as the "pick-up factor" to use for hot water boiler systems. This is reflected in the "Net" BTUH figures. So boilers actually have three BTUH ratings. They have Input BTUH, Gross Output BTUH, and Net Output BTUH ratings, where "net" incorporates the pick-up factor.
So for a boiler with 80.5% "Gross" efficiency (the measure of heat remaining after stack losses are accounted for) the all important "Net" efficiency would be:
80.5% divided by 1.15 = 70% (the overall efficiency figure that I have computed)
The pick-up factor of 1.15 should generally hold true for hot water boilers utilizing typical copper 3/4" hot water baseboards and/or radiant.
But years ago, for cast iron radiators used with hot water, and when the pipes themselves were also cast iron, due to far more metal mass to be heated, the most commonly reported hot water "Pick-Up Factor" of choice back then was 1.3. This would seemingly imply that if my AHS was connected to cast iron radiators its "Net" efficiency would be:
80.5% divided by 1.3 = 62%
A significant drop! Something to ponder.....
I've been reading about boiler Pick-Up Factors, which are required to account for the necessary heating of all of the piping mass in the system before it can radiate useful heat.
Modern boiler manufacturer literature indicates that they have unified upon 1.15 as the "pick-up factor" to use for hot water boiler systems. This is reflected in the "Net" BTUH figures. So boilers actually have three BTUH ratings. They have Input BTUH, Gross Output BTUH, and Net Output BTUH ratings, where "net" incorporates the pick-up factor.
So for a boiler with 80.5% "Gross" efficiency (the measure of heat remaining after stack losses are accounted for) the all important "Net" efficiency would be:
80.5% divided by 1.15 = 70% (the overall efficiency figure that I have computed)
The pick-up factor of 1.15 should generally hold true for hot water boilers utilizing typical copper 3/4" hot water baseboards and/or radiant.
But years ago, for cast iron radiators used with hot water, and when the pipes themselves were also cast iron, due to far more metal mass to be heated, the most commonly reported hot water "Pick-Up Factor" of choice back then was 1.3. This would seemingly imply that if my AHS was connected to cast iron radiators its "Net" efficiency would be:
80.5% divided by 1.3 = 62%
A significant drop! Something to ponder.....
- Lightning
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I have a tough time agreeing with the pick up factor effect having such a big impact on net efficiency. It implies the heat is lost which couldn't be the case. Heat stored in the metal pipes when the circulator kicks on would be radiated to the house once the circulator kicked off, wouldn't it? Otherwise, where would the heat stored in pipes go?
- lsayre
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This is an evolution in learning for me too. I just came across a site which refuted the "metal heating" aspect of the Pick-up factor and laid the blame for its need squarely on the gallons of water in the system which require heating. Each time a zone is to be heated its water (which has fell to 65-70 degrees between firings) must be re-heated to 160-180 degrees. The pick-up factor accounts for the BTU's required to accomplish this task per my latest reading. More studying is required here.
But obviously if a zones water must be heated, the zones metal must also need to be heated, so my intuition (whereby admittedly intuition more often than not leads to bad science) tells me it must be a combination of the two.
But one thing is for certain. All boilers list three BTUH figures. And from all that I've read, the "Net" figure is the one to match with the homes heat loss requirements for BTUH.
Is it merely a fluke happenstance that 80.5% / 1.15 = 70% ?
But obviously if a zones water must be heated, the zones metal must also need to be heated, so my intuition (whereby admittedly intuition more often than not leads to bad science) tells me it must be a combination of the two.
But one thing is for certain. All boilers list three BTUH figures. And from all that I've read, the "Net" figure is the one to match with the homes heat loss requirements for BTUH.
Is it merely a fluke happenstance that 80.5% / 1.15 = 70% ?
- jpete
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But where? In the basement where you don't live or in the space where you do? When doing heat loss calculations, if the pipes in the basement aren't insulated, the basement is considered a conditioned space. That may or may not be what you want.Lightning wrote:I have a tough time agreeing with the pick up factor effect having such a big impact on net efficiency. It implies the heat is lost which couldn't be the case. Heat stored in the metal pipes when the circulator kicks on would be radiated to the house once the circulator kicked off, wouldn't it? Otherwise, where would the heat stored in pipes go?
The boiler I'll be putting in my new house works like a furnace. It never leaves usable heat in the boiler. After a call for heat, it continues to run the circulators unit the supply water temp is below 90*F.
- lsayre
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Well, whatever the Pick-Up Factor is (though definitively leading to Net BTUH ratings), 1.33 seems to be a popular "pick-up factor" used for steam systems.
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Larry,
Yes, AFAIK the 1.3 (or more) pickup factors are associated with steam systems (rather than the piping material). And I agree with you that the net numbers are the ones to consider in comparisons with heat loss computations.
x2 jpete. Even insulated pipe gives off BTU's into spaces not intended to be heated by the radiation it serves.
Mike
Yes, AFAIK the 1.3 (or more) pickup factors are associated with steam systems (rather than the piping material). And I agree with you that the net numbers are the ones to consider in comparisons with heat loss computations.
x2 jpete. Even insulated pipe gives off BTU's into spaces not intended to be heated by the radiation it serves.
Mike
- Lightning
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Larry, are you arriving at 70% by comparing it to the electric boiler? If so, wouldn't said pick up factor already be on both sides of the equation? If again yes then it would wash and you wouldn't see it, right? Or no?
As for radiating heat to undesirable locations, the pipes should be well insulated so they hold the pick up factor for the next heat call.
As for radiating heat to undesirable locations, the pipes should be well insulated so they hold the pick up factor for the next heat call.
- lsayre
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Yes, comparing it to the electric boiler, and you are probably right in that it had to accomplish the same things.Lightning wrote:Larry, are you arriving at 70% by comparing it to the electric boiler? If so, wouldn't said pick up factor already be on both sides of the equation? If again yes then it would wash and you wouldn't see it, right? Or no?
As for radiating heat to undesirable locations, the pipes should be well insulated so they hold the pick up factor for the next heat call.
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The heat lost in the piping is still inside your thermal envelope and will in some way ease the demand for heat in the "Zoned" areas. Or in the case of an old stone basement the heat lost in the piping will keep the basement from freezing. To me that heat should not be included in any pick up calculations. Now as far as the btus required to bring the boiler up to temp (before circ starts) as well as the btus lost up the chimney after a heat call as the boiler cools Id agree that should be included. Ill add that the heat lost by the boiler at rest to surrounding enviroment should not, IMO, be counted.
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wl, none of the heat you're talking about makes it to the radiators, so it isn't where you (or someone before you) has decided the BTU's need to be for the comfort of the occupants. If putting BTU's into the "thermal envelope" were effective at heating everything comfortably you wouldn't need radiators - just put a big modine next to the boiler and blast all of the gross BTU output into the boiler room. Some small portion of the heat that goes into "unheated", unused and/or inaccessible spaces may wind up being useful, but none of it comes out of the installed radiation when needed. The boiler needs to be able to cover the net output of the attached radiation and the heat that is lost before it gets there. Otherwise it's an underpowered system.
Mike
Mike
- Sting
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Until the doubters above have the opportunity to baby sit a boiler on a load that puts it at the fringe of its total capacity - there will be skepticism.lsayre wrote:
A significant drop! Something to ponder.....
"lsayre" your dead on!
- Lightning
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I see. So would the pick up factor be the same for a system with continuous circulation and steady burn rate as one that calls for heat and then doesn't?
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I think you guys are making this more complicated than it really is. As applied in practice, and as lsayre pointed out, the pickup factor is an allowance for distribution system losses that prevent BTU's coming out of a boiler operating at full power from turning into BTU's entering the installed radiation. If you don't allow for that, the boiler could wind up being sized too small to generate enough BTU's to fully power the installed radiation. The actual losses undoubtedly differ due to many factors - e.g., underground piping serving an outbuilding installation. That doesn't mean there is a specific value known to be exactly correct for systems with a given set of characteristics.
Mike
Mike
- Rob R.
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The pickup factor is an allowance of boiler capacity to get the distribution temperature up to operating temperature. With steam, you are heating the piping. With hot water, you are heating the piping and the water. Even if your distribution system is very well insulated with minimal loss, if it is at nearly room temperature when there is a heat call, the boiler must first warm (itself) and the piping before you get any heat in the living space. It doesn't change the efficiency of the boiler, but it can change the efficiency of the system.
One more thing, most of the old boilers had tankless coils to produce domestic hot water. The old boiler books allowed for a certain amount of domestic hot water without adding any extra boiler capacity - the pickup factor already figured in the size was adequate, up to a point. If you are producing domestic hot water by a means other than the boiler, one could argue the pickup factor can/should be reduced.
It doesn't have to be. Some of the "modern" boiler guys that install high efficiency boilers know that they can get away with a smaller boiler if constant circulation is used. A good example is a house with a old gravity hot water system. Thousands of pounds of cast iron and water...and they hook it up to a gas boiler the size of a beer cooler, and let the circulator run all the time (boiler temperature adjusts based on outside temperature). This results in great economy and steady temperature...but if the home owner decides to turn the heat down to 60 on a cold night and back to 70 in the morning, it will could take until lunchtime to get back to 70.Lightning wrote:I see. So would the pick up factor be the same for a system with continuous circulation and steady burn rate as one that calls for heat and then doesn't?
One more thing, most of the old boilers had tankless coils to produce domestic hot water. The old boiler books allowed for a certain amount of domestic hot water without adding any extra boiler capacity - the pickup factor already figured in the size was adequate, up to a point. If you are producing domestic hot water by a means other than the boiler, one could argue the pickup factor can/should be reduced.
- lsayre
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I just came across some reading that indicated that a Pick-Up Factor of a whopping 1.56 was commonly assumed prior to about 1945-1950.