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your thoughts

bobbyg_13 Member Posts: 3
if using outdoor reset.

the modulating unit will run longer at a lower rate. This will give it a better seasonal (I'm not 100% positive this is the right term) efficiency.

The fixed firing rate will equal modulating at design conditions only (with no zones) but will shorten run time with lower heating load. (decreasing seasonal efficiency).

my .02 worth.


  • Mitch_4
    Mitch_4 Member Posts: 955
    Scenario a buddy and I debated, looking for your views

    Here is the scenario.

    2 houses identical construction, for this mental exercise, exactly the same in design, materials etc.

    Heating system is the same (baseboard, rads, radiant floor your choise, the issue that in both homes the same.

    here is the difference.

    1 has a mod con 90% efficient boiler, the other a 90% efficient fixed input.

    Both homes are geographically the same so the degree days, solar gain etc is the same.

    Which one (if any) will save more in gas use and why.

    I will not tell you my point until later, and the data has been supported in actual tests.

    ready, set....GO!!

  • tim smith
    tim smith Member Posts: 2,401
    Depends on the type of emitter.

    I would say if it was all low temp radiant, there would be little difference in useage as both boilers would be set for a low temp condensing mode all the time. If it had basboards, the required fixed temp for this would probably be 160+ so out of condensing mode all the time where the mod/con would be able to weather adjust and be in condensing mode problably 80 to 90% of the time. That's how I see it. Also?? why would any body use a fixed condensing boiler when they come with reset built in always as far as I can tell. Tim
  • Mitch_4
    Mitch_4 Member Posts: 955
    Fixed input

    I use the smaller Olsen fixed input condensing boilers (dunkirk quantums in Canada) for single zone systems like one baseboard loop..still fighting the uphill battle here regarding Zoning.

    I get the better equipment in, and add ODR to keep it in condensing mode as long and as often as possible.
  • Ted_9
    Ted_9 Member Posts: 1,718

    What about the indoor temp.? Are you saying that they are the same?
  • Mitch_4
    Mitch_4 Member Posts: 955
    the only difference permited in the scenario is

    the boiler, one fixed, one mod con.

    If you use ODR, it is on both
    Zoning same for both


    Kinda like parralell where modulating is not the same family, one door opens, so does the other same time, same solar gain..everything but the boiler
  • jp_2
    jp_2 Member Posts: 1,935
    effeicency achieved by low delta T

    lower the delta T the higher the efficiency, thats what the mod/con can do and what the on/off boiler can not do.

    greater delta T's mean higher energy input needed, this creates higher flue temp which equals higher losses.
  • Mitch_4
    Mitch_4 Member Posts: 955
    remember they are

    both 90%. Delta tee is a design thing not a boiler feature.

    Otherwise I can get a cast iron boiler to work on 5° delta tee, making it as efficient without the cost...just a hell of a lot more zones..

  • jp_2
    jp_2 Member Posts: 1,935
    ok then, not enough info

    what have you and your friend decided the running temp will be on the non mod con boiler? 140F, 160F, 180F ?

    remember a modcon without outdoor reset is only half way there.

    delta T: I see it more as a controls function.
  • Tom Blackwell_2
    Tom Blackwell_2 Member Posts: 126

    The mod-con , if set up correctly, will maintain an exact indoor temperature at all times. The full-on unit will control the temperature in a sawtooth fashion, so that part of the time the temperature is less than the desired setpoint. Less average temperature equates to lower heat loss (assuming no overshoot). Assuming equal efficiency, less heat loss is less gas used..How's that for twisted logic?
  • jp_2
    jp_2 Member Posts: 1,935
    true, but

    then the house temp is lower than the other house, so the test is not equal.

    also, think more about how this would happen, remember the tstat is a single point location in the house, almost impossible to say that this type of control would not over shoot or that areas farthest from the tstat would be higher in temp than at the tstat, so in fact you have overshoot.

    more yet, that on/off boilers temp will also cycle in bigger variation, efficiency will not be constant.
  • Mitch_4
    Mitch_4 Member Posts: 955
    I will give this the weekend and post what our debate

    resulted in.

    The ONE factor that is different is fixed input versus modulating. Both are condensing type boilers.

    If you have multi zone, then both do, ODR then both do. If there is one stat in the living room. Both do.

    The boiler is the only difference.

    Going back to stripping wall paper to keep the little lady happy, redoing the bedroom while she is away with her sister for girls week out. (hope she wins at the Casino, this is costing me lol).

    Have a good weekend all...
  • Paul Fredricks_6
    Paul Fredricks_6 Member Posts: 88

    Not that the house is cooler, but the other house over shoots, so sometimes they would be the same temperature. I think you'd have to figure that the two units cannot operate the same, and the less/no overshoot is the benefit of one.
  • Brad White_185
    Brad White_185 Member Posts: 265
    All other things being equal

    (and this includes the mass of the boiler and output relative to heat loss), I would say modulation would save more. The ramping up with each cycle (bang-bang control) is a drag compared to full modulation. Temperatures spike with each cycle too. If no buffer tank that becomes a drag on the system also.

    I have a condensing but non-modulating Monitor MZ and would gladly trade modulation for condensing.

    That said, I am very curious what you are finding out!
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    It's all going to depend on the thermostat habits of the occupants.

    If both are kept at a steady temp (at low as possible for the required comfort level), I would expect the modulating boiler to consume 20% - 35% less fuel over a "typical" full heating season. (This assumes that the non-modulating boiler is a true condensing boiler like the Monitor MZ.)

    If deep and frequent setback is used (particularly different setback schemes in different zones) I would expect the difference to diminish and perhaps even disappear. (The more the modulating boiler is forced into setback recover mode, the less fuel consumption difference).

    With modest (say 4-5F setback for say eight sleeping hours), I'd expect 15%-25% fuel savings with the modulating source. If the modulating boiler used follows the "setback by dropping the supply target" scheme, expect the higher end; if it merely stops firing completely until the room temp drops to the setback point, expect the lower end of savings.

    Emitter type--remember we're talking about systems identical except the boiler--will have very effect. It's only when you compare higher-temp emitters (say fin baseboard) against lower-temp emitters (say highly conductive radiant floors) that the emitter really begins to matter with condensing heat sources.
  • Doug_7
    Doug_7 Member Posts: 210
    The ModCon will save more gas

    The ModCon will save more in gas use because it will run at a lower temperature. It can modulate down to a lower firing rate and flue temperature for the same average heat output as an ON/OFF boiler. The ON/OFF boiler must run at a higher temperature when it is ON to average out for the times that it is OFF.

    The 90% efficiency rating is just one point on an efficiency curve. There is no ONE efficiency number - there are many different efficiencies, which depend on firing rate, return water temperature and flue temperatue - which are expressed as curves.

    On milder days the ModCon can modulate down to a lower firing rate and flue temperature to match the heat load exactly, whereas the ON/OFF boiler must still run at a higher temperature.

    This is shown in the published boiler efficiency curves for several manufacturers. ModCon boilers have what is called an "Inverse" efficiency curve - The boiler efficiency goes up as the firing rate goes down.

    See attached Vitodens efficiency curves. Following the blue line, this boiler will be 89 - 90% efficient at full load, but the efficiency improves to 96 - 97% when it modulated down to 40% of full load. The boiler is more efficient under partial load than it is under full load.

    At least that is what the boiler manufacturers published data says.

  • Driving with the accelerator pedal screwed and glued to the...

    FLOOR is not a good way to drive. Lot's of jack rabbit starts and stops. Not good on ANY piece of equipment. Modulating the speed of the vehicle with the ignition switch is a tricky proposition to say the least during part load considerations.

    With full modulation, it's like having cruise control and computerized fuel system all in one package. With the newer boilers, they're even better yet because they can diagnose the caloric content of the fuel and trims air and fuel to optomize "operating" efficiency. O2 sensors have been controlling air fuel mixtures in the automobile industry since, forever...Why not boilers.

    I too have proof of replacing medium high (85 to 90%) efficient appliances with modcon technology and am seeing fuel reductions of around 24 to 30 percent depending upon how much overkill they have. And how they're being operated.And how (IF) they were set up initially. The best piece of equipment is only as good as the person who commisioned the system. And will work only as well as the technician responsible for its long term care and maintenance. Moco is not for everyone. They MUST be serviced to maintain efficiency.

    My vote is moco. (ModCon is a trademarked name :-)

  • Steve Eayrs_2
    Steve Eayrs_2 Member Posts: 56
    IF doing multiple zoning.....

    and since most places where I am at, involve lots of zoning, then I would have to say the modcon would winn.

    It would be able to more accurately match the load, and not run shorter inefficient cycles.
    Unless I'm missing something.....If you have a 20mbtu load and your operating at 80mbtu, your sending more up the stack at 80mbtu, than operating at 20mbtu. So the question would have to include 90% of what input?

    The reality is both would not be operating at their max. or equal, or at 90%. The modcon would be able to match the load, and therfore have a bigger Dt, or cool enough returns to condense more often, while a fixed would quickly overshoot, or end up short cycling the burner more often.

    Even with a single zone I believe a modcon would win. Or am I wrong? Am interested in what you've found, and sense you wouldn't bring the question up if it was this cut and dried.

  • CC.Rob_6
    CC.Rob_6 Member Posts: 7

    Cool question. I'll take some guesses.

    On indoor temperature. I have a fixed-fire boiler that uses full ODR with indoor feedback. My space temp does not sawtooth. The supply temp and differential are continuously adjusted to match the heat loss (or gain) to hold setpoint. I don't see how a modcon does that particular chore any better. They would be the same.

    On fuel use. Not a clue. I suppose one could figure out how many BTUs over what time interval are consumed by a fixed fire appliance and a modcon, to supply a given BTU loss. Consider a simple case:

    100k fixed input boiler. Fires 3 times for 5 minutes in one hour: 1.5 million BTUs consumed.

    100k modcon, at 4:1 turndown. Fires continuously at 25k input for one hour: 1.5 milion BTUs consumed.

    So I dunno. Fuel savings would depend in part on how quickly the fixed-fire appliance achieved maximum combustion efficiency. Among other things.

    On cycling. No doubt the modcon wins. That said, a fixed-fire appliance with moderate to high mass and decent water volume probably gets some pretty good mileage out of its firing cycles. Mine seems to.

    This is a very interesting question and I look forward to "the answer." I would be skeptical of results, however, unless the buildings were physically right next to each other and all construction was monitored to ensure direct comparability for such tests. The Canadian energy test houses, for example.
  • Tony_23
    Tony_23 Member Posts: 1,033
    Modulation is the key

    They both condense, have ODR, hit the same target temp. The fixed rate stops heating water when target temp is reached, the modulator throttles back. It doesn't modulate on ODT, it modulates on output water temp. ODR controls OWT not firing rate.

    I've replaced fixed rate Heatmakers with modulating Endurances and seen 25% reduction in fuel, more than once.

    I'd rather have modulation than condensing w/o modulation.
  • Mitch_4
    Mitch_4 Member Posts: 955
    We reached

    no resolution despite "evidence". My aurguments winning more points, but he refuses to capitulate on all fronts. (;-o)

    Thanks for all responders, and I look forward to the additional comments after you read this.

    We both concur that the COMFORT level will be greater in a mod scenario, but comfort is not consumption. Nor was it the topic of debate..

    My position was that the mod wins, and that is more evident in a multi zone scenario. Energy must be used to heat the exchanger before it transfers heat to the medium, it saves fuel to do this once in a modulating boiler, the bang bang must do it every time.This is wasted energy that does not heat the structure..

    His position was that heat loss is all. Heating plant makes no difference. If you lose heat you must replace it.

    If the structure loses 50,000 btu's / hour, then it takes 50,000 btu's to heat, whether bang on/off, or modulating down and running longer. He feels most high efficiency boilers, the energy required to heat the exchanger is practically negligible. He stands firm on this in anything 2 zones or less if approximately equal sized.

    The "evidence" he cited is that NRCan (Natural resources Canada) has done this exact test (although with forced air) and they were the SAME!! In fact the fixed unit with a PSC used a tad LESS than the ECM (gas only, electrical was cheaper with the ECM) because the heat generated by the motor is significant enough to save gas. (How do you factor THAT into heat gain?!?)

    The NRCan test esentially proved that heat loss is all, all other features improve comfort. I have no evidence to support me so he feels he wins even though he concedes some points on multizones (however he says a buffer tank will nullify the mod boilers advantage there too and is less techno!!)

    My buddy will agree that in a multi zone system (3+)the mod boiler will save fuel as it can match the load, but remains steadfast that a single zone system, will be a wash between them, and properly done a buffer can still beat the mod in a multi zone without all the fancy technology.

    I say mod in both, but I can see his point in a single zone systems ONLY. In fact...In a one zone system, the savings to me are slim enough to not really justify the additional expense of modulation technology, and I do a brisk business with fixed input & ODR for these systems, based on the payback on modulating will be way longer than I feel is justified.

    the NR Can test seeems to bear true only for single zone as that is what most forced air is.

    This similar to that old debate here of FA or HWH. if both were 90% efficient it should be the same costs.. where HWH saves is because of comfort. you can set the stat lower in HWH, decreasing the delta t inside to outside the structure. but if both systems were to maintain the same temp, theorhetically it should use the same fuel. (But you would be uncomfortable inthe HWH structure cuz it'd be too da*n hot..

    again the Q was costs though, not comfort level.

    Brad W and Mike T, I am particularly interested inyour thoughts as I truly respect your opinions and views as some of the most intelligent and well thought out here on "The Wall"

    Any other supporting evidence that can be dredged up would be appreciated as well.

    Now that you know our positions and the "evidence" what are your thoughts?

  • CC.Rob_4
    CC.Rob_4 Member Posts: 37
    some issues complex...

    ...others less so.

    As I mentioned below, I don't see how a modcon provides greater comfort. I respectfully suggest that comfort is about control, not heating plant.

    Heating of a bang-bang hx will probably depend in part on how massive the casting, and how well insulated the jacket. My particular hunk of iron, for example, is reasonably massive and well-insulated. It doesn't lose heat very fast at all. Possibly even neglible amounts depending on the spacing between firing cycles.

    As to the other observations, I think you've done an excellent job highlight many of the important uncertainties that contribute to the lack of a definitive answer. As important as energy use is, I find it very surprising there are not more objective tests of overall efficiency, assessments of lifecycle costs, etc.

    This thread is a great idea. Interested to see what others think.
  • Doug_7
    Doug_7 Member Posts: 210

    Mitch - It depends on whether you are engaging in a real-world argument or a theoretical one.

    Two different boilers that have the SAME 90% efficiency will use the SAME amount of gas for the same heat load. In this non-real world theoretical arguement your friend wins. But this is a silly argument that distorts the truth.

    In the real world, boiler efficiency is NOT a single number - it is an efficiency curve and the efficiency varies with load. See manufacturers literature.

    In the real world, the efficiency of a ModCon boiler increases as it modulates down to accomodate a lower heat load, whereas the efficiency of an ON-OFF boiler decreases when it goes ON-OFF to accomodate a lower heat load. If you chose to ignore this fact you are ignoring the real world and the primary advantage of ModCon boilers.

    So your friend wins the theoretical argument - two different boilers with the SAME efficiency will by definition use the SAME amount of gas for the same heat load. But it would be a big mistake to conclude from this that there is no real-world advantage to ModCon boilers.

    For those of us who live in the real world, the ModCon will use a lot less fuel than an ON-OFF boiler and you would win the arguement.

    ModCons Rock.

  • jp_2
    jp_2 Member Posts: 1,935
    can lead a horse to water but......

    mitch, take a good look at the vitodens efficiency charts above that doug has.

    says a lot about efficiency and delta T.

    think about a simple situation. you are keeping a pot of water at 100F on the stove. one the burner is set at 102F and the other you switch on to high then back off before water over heats. burner gets red hot then you switch it back off.

    which seems more efficient?

    also, why zone with a mod con?
  • Depends

    A modern "well insulated" house under 2000 sq. in the Hudson Valley or New England is most likely never going to need the upper end output of a say a vito 6-24. Seems to me that the smaller mod cons in the NA market are sized to meet minimum dhw recovery rate expectations, as opposed to matching the heating loads of energy smart (modestly sized) residential structures. For this reason a well designed constant circulation system may never see the full modulation range of the boiler in heating mode. In-fact on/off operation at minimum modulation is often the most common mode of operation for structures like this, as the majority of the heating season will involve loads that are below minimum output.

    My own 1400sq foot house has a heating load which has never exceeded the minimum modulation of my Vitodens 6-24. I know this because I have adjusted the heating output potentiometer (blue dial at the back of main board) to it's minimum setting (25MBH).

    I feel I'm achieving very high efficiency by using a steam control: odr based interval (timer) as opposed odr temperature modulation. Works like a charm, no closely spaced ridiculously short burns as was the norm before this hack.

    My theory is that I am getting better efficiency because the load and soak behavior of my high mass emitter (mostly slab). I am still using constant circulation but by the nature of this variable interval based odr firing the slab cools down between firings (however not in a way that effects room temps noticeably) this means that at the next firing there is more "soak" space available and thus lower return temps and greater condensation,( I'm seeing real world 30deg deltas on 110deg supply temps) Although I have great respect for the euro style variable temp. constant circ. design approach, I do wonder if load and soak cycles have the potential for higher efficiency in some circumstances. My guess is that your experiment may contradict our assumptions about modulation, by virtue of what I'm describing.

  • CC.Rob_6
    CC.Rob_6 Member Posts: 7

    and then some.

    I have a 2100 sf house that I would describe as moderately to well insulated. Nothing fancy. It has a functional lower limit for input of about 6500 BTU/hr. (For heat loss, that's around 55F outdoors, and at these conditions, solar gain allows a lengthy coast into the overnight.) At design of 5F, the loss is around 33k BTU/hr (~16 BTU/hr/sf). Probably 95% of the heating season, I need about 15-23k BTU/hr.

    It would be nice to have a modcon that went from, say 5-50.
This discussion has been closed.