Navies NCB240E for retro install using two existing heat pumps

Discussion in 'Boiler Forum' started by Chris Newman, Jan 13, 2018.

  1. Chris Newman

    Chris Newman New Member

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    Jan 13, 2018
    Location:
    North Carolina
    I have newer home built less than 10 years ago with two 2 ton heat pumps completely independent of each other one controlls the upstairs (zone one 950sq ft)the other for the primary zone down stairs which is 1100 sq ft, during the coldest weather my heat pumps can’t keep up and pretty much runs all the time in emergency strip heat making my meter look like some rendition of a medivac chopper gone awol!
    My hot water is also supplied via 40 gallon tanked electric heater, my question is could I possibly remove the electric heat strips from my air handler and replace with the hydronic heat exachngers (water to air) and be able to heat such large zones with just those two supply areas as well as heat my hot water, and possibly save money of course off my electric the Navien will be using LP.
     
  2. Dana

    Dana In the trades

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    Jan 14, 2009
    Location:
    01609
    You can definitely run a hydro-air handler off a Navien, but LP is going to be a lot more expensive than running a heat pump. Most of the time setting it up for dual-fuel would require setting it up as an either/or situation, not running them both at the same time. Feeding the hydro-air coil with 90-100F output from the heat pump would make for a very mediocre delta-T on the hydro-air coil- it won't be as efficeint or effective as it might seem.

    It's almost inconceivable that a 2 ton heat pump in reasonable functioning order won't keep up with the heat loads of ~1000 square feet of conditioned space without engaging heat strips until it's in the mid-teens or lower. Assessing the system and possibly re-commissioning it would be the best bet. If the heat pumps are in good working order and your heat loads are really that high you'd be better off spending the money on blower-door directed air sealing, and retrofit insulation. If there are any single-pane windows that are still too good to scrap it's worth investing in low-E storm windows (which pay-off faster than clear-glass storms.)

    If the ducts for the second floor are outside of conditioned space, in an attic above the insulation or under the insulation layer in a pier foundation or vented crawlspace the air handler may even be increasing the heat load substantially due to duct imbalances, leaks etc driving air infiltration to stratospheric levels. Sealing the ducts and balancing flows, getting it down to where the room-to-room pressure difference to less than 0.03 water inches under all operating conditions, room doors close or open would be worthwhile. Sealing duct seams and joints, with the appropriate materials (duct mastic is your friend!), and caulking the duct boots to the walls/ceilings/floors with polyurethane caulk, and duct-mastic sealing the duct boot seams is also part of re-commissioning the heat pump systems.

    What are your insulation levels, window types, etc? Is it a full basement, crawlspace or pier foundation?
     
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  4. Chris Newman

    Chris Newman New Member

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    I’m not quite sure that you followed my question let me try to explain a little better, I’m not so much concerned about the difference in cost from electric to propane especially if it’s close to an even swap for a warmer heat compared to the heat pump I was just wondering if two heat exchangers one in each air handler could supply sufficient btu’s to satisfy our heat demands and what it would take to do so I know the CFM ratings of our units but what I do t know is how GPM and CFM and supply temperature translate to BTU’s and how many BTU’s are required for my situation, ok now about the tether house all double pane low e glass attic space is R38 blown in Fiberglass all exterior walls are insultated with r19 batts as well as floor system on crawl space which is closed and encapsulated with dehumidifier which is where our air handlers are located all duct work is in crawl for lower level, upper level is ran through interior chase to attic space, attic space duct work is round metal insulated with fiber glass wrap with blown in surrounding that so all in all a pretty tight house although exterior door has some air loss.
    I’m hair hoping to switch to the propane navien boiler for emergency heat in case of power loss and use it instead of the electric heat strips when our temps drop in the single digits, an added bonus is the domestic hot water supply,
     
  5. Chris Newman

    Chris Newman New Member

    Joined:
    Jan 13, 2018
    Location:
    North Carolina
    As an added note I’m trying to compare this system to a wood fired boiler made by central boiler.
     
  6. Dana

    Dana In the trades

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    Location:
    01609
    I understood that you wanted to use both the hydro air and heat pump on the same ducts- yes? That doesn't work well with both running at the same time, but it's possible to set up controls to switch from one to the other based on the outdoor temperature. It's dead-easy to design the hydro-air to run off a tankless water heater.
    But I also understand that almost ANY 2 ton heat pump has enough capacity at +17F to heat a be able to heat a ~2000 square foot house built to IRC 2006 code minimums without engaging the auxiliary heat strips, and you have TWO of them! So it's an open question as to why they aren't cutting it. If the house is reasonably tight (not super tight) and the heat pump systems are reasonably (not perfectly) designed and implemented you should have gobs of spare capacity even at 0F without the heat strips. Either the house or the heat pump systems (or all of the above) aren't performing up to snuff.

    Ducts in the attic put a lot of holes in the pressure boundary of the house, which could be adding a lot of extra load if the ducts leak or the duct system isn't well balanced. An Energy Star system would need to have less than 3 pascals (0.012 water inches) of pressure difference between adjacent rooms when any air handler (0r both) are running, whether the doors are open or closed, and would need to be tested for duct leakage and that pressure difference/balance upon commissioning the system.

    To know how much heat you need, run a Manual-J heat load calculation on the house using aggressive rather than conservative assumptions (per the Manual-J) instructions, see what you come up with. This online calculator tends to shoot a bit high (most do), but it's better than most quick & easy load tools, a good place to start. Then compare the load numbers to the extended temperature capacity tables for your heat pumps. (Four tons of compressor for a 2000' house is usually extreme overkill, for both heating and cooling.)
     
  7. Chris Newman

    Chris Newman New Member

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    I don’t want to run the heat pump and the Hydronic simultaneously I want to supply all my heating with the navien ncb240e Jair by utilizing the air handler to move the air so fan only function and have the boiler handle the entire heat load?
     
  8. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    The NCB240 has enough output to heat your house even when it's -100F out, but the existing ductwork probably isn't capable of delivering the necessary flow for 100% of the maximum firing rate output.

    All solutions start with a room by room, zone by zone heat load calculation. Without it you're designing in the dark.

    With the subtotal zone heat requirement numbers it's possible to select a coil that can deliver ~1.4x that much heat with an entering water temperature of 130-140F at reasonable flows at the air handler's rated cfm. To do this right using the heat pump's air handler is really an engineering problem, a with lots of parts that interact. It's easier to buy an already engineered well specified air handler with a hydro-air coil and design the system around it, playing with the temperature settings on the NCB240, using a programmable smart pump (they're cheaper than you think), for setting the flows. To get condensing efficiency out of it will require boiler output temperatures under 140F, and air & water flows sufficient to get at ~15F or larger delta-T on the water side of the coil, and for comefort you'll want at least a 50F-60F delta-T on the air side of the coil (~65F in, 115F-125F out, into the ducts.)

    Some nakpin math that you'll need:

    Every cubic feet of air has a thermal mass by volume of 0.018 BTU per cubic foot per degree F. So to raise a cubic foot of air by 60F requires 0.018 x 60F= 1.08 BTU. Every cubic foot per minute of 60F rise is a heat rate of 60 minutes/hr x 1.08 BTU/cubic foot= 65 BTU/hour. (If you raise the temperature change it'll be higher, if you go for a lower temperature change it will be lower.) So an air handler that's delivering say, 750 cfm it capable of delivering 750 cfm x 65BTU/hr= 48,750 BTU/hr at 125F.

    A BTU is the amount of water required to raise a pound of water by a degree F. Water weighs 8.34 lbs/gallon, so a gallon per minute of flow is 8.34 x 60 minutes/hour = 500.4 lbs/hr. Assuming a flow rate of 3 gpm (won't require a monster-sized pump, and won't ruin the NCB240) it'll then be moving about 1500lbs/hour. To deliver the 48,750 BTU/hr out of the coil would then require a temperature change of the water of of 48,750/1500 = 32.5F, which is a reasonable delta-T across the boiler. (It'll want to be between 20-40F. Lower than that can lead to lower efficiency, higher can stress the boiler.) So with 135F output water and 105F return water (a 30F temperature difference) you should be able to get the requisite air temperature out of the air handler, and at 3 gpm it would deliver 1500 lbs/hr x 30F= 45,000 BTU/hr. (Which is more heat than my 2400' 95 year old 2x4 house + 1600 of conditioned basement needs at -10F outside.) Raising the boiler's temperature will increase the heat transfer. Raising the flow rates will too, but this is roughly the range you're shooting for.

    This is the napkin math version- reality is considerably more complicated than that, but the napkin math doesn't lie. It's possible to hack and tweak a system into satisfactory performance if it's at least somewhat in the range. It may not be the most efficient or the most comfortable, but it'll work.

    You may find the various links and discussion in this thread (on another forum) useful:
     
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  9. Chris Newman

    Chris Newman New Member

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    Jan 13, 2018
    Location:
    North Carolina
    Thank you for quick and informative respsosnses Dana, unfortunately I’m a lowly North Carolina GC and Public utility contractor I’m capable of doing the napkin math but will take me some time if you were to ask me about scour velocity pump sizing flow rates soil load rating LTAR for on site waste water treatment or friction loss in a given size pump and run length I’d be your man lol, but this is a foreign territory for me although I love trying to figure out these things it’s a curse I live with, So in order to make it easier for me as you are obviously much more intelligent than me in this field by about 50x it would seem, where should I start and this is what I’ve gathered from your response thus far and will print them out and Read them over several times as to not waste your time,

    1. Room sizing in cubic ft or sq ft?
    2.CFM output of my air handlers
    3.Shoukd I check incoming water temp.
    4.Is the Navein NCB240 E the right Choice
    5.im doing the install with the help of a boiler maker friend. So if I could give you my layout a schematic would be amazing the two air handlers are side by side.
    6. Would it make more sense to go with a wood fired boiler or wood furnace. Although there both boilers with essentially the same installation in regards to the heat exchanger in the plenum. Obviously most cost effect if I can supply the wood. And it would provide domestic hot water via a water to water heat exchanger my neighbor has one and it’s amazing although I think he needs a tempering mixing valve or already I would because of my three year old daughter.
    7. Our utility provider is astronomicaly expensive compared to another provider just down the road that’s not available to us would a price per kWh help in any way and is it possible to BTU to BTU which would be more cost effective compared to LP.
    8. I under stand that it would be much easier to buy another unit deigned around a hydronic system but much more expensive, although this is not how wood fired boiler installers do there installs they are all retro by just putting a heat exchanger in the plenum hooking up a recirc pump to your thermostat and fan and call it a day, what’s the difference between that and a conventional boiler with the exception of the obvious efficiency of the wood fired boiler is not so much of a concern, and can he accomplish this just by running 200 plus degree water through an insulated loop.
    9. My end goal use as little electricity as possible propane may not be the best answer here not sure.
    10. Would be useful to have a home energy consultant come check some things out.
    11. THANK YOU VERY Much your awesome!
     
  10. Chris Newman

    Chris Newman New Member

    Joined:
    Jan 13, 2018
    Location:
    North Carolina
    Looking at your math someone told me it requires 4000 BTU of heat per 100 square foot is that wrong?
    Which would equate to almost 400000 BTU’s that did seem crazy though in. 2000 sq for home.
     
  11. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    The volume or floor area have nothing to do with heat loss.

    Heat loss is primarily a function of the area of the exterior surfaces of the house, the temperature difference from indoors to outdoors, and the R-value of the different surface types (eg: code-min low-E windows are about U0.35. R=1/U and U= 1/R, so you're looking at R2.9 for the windows, whereas a 2x4/R13 wall comes in at about U0.08-U0.01, or R10-ish after factoring in the thermal bridging of the framing, and adding in the R-values of the wallboard siding, sheathing.) The basic formula is:

    U-factor x surface area x temperature difference= BTU/hour

    For the interior design temperature 68F is code minimum, but people often use 70F. For an outside design temperature it's best to use the 99th percentile temperature bin for the local weather history, referred to as the "99% outside design temperature". Of the 8760 hours in a year, only 1% of all hours (about 88 hours) are colder than the 99th percentile bin. Some will use the 99.6th percentile bin, which is a few degrees colder, but there are only 35 hours out of a year that are cooler than that. In NC the 99% outside design temps range from about 15F in some of the mountainous areas, to the high 20s along the coast. If your 99% outside design temperature is, say 23F and you're designing to 68F indoors, that's a 45F temperature difference, and that what you would use for a temperature difference in the heat-loss formula.

    All surface types (roof, walls, doors, windows foundations etc) all have different U-factors, so those areas all all measured up and applied separately, on a room-by-room basis. The ceiling under a heated space doesn't have a heat loss, nor does a floor over a heated (or at least insulated and enclosed space like a basement that stays over 60F even without directly heating it.)

    Air infiltration is another factor that needs consideration, and is hard to estimate accurately. The cubic feet per hour and thermal mass of air by volume and temperature difference math is the same as in the hot-air heating example above, only you're using the design temperature difference and a WAG on the actual air leakage (most software grossly overestimates infiltration losses on newer tighter houses.)

    Then, start subtracting off all of the internal heat sources, such as the 24/7 plug loads such as refrigerators, DVRs & cable boxes, etc. and subtract off 230BTU/hr per sleeping human.

    More can be found here.

    Even though heat loss is not a function of floor area, at a temperature difference of 45 degrees most tight 2x4 framed houses with R38 in the attic will have a load/area ratio of between 10-15 BTU/hr, and 2x6/R19-ish houses would come in around 8-11 BTU/hr per square foot of conditioned floor area. So for a 2000 square foot house expect the load numbers to come in at about 20,000 BTU/hr @ +23F outdoors, 68F indoors give or take 3000 BTU/hr. Even pretty crummy 2-ton heat pumps have that much capacity, ergo the mystery of why FOUR tons of heat pump can't keep up in your house without engaging the heat strips. Something is really wrong with either the system design/duct layout, or the refrigerant charge on the heat pumps, or the house is just plain leakier than you think it is (or all of the above.) But using a BTU/hr per square foot of conditioned space is a truly lousy way to estimate heat load, with room-by-room exceptions to the average that can be well over 2x the average.

    The labeled heating efficiency of heat pumps is expressed as HSPF (heating season performance factor), the units of which are BTUs per watt-hour (x 1000= BTU /kwh.) The lowest legal HSPF efficiency for heat pumps of a decade ago was about 7, or 7000 BTU/kwh. Best in-class newer cold climate ductless heat pumps can deliver twice that much. Oversizing the heat pumps gives more capacity at cooler temps, but reduces the as-used efficiency due to excessive cycling during periods of low to moderate heat load. It takes about 8-10 minutes of run time for the heat pump to hit it's steady state efficiency for the real time indoor & outdoor conditions, and if it's satisfying the thermostat in 15 minutes, most of the run time was at lower efficiency. Every spin-up of the compressor uses some amount of power that you never really get back, so the total number of cycles as well as the duty-cycle matter.

    To compare costs, use the nameplate HSPF on the heat pumps and your local electric rates to come up with the cost of what it takes to put a million BTUs (MMBTU) into the house. A condensing boiler like the NCB 240 delivers about 87,000 BTU/gallon of propane into the house, so it takes ~11.5 gallons per MMBTU, then multiply by the price/gallon to come up with $$/MMBTU. Even in high priced electricity markets it's usually cheaper to go with heat pumps than propane. And if the efficiency problem is the duct design and air leakage, using the same system for heating would suffer the same efficiency losses with hydro-air propane as it does with the heat pumps.

    Outdoor wood boiler can definitely deliver the heat, and can often be cheap to operate where scrap wood is cheap/free, but the are a very substantial local air pollution source, much more so than EPS rated wood stoves. To minimize the pollution with a wood boiler requires loading up only enough fuel so that it will burn completely before the boiler's temperature limits are reached, which requires either a lot of smaller loads/burns, or a large buffer tank of water for thermal mass to keep the temperature from being reached quickly.

    Don't just dive into a hydro-air design project until you calculate the heat loads, and figure out why the heat pumps aren't keeping up, have verified the air tightness of the house, the ducts, and the balance of the duct design, or you'll end up where you started. A 2-tube hand held manometer with 0.01 water inches resolution can be had for $50-150, and can be used to track down room-to-room pressure differences, and room to outdoor pressure differences. An Energy Star house would have pressure deltas no greater than 0.012", but for retrofit fixing getting to at least under 0.03" would be a good starting point. This may require a lot of duct-boot sealing and creating bigger return paths for doored off rooms that have only supply ducts, but it's usually possible to get to reasonable balance without tearing the whole house apart.

    For using a NCB240 or wood boiler for supplementary heat or even as a standalone system it's better to just skip the highly suspect ducts and install heat emitters in all the rooms you care about. Fin-tube baseboard is pretty cheap, and if you calculate the room by room heat loads, if you use a ratio of 350BTU /hr of design load for every foot of running baseboard you would be able to run in condensing mode most of the time, and set the outdoor reset curves (which adjusts the water temp at the boiler in response to outdoor temperature) to ensure that is going to work. The minimum burn rate of the NCB240 is about 17,000 BTU/hr, and it'll deliver 95% efficiency when the boiler output is about 125F, a temperature at which fin tube baseboard is delivering about 200 BTU/hr per linear foot. To keep it from short-cycling itself into lower efficiency/higher maintenance it's best to install at least 17,000/200= 85 feet of baseboard per zone. You can cheat that a bit, but if you go too far it will be doing 10+ ignition cycles per hour, with burn times measured in 10s of seconds, all of which is lousy for system efficiency and the longevity of the boiler.
     

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