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Mod/con boilers for old houses

I would appreciate some technical info on a simple question about the use of Gas fired Mod/Con boilers when used on old houses with Cast iron rads requiring a minimum of 150-160deg water temp to provide enough heat for rad output. I know enough that these new boilers when temp gets about 160deg they no longer perform in the 92-95% efficiency range. Is this true and what is solution, also related to Copper fin which needs a minimum of 180deg without adding more baseboard to compensate for different output. Thanks for info.


  • hot_rod
    hot_rod Member Posts: 21,874
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Icarus
    Icarus Member Posts: 143
    edited December 2019
    From local experience, it depends to some extent on location and local climate. If you CI rads NEED to run full bore ~180f just to keep up than that puts it out of the Mod Cons sweet spot.

    That said, with an outdoor reset mod con, if a lower temp can keep the house warm MUCH of the year, the net result will/could be higher net efficiency. Even running at less than peak ef a mod con is still pretty efficient relative to an older cast iron single stage boiler. Clearly if you can run it at condensing temps MUCH of the time you would still be ahead IMHO.

    That said, this is a case for a good quality analysis done by a competent professional. It has been my (limited) experience that old CI boiler guys (not always old, just familiar with the “old technology) are not real keen on Mod cons simply because they don’t really understand them, don’t generally sell them etc, and are all keen to sell you a new CI boiler that they know and understand. I get that, and no offense to these guys. I myself stick with certain dated technologies simply because I know it well, I have long institutional memory about it, and I have a treasure trove of spares.

  • Ironman
    Ironman Member Posts: 7,352
    We marry mod/cons to CI radiator systems all the time. It's our bread and butter.

    I've never seen a CI radiator system that needed more than 160* SWT at design conditions (coldest night of the year). Most of the season, they operate well below that with a properly setup ODR control that's incorporated in mod/cons.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    Jolly Bodger
  • flat_twin
    flat_twin Member Posts: 350
    How tight is the house in question? Any improvements to prevent air infiltration and heat loss will greatly help to lower your required supply water temperature. My own experience with an old house, cast iron radiators with big pipes on a single zone tied to a modcon has been awesome. Not sure where you got 150-160 degree supply water minimum required. A modcon with outdoor reset programming will vary the supply water temps as needed to just maintain indoor thermostat setting. In our case that's a range of 87 to 140 degree boiler water to cover outdoor temps from 63 to minus 20 degrees. Also no nighttime setback. Leave the thermostat at 70 or wherever you're comfortable. A heat loss survey of your house and a calculation of your radiators total output would help determine if a modcon is right for your home.
    Jolly Bodger
  • Jolly Bodger
    Jolly Bodger Member Posts: 209
    Also, the now RWT as the CI rads are heating up will be in condensing range.
  • TAG
    TAG Member Posts: 755
    When I did my huge 3 story stone victorian w/ ODR over 25 years ago I was surprised with both the low temps required and the increased comfort throughout the house. We took out twin WM boilers and put in Buderus CI. Had to be really cold to get above 150. A properly sized mod con would be perfect today

    Often wondered how they sized radiators in a house like that -- it was originally coal gravity (that's what I was told). Had it been w/ pumps the radiators would have been smaller -- this is what the old guy who did all the layout told me.

    Actually -- with ODR in some ways you are going back to how the really old systems originally operated. Coal was always "on" and the output was controlled by the fuel.

    I took out what copper fin was in that house. For cost of install it's often factored with very hot water so the lengths of fin are reduced and will not give you enough output at lower water temps when it gets cold.

    It's all math ..... The hard part is finding some one able and willing to do it .... I put in panel radiators is areas w/o heat and to replaced the fin tube. You can also add length to the fin.
  • Paul Pollets
    Paul Pollets Member Posts: 3,656
    Old radiator systems and mod cons are a good fit. In most cases, radiators were oversized and allow for lower temperatures. Sizing the existing radiators and cross checking the output is critical. Always install a new TRV on the radiator supply and a variable speed smart pump. Some conversions done in Seattle had a 40% decrease in fuel costs using a mod-con replacement vs. cast iron. The mod cons have higher maintenance costs, but are certainly more efficient.
  • Tinman
    Tinman Member Posts: 2,808
    Yes, I agree they are a very good fit. Do a room by room EDR calculation and compare it to the room by room heat loss calculation. You'll want the room radiation to be oversized evenly across all rooms otherwise the room with the less % of radiation will be the tail that wags the dog.
    Steve Minnich
  • hot_rod
    hot_rod Member Posts: 21,874
    If you want to crunch the numbers that accurately you can download BIN data for your area and determine how many days at what temperature are typical over a period of years. That would help determine if or when you might be at or below design day conditions.

    A high mass system may even flywheel through a below design condition without any noticeable drop in indoor temperature.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream