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CI boiler efficiency

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hydro_newbie
hydro_newbie Member Posts: 37
There have been a few threads that touched on the idea that a mid-efficiency (~85%) non-modulating, non-condensing boiler will only achieve that efficiency during design day, when the boiler is able to run for a reasonable period of time (10min or so is often quoted as a minimum to avoid drastic efficiency losses).

I’ve been interested in getting some data to back up this information, but haven’t been able to find much.
It sounds like a lot of the lost efficiency is assumed to be in the form of heat used to heat up the large amount of cast iron and water in the boiler. That heat is then lost either via the jacket or up the flue when the boiler is off (jacket losses would occur when the boiler is running too).

I’ve tried to estimate this loss by taking some info on the boiler (water content, weight) and pretending it’s a cast-iron radiator. Assuming the boiler is in unconditioned space (e.g. basement), and that space is at the same temp as outside, I can make some calculations for how much heat is lost when the boiler is running and sitting idle.

The numbers I get are around 14%, not counting heat lost up the flue. I’m assuming a direct-vent CI boiler, so have ignored off-state flue losses for now.

While an additional 14% from the 85% starting point is pretty bad, that also assumes the boiler is basically a CI rad sitting in a basement that is at the same temp as outdoors. That seems quite pessimistic to me.

Any thoughts? Just trying to put some numbers to this. Every situation is different, and I can share the specifics of my assumptions, but was just wondering what folks think of the thought process (other than too much thinking! :smile: )

Thanks!

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 23,428
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    There isn't much wrong with the thought process -- as a starting point. But you need to be careful about overthinking things!

    That said, a couple of points. You could consider a boiler to be a lousy radiator, that is true, but it really is a lousy radiator. First off, you need to be using the surface area of the boiler casing to calculate your heat loss -- in the case of my Cedric, for instance, that's about 60 sq. ft. The mass of the boiler and the water in it isn't significant for heat loss rate -- although it is for heat content (more on that in a moment). Then you also need to take into account the insulation on the boiler, if any. Again, to take my Cedric for example, the insulation is sufficient to keep the surface (radiating) temperature down to about 120 or so, at the most. Using those two numbers you can estimate the heat loss from the boiler while it is running (in the case of Cedric, that comes to perhaps 7,000 BTUh -- about 2%). However, strictly speaking, that is not a loss for the total building, unless the boiler is outside (Cedric isn't) as that is 7,000 BTUh which isn't lost through the floor from the ground floor, which otherwise would be (and, of course, if the basement is heated at all, it's no loss at all).

    There is a somewhat similar loss -- with similar considerations -- when the boiler recovers from a cold start. You could calculate that from the mass of the boiler iron and the water in it. You could also do it the easy way: time how long it takes from a cold start to produce steam, and take the burner heating rate and multiply. Again, using my Cedric as an example, that works out to somewhere around 30,000 BTU. But... that most of that heat is returned to the basement when the boiler cools down again. Is that a loss? That's much the same debate as in the above paragraph on jacket losses.

    In fact, when you come right down to it, the only non-recoverable losses are the flue gas losses. This, of course, is where the condensing boiler has the advantage, since it recovers the latent heat of the water vapour from combustion, while a non-condensing boiler doesn't.

    I might point out, also, that the material of which the boiler is made -- cast iron, steel, what have you -- is not relevant.

    Have fun! This sort of calculation is entertaining -- but becomes complicated pretty quickly!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Solid_Fuel_Man
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,613
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    agree wit @Jamie Hall . Heat lost to the basement is still heat in the building so not a total loss. Flue loss is a goner
  • hydro_newbie
    hydro_newbie Member Posts: 37
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    Thanks Jamie, Ed, good points. Agreed that things can get complicated pretty quickly...and everyone's situation is different. Which I guess leads to the 'it depends' response a lot. But also makes for an interesting line of work. :smile:
  • Gordy
    Gordy Member Posts: 9,546
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    Even flue loss can be tricky. Is the flue vented into an exterior masonary chimney, or an interior chimney. How much flue pipe is exposed in the basement before it hits the chimney.

    I also think the 10 minute rule for flame stabilization could vary depending on the size of the boiler. The bigger the longer it may take.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    One way which I've thought of to quantify real world flue loss would to measure actual heatloss at the flue top (on the roof). This would have to be cfm of air and the temp of that air. The only loss not accounted for would be that lost to the attic space if applicable. Barometric damper allowing building air to be pulled out would also be accounted for in this type of measurement.

    My guess....its much higher than most think. Worst would be after a DHW call and all that iron/water keeps the flue warm up and out! This is where the heat purge (EK style) recovers some of that energy. In my custom panels I build for such things it is simple to do with relays, but in the non-heating season still lossy.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • bob eck
    bob eck Member Posts: 930
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    What about the Weil McLain GV90+ this boiler AFUE is above 90%
    Vents with PVC
    There is boiler efficiency and there is system efficiency.
    You can have a high efficiency gas condensing boiler and if not installed and pipe the correct way and installed on a system where you can heat the house with low water temps you would be better off installing a cast iron 84% - 90% AFUE gas boiler.
    You can install some Burnham cast iron gas boilers and do outdoor reset with them for jobs where you will need a higher water temp to heat the home.
  • hydro_newbie
    hydro_newbie Member Posts: 37
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    @Solid_Fuel_Man , you might find this link interesting:

    https://av8rdas.wordpress.com/2016/03/20/retrocommissioning-findings-reducing-boiler-purge-cycle-losses/

    It’s pretty long, but they took measurements of flue gas temp over a long period of time (at short intervals), and used that to estimate losses to due purge cycles. In this case the boilers had a combustion air fan, so I think there isn't much airflow during the off-cycles. They didn’t measure flow rate during the purge cycle, instead assuming it was related to the required flow rate for efficient combustion.

    Not sure how good the method is, but it was the only thing I could find on the web for estimating purge cycle losses. I used it for my situation, and it seems the loss is negligible. But still just guesswork I suppose.

    If there is a continuous draft up the flue, I could see that starting to eat into things as you suggest.

    Interesting point about DHW production during the off-season. There the heat left in the boiler at the end of the call would definitely be lost or worse.
  • Tim McElwain
    Tim McElwain Member Posts: 4,628
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    If I am not mistaken the GV90 is a condensing boiler and is basically the old Weil- McLain Gold with a designed added heat reclaimer on the side which then becomes a condensing boiler.
    GordyZman
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    Yes Tim, you are correct. It's kind of goofy looking. But it has a proven track record. I was at a WM show and the reps were practically drooling over the GV90+

    They claimed it was the best of both worlds. Have yet to see one installed.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Tim McElwain
    Tim McElwain Member Posts: 4,628
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    I also got similar feed back from a former Weil-McLain rep who also did training for them. He said it was the best residential boiler he had ever seen.
  • megajennyposts
    megajennyposts Member Posts: 4
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    Your thoughts are worthful for what a boiler needs to be. Thanks for the post.