Boiler Sizing

Discussion in 'Boiler Forum' started by Drewski123, Sep 12, 2013.

  1. Drewski123

    Drewski123 New Member

    Messages:
    47
    Location:
    Seattle
    Hi Everyone. I am planning to replace my current heating system which is a conventional oil fired furnace (duct system), with a gas fired boiler (hydronic system) and have some boiler sizing questions.
    The new hydronic heating system will contain Wirsbo Joist Trak 1/2†heat transfer panels under the main floor (1,300 sq.ft), and hydronic baseboards in the basement (900 sq.ft). The hydronic baseboards will be installed at a later time once the basement is finished (within 2 years) so the entire system will have to be prepared to accommodate the basement. I am planning on having 4 zones; 3 on the main floor, and one for the basement.

    I had an estimate with heat loss calculated for the main floor by a reputable hydronic heating company, here is the summary:
    Total Load: 20,600 BTU/H
    Total Radiant Load: 13,821 BTU/H
    Total Supplemental Load: 6,779 BTU/H

    Well, I had another estimate conducted by a plumber specializing in radiant heating. This contractor wants to use gas fired Navien CH-240ASME - 199,000 BTU/H boiler.
    Here is my question: Knowing that I will be finishing the entire basement within the next 2 years, is the boiler size an over kill for my house?
    Thank you for your help.
    Andy
  2. Dana

    Dana In the trades

    Messages:
    2,805
    Location:
    01609
    I'm not sure why the -240ASME and not the 180ASME, which is somewhat better matched at low fire to your actual heat load. Do you have a major gusher-shower that needs to run simultaneously?

    The fact that it's cut up into 4 zones could be an issue for any boiler- you're probably better off with a Polaris tank-type HW heater. The min-fire output of the 240 ASME is about 19,000BTU/hr, and your smaller zones (maybe all of your zones) are individually less than 1/3 of the total output, probably under 1/4 of the output, and there isn't enough thermal mass in that type of radiation to keep it from excessive cycling. With a Polaris and a heat exchanger the thermal mass of the tank keeps it from short cycling no matter how small the sipping of zone calls are, yet it's plenty of burner & buffering mass for both hot water and heating needs.

    With your radiation and micro-zoning any of the Navien combis would likely be seeing a dozen or more ignition cycles & flue purges per hour during the heating season, which is hard on the equipment, and bad for efficiency.

    A ~7K heat load on a 900' basement sounds a bit on the high side at Seattle's outside design temperatures. is the foundation insulated, or is it a walk out, with a bunch of windows? (I have 1500' of basement and a design temp 20F colder than yours, but only a marginally higher heat load for the basement.) With R15 continous foam on the foundation walls (Or an inch of EPS between the concrete and an unfaced-batt insulated 2x4 wall, no interior vapor barrier) and an insulated & air-sealed band joist/foundation sill you'd probably be comfortable during the heating season even without the baseboard zone. But it takes on the order 100' of baseboard to deliver 15,000 BTU/hr @ 120F boiler output, and if you cut that length by half you'd be hitting the short cycling range for the Naviens.
  3. Drewski123

    Drewski123 New Member

    Messages:
    47
    Location:
    Seattle
    There is only me and my wife in the house and we have only one bathroom plus a second will be added in the basement in the near future. We want to make sure that we will not have any problems with hot water while taking showers and having a washer running at the same time.

    Dana, would you also recommend installing 3 zones, instead of 4? You mentioned that with 4 zones there would be some issues, can you give me more details on the issues?
    We have 3 bedrooms upstairs, open living space with dining area and a kitchen. My initial plan was to have 3 zones upstairs and one in the basement. First zone would control the master bedroom, the second one would serve the other two bedrooms, the third one would serve the living room/dining area and a kitchen. The forth zone would serve the entire basement. If it makes sense to go with 3, how would you combine the zones? Would you have all bedrooms under one zone and the living space with dining and kitchen area under a second and the basement under a third zone?

    In regards to ~7K heat load for basement; once the basement is finished, it will have insulated walls (exactly what you have described). The basement is about 80" tall with quite big windows. I am planning on having 3 baseboards installed in the entire basement. One in the media room (separated with a door from the other space) and two in the remaining space outside of the media room. The basement will definitely have lower set temperatures then the rooms upstairs.
  4. Dana

    Dana In the trades

    Messages:
    2,805
    Location:
    01609
    To take a stab at getting the hydronic zoning "right", at a minimum you'd have to calculate the total volume in the PEX on each zone, and make sure that it's sufficient to keep the boiler from short cycling at min-fire. Half inch PEX has only 0.92 gallons ( 7.7lbs) of water for every 100' of tubing. At 8" o.c. tubing spacing a 400' master bedroom would have only ~600' of PEX, or 46lbs of water.

    Assuming the combi boiler runs a 7F high/low differential around the output setpoint temp, that 46lbs of water takes only 7F x 46F= 322 BTU to heat up, and at 19,000BTU/hr of output that only takes 322/19,000= 0.017 hours, or 0.017 x 60= 1 minute to heat up, which is a fairly extreme short-cycle. Ideally the minimum burn time at low low output temperatures using the outdoor reset function would be 5 minutes, but 3 minutes wouldn't be a disaster. With 1300' of total space on the first floor I'd hazard that you have at most ~1500-1800' of tubing, or 115lbs or more of water, which would deliver tolerable minimum burn time as a single zone, unbuffered.

    Run the math on the tubing of the three-zones as-configured, then how to combine them to deliver at least three minutes of burn on their thermal mass alone, which would allow you to run 100F or lower water temps for high-90s efficiency when it's not all that cold out. At 8" spacing it's 1.5x ratio of tubing-length to floor area, and 7.7lbs/100' of length.

    In the basement you'd be far better off with panel radiators than with baseboard, to keep the boiler from short cycling. It only takes about 20' baseboard to deliver the ~6800BTU/hr at the 99% design temp with 140F water, but it's output at 120F is only ~4200BTU/hr, and you'e have 15,000BTU/hr of excess heat being dumped in. Assuming 50' of 3/4" copper on that loop (including the fin tube itself) you're looking at 10lbs of water for thermal mass- the thing will short-cycle like crazy with burn times well under a minute when only that zone is calling for heat.

    Using a condensing tank heater the minimum burn times are determined by the thermal mass of the water in the tank, which is substantial, and it's internal controls. With a heat exchanger for the heating loops you can micro-zone it to your heart's delight, and it doesn't affect the math.

    But at Seattle's utility rates, at a lower installation & operating cost, heating the main zones with ductless mini-split heat pump technology may make more sense, probably a 3-4 head 1.5-2 ton multisplit. Take a look at the room-by-room load numbers. Something like the Mitsubishi MXZ-3B24NA or Daikin 3MXS24JVJU or Fujitsu AOU24RML1 can handle three zones using a mix of interior head/cassette types, and can deliver over 27,000BTU/hr of heating. With mini-duct cassettes it's possible to "share" the load between two adjacent low-load zones. I'm sure the main zone has a heat load over 7000BTU/hr, and maybe the master bedroom too(?), which is enough load to rationalize it's own separate zone head. to get four-zone systems you have to bump up to a 2.5 or 3 ton compressor, which is oversized for your heating loads- enough to affect efficiency on the high end. Ideally the heads would be sized no more than 1.5x oversized for the individual zone, an the compressor no more than 1.5x the total heat load, which is why a 24000BTU compressor (typically 30,000BTU out in heating mode) is a better choice if you can make it work. Installation costs will vary, but it'll be WAY under the cost of a professionally installed radiant floor solution.

    In a Seattle climate better class ductless technology delivers over 10,000 BTU/kwh, and you have some of the lowest electricity prices in the US. Like politics, all utility rates are very local, but last I looked natural gas in Seattle was on the order of $1.20 therm (100,000 BTU source-fuel, 95,000BTU if burned in a condensing boiler), or about $12.60/MMBTU (plus the electricity for pumping costs), and electricity was about 9-10 cents, which used in a better-class ductless costs $9-10/MMBTU.

    What you'd be giving up is the barefoot comfort on the coldest days, but unlike ducted heating solutions, ductless systems are variable speed and very quiet, delivering VERY stable room temps, and are in no way comparable to hot-air furnaces. They provide super quiet high-efficiency air conditioning too.

    For probably more money than ductless or radiant you could go for a 2-ton Carrier Greenspeed variable-speed ducted heat pump system for comparable comfort and efficiency, but you'd probably need to go with all new ducts, but I'm not sure if you can micro-zone it the way you can with a ductless approach.
  5. Drewski123

    Drewski123 New Member

    Messages:
    47
    Location:
    Seattle
    Dana
    Here is some additional information:
    Radiant Tubing Volume: 15.1 gallons
    Total Flowrate: 3.2 GPM
    Total Assigned Circuit Length: 1,640 ft

    Here is the summary of the project. I appericate your responses.

    I've started reading about the panel radiators that you have mentioned, so I may go with them instead of baseboards. I was not sure of the difference so I google it: "Steel Panel Radiators are the better product when it comes to design, flexibility and operation. They are more expensive with respect to initial cost, but overall the operating expense of your heating system will be less then would be expected with a traditional baseboard system"

    Project.jpg
    Last edited: Sep 16, 2013
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,889
    Location:
    New England
    If that's over 1600' of pex with four zones, I hope those zones have multiple loops and a manifold! TO keep the head within reason, you want to limit a loop of pex to about 200' max (in 1/2"), otherwise, first, it doesn't flow well, and second, by the time things circulate, the end of the loop has cooled off making heat transfer less efficient.
  7. Dana

    Dana In the trades

    Messages:
    2,805
    Location:
    01609
    If you're running with a maximum temperature of 110F as indicated in that sheet, you'll need at least 50 feet of fin-tube to deliver the ~6800BTU/hr. But 50' of fin tube + associate plumbing it only has about 10lbs of water in it, and it will still short cycle even at design condition with 19,000BTU of boiler output, and still short-cycle like crazy at temps below 110F.

    If you have room for 100' of fin-tube you'll have better balance with the boiler output, and since fin-tube is really flaky at temps that low, it would ensure that it actually delivers the heat. But that's a lot of fin tube.

    Panel radiators have more thermal mass, which addresses some of the short cycling, but if you're running 110F water you'll need more radiator. Running the whole thing fixed temp at 130F output would work- you'd still be in condensing range on the return water temps, and you'd get design day heat out of a single 2' x 6' biasi radiator, with ~42lbs of water-equivalent thermal mass. At 130F fin-tube delivers ~250BTU/ft, and while it would deliver the 6800BTU/hr of design heat with only 27', but it's so low-mass (6lbs of water-equivalent) that it is guranteed to short-cycle on the remaining 12,200 BTU/hr of boiler output.

    Both thermal mass, and the output of the radiation on that zone at the given water temp relative to the boiler's minimum output count. Fin tube output with temp looks like this, but it's all low mass. Panel radiators have substantial amounts of steel- multiply the weigh x 0.12 to get the approximate water-mass equivalent, but they also carry at least some amount of water, all found in the spec. The output of either at 130F will be about half what they deliver at 180F. At 110F water temps it'll be about a quarter the 180F number.
  8. Drewski123

    Drewski123 New Member

    Messages:
    47
    Location:
    Seattle
    It's been a while since I've posted anything but at this point I am half way through the project. That having said, all loops are installed together with manifolds. Here is the problem. I do not like my contractors plans for the final installation within the mechanical room.
    I am not a pro just a DYI person, but hired a contractor to finish the mechanical room. Here are the few components that I wanted him to install:
    - Navien 180 NCB Boiler
    - Grundfos Alpha Pump
    - Two Gurndfos Zone Valves (there will be 2 zones with slightly different water temperatures)
    - Taco 3 Zone Control Box (the third zone will supply future basement radiator panels)
    - Mixing valves; would you install one if the water temperature differs by 10 degrees?

    here is what he suggested:
    - agreed on the boiler
    - no need for pump since the boiler has second pump that will supply water to manifolds. Is that true, is the pump sufficient enough?
    - no need for zone valves
    - no need for mixing valve since the water supply temperature will be the same for the two manifolds (he suggests a 130 degree water supply) and will be enough for future panels in the basement
    Also, he suggested not to use the zone control box, and instead, to use zone valves

    Am I missing anything? I understand that he wants to go the cheap way and said that the more things you put in the system the more problems I will have in the future.

    Please, help!
    Last edited: Mar 17, 2014
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