Navien Tankless Water Heater Comments and questions

Discussion in 'Tankless Water Heater Forum' started by willl, Dec 28, 2008.

  1. JimmyTony

    JimmyTony New Member

    Messages:
    4
    Location:
    Bend, Oregon
    New Construction

    Hey Guys,
    Im getting ready to build a new house and was looking at in-floor heat using a tankless combined with a small tank to act as a "shock absorber" to keep the tankless, from cycling on all the time. In reading this thread I've come across some great posts by Dana and some others about this kind of system. I'm not a plumber, and could use some help getting a good mix of cost and efficiency in putting a system together. Any thoughts would be appreciated, Thanks!
  2. Dana

    Dana In the trades

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    2,785
    Location:
    01609
    This really should have been a new thread, eh? (It has nothing to do with Navien, other than that Navien doesn't void the warranty when their units are used for space heating.)

    Every reasonable heating system design starts with a good heat-loss estimate/calculation.

    If you build the radiant in to a concrete slab, it doesn't need a buffer tank.

    There are cheap ways of doing radiant with a tankless, but if your design condition water temp requirements are low enough, there may be better options.

    In climates as mild as Bend, for new construction, you have the option of spending the money on high efficiency building envelope rather than a high heating system. If you target design condition heat loads to be <<20KBTU/hr (not tough to hit for 2000-2800' house in Bend's climate, but it has to be designed), at which point you can have your cake and eat it too, since you can then heat & cool the place with a highly efficient ductless mini-split. (Even if it's not as cushy as radiant floors...) If you're not on a gas main and would be otherwise be using propane, it would be practically INSANE not to use an R410A refrigerant heat pump for primary heating in that climate. (If propane-fired radiant, even R410A air-to-water air source heat pumps start looking pretty good in terms of 10 year NPV compared to condensing propane, even though they're far more expensive than mini-splits.)

    Consider building to Zone-5 specs in the table 0.2 on p10 in this document, for starters:

    http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

    Note that those R values are not center-cavity, but rather "whole wall" R with thermal bridging of the framing included. An R30 wall isn't a 2x10 studwall with R30 batts- when the thermal bridging of all framing is factored in that would come out at ~ R21-R22 for homes with "typical" or "average" framing fractions. But a 2x4" wall with R13 batts or spray cellulose + 3.25" of exterior polyisocyanurate rigid-board is ~R30, with typical framing fractions.

    Air infiltration is a LARGE factor in total heat load- designing a continuous primary air barrier on all 6 sides of the cube and having a Konstruction-Kommandant to enforce air sealing is critical, as is blower-door testing & remediation on the main shell as soon as you have the windows & doors installed. Typical pretty-good construction comes in at ~ 10 air changes per hour @ 50 pascals pressure (ACH/50), the standard leakage test. The IBC 2009 standard specs out 7 ACH/50 or less, which is usually achievable as a post-construction (read "post test-failure") retrofit. But to be very efficient you need to be under 3 ACH/50 and under 1.5 is better, and relatively easy to hit, if you have a plan and execute on it.

    Air sealing is by far the most cost-effective envelope performance upgrade you can do- a well insulated wind tunnel is a waste. Put a bead of caulk or acoustic sealant under & between stud-wall plates, foam seal & gasket foundation sills, caulk every sheet of structural sheathing to the studs, etc etc. It's cheap & quick, but it has to be consistent. On upper floor ceilings use OSB or ply on the underside of the joists/truss-chords (you'll need it to hold up the 20" of cellulose without bowing), and detail it similarly as an air barrier. Don't mess around with stuffing fiberglass in around window framing either- use the appropriate compliant foams.

    Only use insulated doors. Don't use sliding doors- they all leak like crazy with age (some even when new.) Swinging patio doors/french doors can be made to seal better. (But see notes about minimizing glazed area.)

    Minimize the total glazed area except where passive solar gains have been site-simulated and optimized. Every square foot of U-0.34 pretty-good window is an R3 hole in your R30 wall, with literally 10x the heat loss per square foot. Size & locate them for daylighting & egress needs.

    Use fixed (non-opening) windows where you don't absolutely need to open a window- they leak LOT less air. Where they must open, use casement & awning types, since they leak less air than double-hungs & sliders, and they give more egress & ventilation cross-section per square foot of glazing too.

    Avoid recessed lights, particularly those that would penetrate into attic or cathedral-ceilings. Even IC rated air-tight versions aren't usually all that air-tight, and make thin spots in the insulation.

    If taking the foam-clad framed building approach, a LOT of money can be saved by using reclaimed roofing insulation from commercial re-roofing jobs. An overcoat of R18-R24 iso or eps comes in at well under $5K for most reasonable-sized houses, which roughly triples the whole-wall R-value of a 2x4 fiber-insulated wall, and more than doubles that of a 2x6 wall. (If virgin stock it could easily hit $12K+.) Going to an air-tight R30 with glazing reduced to under 15% of floor area (as opposed to the ~18% new construction average) can cut the heat load of a house down to 1/3 or less of a typical-leakage typical glazing fraction code-min house, without having to live in dank darkness.

    To see what foam clad timber frame looks like, check out the retrofit the section titled "An architect works on his own house":

    http://www.greenbuildingadvisor.com/blogs/dept/musings/visiting-energy-smart-designers-and-builders-maine

    (Note the 1-part expanding foam in the pictures that seals the seams of his 6" of reclaimed iso board.)

    Other foam-cladding retrofits can be seen here: http://thousandhomechallenge.com/case-studies

    If you ARE on the natural gas grid, running low-temp radiant with a condensing water is roughly comparable to heating with a mini-split for a low-heat load house, but you pay quite a bit up front for that extra-cushy warmth underfoot. There's no payback on it, other than the "aahhhhhh" factor when it's 10F outside.
  3. JimmyTony

    JimmyTony New Member

    Messages:
    4
    Location:
    Bend, Oregon
    Thanks for the reply Dana, and the info.
    I should provide more info. Radiant appeals to me because I can get it done in a slab for cheaper than a crawl space with a finished floor on it plus a separate heating system like a conventional forced air system. I know thats not the case for everyone, but it is for me. The ductless split is intrigueing but we have pets and allergy sensitive kids, so I was trying to take out the air movement from the equation. (dander and dust). Also, an aquantanace who had a split system installed says the moving air always feels a bit cold. Could be just her I guess. Also, I wanted to use the tankless for houshold water as well, so I thought thats where a buffer tank would come into play. i.e. prevent hand washing from forcing the tankless to kick on every time, not to mention the cold water sandwiches. As I say, I am not a plumber, but I'm trying to educate myself so I can make the best choice.
    Yes, an air tight envelope is part of the plan, likely a low density foam/ cellulose hybrid for starters, with major caulking, so I feel pretty good about starting with as reduced a demand as possible.
    We do not have the option of ntural gas, so propane or electricity are my options. I worry about the volatility of propane prices more than electricity prices in the NW, Propane is about $2.40/ gal and electricity is about $.10 /kwh which is about a wash before comparing efficiencies, but I also don't know if efficiency cclaims can be compared between propane units and electrical, and if they can be believed, especially heat pumps. I guess I've started to question all the assumptions I'd made while planning the house. I suppose thats good because I'm not locked into one idea/ solution/. But I need to come to a decision here pretty soon, and got to this site after my plumber laughed at my idea of doing infloor heat with electricity. He suggested propane and swears by Navien tankless. Thats the system he used with his in-floor system in the same neighborhood. What I've read here so far makes me lean more towards a Noritz or Rinnei, but obviously I'd have to committ to propane. The one thing I know for sure is that I don't know enough, so thanks again for the info and if you see a good heating solution somewhere in all this let me know.
  4. Dana

    Dana In the trades

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    Location:
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    With $2.40 propane as your fossil fuel and 10 cent electricity your heating cost will be at least 2.5x that of doing it with heat pumps.

    At $2.40 per 91000 BTUs and a 90% average burner efficiency that works out to about 10 cents/kwh, delivered- heat, but with a heat pump you'd most likely get a COP of 2.5 in winter, better than that in the shoulder seasons.

    With a continously variable interior unit the air movement issue is very slight (some use 2-speed AC motors, but the better Mitsubishis all use variable DC) and you can bump up the temps 2-3F for higher comfort and still be ahead. Placement of the interior units where they won't be blowing directly on you (at any speed) is also an important comfort factor.

    Ecotope (a consulting company in Seattle) has an ongoing study of primarily retrofit heat pumps all over the PNW commissioned by the BPA, some of which is available on online if you want to seek it out. You'll get a COP > 2.5 in Bend if you size it right, and at least 2 even if you don't.

    There's no particular cost-advantage to going with a propane tankless with a separate tank for HW compared to a condensing boiler + indirect. With a radiant slab and an even modestly high-R house your heating water temps on design day will never exceed 100F, and you'd need 120F+ for the tank.

    Design the house for the minimum heat load, THEN decide what mechanicals make the most sense. But there are 2-ton mini-splits heating high-R homes in much cooler climates than Bend, that cost less up front than a propane tankless + tank + radiant floor.

    With a Daikin Altherma air-to-hydronic heat pump and a low-temp slab you'd probably average a COP of ~3 in Bend, maybe even a bit more, but it'll be at least 2x the upfront cost of a mini-split.
  5. JimmyTony

    JimmyTony New Member

    Messages:
    4
    Location:
    Bend, Oregon
    You mention combining a boiler and an indirect. Whats an indirect?
  6. Dana

    Dana In the trades

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    2,785
    Location:
    01609
    [​IMG]

    The coil inside the tank contains heating-system water, not mixing with the potable hot water. The indirect is usually operated as a separate heating zone, often a "priority" zone, inhibiting other heating zone calls until the hot water tank's call for heat is satisfied (that way you get 100% of the boiler's output applied to the hot water, much like a tankless.)
  7. JimmyTony

    JimmyTony New Member

    Messages:
    4
    Location:
    Bend, Oregon
    Huh! Can you hook it up to a tankless (cheaper) instead of a boiler and have the tankless directly heat the floor, and the indirect supply the potable side? I don't know the efficiency of indirects but tmaybe this could be an economical choice as far as up front costs and monthly costs.
  8. Dana

    Dana In the trades

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    2,785
    Location:
    01609
    In most states it's not legal (and not a great idea where it is) to run potable water through your heating system plumbing, especially at the volumes you have with a radiant slab, which is why you'd need the indirect. Tankless heaters aren't inherently set up and controlled for zoned space heating, and by the time you've monkeyed around engineering your way around it it's not clear there are any savings to be had. If the radiation water temp requirements are essentially the same as domestic hot water temps there are sometimes shortcuts, but with radiant slabs + domestic hot water you're talking dual-temp system. Condensing boilers can tweak double-digit savings out of higher single-temp solutions by using outdoor-reset curves to vary the boiler temp with heating load, resulting in more condensing hours. Navien and Rinnai both make combi-systems for both space heating & DHW with outdoor reset built in, but they're not particularly cheap either.

    And again, at Bend's average winter temps you'd pay less than half as much on space heating if you went with with an R410A refrigerant split-system heat pump solution, no matter HOW good your condensing propane system might be. From a strictly financial point of view you're far better off spending the money on insulation in a new-build not radiant heating, and heating with a (relatively)low cost but high-efficiency heat pump.
  9. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    I always plumb the radiant as a full time inlet to the water heater. Whats the beef? extra warm incoming water to the heater in the winter and removing heat from the slab in the summer. Seems like the best of both worlds.
  10. Dana

    Dana In the trades

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    2,785
    Location:
    01609
    Other than the fact that it doesn't meet code in most places?

    Using potable in the radiation is more corrosive to the heating system components (requires bronze pumps, etc.)

    Any zoned systems may have days/weeks/months of stagnation at tepid temps high enough to promote to potential human pathothens (protozoan & bacterial), which is the primary rationale for codes barring "open" systems. In MA open systems are allowed only if controlled in such a way to guarantee a specified minimum amount of circulation PER HOUR occurs whether the heating system is operating or not. In a tankless system recirculation this would usually cause an ignition cycle & burn, whether it's 95F outdoors or not.
  11. cslee

    cslee New Member

    Messages:
    1
    Location:
    irvine, ca
    Hi all,
    I have some questions of the external recirculating of NR240-A. I have a return line and I've set DIP switch 4/5 to OFF/ON and make sure the 3-way return valve is in the horizontal position. I set the timer to run about 16 hours. Is that all correct?

    I dont feel the return pipe getting hot.

    Another question, since I have the timer running 16hrs. What do I do if I'm on vacation? The tech told me to press the "POWER" button. I thought the "POWER" button on the remote is just for the remote keypad. Does it turn off the heater?

    After a power failure, beside setting the clock, what do I need to do to ensure the heater is on and running?

    Did anyone use a computer UPS with this heater, so heater will fire up during power outage?

    regards,
    C Lee
  12. Surfing Plumber

    Surfing Plumber New Member

    Messages:
    10
    Location:
    Orange County, CA
    Warning: Stay away from Navien!!!

    I installed a good amount of Navien in my life time and I gotta say, 80% call backs from my customers. It is frustrating for both the homeowners and I as it takes time away from my other jobs!!

    There is always something wrong with these Navien, if it's not the flow sensor, it is the mother board.. I am tired of them...Stay away. go for other reputable brands like Noritz or Rinnai
  13. techsavy

    techsavy New Member

    Messages:
    15
    Location:
    ontario
    Hmm...4% difference in efficiency eh?



    Hi nhmaster,

    You claim that tankless water heaters are only 4% more efficient compared with tank type water heaters. Could you provide us your source? From what I understand, even the best insulated tank water heaters have efficiency ratings in the 67% energy factor range. Now add condensing to the tank and you are still below 80%. If you believe this to be wrong information, perhaps you should visit any tank water heater manufacturer's website and look at their efficinecy ratings.

    By the way, a condensing direct vent tank water heater costs more than a condensing direct vent tankless water heater. You said you are in the business, so you should know this. You should also know that nearly every tank manufacture today has gone out and secured a tankless OEM line for themselves. If the technology was not proven, they would not waste their time, energy, or efforts with tankless technology.
  14. techsavy

    techsavy New Member

    Messages:
    15
    Location:
    ontario

    I do not believe that you Paloma unit has a dual flame burner. As of the time of this email, Noritz is the only water heater with a Dual flame burner. They also have an eco burner on their new line of products. By the way, Rinnai is also up there in terms of quality, but I would have to say Noritz is the best of all tankless water heaters. They give you products specific to your needs, so you can definitely find one that suits your needs at a good price.
  15. techsavy

    techsavy New Member

    Messages:
    15
    Location:
    ontario
    I am suprised that you prefer Rinnai over Noritz. If you want to speak about ease of installation, the Noritz concentric vent products are easier to install than Rinnai. No cutting of venting required and no gluing required unlike the Rinnai concentric. Also don't fortget the 25% thicker heat exchanger that will increase product life by 25% compared to Rinnai. We haven't even started on the dual flame burner that helps out with heat exchanger longevity due to more evenly dispersed heat intensity in the combustion chamber.

    I can't help it; I just love the Noritz product line and I am sure that based on my previous posts, you are probably aware that I am pro Noritz. This is a company that has the best of both worlds. Concentric venting for our non-condensing units and PVC636 venting for our condensing units. No matter what your application, you definitely have a product that will work for you. By the way, Noritz went with PVC 636 venting for their condensing units so that you don't have to spend so much more on venting if you have to snorkel due to inadequate clearance of the vent termination to grade.
  16. techsavy

    techsavy New Member

    Messages:
    15
    Location:
    ontario

    By the way, the Center for Energy and Efficinecy has a report out that states that none condensing tankless water heaters performed at highere efficiencies under low flow conditions than Navien condensing tankless water heaters with Buffer tanks. Keep in mind that none condesning units are rated in the mid 80% efficiency range.
  17. techsavy

    techsavy New Member

    Messages:
    15
    Location:
    ontario

    Agreed that temperature control and flow rate are both more important. But which tankless provides both features better than any other manufacturer does? according to the Center for Energy and the Environment, Noritz wins on both counts. Don't get me wrong, Rinnai is a very good unit, but I still like Noritz better. Navien has made some improvements to their water heaters over time, but they are still nowhere near the Japanese manufactures such as Rinnai, Takagi (now AO Smith), or Noritz. As for me, I will stick with Noritz any day.
  18. techsavy

    techsavy New Member

    Messages:
    15
    Location:
    ontario

    I have not personally done any tests, but it looks to me that the efficiency depends on temperature set-point as much as it does on length of usage and firing rate. You would have to look at the effects of temperature set point and of firing rate vs. heat exchanger longevity to identify the optimum usage factors. This of course is not easy to determine. I would immagine that it is not advisable to operate a water heater at the low firing rates for too long or to operate at maximum fire for too long either. Somewhere above 50% burner burn should in my openion be a target range, since all burners will be activated and hence the heat distribution in the combustion chamber would be better thermal control of the heat exchanger pipe coils.
  19. techsavy

    techsavy New Member

    Messages:
    15
    Location:
    ontario

    Where do the others rank in your book? Who is at the top of the crop?
  20. techsavy

    techsavy New Member

    Messages:
    15
    Location:
    ontario

    Actually, you are not very accurate here. Yes, tankless tests are all done in the same manner, which means that Navian's A models are tested in the same fashion as the competitor's brands without a buffer tank. It also means that the Navien A model is tested without the pump in operation, so the advertised efficiency is actually quite a far cry from what it actually delivers in real life tests. You should take a look at the study done at the Centre for Energy and the Environment. According to that study, at below 100000 BTUH per day usage, the Navian A models are actually worse than non-condensing models which are rated with efficiencies in the low to mid 80% range.
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