Is my Navien Combi install salvageable?

Discussion in 'Boiler Forum' started by Mike Bernardo, Jan 6, 2019.

  1. Mike Bernardo

    Mike Bernardo New Member

    Joined:
    Jan 6, 2019
    Location:
    Boxborough, MA
    Our 4 year-old Navien NCB-240 (propane) has been problematic for most of its life. Many parts have been replaced and is currently acting up again.

    I was at my wits end and about to replace it with a different manufacturer, but as I learn more about mod/cons I'm starting to think maybe our heating system is not designed correctly and/or the 240 is oversized. My hunch is that this is at least partly to blame for the equipment failures we've been seeing.

    Hoping I can get some advice from the group here on how to adjust or replace our system. Our Navien-certified contractors have been good at replacing parts, but lack advice on system design. (Recommendations for good hydronic system contractors in New England also welcome.)

    Our setup:

    Late 80s ~3000sqft 2-story "Acorn/Deck" style home in MA. Lots of big windows in main living area.

    1st floor Main zone:
    Living, Dining, Kitchen, Kids room
    Baseboard: Mix of 40.5' of Runtal UF-2 and 28' of fin-tube baseboard
    Mitsubishi MSZ-GE18NA-8 in Living/Dining

    1st floor Master BR zone:
    Master BR, Master Bath
    14' Runtal UF-2 in BR, 6' in Bath
    Mitsubishi MSZ-GE09NA-8 in BR

    2nd floor zone:
    2 bedroom, 1 bath
    1 zone baseboard: Mix of 4' Runtal UF-2 and 37' of fin-tube baseboard
    Mitsubishi MSZ-GE06NA-8 in each Bedroom

    Basement zone:
    Partially finished, almost entirely under grade (~550sqft)
    1 zone 20' fin-tube baseboard

    No outdoor sensor on the Navien. As far as I know, it has been set to 180 supply temp the entire time.

    The Navien is currently occasionally throwing error E109 (air flow related error). Techs have replaced the fan, venturis, and ECB. Still E109. Latest advice from Navien tech support is to replace the air flow sensor and entire wiring harness. I have not done this yet as I'm usually able to get the unit to re-start by unplugging it for a bit and re-plug it back in. At this point I'm hesitant to throw more money at this unit.

    My guess is that our use pattern on the Basement zone is the main culprit for the Navien breakdowns. It is a home office for me, so during the day it is usually the only zone operating. While working, I either turn down the 1st floor thermostat or turn on the Mitsu in the living room which keeps most of the 1st floor at temp.

    When running Basement zone only, I have noticed it short-cycles the Navien like crazy. Once the fin-tubes are heated up, there is practically no temperature delta. Water goes out at 165 and comes back at 164/165. During heat calls on this zone, the Navien starts up and burns for 3-4 seconds and immediately shuts off. It waits the 3-minute cycle time and starts again, continuously. I have tried turning the zone circulation pump to it's slowest setting but that seems to have zero effect on temp delta. Mentioning this to the boiler techs a couple of times, they just shrug it off as "normal". Am I correct to assume this is catastrophic for the boiler to run this way for several hours a day?

    The Navien seems to run the 1st floor pretty well. For example, today it's about 30-40F outdoors. I have set the supply temp to 170 and the Navien has spent a several hour-long burn getting it to 70F. Steady temp delta of 167->156F

    2nd floor baseboard zones almost never used. Mitsus do a good job keeping the 2nd floor at temp on their own. Similarly, baseboard in Master BR zone almost never used.


    How would you improve the current setup? I see a few options:

    1.) Replace the Navien with a different combi manufacturer. My contractor suggested a Lochinvar or HTP.

    Without adjustments to the Basement zone, this seems like a costly+bad idea and likely to cause the same short-cycling problem I have now.


    2.) Improve the radiation of the Basement zone.

    The Navien is clearly oversized for running this zone in isolation. Would going to bigger/more radiators help? My thought was to replace the 2 10' runs of fin-tube with bigger runtals (maybe UF-4 which is 930 BTUH/ft)

    https://runtalnorthamerica.com/residential_radiators/baseboard_uf.html


    3.) Change zone usage pattern and setbacks:

    Currently we have Nest thermostats keeping the house at 68-70 while occupied and going down to 65 during the night and when we are away. We could add the outdoor kit to the Navien and try to move to a constant low-supply temp setup. This way, the basement has less of a chance of running in isolation since the 1st floor zone would be running more often. My concern is that this will increase fuel usage? It also negates the use of the Mitsus for heat?


    4.) Move to something other than a mod/con boiler:

    What are my options here? Clearly this will be costly, but perhaps makes sense if this mod/con is a lemon and/or the wrong type of boiler for my setup.


    Other ideas? Thanks!

    Mike
     
  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    The MBR zone with 14' Runtal UF-2 in BR, 6' in Bath isn't dramatically better than the 20' of fin-tube in the basement zone. Neither is capable of emitting more than about half the minimum fire output of the NCB 240 even at 180F entering water temp. I suspect the MBR zone calls for heat much more often than the basement zone, even if it's a walk-out basement (?).

    Running it at lower water temp would induce even more short-cycling, since the heat emitted by the zone radiation would be even less. It takes about 75-85' of baseboard or UF-2 to emit the full 17,000 BTU/hr min-fire output at condensing temperatures. That would require operating the whole house (or at least the whole first floor, including the MBR) as a single zone. Even HTP's lowest min-fire UFT-080W boiler (not a combi) at a min-fire output of 7600 BTU/hr needs at least 35 feet of baseboard (or equivalent) per zone to not short-cycle on zone calls at condensing temperatures. For the napkin math on the problem you're up against, read this bit o' bloggery. The fundamental problem is you don't have sufficient zone radiation or thermal mass in the system to run a low-mass combi boiler. A tank-type combi can work though.

    HTP's EFTC-199W combi would have the same problem, since it's min-fire output is about 5% higher than the NCB 240. I've never looked at Lochinvar's combis, but any low-mass combi has this issue, when sized big enough to serve more than one bath.

    Are you using the ductless heads for heating as well as cooling? If you're not running them for heat in the winter it would be easy to run a whole house heat load calculation based on wintertime propane use. Most 3000' houses of that vintage would come in under 40,000 BTU/hr @ 0F unless it has an unusual amount of window area or large air leakage/infiltration. Your 99% outside design temp in Boxboro is about +5-6F (like Framingham or Worcester).

    At the high cost of propane it may be rational to go with cold-climate "Hyper Heating" ductless (not -GExxNA) systems, even at 20 cents/kwh.

    With a better handle on your heat load we can estimate the water term requirements at design conditions, and how much burner it takes to heat the place. HTP's smallest Versa Hydro, the PHE130-55 (130K burner, 55 gallon tank) tank type combi would almost certainly cover it, provided you don't need water hotter than 160F to heat the place (which is probably the case.) The thermal mass of the tank and the 3:1 turn-down ratio of the burner makes it essentially short-cycle proof.

    But you're talking $6K just for the Versa, even before getting into the installation costs, and you're still burning pricey propane. If you're going to spend north of $10K for a solution it's worth running a room by room Manual-J load calculation (get that from an engineer or RESNET rater, not an HVAC contractor) and figuring out a reasonable cold climate heat pump solution. I've seen houses that size & vintage do just fine on 4 tons of ductless (for about $15K, before MassSave rebate incentives), but if going with a multi-zone ductless solution it's critical to have the accurate room load calculations to achieve the comfort & efficiency you're looking for, since the heads don't modulate with load (the way single-zone mini-splits do.) Heads that oversized for their room/zone loads can end up overheating/overcooling rooms even when "off", since they still have refrigerant going through the coils when other zones are running.
     
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  4. Mike Bernardo

    Mike Bernardo New Member

    Joined:
    Jan 6, 2019
    Location:
    Boxborough, MA
    Great info! The Versa seems like it's worth investigating.

    I'm baffled at why HVAC contractors even suggest installing low-mass combi boilers for whole-house heating?? It seems that any combi boiler sized big enough for domestic hot water is guaranteed to short-cycle a zone somewhere? Doubly baffling in my case since the same contractor installed both the boiler and radiators for the basement zone!

    Ok so the napkin math for baseboards roughly goes like this:

    at 180F water (600 BTUH for UF-2, 570BTUH for fin-tube):
    1st floor: 40,260
    MBR: 12,000
    2nd: 23,490
    Basement: 11,400

    at 120F water (228 BTUH UF-2, 200 BTUH fin-tube):
    1st: 14,834
    MBR: 4,560
    2nd: 8,312
    Basement: 4000

    My thought with trying to go to a constant lower temp vs hi/lo setbacks was the theory that the smaller zones would have a greater chance of running concurrent with the 1st floor and thus "piggyback" off of higher mass of that zone, reducing short-cycling. In this scenario, even at 120F avg water temp, as long as the 1st floor and one other zone operate simultaneously, we are over the min-fire output of the NCB 240. Does this logic make sense, or am I confused?

    I know the NCB 240 has a setting that sounds like it will reduce the overall burn rate. I'm referring to parameter "F" in the manual (Space heating max heat capacity, 50-100%) Is this an option to turn down to effectively make it a "smaller" boiler? There is a corresponding setting "J" for DHW.

    We DO use the minisplits for heating. They are on in all the bedrooms, resulting in almost no baseboard use in those rooms. This is why the MBR zone doesn't trigger the short-cycle problem in the same way that the basement does. (We actually wired the Nest thermostat in the MBR to use the minisplit and baseboard as primary/secondary heat source, respectively. It mostly works great, with the exception that the master bath stays quite cold in the winter)

    Which "hyper-heat" product were you thinking of? The model we have now does a great job extracting heat even down to the low teen outdoor temp.

    My concern with a ductless solution for the rest of the house is that we'd have to add a bunch more indoor units. As it is now, the sole 1st floor mini-split leaves some areas quite cold when used for heating due to the floor plan layout. The baseboards are actually laid out pretty well and the house feels most comfortable when using baseboards. It's not a big deal during transition months, but during the coldest months it can be a problem.

    I'm not sure the tank combi will work either since I'm skeptical of 160F water being able to heat the place. Having tested bumping the NC 240 down to 170F this past weekend, it seems to take a *long* time to get to temp. We have a lot of big windows.

    In any case, it seems like the next best step is to get the Manual-J analysis to really understand what the house needs and hopefully this will clarify the tradeoffs.

    When we installed the boiler a few years ago, we were paying $2.50/gallon for propane. Now it's about $3.50. Our electric rate is $0.12/KWH. How do I figure the fuel consumption of the combi?

    Thanks for your help!
     
  5. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    >I'm baffled at why HVAC contractors even suggest installing low-mass combi boilers for whole-house heating??

    Very few rocket scientists are wasting their careers installing boilers. It seems most boiler installers in MA never took hydronic design classes, learning most of the craft on the job using rules of thumb working on cast-iron boilers. They may be competent gas fitters & plumbers, but are really in over their heads on low-mass systems.

    Setting the limiter on the max burn rate of the NCB doesn't change it's minimum firing rate. It's probably never burning at it's 50% firing rate as currently configured.

    Running it at low temp will increase the amount of overlapping of zone calls, but even with ALL zones calling for heat it can barely emit the minimum firing rate at 120F AWT.

    You have enough radiation to emit 87,000 BTU/hr at an average water temp (AWT) of 180F, but that's probably more than twice your actual heat load. You can probably cover the entire load with an AWT of 130F, which would be on the edge of condensing. If you want to test the call overlap issues, set it for a fixed temp output of 135-140F, which may be enough to heat the house, and may have sufficiently low return water temp to hit 90% efficiency with a bit of condensing. It really depends on the ratio of the heat load per zone to the radiation per zone- if the smaller zones are over-radiated compared to the bigger zones the overlap can be pretty good, and you can run it on outdoor reset with very little short cycling. But if the main zones are over radiated for their zones compared to the smaller zones the short cycling could be worse.

    Using wintertime-only fuel use for the calculations the heating fuel use dominates the water use, and the water heating fuel use offsets the solar gain error. But it'll get you pretty close to the real heat load, even with those errors.

    The "FHxxNA" series single mini-splits or "xxx-NAHZ" multi-splits all have more capacity and higher efficiency at low temp, and have a fully specified capactity even at -13F. This is due to the vapor injection compressor technology that their other heat pumps don't have. Sized correctly they will hit their HSPF numbers. The HSPF is the BTUs per watt-hour delivers, so an HSPF of 10, is 10 BTUs/watt-hour, or 10,000 BTU/kilowatt hour. At 180F the propane burner is delivering about 87% efficiency, so out of every gallon you get 0.87 x 91600= ~ 80,000 BTU per gallon. So the output of 10kwh of an HSPF 10 ductless uses 8 kwh to deliver the same amount of heat as a gallon of propane. At 12 cents /kwh that's the equivalent of $1.20/gallon propane, about 1/3 the cost of $3.50/gallon propane. The FH09NA runs an HSPF of 13.5 (13,500 BTU/kwh) and can deliver almost 11,000 BTU/hr at your likely design temp. With 12 cent electricity it's equivalent to less than buck-a-gallon propane.

    What compressors are driving your GExx heads? Look up it's submittal sheet for the HSPF numbers, but it's probably considerably cheaper heat than the NCB.
     
    Last edited: Jan 8, 2019
  6. Mike Bernardo

    Mike Bernardo New Member

    Joined:
    Jan 6, 2019
    Location:
    Boxborough, MA
    By my math, all zones running at 120F water = 14,834 + 4,560 + 8,312 + 4,000 = 31,706 BTUH. Isn't this well over the 17000 min fire rate of the 240? Even 1st+basement = 18,834. What am I getting wrong?

    Thanks for that fuel use comparison. Seems likely that my current mini-splits will be cheaper than current propane rates. I'll have to dig up the spec sheets.
     
  7. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    OK, so with literally ALL zones calling for heat it's well above the min-fire output of the boiler. But combining the MBR and first floor is just barely more (which is what I was thinking, but didn't edit correctly.) The heat loss & solar gain characteristics of top floors & basements are pretty different from first floors- combining all into a single zone doesn't usually work, but combining the first floor might work, but the basement & upper floor zones would still cycle quite a bit at condensing temps.

    Just the model number of the multi-split compressor is enough. With two GE06 and a GE09 all on one compressor you're probably looking at the MXZ-3C24NA, or -3C24NA2 ith an HSPF of 9.8 (almost 10). They were "rated" 14,000 BTU/hr @ +17F which is the modulation level at which it's efficiency was tested. It's capable of 19,600 BTU/hr @ +17F at max capacity, but at lower efficiency. Looking at the capacity derating curve on page B39 of the engineering manual it's only delivering 40% of the nominal 25,000 BTU/hr heating BTU (=10,000 BTU/hr) when it's 3F outside.

    Even at it's lower efficiency at single-digit temps it will still deliver cheaper heat at +5F than $3.50 propane in a condensing burner, but 10,000 BTU/hr is not going to fully heat a 3000' house at +3F. It's efficiency at that temp running full speed is about half what it is when modulating at part load, figure 5000BTU/kwh, taking 16 kwh to be equivalent to a gallon of propane, making it more like $1.90-$2 /gallon propane. (Note the arithmetic error in my prior cost comparison: 12 cents/kwh x 8kwh - $0.96/gallon, not $1.20 gallon. I need more sleep or more coffee, maybe both. :))

    By contrast the 'Hyper Heating" -3C24NAHZ still has the "full" 25,000 BTU/hr nominal capacity @ +5F, which is at least a substantial fraction of your actual heat load at that temperature, and a slightly higher HSPF of 10.0.

    Either way any heat you're getting from the mini-splits is way cheaper than what you'd get with the NCB. Since the ductless heads sense the room temp by the temperature of the incoming air at the head there is an ever increasing offset between the stratified air temp at the head vs the average room temp. Turning the wall thermostat for the baseboards down and just bumping up the temps of the mini-splits when you're cold, ignoring the temperature number will get the most out of them. Even if it says 85F on the remote and it's only 7oF in the room when it's 20F or colder outside would not be an indication of something wrong with the ductless. As long as it's keeping up with your desired room temp it's fine, and cheaper than burning propane.
     
  8. Mike Bernardo

    Mike Bernardo New Member

    Joined:
    Jan 6, 2019
    Location:
    Boxborough, MA
    Thanks again for the info! Looks like I've got more homework to do on this newer ductless tech.

    You mentioned earlier that multi-zone heads don't modulate with load. Is that a characteristic of any multi-zone solution in general, or specific product line? I had thought that my current indoor heads *do* modulate, but maybe I'm just observing changes in fan speed?
     
  9. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Only the higher end and much larger commercial building multi-zone heat pumps have zone by zone variable refrigerant volume modulation. (Daikin is the real pioneer there.) On residential systems the compressor steps up/down depending on the size of the heads that are calling for refrigerant, but the heads themselves don't modulate when on a multi-split compressor. The do bump around a bit with the blower speed when set to "automatic", but the output is still in a fairly narrow band around it's nominal heating or cooling range.

    If the blower speed is forced to "low" the excess refrigerant gets shunted off to the other zones, even if they're nominally "off", and the system will even turn the blowers on in the "off" heads briefly to avoid loading up the input side of the compressor with liquid refrigerant.

    The GE06 is nominally 6000 BTU/hr cooling, 7200 BTU/hr heating, the GE09 is nominally 9000 BTU/hr cooling, 10,900 BTU/hr heating when married to a multi-split, but when on a single-zone compressor it has a range, with a max capacity higher than the nominal, and a minimum capacity much lower than nominal, which allows to run continuously at low blower speed modulating it's output in response to the actual immediate load. At part load the efficiency is quite high compared to cycling on/off at the nominal level, which is why it's more than 10% more efficient than when driven by a multi-split.

    The actual heating & cooling loads of the bedrooms are probably quite a bit lower than the nominal output of a GE06, let alone a GE09, but with the doors left open it can be supporting more load and deliver higher efficiency than when cycling on/off. But as the capacity of the 3B24NA or 3C24 compressors fall off the heads will of course be putting out less heat. (you can't get 10,900 BTU/hr + 2x 72oo BTU/hr out of a compressor that's only capable of about 10-11,000 BTU/hr when it's +5F outside. But it probably has enough to cover the full loads of those three rooms with a bit to sparer. (The heat load @ +5F of my master bedroom in my 1920s vintage bungalow is less than 3000 BTU/hr, the other bedrooms are in the 2000 BTU/hr range. Only the large upstairs "guest quarters" room with it's own bath would call for a GE09 at it's full nominal output, and would almost be covered by a GE06. )

    The Mitsubishi "FH" series single zone ductess with the better compressor types are pretty good, delivering their full "nominal" output all the way down to +5F and still have a specified output at -13F (-25C). Not to be outdone, Fujitsu's single zone mini-splits all have the cold-climate type compressors, but only those ending with "H" have the pan heater and deicing controls for extensive use during very cold weather. The "XLTH" (extra low temperature heating) versions of their multi-zone versions are comparable Mitsubishi's "-NAHZ" version multi-splits, with the right compressor design and de-icing capabilities.

    During cooler weather frost forms on the coils, and the unit will automatically turn around to air conditioning mode periodically to melt the ice off the coils. The bottom pan of the sheet metal is set up with a drain to let that water out, but when it's REALLY cold it can re-freeze in the pan, or plug the drain with ice. With your current multi-split you may need to check to ensure the drain on the outdoor unit's pan doesn't get plugged with ice if it's going to stay well below freezing for a couple of weeks. The cold climate versions have the controls for a pan heater (sometimes sold separately) to heat up the pan during defrost cycles to let the water drain away even when it's in negative double-digits outside, but yours doesn't. If the drain gets plugged up the ice can build up in the bottom pan high enough to interfere with the fan or even the coil. Most weeks there will be at least some thawing hours but if you see ice building up it's usually only a few minutes work with a 1500 watt hair dryer to clear the drain &/or pan, and it'll usually take several days of defrost cycles to create a problem, or even week if it's below 10F. (There usually isn't much water in that extra-cold air to create frost on the coil.)

    Fujitsu's mini-ducted mini-splits are worth a mention here. They're pretty efficient- more efficient than the 3C24, and have a more powerful blower motor than most of the competition. The 1.5 ton version puts out more than 18,000 BTU/hr even at -4F. If there is anywhere to run ducts inside the insulation & pressure boundary of the house (= not in a vented attic, above the insulation), that might be capable of heating/cooling your first floor zone (TBD) even if the rooms are doored off from one another. The installed cost is going to be higher than wall-coil types (due to the duct design & implementation), but there are whole houses being heated/cooled by these.

    This guy in Reading MA is planning on heating/cooling his second & third floors with a mini-ducted Fujitsu after he finishes air sealing & insulation projects currently in progress. He made this graphic plotting the output capacity of the -18RLFCD against his calculated (and since revised downward) load numbers across temperature. The green line is the capacity, the others are the loads for second & third floors:

    [​IMG]

    The only down side to that series is the lack a pan heater for clearing defrost ice, but in your/our climate that's still manageable, whereas in Green Bay or Winnipeg you probably won't want to be out there with a hair drier clearing ice for a half hour every week or two in -10F weather. :cool:
     
  10. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Does Littleton Electric Light & Water have a special rate for customers heating with electricity?

    It looks like in ZIP code 01719 MassSave will rebate ductless heat pumps to the tune of $1000/ton if displacing propane heat, $1600/ton if the ductless equipment appears on the NEEP cold climate heat pump list when displacing oil or propane.

    This is fatter than the rebates available a couple of years ago, when I got some friends of mine in Vineyard Haven into 4 tons of Fujitsu multi-split (for about $15K prior to incentives, which was less than what they'd been quoted for a propane furnace + split AC by a good $10K) !

    It's worth getting a third party room by room Manual-J for specifying the equipment. Between the savings and rebates you can probably take out a loan and it'll pay itself back in reduced net average monthly cash flow if sized correctly. You might even qualify for the seven year 0% MassSave loan if doing the whole house (as the folks in Vineyard Haven did.)

    Given your much lower than MA average electric rates and the subsidies there's really no point in going forward with investing anything in the Navien. Mothball it politely as backup should your heat pumps fail, but there's no point in using it- test it once or twice a year just to keep it running- you don't want the pumps or valves to seize up. For hot water you can get a plain old electric tank, or for more money a heat pump water heater (which also qualifies for rebate from MassSave, but read the fine print- it might not apply if you've been heating water with propane.)
     
  11. jac04

    jac04 Member

    Joined:
    Dec 11, 2013
    Location:
    Connecticut
    I have a few ideas to help you use what you've got. I have a Navien NCB180E. I think the regular NCB units have the same features, so ...

    You can put the unit into Error Checking Mode, select "FAN", and the unit will do a self-diagnostic on the fan and APS (air pressure sensor). Have both the fan and APS been replaced? If not, they are fairly inexpensive parts, easy to get, and very easy to replace by yourself.

    To help with the short-cycling, here are my thoughts:
    1) If the supply temp set point is 180F, then the default burner on temp is 175F (M=5) and the burner off temperature is 184F (L=4). M can be set anywhere between 5 and 54F. Try setting M=50. This means that the burner won't fire until the supply temp gets down to 130F (assuming the set point is 180F as you mention).

    2) The default Max Heat Capacity for Space Heating is 100%. This can be set as low as 50%. Try setting this lower, like 50%. This means that when the burner fires on a call for heat, it will be limited to 50% of its maximum firing rate.
    This will result in longer burn times when used with 1) above

    3) If you can, open all of your zone valves so that all 3 zones are open no matter what zone(s) is calling for heat. I have 2 zones, and Zone 2 is very small. I have found that keeping Zone 2 open all the time works great (for me). I set the Z2 t-stat for 58F, but keep the zone valve open all the time, and it keeps the room temperature where I want it.
     
  12. Mike Bernardo

    Mike Bernardo New Member

    Joined:
    Jan 6, 2019
    Location:
    Boxborough, MA
    @Dana : Thanks for the heat pump tips and explanation. The modulation behavior makes sense now and I can see why it's important to size these correctly for the room.

    Good tips on the rebates. Unfortunately they're only valid for customers of the bigger utility companies. Our little municipal utility tends to never be on the list. I'll have to check with them directly to see if there are any incentives.

    I've got some calls out to some RESNET-affiliated raters for a Manual-J and you've sold me on getting that before committing to any new equipment. I'd welcome recommendations if anyone has experience with someone they like in MA.

    I think with the addition of 2 more indoor ductless heads we might be good to rely on that 100%. What's the best practice for set points on these? Should they just be set to constant temp and run *continuously*? I guess old habits of setting down thermostats dies hard. (Also my investment in Nests throughout the house is not looking like it was a wise decision. :()

    Another remaining piece will be to figure out how to heat the cold tile floor in the baths. (Master bath has subfloor access from the basement, so I'm thinking maybe some kind of retrofit radiant might work)

    Assuming the Navien can still hobble along, any reason not to continue using it for DHW? Also, I was thinking maybe a good temporary hack I could put in place until it kicks the bucket would be to install a buffer tank for the smaller heat zones. (If I switch to 100% ductless and the baseboards truly become "emergency" use only, then I'll grant this will be moot.)

    @jac04: Yep contractors replaced the fan recently. Next step was to replace the APS and entire wiring harness. I won't tell you what the they wanted to charge me for that. I ordered a new APS myself out of warranty and just replaced it yesterday. Super simple. So far so good - no codes yet. Annoying that Navien tech support *really* doesn't want to let homeowners touch these things.

    Thanks for the settings tips! Yep I've got all those same parameters available. Settings M and N finally make sense to me now! I didn't realize they were a delta from the set point. I will have to play with these and see what happens.

    Unfortunately I can't easily play the same trick with the zone valves. My rig was set up with individual pumps for each zone. Maybe there's a way to rig the thermostat relay do achieve the same thing...
     
  13. jac04

    jac04 Member

    Joined:
    Dec 11, 2013
    Location:
    Connecticut
    The Navien manual isn't very clear when it comes to defining some of the parameters, that's for sure.

    As far a Tech Support, try calling early in the morning. That way you will be sure to get someone in New Jersey and not California. The NJ people seem to be more knowledgeable. I have found homeowner support to be hit -or-miss. Sometimes the Techs will talk to you. If they ask if I'm the homeowner, I always tell them that I'm an Engineer trouble-shooting the unit for the homeowner. Not really telling a lie, I just happen to be an Engineer, and am working on the unit for myself.

    If you don't have the Service Manual, it has a bunch of good info in it. Go here to download it:
    https://www.rogerssupply.com/ASSETS/DOCUMENTS/CMS/EN/Navien NCB-E Service Manual (v1.4).pdf

    Also, you asked about being able to set the max firing rate "J" for DHW . Yes, "J" is the DHW max firing rate and it can be set between 50 - 100%.
     
    Last edited: Jan 11, 2019
  14. Dana

    Dana In the trades

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    Jan 14, 2009
    Location:
    01609
    I punched your ZIP code into the MassSave, and it did look like you're covered, but check with MassSave first (your local utility isn't necessarily in the information or financial loop here). The bigger utilities still sponsor this sort of thing, since they're under the gun to reduce carbon emissions, and converting from propane to heat pump buys them some of that, even when they're not your local utility.

    Minisplits (even multi-zone) run more efficiently with "set and forget" approach to the temperature setting. When it's in the 30s outdoors, modulating at part load they can be literally twice as efficient as when running full tilt, which more than erases any savings from having the house at a lower temp during setback. (See figure 5, p10, p18 in PDF for the COP at different modulation levels and temperatures.) That difference shrinks when it's below 20F, but it's still there. Your mean wintertime time is about 30F, maybe 25F in January (or at least most Januarys- it's been warmer so far this year.)

    As the outdoor temperatures change the temperature number you set it to may not reflect the average indoor temperature, since the heads sense the room temp by the intake air, which is at the top of the head. With any sort of room stratification it'll be off at least a few degrees, when it's really cold or really hot outdoors. There are wired and wireless remotes available for most Mitsubishi & Fujitsu heads if that bothers you. Most people just bump the temp up when they're cold, down when they're too warm, and learn to ignore the number. As long as it's not more than a couple of degrees per bump it doesn't change the average efficiency much, but even a 5F change can make a measureable difference with a fully modulating single zone mini-split. (The effect is less pronounced with multi-zone, but still measurable.)
     
  15. Mike Bernardo

    Mike Bernardo New Member

    Joined:
    Jan 6, 2019
    Location:
    Boxborough, MA
  16. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    You're right, of course.

    Even without rebate incentives the internal rate of return on this investment would be extremely high, with a net present value in positive territory in fairly short years at any discount rate. (If my 401K were anywhere near that good that I would have retired decades ago! :) )
     
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