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Fuel waste if boiler size exceeds capacity of radiators?

frogpondfrogpond Member Posts: 14
My current gas boiler is massively oversized – 175KBTU/hr. Using the EDR calculations kindly provided by someone on this forum, the maximum output of my radiators is about 75K BTU/hr. Putting aside for a moment that the heatload calculation for the house is even lower, what happens to the extra 100K that the boiler tries to put out but that the radiators simply cannot radiate? Is that all wasted fuel?


  • STEVEusaPASTEVEusaPA Member Posts: 4,547
    The waste is in the short cycling, and even comfort. The boiler quickly runs up to limit and shuts off.
    If the heat loss is less than what the radiation can put out, then you have the opportunity to put in a properly sized boiler, and use a lower supply temperature.
    And if it's a modulating/condensing boiler, even more savings.
  • frogpondfrogpond Member Posts: 14
    Okay, the short-cycling issue makes sense to me. It's not a modulating boiler, so when it's on it's using enough fuel to fire at 175K. But I think my question is a little different: even while the boiler is on, if the radiators are only capable of pushing out less than half of that heat, what happens to the rest of the energy being consumed by the boiler?

    To use an analogy: I guess I'm wondering if it is like a pump trying to pump a greater volume of water through a narrow pipe, and the pipe can only handle half of the volume. The pump is still giving it its all, using enough energy to pump 2 GPM even though the pipe can only carry 1 GPM. The result is that the pump is wasting any amount of energy beyond what it would take to pump only 1 GPM.

    Isn't it the same sort of issue if the boiler is using enough gas to produce 175K but the radiators just don't have a throughput of more than 75K? And if so, is there a formula to calculate how much extra fuel is being wasted?
  • Jamie HallJamie Hall Member Posts: 13,897
    Not really. The boiler short cycling -- and I presume you are looking at steam, not that it really matters -- is actually "modulating" the boiler output to match what the radiation can put out.

    Depending on a whole host of factors, it might be possible to determine the reduction in efficiency -- and hence the amount of extra fuel -- but it isn't simple by any means. Not, if the short cycling is managed well, is it all that much of a hit -- a few percentage points.

    There have many -- and occasionally somewhat vigorous -- debates on how best to manage a seriously oversized boiler, and in many ways, oddly, it comes down to the combination of the boiler and the radiation. In some ways, the ideal would be to manage the burner on/off on so short a cycle that the temperature of the water (for hot water heat) never varied much or, in the case of steam, that the residual heat in the boiler metal kept the boiler steaming (it will), in either case giving the burner just enough of an on time to keep it that way.

    Most burners don't like to do that. So... if the boiler isn't insanely oversize, one can run a relatively short cycle without real problems. In the case of water, allow the circulating temperature to droop a few degrees -- perhaps as much as 10. In the case of steam, allow the pressure to drop some (in a vapour system, for instance, from 6 ounces per square inch down to 2). The objective being to never actually let the boiler cool off -- because that's where you lose efficiency. On the other hand, if one has really massive radiation (big cast iron radiators!) and a really seriously oversized boiler -- such that the "on" time is less than the "off" time of the cycle, perhaps -- it may make more sense to let the residual heat in the radiation be the guide, and force the boiler to stay off, once it starts to cycle, until the radiation -- not the boiler -- has cooled somewhat (that won't work if you need all the radiation going at full capacity, but that is a vanishingly rare situation). A delay time on letting the boiler turn back on of 10 or even 15 minutes -- if the thermostat is still calling -- may work well.

    I hope this helps at least a little bit...
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • frogpondfrogpond Member Posts: 14
    That is helpful. The takeaway, I suppose, is that there may be some extra fuel wasted, but it is not as bad as I initially thought and probably not worth calculating. Thank you for your thoughtful reply.
  • JUGHNEJUGHNE Member Posts: 7,455
    Perhaps if you thought of this as city traffic.
    You have a traffic signal at each intersection, the lights are timed for 20 MPH.
    But at the first green you speed away at 40 MPH.
    Too fast to catch the next have to brake and stop.
    Take off again at 40 MPH....same thing for the next light. All the way across town. (Or thru most of the heating season)

    But if you were a boiler going 20 MPH and caught all the greens,
    (that is just the right size for the job) you would never have to brake or stop. (But you get to your destination....keeping the house warm enough)
    Just steady constant speed, avoiding the braking and restarts.
    And avoiding the inefficiency of start and stops.

    In the real world the only way the boiler could run constantly is with burner modulation. But if on/off burner and sized close to the heat loss the run time would be increased. If sized to the coldest day of the year then one would expect constant firing on that day......a rare happening as there are only a finite number of boiler sizes and many variables for that design day temperature.
    So it is the right size for a small percentage of the season.

    My truck gas bills reflect this... 460 cubic inch V-8...7.7 liter.
    Town mileage is 5-7 MPG.....down the highway it does the best at almost 10 MPG. Fortunate to only drive 3000 miles per year.
  • Jamie HallJamie Hall Member Posts: 13,897
    Not a bad analogy, @JUGHNE -- although it can be stretched a bit further. Suppose your car had only two possible engine settings -- on, full song, and off. If you were clever, you could switch it on just long enough, at intervals, to maintain the required average speed. You could lengthen the intervals of off and on, and have more speed variation around the average, or you could switch it on and off more often, and have less. And never touch the brake, all the way across town!

    You'd have to try various combinations to see what gave the best economy (for reference, if you can find it, back in the '50s there was a Mobil crosscountry economy run -- which used similar techniques!).

    And further yet -- a surprising number of World War I airplanes did have engines which were either on or off -- no throttle -- and that is exactly how they were flown. Go to Old Rhinebeck Airdrome in New York state to see some.

    I sympathize with you on the truck -- one of mine has the 454 Chevy V8, and the mileage isn't a whole lot better than your 460...
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • frogpondfrogpond Member Posts: 14
    Haha a very thorough analogy. Thanks again guys.
  • STEVEusaPASTEVEusaPA Member Posts: 4,547
    @ChrisJ disagree again?
  • GilmorrieGilmorrie Member Posts: 127
    Here is an axiom - almost universally true, despite what boiler salesmen will say. If your boiler runs reliably, you can't economically justify replacing it to achieve fuel savings. Even if the replacement boiler miraculously puts out heat without burning any fuel, the payback period will be too long. Do the math.
  • ChrisJChrisJ Member Posts: 11,344
    @ChrisJ disagree again?
    Just making sure you're paying attention.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • JUGHNEJUGHNE Member Posts: 7,455
    Jamie, are you familiar with "hit and miss" engines, the flywheel carried them thru a few revolutions until the governor sensed a drop in speed and then would fire a "pop"?

    Then on the subject of ROI.....I made my 16 year old grandson calculate (paper & pencil BTY) my MPG as we filled up. He said I should update and would save so much money on gas. So we discussed this....a new van might get twice the mileage for an investment of maybe $30,000..... minimal....then 7 % sales tax.
    So I spend about $1000 a year on gas.....I might save $500 a year.
    The first few years the new van plate tax would approach that amount.
    Plus the $2100 sales tax the first registering.

    In 2007, I paid $2250 for this 1984 1 ton retired ambulance van. Every thing is heavy duty, always stored inside, 69,000 miles.
    Best thing is that it has the 10" roof extension.....stand up :) .
    Today the grandson found out what sales tax and plates cost, and annual tax on a newer car....but he gets about 30 MPG.

    I am thinking that a new boiler has about the same financial situation. Especially to go from the old 80% CI to 95% mod con.

    ChrisJ, so now we know who is throwing up the "disagree" tag. ;)
  • Jamie HallJamie Hall Member Posts: 13,897
    Yes indeed -- hit and miss engines! The basic principle we're talking about!

    And you are so right on the economics -- which is why I still have, and run, that '94 K2500 I mentioned!
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • ChrisJChrisJ Member Posts: 11,344
    @JUGHNE I haven't averaged less than 40mpg on a tank of gas since 2012 and I drive 20000 miles a year.

    30 would upset me a lot.

    Also never owned a hybrid yet
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,571

    I agree the numbers don't lie
  • EdTheHeaterManEdTheHeaterMan Member Posts: 1,063
    edited August 27
    The cost of ownership is used to justify the difference between the higher efficiency and the standard system. This is assuming the boiler is being replaced anyway. So now you are only justifying the difference between the two systems.

    Back to the original poster. Is your current 50-year-old system economical to operate? How many Gallons per year?

    Now if the oil burner is in the ash pit of the 50 YO coal conversion type boiler, you can raise the flame to the originally designed location. When the "Dead Man" had a picture of it on the drawing table, the coal fire was near the middle door. If you put the burner there and plug up the hole at the bottom of the boiler, (where the ashes fell to the bottom) I have seen dramatic savings because the 80% flame is closer to the water and makes steam faster.

    Has it been determined if this is a steam or hot water boiler? Gas or Oil? A 50-year old boiler could be a modern style package boiler or it could be a leftover pre-WW II coal-repurposed with oil burner.

    Pictures of the boiler will help us to see if that project is possible.
    I believe the only reason to replace a steam boiler is if it leaks. 50 years is nothing for some boilers. How many of us have pulled out 75 or 100 YO boilers?

    Just saying.

  • EdTheHeaterManEdTheHeaterMan Member Posts: 1,063
    @Jamie Hall there are 2 issues discussed here. "very confusing" @bleeder is talking about Steam and the age v replacing, so a Mod-Con is not an option.

    The original poster, @frogpond is talking about oversized boiler and has not indicated water or steam and/or oil or gas.

    So how do we address this confusion? Maybe we, as experienced members, should ask off-topic questioners (especially with a low number of posts) to begin a new discussion.
  • STEVEusaPASTEVEusaPA Member Posts: 4,547
    Where's the 'head spinning emoji?
    I sent a note to @Erin Holohan Haskell to recommend @bleeder posts get moved to his own thread.
  • Erin Holohan HaskellErin Holohan Haskell Member, Moderator, Administrator Posts: 1,448

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