Navien tankless

[Tankless Adviser]

Yeah, I too get a bit of a laugh out of the "98% thermal efficiency" numbers for Naviens, in much the same way that I do with the overzealous mod-con boiler advocates.

An on-demand hot -water heater almost BY DEFINITION short-cycles on most of it's burns. Anything less than 5 gallons would be an efficiency-robbing short-cycle for tankless, condensing or otherwise, and even though the bulk of the water use is indeed bathing/shower draws greater than 5 gallons, the numbers of short-cycles, the fixed-losses per burn (flue purges & ignition sequences) add up- they aren't anything LIKE 98% efficient, or even 90% in real-world situations, no matter what their EF number is. (An EF test takes 10+ gallons at every draw, enough to signficantly inflate the performance of a condensing tankless.)

What determines the real as-used efficiency is how well the unit manages the fixed losses on short-draws of 2 gallons or less, not whether it's condensing/non-condensing. Depending on real world use patterns and the fraction of large/small volume draws, a non-condensing tankless will run anywhere from 75-80% efficiency, whereas a condensing unit might run 75-85%, no more.

Used as a condensing space-heating boiler a condensing tankless can indeed hit 90%+, but only if the return water entering the tankless is below 110F. In order to hit 98% the return water would have to be under 70F. While not-too-likely in a heating system, 98% efficiency possible for high volume hot-water draws, since the water from street is typically well below 70F. But the average efficiency as a hot-water heater will always be considerably lower than it's steady-state thermal efficiency.

Tankless HW heaters see a huge number of ignition & flue-purge cycles, robbing efficiency as well as wearing out some of the sub-systems. Even a small well-insulated buffer tank can cut the number cycles by more than half and increase the overall efficiency (while getting rid of the "cold-water sandwich" issue) for the efficiency cost of a small standby loss (less than 1%). From a fuel-saving point of view it's not always cost-effective for just water-heating, but in a combi space-heating/DHW system it can be. (Systems using a reverse-indirect like a ThermoMax or ErgoMax as a heating system buffer while acting as a DHW heat exchanger tends to work well for homes with low/moderate design-day heat loads of 25-75KBTU/hour. But if it's an already high mass low temp radiant-slab heating system where lower than 110F temp heating water is typical the reverse-indirect-as-buffer approach is less than ideal.)

The best selling point for a condensing tankless is when it can use a cheap PVC vent stack instead of stainless/Z-vent for the standard-efficiency model. In installations with long vent runs the installed cost of a condensing tankless + PVC vent sometimes works out to about the same or less than a standard-efficiency unit + Z-vent. In those cases taking the (very modest) efficiency savings offered by the condensing unit seems fair.

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.

[Tankless Adviser]

JM, a few thoughts to ponder:

How many Rinnai's do you install per year? Does not the Rinnai have better water temperature control and higher flow rates? Doesn't it cost a couple hundred bucks less and have better tech support?

For efficeincy, if you have a car that gets 30mpg and one that gets 35mpg, and they are both in the driveway "OFF" which one is getting better milage?

Now I get the PVC vent thing, however if the venting is up and out the Rinnai is still less expensive with an easy push joint vent kit and a much better hot water source. These things only run for minutes a day, the rest of the time they are off. I think 90+ is over rated for tankless. Seems to me that tgemperature control and flowrate matter more.

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.

[Tankless Adviser]

80+ is (quite literally) overrated for most tankless in most installations due to the particulars of the EF test: 10.2 gallon minimum draws are nothing like real-world use, and the short-cycle losses for sub-2 gallon draws erodes efficiency dramatically. In real-world draw profiles 0.82EF type tankless units hit in the mid-70s, condensing versions scoring 0.90+ EF numbers run in the high-70s/low 80s. The higher thermal efficiency isn't worth (much) extra for. (Indeed, you'd get better return in fuel savings on drainwater heat recovery on the main shower for the price-delta on the high/mid efficiency units themselves, vent costs not included.)

Steady-state thermal efficiency is a theoretical upper limit, but not a good measure of the real-world performance. Those with tight controls over flue purge cycles losses will do better than those that don't. Those with small header tanks to mitigate cold water sandwich issues that also inhibit fire when the header is above a minimum temp will also suffer fewer cycles, and thus lower purge-losses from multiple low-volume draws, etc. Steady state thermal efficiency or raw combustion-efficiency numbers are only relevant for very large or continuous-draw duty (pool heating, anyone?), not domestic hot water for typical household use. In space heating, commercial laundry/car-wash, health-club showers, or home solar-backup you might beat 90% with a condensing tankless, but not very often (if ever) in straight-ahead DHW apps.


BTW: A a question for any of you who have taken (or will take) the tech courses from various tankless vendors: Is there a modulation level where they typically peak out on raw combustion/thermal efficiency? Do any vendors share any of that data?

Condensing versions likely peak at lower fire in 25-35% of full-fire range (like most mod-con boilers), but I suspect the single heat exchanger types may do better at the high range (like copper-tube boilers), and may be under 80% for raw combustion efficiency at the low end of the modulation range, but 85%+ in the mid/high range. I could as-easily believe the designs have been tweaked to provide peak efficiency at some other burn level, but haven't been able to find test data (which I'm sure is quite proprietary when tested by the manufacturers). Can anybody either confirm/disabuse me of those guesstimates?

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.

[Tankless Adviser]

I'm going to add to this topic again. when they first hit our local central california coast market, I liked them but the problem of low flow cut out became a problem and changing boards became a regular task for us. What did become a problem was the issue remained after the 1 year labor ran out and Navien would not honor replacement of known problems with these by paying to have these replaced.
Considering the near non response of the sales distributor and the full mailbox of tech support on weekends, I wouldn't recommend this line for any reason. The recirculation pump feature in the A models can not be set up with a remote on demand feature and the pump has limited head capability. I challenged the sales rep how 98% efficiency is achieved with temps at 120 or lower at low flows, he couldn't answer that and has quite touting this at trainings.
We've been installing tankless for over 30 years and have seen our share of what works and what does not. Navien has failed to realize service makes or breaks a company and in my book they lay near at the bottom with Bosch.

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

[Tankless Adviser]

Tankless testing (all brands) follow a standard testing procedure for determination of combustion efficiency and EF.
10 Code of Federal Regulations Section 430.23.
In a nutshell its a series of draws at different flow rates and gallons.

So if Navien is misstating efficiency so is all others.

I wont argue about what it takes to get to it, yes it involves cooler inlet water, but so do the 80%ers to achieve 82-84%.

Whats the big deal with changing a part, make it part of the charged annual service

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.

[Tankless Adviser]

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.

This is actually not the way to go about this. Your tankless is not a boiler and therefore cannot be used in the manner described. Your tankless must heat domestic water directly. You can add a heat exchanger to heat the space indirectly, but keep in mind that there are rules guiding how much space heating can be done using a water heater. see the following link http://www.noritz.com/u/plumbing_dia...83_dhw_rad.pdf

[MEE]

in the spring i helped remove an old corroded boiler from a house with about 12 bedrooms and 4 bathrooms. the boiler and 2 water heaters supplied DHW and radiant heat. we
replaced it with a Takagi tankless to run the DHW for the summer months with the intentions of plumbing it for radiant heat before winter. Well, the home owner found a Navien
CH240 to do the job instead. I have to say that i liked the Takagi. It installed easy and did provide almost instant hot water, and was simple to operate. We are however
experiencing problems with the Navien. It keeps giving us error codes for low pressure just running one sink for less than 2 minutes, and we can't adjust for elevation with a dip
switch like the Takagi. I have read about 2 years worth of posts on this subject and frankly i have some mixed emotions on the whole tankless concept. Anyone have any
suggestions?

[Dana]

Tankless Adviser: Firing rate is only a secondary factor on the raw combustion efficiency, and setpoint temp is third-order factor for tankless units. Incoming water temp is primary, and it's ALWAYS going to be well into the condensing range in DHW mode, but can be much warmer/less efficient in space heating applications with high return-water temps.

In DHW mode whether it's at min-mod or full-fire a condensing tankless will always be north of 95% steady-state, whether the output temp is set to 40C or if it's set to 60C. There will always be condensing going on in part of the heat exchanger that contains water under 50C. But flue purges on short-draws eat into efficiency considerably, since every purge extracts the same amount of heat out of the the heat exchanger whether you just took a 20 minute shower, or whether you just rinsed your hands. On the former that loss is a negligible fraction, on the latter it's a good chunk of the total burn. The US DOE EF test over-rates tankless units relative to tanks, since the use profile is all long draws.

I'm curious about the test data on the mini-buffered Naviens- is that Centre for Energy and the Environment available online?

[Tankless Adviser]

Tankless Adviser: Firing rate is only a secondary factor on the raw combustion efficiency, and setpoint temp is third-order factor for tankless units. Incoming water temp is primary, and it's ALWAYS going to be well into the condensing range in DHW mode, but can be much warmer/less efficient in space heating applications with high return-water temps.

In DHW mode whether it's at min-mod or full-fire a condensing tankless will always be north of 95% steady-state, whether the output temp is set to 40C or if it's set to 60C. There will always be condensing going on in part of the heat exchanger that contains water under 50C. But flue purges on short-draws eat into efficiency considerably, since every purge extracts the same amount of heat out of the the heat exchanger whether you just took a 20 minute shower, or whether you just rinsed your hands. On the former that loss is a negligible fraction, on the latter it's a good chunk of the total burn. The US DOE EF test over-rates tankless units relative to tanks, since the use profile is all long draws.

I'm curious about the test data on the mini-buffered Naviens- is that Centre for Energy and the Environment available online?

Set-point can play a very important role based on the design of a tankless water heater. Some tankless water heater manufacturers are known to use bypass valves (controlled or fixed) to assist with heat exchanger protection and temperature stability. If the bypass is controlled, setpoint will determine if the bypass is partially opened, fully open, or fully closed. If bypass remains fully closed, then set-point as you say becomes less relevant.

- Agree on efficiency drops on space heating mode due to high return water temperature being too close to or depending on application above dew point.
-Problem with the Navien tankless water heating units with buffer tanks is that the bufffer tank's recirculation pump when activated mixes heated water with cold water and increases inlet water temperature. I have not looked at the pump curve in the units, but since tankless water heater manufacturers typically require 2 GPM flow through the heat exchanger in recirculation mode, I can't imagine that they would recirculate the buffer tank with anything less. Here again, the set-point will affect the efficiency of the unit as the higher the setpoint, the higher the temperature at which water in the buffer tank will be recirculated into incoming cold water line and hence into the water heater. If Navien used Takagi as a reference point, then the buffer tank will always be recirculated on hot water demand, and as such, the inlet water temperature will always be greater than the city supply. I can not confirm this, as I have not done any tests on Navien units, but I am very familiar with Takagi.
- A problem with the Navien CH combi boilers, is that it is a boiler first and a water heater second. This means that it essentially operates as a boiler and hence the domestic hot water does not flow through the heat exchanger. As a result, even in domestic hot water mode, the system will be hard pressed to condense. This is partially why the Navien combi-boiler has lost its energy star rating (visit Navien's website to find out more about their energy star rating for combi boilers). Here again, setpoint will be a factor, as the return water temperature on the boiler side will be influenced based on closest approach temperature of the flat plate heat exchanger employed for domestic hot water production.

[Tankless Adviser]

in the spring i helped remove an old corroded boiler from a house with about 12 bedrooms and 4 bathrooms. the boiler and 2 water heaters supplied DHW and radiant heat. we
replaced it with a Takagi tankless to run the DHW for the summer months with the intentions of plumbing it for radiant heat before winter. Well, the home owner found a Navien
CH240 to do the job instead. I have to say that i liked the Takagi. It installed easy and did provide almost instant hot water, and was simple to operate. We are however
experiencing problems with the Navien. It keeps giving us error codes for low pressure just running one sink for less than 2 minutes, and we can't adjust for elevation with a dip
switch like the Takagi. I have read about 2 years worth of posts on this subject and frankly i have some mixed emotions on the whole tankless concept. Anyone have any
suggestions?

If you followed the installation diagrams published in the Navian product installation manual, then you could be up for some challenges right off the back. Notice that the pump on the Navien CH combi-boiler is on the boiler loop outlet inside the unit. Notice also that they recommend placement of the expansion tank at the boiler outlet, which means that the pump will theoretically not be able to increase pressure above static pressure. While this is only one problem with the diagrams, there are more, so I suggest getting your hands on a proper diagram and use that as your guide.

[Dana]

Bypass open or closed, the setpoint doesn't affect the steady-state (or as-used, intermittent) efficiency of the unit (even though it may be important for temperature stability.)

The mini-buffered units only suffer a standby loss during their programmed time-of-day active periods, and the hit from lower combustion efficiency for heating that liter or two of water is probably less of an issue than the flue purge losses during buffer-temp-maintenance-only burns (the flue purge loss on very short draws will still dominate the loss figure in most use profiles.) On longer draws the combustion efficiency is still determined primrialy by the incoming water temp, and it's barely affected by the setpoint of the mini-tankL: Mixing the recirculation tank-let with incoming water still yields a temp entering the HX is deep into the condensing zone after the first liter of draw, even if you have it set to 60C. The steep part of the condensing curve in a natural gas burner typically starts at ~87% @ ~52C, and you're already at 95% or more with 40-45C incoming water, climbing only slowly to 98% as it falls to 30C & lower. With 10-15C water in from the street it it dilutes that 50-60C water well into the condensing zone quickly.

I've been mostly un-impressed with the design of either the Rinnai or the Navien CH combis. I'm not surprised that the Navien fails to condense much under DHW-only loads (but then again, neither do most mod-con + indirect systems as-operated, since they tend to set the indirect tank temp too high.)

I'd still like to read the Centre for Energy and the Environment piece though, if it's available online. (If not available on the web, ping me via PM message.)

[zl700]

"This is partially why the Navien combi-boiler has lost its energy star rating (visit Navien's website to find out more about their energy star rating for combi boilers"

Anothe misleadingng untruth from you sir, Mr Tankless Adviser

Since there is no combi listing on DOE / Energy Star, Navien listed their combi as a water heater.
With recent DOE tightening and failure to create a a combi boiler classification, combis such as the Prestige, Challenger and a few others were moved over to boiler classificarequiringreing ASME and CSA 4.9 testing to name some.
Check the boiler listing where it is Energy Star listed.

Centre for Energy and the Environment is online and appears to be written to favor tanks.
http://mncee.org/Innovation-Exchange...ural-Gas-Tank/

"If you followed the installation diagrams published in the Navian product installation manual, then you could be up for some challenges right off the back. Notice that the pump on the Navien CH combi-boiler is on the boiler loop outlet inside the unit. Notice also that they recommend placement of the expansion tank at the boiler outlet, which means that the pump will theoretically not be able to increase pressure above static pressure. While this is only one problem with the diagrams, there are more, so I suggest getting your hands on a proper diagram and use that as your guide."

You fail to understand the purpose of a boiler pump which in most cases is not the system pump, look at other combi units with internal circulator.

MEE if your getting low pressure codes on a CH you either didnt hook up the automated feeder to water connection, or installed a PRV which wont let it get to fill shutoff, pressure switch is downstream of outlet of internal circ. Factory setting is 17 PSI, circ creates 6 so its static 11 + 6 = 17 for fill shut off (Circ always runs when feeding water, for auto purging) this setting is adjustable from 12 to 30 PSI on remote.

[Dana]

That document is consistent with (and references) laboratory work done over the past decade by the Davis Group in CA, so I'm not too surprised.

Favor tanks? Not really- more like favoring the facts (as meticulously measured in-situ rather than presumed in a laboratory or rating agency test protocol.) Tankless units definitely save fuel, but the benefits are far less than implied by the DOE EF test numbers or steady-state performance numbers.

Efficiency is rarely the primary reason people give for going tankless in the first place. Not running out of HW is #1 most-cited, space savings is #2.

The standby power is substantial on some of them, which makes me think the zero-electricity cheapo Bosch 1600H would be a better choice in some applications. (It'll support one shower flow at a time in a northern Maine winter, but not much more.)

[Willowtree]

Just the facts men. I purchased a Homemaster instantaneous hot water heater in 1979 as a DHW backup for my evacuated tube solar collectors. It was on a closed loop w/ treated dionized water. I never flushed the system. I removed it when I sold my house in 2005. That's 25+ years! It was working fine and I gave both the solar system and the instantaneous hot water heater to a friend who currently is using it. Sounds remarkable but it is a fact. I currently have Navien double units paralleled to provide DHW water for a six unit apartment house. We do experience the thermal lag inherent w/ all instantaneous systems but the tenants have been understanding and no complaints. These units are more sophisticated so I do expect potential problems. Finding a trusted and supportive installer is key. I must say that a few of the forum contributors would not be on my list of repairman. The systems do require more knowledgeable technicians. If you like the easy/simple and less efficient life then stick w/ tanks. This applies to both installers and home owners. There is plenty of business out there for all of us.

[Surfing Plumber]

I have installed Naviens before and let me tell you, there are A LOT of problems with these heaters. Which is fine because I know not everything is perfect. But to find the support and parts from Navien is almost impossible. They will give you the run around and takes a lot of convincing to get the part(s) needed to fix the heater for customers. I am sick of dealing with them.

I now install Noritz, their technicians are great, warranty is awesome (even with controlled recirc system). I highly recommend going with Noritz, their new condensing PVC line up is great! 3" PVC and up to 60+ feet in vent length!!!!