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Modulating boiler efficiency when oversized

sheropod2 | Posted in Mechanicals on

Do mod-con (or modulating) boilers lose efficiency if they are over sized? I expect most boilers installed without a manual J calculation are more than what’s needed, but I’ve been told heat load calculations are irrelevant with these boilers.


  1. Yupster | | #1

    The answer is, of course, it depends. :) The amount of oversizing, the radiation it serves, whether there is a buffer tank in the system, etc. The bigger factor that is often overlooked is return water temperature. If it isn't below 130°F, your boiler will never enter condensing mode, meaning it will never see that rated 90%+ efficiency, dropping you somewhere around 85%.
    The modulation of the boiler is a tool to use, not a magic wand to cover not doing a proper design. If someone is telling you they don't need to do any design, don't hire them. They are missing some critical education.

    EDIT: See the article about sizing at this link for more information

    1. sheropod2 | | #4

      Thank you! I knew about the return water temperature, but I don't believe I've seen a temperature gauge on the return. I've also never known anyone to use a manual J calculation to size a boiler. Out here, radiant slabs are the standard. As the daughter of an engineer, I have religion about design!

      1. Expert Member
        Dana Dorsett | | #5

        >"Out here, radiant slabs are the standard."

        The thermal mass of the concrete offers some, but not complete forgiveness on oversizing. It's still possible to short cycle a slab radiant system, especially when broken up into multiple zones.

        Where is "...out here..."?

        1. sheropod2 | | #7

          Taos, NM. Aka, heaven on earth!

  2. Expert Member
    Dana Dorsett | | #2

    >"I’ve been told heat load calculations are irrelevant with these boilers."

    You've been sold/told a load of crap then.

    The modulation ranges aren't infinite- on some boilers it's only a 3:1 turn-down ratio (which if 3x oversized for the 99% heat load means it never, EVER modulates more than 87 hours in an average year.)

    The load calculation is relevant in two ways:

    1> The load /radiation ratio has to be such that it can keep the place heated at condensing temperatures most of the season, otherwise it's only truly high-efficiency during the shoulder seasons.

    2> The zone-by-zone load (and radiation) has to be sufficient to not short-cycle the boiler at it's MINIMUM fire rate.

    If there is sufficient radiation on each zone that it won't short cycle at condensing temps it can still be pretty efficient, but right-sizing would still be recommended.

    If it's short cycling it will be blowing away a fixed amount of heat with every flue purge and ignition cycle, and if it's short cycling a LOT it can easily operate at lower efficiency than a right-sized non-condensing boiler that has more thermal mass.

    About a decade ago I had a contractor trying to sell me on the merits of a ~150K modulating condensing boiler for my house, where the 99% heat load is between 35-40K. The turndown ratio on that boiler was about 4:1 , which means that even at the 99% outside design temperature it was just hitting the range at which it would modulate. When I pointed that out they simply went away and never called back (and I sure wasn't going to chase them! :-) )

  3. sheropod2 | | #3

    I love you guys! Nobody has better mentors than GBA. Thank you so very much! I want to mail you all cookies!

    1. Yupster | | #10

      I like oatmeal raisin ;)

  4. Expert Member
    Akos | | #6

    This is a very squishy topic. For example, I've used condensing tankless water heaters for combined space and DHW. These are typically 200kBTU but can modulate down to 12k-15k on low fire.

    You can pretty much hook almost any reasonable sized zone up to then and not run into cycling issues (very small zones would still need a buffer tank).

    With these the bigger worry is enough hot water for all the bathrooms, but you still have to watch zone sizing.

    P.S. high mass floors + open tankless setups are a great combination. The slab floor does a good job of tampering the incoming water even in the summer, you can get way more hot water out of a tankless.

  5. sheropod2 | | #8

    The P.S. is a revelation to me. I hadn't considered the effect, but of course the floor tempers the water and reduces the load on the boiler! How cool is that? A friend just built a house with an open tankless setup. She insisted on a soaking tub so I believe this is going to make her a very happy new home owner!

    1. Yupster | | #9

      Due to Legionella risks, potable water and space heating water should never be combined. The hydronics industry has established this and CSA B214-16 warns against this. Some claim the risk is low if you use a timed purging pump with 60°C water but this is your drinking water! Why take a risk on exposing yourself to potentially lethal bacteria. As a professional in this field, I advocate strongly against mixing potable and heating water. Not to mention all your components need to be rated for potable use, you need bronze or composite circulators, you are using an applicance not designed for reheating hi-temp water but rather an appliance meant for large delta-T's, the appliance isn't rated as a heating appliance which can cause code issues in some jurisdictions.
      Add a heat exchanger, any risk gone. If you want to run a water supply through your slab to prewarm your water, by all means be my guest but don't run potable water through your heating system.

      1. Expert Member
        Akos | | #13

        Properly designed open systems work. With the cold water feed through the field, there is no risk of stagnant water. A typical install of around 2000' of pex has 20gal of water, whereas a typical household uses over 40/day, you get more than one water exchange per day.

        All water going to the fixtures still needs to go through the tankless which will heat it up to 150F, which kills any bacteria.

        There is extra cost for potable rated components, but it is not that large when compared to the whole install. Major tankless manufacturers have application notes and diagrams on how to properly set up their unit for space heat.

        With anything there is always ways of messing it up, an open system IS easier to implement incorrectly.

        P.S. For me, the solar hot water pre-heaters / larger stratified storage tanks are the bigger risk.

        P.S.S. This all assumes chlorinated municipal water, an open system with well water is not a good idea.

        1. Yupster | | #15

          Can you provide a manufacturer's diagram showing a tankless hot water heater being piped for a radiant floor heating system? I've never seen one but I don't usually go looking at DHW appliances for space heat. I've seen them for providing heat to a fan coil but that's completely different.
          Tankless hot water heaters are often set to produce water at the temperature it will be used ~105°F. Rinnai has a factory maximum temperature 120°F that you can only change with dip switches.
          A family going on vacation for a week could easily have stagnant 100°F water growing some nice Leigionella and that bacteria can multiply fast. They come home, turn on the shower after a long trip, that nasty stagnant water comes flying out of the shower head, Legionella filled mist floating up. Legionella not killed even if you have your water heater set to 150°F because some water makes it past before the heater is at steady state operation. Susceptible little Billy takes a deep breath and next thing you know, potentially fatal disease. Unlikely, yes. Possible, absolutely.

          Of course it can be done, and maybe the risk isn't that high with a tankless. But for the cost of separating the system with a heat exchanger and an extra circ, I'll take the zero risk option every time. Especially since the smart people who write the codes on this stuff say don't do it.

          1. Expert Member
            Akos | | #16

            On one level, I agree with you, plate HX is simple enough.

            Risk is always one of those squishy topics.

            For example with modern code with mixing valve at the tank and the typical 5 bed 10 bath McMansion. The linear feet of pex in there is less, but with a home run setup and sprawling layout, you can easy end up with 500ft. That is a lot of water stagnant in piping in the rarely used bathrooms. Because of the mixing valve, even when flushed, the pipe will never get hot enough to pasteurize properly. So is that a risk?

            My thinking is, keep the water heater (regular or tankless) hot, run a recirc and mix at the end point. Design should still prevent stagnant water elsewhere, but what happens before the water heater, won't matter much.

        2. Expert Member
          Dana Dorsett | | #17

          >"All water going to the fixtures still needs to go through the tankless which will heat it up to 150F, which kills any bacteria."

          Most condensing tankless water heaters max out at 140F, not 150F.

          Even a 150F flash isn't hot enough for an instant-kill on legionella. Contaminated potable systems are typically sterilized by pumping 160F+ water through them for a period of HOURS, not seconds.

          Storage temps of 140F is high enough kill legionella, but only slowly, and high enough that legionella colonies can't get established in the tank.

          Not all US states will even allow "open" radiant systems due to the potential pathogen hazard from stagnation. In my state it's allowed, but only if water is pumped through every part the system at some minimum duty cycle (the specifics of which I don't remember because I never intend to go that route.)

          1. Yupster | | #18

            It is indeed a risk, which is lots of thought is being put into how to reduce these risks in hospitals and other facilities with similar situations, long DHW runs with stagnant water in them. It's a well known problem being talked about lots and applies to McMansions as well. But DHW also isn't continously kept at the prime temperature range for Legionella growth, whereas a radiant system is. DHW to rarely used rooms will cool off to room temperature, where growth can still happen but not at the rate of a 95°F system.
            I think it unwise to design a system to include that risk when it can be very easily eliminated.

  6. airfix | | #11


    I'm 85% through my build in climate zone 6a. Because of my heat load and zone sizes just about every condensing boiler was over sized. What people here recommended (and advice I took) was to install an HTP Versa-Hydro.

    It has a 55 gallon water tank for domestic hot water with a 130,000btuh condensing boiler. The trick is that it has an internal heat exchanger that you run your radiant floor water through.

    The domestic hot water storage acts like an energy buffer allowing you to have small radiant zones without short cycling the boiler. The radiant taps into the stored energy of the hot water tank. The boiler will only turn on when the set point drops to a predetermined level. I'm not sure exactly where I'll end up but my storage temperature might be 140f and the boiler might not fire until the water temp is 120f.

    The down side is its more expensive (about $6000) and now you don't have an on demand domestic water supply so you are storing hot water again. It does solve the boiler sizing and short cycling issue.


  7. Expert Member
    Deleted | | #12


  8. Jon_R | | #14

    Note that on/off zoning will create over-sizing even on a perfectly sized boiler. A solution is zones that cycle between 70% and 100% on.

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