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Community and Q&A

Troubleshooting a Condensing Boiler

mackworth | Posted in Mechanicals on

A few years ago I bought a house outside Boston with a failing oil based hot water baseboard system.  House was built in 1915, original windows, ~4-5 inches of blown in fiberglass in the attic, and blown in fiberglass in the walls.  House is about 1700 sq feet composed of two floors, 1 zone for each.  We promptly redid much of the second floor, all new Rockwool in the walls, new windows, R62 in the attic and everything air sealed.

We got three quotes to switch to gas from local well reviewed heating/cooling contractors and all of them recommended the Navien NCB-240E.  Unfortunately, I didn’t really understand much about heating/cooling at the time, but I now understand that this system is not a great fit because its way oversized on the heat side and can’t modulate low enough.

I have the outdoor reset installed, but the last two years its been just running at 180 degrees, which I would like to change.  I read but I am not sure I fully grasp a path forward.

For example, my second floor zone is a series run around the entire perimeter of the house.  It’s about 160 feet of copper from the boiler, of which ~58 feet is finned.  I used cool calc and the slant fin tool and both of them came out at about 13,000 BTU as my heat load for that zone and the Navien can only modulate down to 17,000 BTU.  Putting aside that modulation, if I understand this correctly, when its running at 180 degrees, I am probably putting out about 34-35K BTU which is way way too much, causing my boiler to short cycle.

So my question is, do I have any hope in salvaging my ability to use my outdoor reset and get my boiler to condense if only my second floor zone is calling?

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  1. Jon_R | | #1

    Sounds like your outdoor reset isn't set up correctly. Also look into zoning that reduces but doesn't fully turn off zones. Also consider adding a buffer tank to decouple building load from boiler load (ie, reduce cycling).

  2. mackworth | | #2

    Yes Jon R, my outdoor reset is currently off and while investigating how to turn it on this fall, I realized that my second floor zone seems too small for the boiler. The buffer tank seems like an interesting idea. If anyone is reading this around the Boston area and knows a good company that I could have come and evaluate this, let me know. The company I used did a great job with installation craftsmanship (according to my retired plumbing neighbor), but I am not sure I trust their judgement on resolving the sizing issue.

    There was also some comments in that thread by Dana Dorsett that mentioned in that scenario with 15K BTU load, that only 2k of wasted BTU wasn't ideal but also not a huge deal, but in my case it would be more like 4-5k BTU, but I think that assumes I wouldn't be able to run lower than 130 degree water.

  3. Expert Member
    Akos | | #3

    With a two story structure, most of your house heat comes from the main floor. The generally means the main floor zone runs more often.

    In your case, the simplest solution would be to interlock the 2nd floor zone to the main floor. This way, the 2nd floor zone can only call for heat if the main floor is running but the main floor can run independent. This still gives you independent control of temperature in both places without having to worry about short cycling.

    This assumes the main floor has enough radiation capacity or has high mass rads to avoid short cycling from outdoor reset.

    This should be a fairly simple re-wire on the zone valves/zone controller.

  4. walta100 | | #4

    Ask your service tech if it would be possible to use a smaller oil jet that would reduce the BTU output. There is a limit to how small you can go.


    1. joshdurston | | #5

      With some careful tuning I think you can get even your second floor zone to run smoothly. Might not get a lot of condensing though. You'll gain some efficiency by reducing the temperature from 180 even if you aren't condensing. You should be able to reach equilibrium some where between 130F and 150F. If you go below that the emitters wont release the heat fast enough causing the temperature to rise above set-point and the boiler will cycling off.

      You should be able to setup your on/off differential to allow some temperature rise above set point without immediately shutting off. (so the boiler will start at say 10F below set point and run until 10F deg 15F over.) So make sure that if your Outdoor reset goes down to 120F, that the differential allows it to float to 140F before shutting off. The idea is that you want to reach equilibrium rather than shut off and short cycle.

      You may find that you have to be conservative how low the ODR goes. If you add mass with a buffer tank you can reduce the temperatures and still have reasonable cycling.

      A small buffer tank would go a long way.
      Even a small electric tank installed as a two pipe buffer would be good since the ports are smallish.

      Also, make sure you aren't over-pumped, that will drive your return water temp upwards.

      The parameters to adjust are:
      - the burner-on temp (set to 10F-15F, default is 4F)
      -burner-off temp (set to 10F-15F, default is 5F)
      -Anti-Fast cycling time (set to 10 minutes, default is 3)
      -reduce the max space heating capacity to 50% which is as low as it goes.
      Leave the boost feature off.

      Set your outdoor reset to 1. (fin tube) to start. You may be able to tune it downwards.

      1. mackworth | | #7

        Josh Durston Thanks for the detailed reply. Sorry for taking so long to reply, I really appreciate it. My newborn has me busy :)

        So I did a heatloss calc on my first floor, which has a ton of windows and I haven't replaced them yet, so its much leakier than the second. The BTU requirement is about 28K as is. Once I redo the windows but take my unheated 3 season room into the envelope, it's about 32k.

        I already changed my water temp to 145, although now that I did the calc, I think I have about 72 feet of baseboard on the first floor, so I may need to increase it up to 150-155 to make sure I can adequately get the first floor.

        Using that calculator, it seems like you are right that even a small tank would help the second floor zone. Something as small as <25 gallons would work.

        I will check the pumping. I need to see if the Navien provides the flow rate in its menu. I know that each zone has a pump, and I think they are both set to low but I will double check.

        1. joshdurston | | #8

          You boiler has a built in primary pump, I'm not sure if it's variable speed or not.
          But I would recommend installing a system supply sensor to measure the blended supply rather than just the boiler discharge. That way if the secondary flow rate is higher than the primary the boiler can compensate. I would leave the zones on low if they heat fine, the boiler flow is determined by the internal pump not the zone pumps. The zone pumps only need to move enough water to heat the zone.

          There are various balancing devices you can install if your flow is too high. High flow will heat fine but you will sacrifice boiler efficiency since your RWT will likely be coming back hot. You can throttle hand valves a bit to experiment if it seems like your flow is high. Note you will only have a high delta at high SWT temps and loads. As you lower the temperature the delta will reduce as well (assuming constant flow).

          You really want to get your RWT as low as you can (120F or lower). I might even try running your boiler in RWT control mode (instead of supply). (It will still constrain the supply temp to a high limit.) There is a dipswitch to change this (the fin tube outdoor reset setting of 101F-147F RWT is probably a good starting place). Running off RWT kinda compensates for having too much flow.

          Akos' suggestion about interlocking the second floor to only run while the first is calling is a good idea. It may let you run much lower temperatures without short cycling the boiler. But I would play with the boiler thresholds and cycling parameters a bit first, to see if it runs ok without cycling.
          Ideally you want the boiler to run for the duration of a heating call, or if that's not possible close to 10 minutes would be reasonable target. Your min water temperature will probably be determined by short cycling (unless you add a tank), and your top temperature will be determined by your highest load on a cold day. Reduce both until your run into short cycling or comfort issues.

    2. mackworth | | #6

      Hi Walta, to clarify, I have a gas boiler, I switched away from Oil.

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