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Getting Correct Efficiency Out of a Mod/Con Existing System?

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Rizz861
Rizz861 Member Posts: 52
I was interested to see how you guys are achieving proper efficiency with a mod/con boiler on an existing system. On a new build it should be easy to get this because you control all the design aspects. But what about an existing system? I’ve been told the typical 180F out 160F return approach will just be a waste of money, doubling the price of a boiler, and often times costing more money to run than a traditional boiler. I have a general idea of the concept of returning water back to the boiler at 140F and below to encourage condensing, but I’d like to learn more about the design aspects of these systems. What are you guys doing to lower the return temperatures to encourage condensing of the boiler? I’ve been told dump zones will aid in this process. Are there other ways to achieve this? Any reference to any good manuals/books/PDFs on this topic would be much appreciated. Thanks!!!!

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  • Hot_water_fan
    Hot_water_fan Member Posts: 1,846
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    The Caleffi Idronics journals are the place to look. Remember: the highest temps are only needed a few days per year. You can condense the majority of the year even if you need 180 on the coldest day. Add in over radiation in existing homes plus delta Ts > 20 and it all comes together.
    GGross
  • GGross
    GGross Member Posts: 1,045
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    adding dump zones will just add to the total load and result in higher fuel usage, the best way to lower return temp in an existing system is to lower the supply temp out to the emitters. Generally existing systems already have emitters that are oversized, and while they might not be set up for very low temp, you can still lower the temp quite a bit. Also take in to account that if you are always feeding the emitters 180 with a cast iron boiler whether it is +40F or -10F that you can in fact be condensing the majority of the heating system, that 180 supply may still be too high even on design day.

    There are two general methods to achieve peak efficiency from an existing system. The first is to use trial and error, adjusting the heating curve on the boiler from as low as you could imagine, right up to where it is comfortable. training customers on simple use of the control and what the results can be for efficiency can be helpful here, I do know several contractors that do a couple follow up calls on mod/con installs to adjust the curve. The other method is to actually do a full manual J on the home, this should be done anyway. Once you know the actual heat loss of each room, you can measure the existing emitters, this will allow you to figure out the correct highest temperature needed for your boilers heating curve. there may still be some trial and error involved to dial it in perfectly.

    In short it requires some extra work, and there are many contractors who don't want to do that extra stuff, so they tell you the mod/con is a waste of money and install a cast iron
    mrhemi
  • psb75
    psb75 Member Posts: 833
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    Dont forget the MODULATING feature of a "mod/con" boiler--a good thing even when return temps aren't condensing.
    The obvious way to lower your return temp is to lower the supply temp by LOWERING your house's HEAT LOSS. Start with the "low hanging fruit": 1. air sealing, 2. more/better attic insulation--in this order.
    GGross
  • dko
    dko Member Posts: 589
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    You can lower temps and make the system as efficient as you want, but managing a customer's expectations is another checkpoint.

    You set up a nice efficient system with odr, but then customer complains he's not being blasted by heat in the bathroom. What do you do? Disable odr and let it fire at 180?

    A set and forget system is the most efficient, but that is also one of the hardest selling points in colder climate areas. Most don't want to be merely "comfortable" they want to feel the heat radiating around their body until they sweat and turn down the stat.

    Discussions with the customer regarding this must be had before offering mod cons.
  • john123
    john123 Member Posts: 74
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    I understand that the lower the return temp, the greater the condensation, the greater the efficiency and the lower the cost. But what does the curve look like: is it linear? do you proportionally get more efficiency by lowering the return from 135 to 130 or from 130 to 125 or from 125 to 120 etc. etc.

    An other reason for the question is: is there any real advantage in keeping the maximum return at say 135 if you are only going to get a very minimal condensation there? OK so you keep the return at 135, the temp goes down in the house, you feel cold and put a little electric heat in the room you are in (like your bedroom at night). you do all this to keep the boiler condensing. Does it really make a difference at a return temp of 135 or 130 or 125 or are there diminishing returns at these temps?
  • dko
    dko Member Posts: 589
    edited March 2023
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  • psb75
    psb75 Member Posts: 833
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    I don't know the curve. The more it can condense, the more BTUs it 'squeezes' out of the exhaust. I just enjoy that these boilers can use plastic exhaust pipe and its nice to know it isn't blowing unnecessary BTUs out of the house. I like watching the Viessmann cruise along at 99°F supply temps this time of year. You have to have the right amount of radiation capability in the house. This AIN'T gonna happen with fin-tube baseboard!
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,846
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    I understand that the lower the return temp, the greater the condensation, the greater the efficiency and the lower the cost. But what does the curve look like: is it linear? do you proportionally get more efficiency by lowering the return from 135 to 130 or from 130 to 125 or from 125 to 120 etc. etc.


    The Idronics journals will answer all of these questions :smile:

    An other reason for the question is: is there any real advantage in keeping the maximum return at say 135 if you are only going to get a very minimal condensation there? OK so you keep the return at 135, the temp goes down in the house, you feel cold and put a little electric heat in the room you are in (like your bedroom at night). you do all this to keep the boiler condensing. Does it really make a difference at a return temp of 135 or 130 or 125 or are there diminishing returns at these temps?


    The goal is that the temp in the house never goes down - the water temp changes as outdoor temps change to maintain the setpoint. No electric resistance needed. Lower return temps are better but there is a max efficiency of course. However, there's seldom a real reason to stop at 125 vs. 120 (hydro-air might be an exception).
  • GGross
    GGross Member Posts: 1,045
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    @john123

    Don't think about this like you are trying to lower return temperature, and instead think about it like you are trying to lower the supply temperature to exactly what is needed. You do not want to lower temp to the point you are using electric heaters, that would waste all the efficiency you have gained, and lower comfort. Also you are not trying to maintain a set return temperature, you are not trying to maintain any set supply/return temperature, that correct value changes with outdoor conditions and your boiler will adjust the supply temp based on those conditions, that is where trial and error comes in with outdoor reset heating curves. With some knowledge of a system you can find a good starting point and fine tune it to exactly what you need, that will help squeeze the most efficiency out of the boiler without altering the emitters.
  • Jamie Hall
    Jamie Hall Member Posts: 23,280
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    May I also point out that if your radiation is arranged in parallel, preferably reverse return, tweaking (well, maybe really twisting!) the flow rate has a big effect on return temperatures. Don't overlook it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 22,131
    edited March 2023
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    I propose a properly tuned, sized and installed mid con will always run in the high 80% range, into the 90 when condensing

    In zoned systems, micro load systems it is quite possible a brand new cast boiler slips into the 70 if it short cycles as many do.

    Lowering the homes load is the best money you can spend. Then a load calc, size the equipment to the actual current load.Next best would be increasing heat emitters so you never require over 120 supply. Thus puts you in mod con and heat pump territory 

    if you want to crunch numbers first use Idronic issue #
    The weather data on hours if occurance could show when you are running in condensing mode

    https://www.caleffi.com/sites/default/files/file/idronics_25_na.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Rizz861
    Rizz861 Member Posts: 52
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    Thanks for the pointers guys
  • flat_twin
    flat_twin Member Posts: 350
    edited March 2023
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    There are guides to figure the output of old radiator systems at various supply water temps. For my house the total radiator output at 180 degree supply is 106k btu. Calculated heat loss is 58k btu. See how this over radiation allows the system to provide plenty of heat at much lower supply water temps? Our modcon runs between 88 and 140 degrees per outdoor reset. It's just a lucky coincidence courtesy of new doors / windows, insulation, that our modcon is always running in the condensing range for space heating.

    Sizing Cast Iron Radiator Heating Capacity Guide.pdf