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energy savings

I have a client who wants me to estimate of the potential savings a new mod- con could deliver over the dying old slant fin atmospheric vent gas boiler they now have.

The house is 1970's 2x4 construction maybe 35 plus btu/sq. I am planing to propose trv equipped panel radiation sized for 150deg. at design. To replace and in some areas assist the existing base. The user plans significant wood supplement. I expect the trvs to be especially useful in helping to prevent over heating in the rooms effected by the wood-stove. Outdoor reset probably with sensor in non trv'd reference room far from wood stove(automatic curve adjustment). Probably Tekmar 270 paired to TT prestige. Direct piped, (researching smart circulators) 40-50 gallon indirect to replace gas atmospheric tank heater.

General experience seems to indicate that a %25 percent savings could easily be realized by these sort of improvements. However boiler AFUE's don't really prove this level of savings. I have explained the value of building envelope improvement in addition to mechanical improvement, and the variables and difficulties of making absolute predictions of efficiency improvements. I'm looking for some documentation to support the observation that low temperature systems of this nature can realize gains beyond boiler plate percentages.

Any opinions, does anyone think I would be misleading them by suggesting potential gains of this size?


  • Empire_2
    Empire_2 Member Posts: 2,343

    I'm sure Brad will see this and help you out. He is good at this kind of thing.

    Mike T.
  • hr
    hr Member Posts: 6,106
    The energy calculator on Siggys

    HDS software or Heat Load Pro software is a good way to show comparisons. www.hydronicpros.com for free demo. It will compare gas, oil, lp, electric, and even firewood options.

    hot rod

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  • Brad White
    Brad White Member Posts: 2,393
    Someone call my name? :)

    Mike T. -thanks for your confidence.

    Scott- one danger we all have is in making promises so couch the potential outcomes in language that is soft enough such as "weather dependent" and being based on certain operating parameters. Not to dodge the issue but I would not want them coming back to you because of missing a specific number.

    That said, 25% does not seem unreasonable. Keep in mind the AFUE number alone is a snapshot, not reflecting the difference between a so-called 80% cast iron boiler and a properly sized mod-con. For example, your "80% AFUE" cast iron boiler might have been 200% over-sized and never saw 80% but was more like 55%. Starting, stopping, warming up, cooling down, with or without constant circulation, who can say?

    The ModCon, properly sized, will ramp up and down becoming the boiler it needs to be so long as the load is above it's minimum modulation point. Cruise control versus "accelerator/brake".

    The fact that they will use supplemental wood-firing roughs up the comparison basis a good bit but will certainly help the numbers. TRV's are the best investment for the dollar for all the reasons you state.

    All things being equal, yes, 25% is a save number for saving on their base fuel use. I think they will be surprised that it will be better but with the wood fuel, they may not be able to separate that out.

    Bottom line, comparing AFUE as an annual fuel comparison is of little value.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • PS_3
    PS_3 Member Posts: 28

    As Brad stated, I would be careful quoting hard numbers -"Individual results may vary". It's apples and oranges, but for what it's worth, I've saved ~ 40% (adj) on my gas bill since replacing my old beastie with a mod-con, but I also kept the old CI radiators. Also, I had a much higher heat loss(mid 50's), and the 1940 boiler was approximately 5X oversized. You state a design load of ~35 - that is pretty low. There is only so much you can save.
  • modcon efficiency

    Hello Scott,

    Since your client will be using wood, you cannot predict his savings. Even in a direct change-out weather, operation, installation, radiant terminals and many other factors make the outcome or return on investment in retrofit jobs really impossible to predict. Better still you do not need to.

    Simply explaining the concept of heating say 300# cast iron versus 25# stainless steel or aluminum, to a space heating temperature can make your point. You can call his attention to stack temperatures in the 100F range instead of 300-600F. You could go on to show your customer his neighbor's house which will probably reflect the 20 year trend in the bulk of the heating industry towards light weight (one might even say "low-mass") condensing furnaces which by the way are PVC vented.

    Sell the sizzle with confident that their new ModCon will give them a guaranteed "tax-free" return on investment.

    I like to let my clients come to their own conclusions, take ownership thereof and never settle for less than I can offer. But what do I know?



    PS. I would be careful to properly load your new ModCon. TRV's can lead to short cycles, obviously very bad for operating efficiency.
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