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mod-con theory/design question

rhl Member Posts: 93
So flue gases condense when the return temperatures are below 130 degrees. let's suppose that we have a mod con boiler with high temp convectors, returning say at 150 or 160 degree water. Couldn't we achieve the desired high efficiency, by just simply mixing in fresh cold water from the street? I mean, this costs money, but, i wonder how much that might cost relative to savings achieved?


  • Gordy
    Gordy Member Posts: 9,514
    Typically if it’s a fresh design you would size the emitters to accommodate the load with lower water temps.

    In a install with existing emitters which are usually oversized due to envelope upgrades in older structures, or designers installing more than enough emitter. Water temps can be lowered.

    Usually the highest temps needed are during , and below design temp conditions. Which is a small percent of the heating season. At warmer outdoor temps the supply water temp can be reduced into condensing range.

    This where outdoor reset has the advantage. It decides what water temp is needed to meet the load of the outdoor temp.

    Your idea introduces fresh water into the system. Not good for the components, and wasteful.
  • VAsparky
    VAsparky Member Posts: 23
    edited January 2019
    Yes, introducing cold water from outside the system would increase the boiler's efficiency, but what we're really interested in is the efficiency of the entire heating system; that is, the amount of heat delivered to the building for a given amount of fuel.

    Remember that a hydronic system is a closed loop, like the cooling system in a car - it can only hold so much water. Adding fresh water means that "old" water must be discarded - and that's water that we've already paid to heat, so essentially we'd be throwing away fuel.
  • sunlight33
    sunlight33 Member Posts: 356
    it's like dipping the return pipe in an ice bucket, you end up paying more for energy for making the ice.
  • ced48
    ced48 Member Posts: 468
    You would spend a whole lot more money reheating the water back to 150 degrees than you would save by trying to increase efficiency this way.
    GordyHarvey Ramer
  • Gordy
    Gordy Member Posts: 9,514
    edited January 2019
    Typically you gain about 3-4% with condensing once you get below 130. If you can get return temps down in the weeds with radiant emitters then the efficiency goes up a bit more.

    Where you really gain is return temp in the mid 70’sand lowest modulation. I’d say lower than 70’s but that is not real world unless it were say a deep setback zone. Or a garage/workshop kept in the 50’s.
    I think modulation trumps the condensing in efficiency.

    The last graph is more what you see with modulation range of a typical 5:1 turn down.
  • SuperJ
    SuperJ Member Posts: 605
    edited January 2019
    Best thing is to design and commission the system to ensure it runs at appropriate temperatures and deltas to get the most out of your equipment.

    You gain 5-15% from low return water temperature (assuming same equipment). I think best case is a savings of up to 30% if you're switching from a particularly atrocious setup with a natural draft boiler to a properly setup modcon. (old equipment, and problems can skew this higher but that's not a fair comparison).

    Efficiency only gets you so far, eventually you have to lower your heat load (tighter construction, better insulation, tighter schedules, etc.) to see further gains.

    Better to have 80% efficiency with a 50kbtu heat load, than 95% efficiency with a 80kbtu heat load. So if you have a leaky house fix the leaks, don't just put a more efficient boiler in.

    You need to look at overall system efficiency, not just the efficiency of individual components. (This is something the steam guys like to point out since they have no pumps but slightly less efficiency boilers). Sometime top system efficiency occurs when a specific component is operated a little less efficiently. A good designer keeps an eye on all the significant variables and produces a balanced effective design.

    Financial efficiency is another thing too, sometimes you take a small hit on thermal/combustion efficiency to save a lot of money once you get into diminishing returns.
  • Gordy
    Gordy Member Posts: 9,514
    One thing to remember. Low return water temps are the rewards of an efficient system design.

    To “make” the return temps lower than what the system returns is only increasing the operating cost. Both in how you lower the return temp, and heat the water back up.

    If you wanted free lower return temps just pipe the return pipe so it runs outside, and back into the boiler loop. It costs nothing, but some extra pipe, and freeze protection :)
  • rhl
    rhl Member Posts: 93
    right, that makes some good sense. thanks for the sanity check. also -- i own that book now.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666

    Couldn't we achieve the desired high efficiency, by just simply mixing in fresh cold water from the street?

    Yes, but it has many problems, one of which is diluting any water treatment or anti-freeze you may have in there.

    Here is what I hope is recognized as true, but a joke. Install a snow-melting system in your driveway and side walks. Run the return water through this system before it returns to the boiler.

    Of course, there will be zoning and pumping problems with this. And the expense of putting that heat into melting the snow if you had not intended to do that. But at least you would see where your heating money was going.

    If I remember correctly, you may raise the efficiency from 85% for a conventional boiler up to 95% with a properly sized modern mod-con boiler. But it seems to me that at most half of that saving is due to the condensing, and more than half is due to the use of outdoor reset with which most (if not all) these boilers are equipped.
    JUGHNE Member Posts: 8,945
    Or with that in mind, how about running the return thru a HEX that would preheat DHW.
    Might work if there was a small constant demand for DHW.

    Or just another pipe dream?? :)
  • rhl
    rhl Member Posts: 93
    oh i see, yes, deliver any extra heat to a hot water tank, which may serve some useful purpose, at least if you lose the heat out of that, it stays inside the building..
  • rhl
    rhl Member Posts: 93
    I suppose that thought just suggests why not stick a buffer tank in the system in the first place..
  • Henry
    Henry Member Posts: 988
    Please note firstly that mod-con are more efficient than standard boilers at higher temperature. Here in the cold White North, boilers operate on average 1000 hours during the winter and according to the utilities only 100 hours at maximum temperature. of 180F for copper convectors. I transformed 7, 8 unit apartment blocks that had steel fin convectors (180F design). The savings were between 35 and 55%. Why the difference? The occupants.
    JUGHNE Member Posts: 8,945
    The buffer tank would not remove any heat from the system, IMO. Need to dump the BTU's somewhere.
    That is why more radiation uses up more of the heat and allows you to run lower temps on the supply to get the same level of comfort delivered.
  • Gordy
    Gordy Member Posts: 9,514
    Let’s just put some numbers to it.

    Let’s say you go to the trouble of unnaturally lowering return temps. Let’s say you gain 3% efficiency.

    So for every 1000.00 you spend on fuel that’s saving you 30 bucks. Really that number is inflated, because in the warmer part of the heating season chances are you would be condensing with naturally occurring return water temps.......