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Condensing boiler vs. furnace efficiency

igrigos | Posted in General Questions on

I’ve in the process of comparing different heating systems and had a question about condensing furnaces and boilers at cold outdoor temperatures.

The way I understand condensing boilers to work, is that you enter condensing mode when the return water temperatures are below 140F (lower RWT means better condensing operation and better efficiencies). A condensing boiler should also be utilizing outdoor air reset in order to keep RWT low and maximize efficiency. During colder temperatures, the SWT is usually higher (180F) and the boiler doesn’t operate in condensing mode (and as a result is probably operating with an efficiency closer to 80% than the listed 96%).

Now with a condensing furnace, I feel like the supply and return air temperatures are pretty fixed, regardless of the outdoor temperatures. Does this mean furnaces operate in condensing / higher efficiency mode at all times, independent of outdoor air temperatures? Do furnaces have OAR capability?

I haven’t been able to find much out there concerning this, and it seems like it could have a sizable effect on the efficiency of the systems during cold weather.

Thanks,
Isaiah

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    >"The way I understand condensing boilers to work, is that you enter condensing mode when the return water temperatures are below 140F"

    What fuel condenses at an EWR of 140F?

    With propane fuel the EWT at the boiler needs to be about 130F, with natural gas it's a bit cooler than that. In practice don't assume condensing efficiency of either to start taking off appreciably until it's coming in under 125F.

    >"During colder temperatures, the SWT is usually higher (180F) and the boiler doesn’t operate in condensing mode (and as a result is probably operating with an efficiency closer to 80% than the listed 96%)"

    Even at 180F output condensing boilers usually hit north of 85% efficiency. To hit the AFUE numbers there usually needs to be enough radiation to deliver the 99% design load heat at an average water temp of 140F, a typical design goal for systems radiators/baseboards. A system that actually needs 180F output on design day won't hit anywhere near the boiler's AFUE efficiency

    >"Now with a condensing furnace, I feel like the supply and return air temperatures are pretty fixed, regardless of the outdoor temperatures. Does this mean furnaces operate in condensing / higher efficiency mode at all times, independent of outdoor air temperatures?"

    Yes, with hot air furnaces air temperatures on the return ducts falls within a narrower range, and is always in a condensing temperature range. But the supply temperature can vary by quite a bit depending on the actual flow rate through the heat exchanger.

    System efficiency of hot air furnaces is impacted by conducted and leaking duct losses, air handler driven outside air infiltration, and higher electricity use to deliver the same amount of heat. Even an Energy Star duct system has higher parasitic losses than a reasonably designed hydronic solution, and combined with the higher electricity use expect something between 5-10% higher operational costs at a given AFUE efficiency or raw combustion efficiency.

  2. Jon R | | #2

    Re: non-condensing operation

    More like drop to 86-90% efficient with a good boiler.

    There is too little to be gained with furnaces having OAR.

  3. igrigos | | #3

    Awesome, thanks Dana and Jon. That's the exact type of information I needed. The quality and timeliness of responses on this site is refreshing.

    I see a lot of multifamily projects going with gas fired furnaces, as they can integrate a cooling coil fairly easily and the cost of gas is so cheap. We've had some success pushing heat pumps, but there's always the issue of equipment being oversized for the space.

    1. Expert Member
      Dana Dorsett | | #4

      When looking at boilers it's also important to look at how much the zone radiation can emit at condensing temperatures, so that you don't end up short-cycling the boiler into low-efficiency and an early grave:

      https://www.greenbuildingadvisor.com/article/sizing-a-modulating-condensing-boiler

      For multi-family units with low heating and cooling loads per apartment, sometimes a right-sized hydro-air coil running off a condensing water heater (that's also serving the domestic hot water) is better than any boiler or furnace solution.

      But in the age of modulating 3/4 ton ducted mini-splits the heat pump approach can be even better.

  4. Karl Brumund | | #5

    Not mentioned, but having lived in both hydronic and air heated homes, the former feel much more comfortable in winter. Especially with radiant floors, but even with cast iron rads.
    Makes summer A/C harder though.

    1. kjmass1 | | #6

      I have a pretty good set up- ductless splits for AC and shoulder season heat in to 30Fs, with steam radiator backup. 50 year old boiler may not be the most efficient, but requires zero electricity and can't beat the comfort during deep winter. No expensive mechanicals to maintain either- it's essentially an on/off switch.

    2. Expert Member
      Dana Dorsett | | #7

      The level of comfort difference between hyrodonic and hot air varies with climate and how lossy the house is, and the degree of oversizing factor of the air delivery systems. Most hot air systems are 3-5x oversized for the design heat load, which means they're noisy, create wind-chill drafts, and run at a very low duty cycle even during cold weather with a hot-blast followed by the long chill- the opposite of "comfort". Right sizing it make a huge difference on that. Right-sizing it also makes a real difference on hydronic systems too, but the comfort hit from oversizing isn't as severe as it is with hot air.

      In a better than code-min house in a less-cool climate can still be quite comfortable, and even a sub-code lossy home with a right-sized modulating or multi stage condensing gas burner can still be pretty comfortable.

      When my in-laws on the other side of town re-did their mid-century modern I insisted they run a Manual-J before selecting new equipment. After some necessary glazing & insulation upgrades they went from a 200 KBTU/hr hydronic boiler serving radiant ceilings and 7-8 tons of commercial building rooftop AC split between two separate units (they sounded like a B36 on a take off roll when operating) to a modulating 60K condensing furnace and a 2-speed 3 ton split AC using all-new right-sized ducts. In both heating and cooling mode the new system is hard to hear with even a 10mph breeze outdoors even if listening for it, and the temperatures are very even & steady. It's not as cushy as radiant floors at the local design temp of +5F outdoors, but it's still pretty nice. The cycling of the high temp low mass radiant ceiling wasn't super-comfortable and could arguably have been tweaked into comfort, but since it was closing in on 60 years of service they opted to avoid the leak risk and scrap the system rather than installing a right-sized modulating boiler.

      In my own 2x4 framed 1920s antique I'm happy to have cast iron rads (micro-zoned) three rooms, and a radiant floor in another (all designed to deliver the heat with 125F water), and are much more comfortable than the zone served by the OVERSIZED single speed hydro-air handler that's serving the main open space. A wood stove in that zone provides much nicer heat than the air handler, which logs more hours during the short cooling season than during the much longer heating season some years.

      1. FluxCapacitor | | #8

        Dana,

        Sorry to get off topic but I’m an aircraft enthusiast so when you referred to the B36 I got a little excited...6 radials and 4 jets, very noisy indeed.

        The loudest I ever heard is the F22 which IMO narrowly beat out the F4 and F111 I’ve heard in full burn.

        If you get bored check out the “Noise” section of the Wikipedia article on the XF-84H experimental fighter.

        1. Expert Member
          Dana Dorsett | | #9

          In the late 1950s/early 1960s I lived about a mile from the north end of the Boeing airport in Seattle (yes I'm really that old) and got to hear a lot of that antique stuff taking off and landing. (Ever been to the Boeing Museum of Flight down in that neighborhood?)

          In raw dB the newer fighter jets might be louder, but in terms of raw abrasive scream some of the Korean War vintage jet fighters such as the F84 were pretty daunting, even when throttled back for landing, and down right scary when climbing at full throttle. From the article description the XF-84H had to have been considerably worse! The windows in that house shook pretty hard sometimes, but none ever BROKE from aircraft noise.

          The first 707 airliners were also pretty loud, but nothing like those vintage jet fighters, and with a very different timbre than the loud prop driven military planes.

          To be sure, my in-laws could not possibly fall sleep, or even have a conversation indoors without yelling when the rooftop air conditioning was running. It was that bad. It took them about 5 minutes to decide they weren't keeping it, but it was about 5 months before it was actually gone. (They went to Maine for that first summer rather than live with it!)

          1. FluxCapacitor | | #10

            Ha. The neighbors must have been happy when your parents replaced that AC.

            My dad is retired Air Force Major so I got to see various stuff.

            Early turbojets definitely had that screaming sound that today’s turbofans don’t.

            And no, I never been to boing museum.

      2. Mike Clarke | | #11

        Hi Dana,

        "(all designed to deliver the heat with 125F water)". That converts to 51.6°C, which I am more familiar with. That water in temperature seems rather high for a hydronic floor, or is that for the cast iron radiators only.

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