Navien Combi Boiler sizing

Discussion in 'Tankless Water Heater Forum' started by cadobe, May 25, 2019.

  1. cadobe

    cadobe New Member

    Joined:
    May 25, 2019
    Location:
    Seattle
    What would be the right model of Navien Combi Boiler for heating a 3 bedroom house of 1550 sqf located in Seattle area?

    The house is a rambler style 1956 build having 2 bathrooms.
    Blow insulated outside walls, the heat loss for 60 degrees temp diff. came up as 45,107. We run Myson panel radiators on each bedroom and bathrooms(2).

    Thanks
     
  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    First, a heat load of 45K @ 10F outside/70F inside is unrealistically high for an insulated 1550' fifties vintage rancher even if it has a full uninsulated basement and only single pane windows, no storms. Reality would be closer to half that, or perhaps ~30K or slightly more if it has an uninsulated basement and mostly single pane windows. What load tool came up with that number?

    For reference, my sub-code 1920s bungalow with 2400' of fully above grade living space with antique single panes and 1980s vintage clear glass storms, plus 1600' of insulated but not directly heated basement doesn't hit 45,000 BTU/hr until it's about -10F outside, not +10F. Your heat load HAS to be smaller than mine unless it's one of those mostly window "Mid Century Modern" type houses, not usually refered to as a "rambler".

    Also 10F is WAY below the 99th percentile temperature bin for Seattle- mid-20s is more likely for Seattle.

    If you have a wintertime heating history on this place, run a fuel-use load calculation using the old boiler or furnace as the measuring instrument. Use only December through February meter reading periods the fall & spring shoulder seasons introduce too much error, exaggerating the load.

    All Navien combi boilers are oversized for your heating load, but at the sometimes glacially cool incoming water temps you can't really downsize much and still have reasonable domestic hot water service. Whether you can make it work with your radiation depends on how much radiation you have, and whether it's operated as a single zone. The critical aspect is how much your radiation (per zone) can emit relative to the minimum fire output of the combi boiler at condensing temperatures.

    To hit 95% combustion efficiency takes an average water temperature of about 115F or so (120F out, 110F back), so look up the output figures on the Mysons at 120F or or 50C. If each zone can emit the full minimum-fire output of the boiler with 115F water it won't cycle much at all. If a zone can only emit half that amount you could be in for short-cycling, which cuts into both efficiency and longevity of the boiler. The napkin-math on that lives here.

    As general rule, low-mass combi boilers are only a good fit for homes with larger than average heat loads, and low to moderate domestic hot water loads. But if you run the fuel use load numbers and measure up the radiation we can figure out what works, what doesn't, and what the reasonable options might be.
     
  3. Sponsor

    Sponsor Paid Advertisement

     
  4. cadobe

    cadobe New Member

    Joined:
    May 25, 2019
    Location:
    Seattle
    Hi Dana,
    The heat load I've got is from a not remember online calculator honestly.

    The house has double pane insulated windows with a not insulated hardwood floor above vented crawlspace.

    Now for the history....:) the first thing I've done by the time we moved in, I just trow away the old/awful/noisy(you name it) forced air furnace. So I cannot get into that calculation.

    The only thing for sure I know is that we heated the house partially ~1200sqf with Myson radiators ( 1: T622-5-12; 2: T622-5-12; 3: T622-5-14; 3: CV22-2600 ) hooked up into a 40 gal water heater of 38,000 BTU which worked like a charm this winter having 72 degrees inside. BDW this winter was a colder one for this area. The highest bill was <$200 in Feb. running water out ~140 degrees and in ~120. The only concern I had was the water heater burner running 40 min almost each 1.5 hour.

    Anyhow taking your word in terms of heat load being to high, you think would be a big difference between the NCB 180-E and NCB 210-E?
    [​IMG]

    Thanks again Dana
     
    Last edited: May 25, 2019
  5. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Gas is metered in CCF and billed in CCF, therms, or decatherms, not dollars. Seriously- run the fuel use load calc using exact meter reading dates and heating degree-days for those time periods.

    Some online calculators are much better than others, and none are as good a professional Manual-J tools used in the hands of a qualified professional. To get reasonably correct results out of LoadCalc or CoolCalc requires being very AGGRESSIVE about the air tightness, R value & U-factor numbers rather than the impulse to conservatively low-ball the R-values and assume high or even moderate air leakage.

    An atmospheric drafted or power drafted water heater runs about 80% steady-state efficiency. So 38K-in means it's only delivering 0.8 x 38K= 30,400 BTU/hr into the water. If in it's worst case it's running 40 out of every 90 minutes that's a 4/9= 44% duty cycle. At 30,400 BTU/hr and a 44% duty cycle it's only delivering 30,400 x 0.44= ~13,400 BTU/hr at most to the radiators. So even if you were only actively heating half the house your heat load is nowhere near 45K.

    Replacing the water heater with a stainless steel modulating condensing commercial water heater would improve the overall efficiency, and would be a better choice than any NCB due to it's self-buffering thermal mass making it impossible to short-cycle. HTP's "Light Duty" version of their Phoenix water heaters has a 76KBTU burner with a 3:1 turn-down ratio.

    It's not the heat load that is too high, but rather the minimum firing rate that is too high, and the thermal mass too low, leading to potential short cycling with lower condensing temperature water. The minimum firing rate of the NCB-180 is 14K, which is probably still more than half your likely design heat load, the NCB-210's is 18K, which is probably more than 2/3 of the design heat load. That is a big difference if it were on the edge of being a good fit, but neither is is even close to being a good fit for your heating load. But the 150K max firing rate of the NCB-180 really doesn't quite cut it for a 2 bathroom house in a location where the incoming water temperature in winter can drop as low as 40F, (which it can in much of King County).

    Even the NCB-210's max of 180K isn't great, but since it's minimum firing rate is the same as the NCB-240's, if you can demonstrate with the napkin-math that it won't short cycle on your radiation that would be the better choice, if you're dead-set on going with a low-mass combi boiler.

    To really get the best comfort & value out a modulating condensing boiler, operating it under outdoor reset control and having a modulation range at the low end that's something like 1/4 the design heat load would be more appropriate. Otherwise you might as well go with a condensing water heater.

    I'm not going to look up the output specs on all your radiators for you, but you should. If you want to total up their output at 140F (I believe they all have a 140F spec) and report back we could use that to infer to expect the behavior of the NCBs to be when the average water temp is 110F or 100F, and with a more accurate load calculation we can also use it to get a good starting point for setting the outdoor reset curves.
     
Similar Threads: Navien Combi
Forum Title Date
Tankless Water Heater Forum Venting Navien Combi for high temperature design Jun 4, 2019
Tankless Water Heater Forum How to Bleed Air from Navien Combi 240? Jan 8, 2019
Tankless Water Heater Forum Navien CH180 combi Error 12e & 48e Jan 5, 2018
Tankless Water Heater Forum Is Navien 240B or Bosch Combi 151 right for me? Mar 7, 2015
Tankless Water Heater Forum Navien CR240 combi seems to be stuck in heating mode, not switching to DHW Aug 26, 2013

Share This Page