I was looking at the Takagi TK-H2 for use in our hydronic system, but the Navien NR-180 has been proposed as an alternative, at a much lower cost. They both have similar efficiencies, both are PVC direct vent, and use stainless steel internals.

Takagi: http://www.takagi.com/download/prod...DV.pdf

Navien: http://www.navienamerica.com/PDS/ft...091208.pdf


Any opinions on the Navien products?

If it's intended to be used as a boiler for a heating system, is there a reason you're going for a condensing tankless as opposed to a modulating condensing boiler?

FWIW (and it isn't worth much) scuttlebutt has it that some of Navien's design talent came from inside Takagi (don't know if that's factual at all...)

Even when the burners an heat exchangers are nearly identical , the control setup is not. HW heater are designed for tightly controlled fixed temp output, whereas mod-cons use load sensing or outdoor reset to determine firing rate/output temp for highest efficiency as space-heating appliances. While it's possible to work around the water heating control nature of the beasts, it's easier from a system design & optimum efficiency point of view to use a mod-con.

Both Navien & Takagi have been pretty good about supporting their HW heaters in combi or space heating installations (unlike some others, who shorten or void the warranties). But ultimately it boils down to have good the support network is near YOU, should you ever need parts/maintenance/system-debugging. The support network for Navien is still a bit behind that of Noritz/Rinnai/Takagi according to folks who install a lot of these beasts. That said, I know people who have had acceptable results with Naviens as backup boilers to solar space heating, yet it's not hard to find people unhappy with any on-demand HW heater in internet searches. But in space heating mode most of the hot water quirks people complain about just aren't in the picture.

Cost wise you're not alway saving much in up-front hardware costs between a condensing tankless and a small to mid-sized mod-con, but the Naviens DO seem remarkably low-cost relative to the competition these days.

If it's intended as a combi, Navien's dedicated combi has some mod-con control features that make system integration a bit easier:

http://www.navienamerica.com/PDS/ftp/CombiGasWaterHeaters/Brochure/Navien_condensing_combi_boiler_091103.pdf

Some installer recommended the Quietside units with PVC vent pipes - just passing that on.

Some?

With a tank-less water heater, you are missing a critical component of efficient, comfortable radiant floors...outdoor reset.

But they are cheap.

Condensing boiler with a plate heat exchanger and ODR are the answer where a ModCon will not fit (rare).

Tank-less "water heaters" used as boilers are a short-sighted option.

It is odd that some of the modulating condensing boilers (like the Quietside) don't include outdoor reset.

All the good ones do.

The condensing boilers that do not offer outdoor reset take advantage of the ignorant and the parsimonious.

How critical to comfort & efficiency is outdoor reset to a high mass low temp radiation like slabs, really? (Not much, methinks.) Floor thermostats w/PID or similar control algorithms and reasonable setup for flow & programmed temp on a condensing Navien or Takagi used solely as a space heating boiler (not a combi) could pretty much guarantee high 90s performance and identical comfort to an outdoor reset approach in most instances. You might squeak another percent or two out of it with a outdoor reset, but not three. With higher-temp radiation outdoor reset would have a more significant effect on efficiency.

Whether the thing will last as long as mod con is a legitimate point of skepticism, but if the design-condition heat load is truly under 30KBTU/hr and you'll never short-cycle it, odds are it'll outlast is hot-water use warranty period. You'd be burning about the same amount of fuel as folks in McMansions with spas, side-spray showers, & soaking tubs do just for hot water, but running 1/10 the number of ignition & purge cycles, lower delta-T, and lower output temp for less thermal stress on the components.

Still, Navien voids the warranty on the NR series when used a space heating boilers though: http://www.navienamerica.com/customer/warranty01_04.aspx?skin=warranty01_02

Takagi on the other hand offers a all-components 5 year warranty on the TK-H2 when used as a boiler, independent of whether it's being run at 15K or at 150K, low temp or high (see page 51: http://www.takagi.com/download/product_manuals/T-H2.pdf ). At your heat load, if properly set up I'd expect it to last 3-5x that long. It's min programmed temp is 100F, and they spec a min flow of 2gpm in heating apps. It shouldn't be too tough to set this up to get ~96% performance in a slab radiant application with 25-30K output at 3-4gpm, with the slab at it's highest design-condition temp, and it would do slightly better when the slab was cooler. With outdoor reset on a mod-con you might bump on 98%, but it becomes a "who cares?" proposition.

OTOH you can probably get a bare-bones dumbed-down (no-enhanced control options, no outdoor reset) Munchkin Contender MC50 for similar or less money (& a better warranty) to do pretty much the same thing. With 110F out & 90F return it tests at 98% running flat out, whereas the the condensing tankless you're kind of guessing the shape of the condensing curves are at 15-20% of full fire. In your situation that's the option I'd be looking at. (Street & auction-site pricing on 'em is in the $1.5k range, sometimes less.)

And why do you want to horse around with a water heater again?

The full modulation really doesn't makes sense for our high-mass application. I've run some numbers of the heat capacity of the main floor and basement:

Basement Slab: 55,000 Btu / °C

Main Slab: 25,000 Btu / °C

This heater will never short cycle, even at full 180Btu output with a call for a 2°C rise in slab temp would mean running for over an hour. I really can't understand how having an outdoor temperature reset changing the water temp would make any differance in a high mass system... What would it do?


Warranty: I notice that Navien only has the NP in the 240 size, not the 180. I'm not seeing the differance in the NR vs NP construction, but there must be something.

Why do manufacturers in general reduce the warrenty period for space heating? Is there problems with the burners running for longer periods of time, or is it a cycling issue?

Lower water temperatures control overshoot and allow the system to run at the lowest operating temperature possible. The lower the operating temperature the lower the fuel bill.

Modulation also assures a nearly full-time pump which increases comfort and makes the system all but frost-proof.

First, the warranty is reduced because the water heater is not a boiler. Those who misapply them are seldom qualified for the extra care needed in design and misapplication.

Second, a water heater is designed for peak loads (about 20min./day).
An 8000 hour life cycle is not in the cards. The heat exchanger being over-fired and happiest with high flow rates.

Where domestic hot water is needed, there is no valid argument against a ModCon and indirect.

Running it off a fixed temp Navien or Takagi it WILL modulate, (starting higher, finishing lower on every burn) and with a floor T-stat w/PID algorithm it won't overshoot any more than it would with a mod-con running outdoor reset. The average loads we're talking here are FAR too low for there to be significant load tracking via modulation with existing mod-cons. The design day load is only ~2x the min- mod of the tiniest mod-cons- about the same as the min-mod of a Takagi or Navien.

The duty cycle won't improve by much with outdoor reset with design-day loads that low. Duty cycle would be determined by the min-mod of whatever got installed and the system design, and reset control would be only a subtle improvement, min-outputs being equal. If the system is designed properly it won't overfire tankless beasts or cycle a them to death, but I'm sure there are folks who can do-in almost any heating appliance, eh? ;-) They're not particularly happiest a full flow or full fire, they're just fine at 20-25% of the rated flow or 15-20% of the rated fire, which is the order of magnitude we're talking here. Sure, running them as a 100K+ or high-temp boilers you'll burn 'em out early, but not in a slab if you baby 'em by the system design.

But given that a bottom-of-the-line Contender MC50 (without outdoor reset) gets you there with none of the warranty or design issues of running with a tankless, and for about the same cash, why bother with a condensing tankless? Since there's not even a domestic hot water load to be concerned with you'll never need a burner as big as a tankless, but you still need to up-size everything to on the fuel & venting sides to support that never-used burner capacity. A 50K Contender is a much better fit to what you're doing here, and even if it were is to cost $500 more, it would be worth it.

I developed a tankless space heating hobby with a $300 trailing-edge Takagi and a willingness to replace it in 5 years (hoping for 10), and a hope that the right-sized-right-priced micro-cogenerator would show up before it dies. If that doesn't happen I'll probably replace it with a Contender or whatever bargain-basement mod-con deals shows up. ( Currently the right-sized micro-cogens exist Marathon EcoPower but are still 3x the installed cost that would make financial sense as an investment, but that may change with higher production and more competition. ) With slab radiation a sub-$2K mod-con MIGHT have paid for itself in the anticipated lifespan of the non-condensing Takagi, but not necessarily, at the temps I need to run. (But if the price of fuel triples next year, I'm there! :-) )

Posted By Dana1 on 29 Nov 2010 04:46 PM
If the system is designed properly it won't overfire tankless beasts or cycle a them to death, but I'm sure there are folks who can do-in almost any heating appliance, eh? ;-) They're not particularly happiest a full flow or full fire, they're just fine at 20-25% of the rated flow or 15-20% of the rated fire, which is the order of magnitude we're talking here. Sure, running them as a 100K+ or high-temp boilers you'll burn 'em out early, but not in a slab if you baby 'em by the system design. 

Our design will be looking at a delta-T of 15-20°F with 4-5gpm flow through the heater. (should work out to around 60K Btu loading on the heater) The flow rate could be increased if needed with the three pump speed settings.


I'll check out the MC-50, but it's looking like it'd be an $800 upgrade from the Navien NR-180. Warranty is a concern, but I still can't figure out what makes the NP-240 design that is rated for space heating and the NR series differant.

4gpm x 15F is ~30K

5gpm x 20F is ~45K

If "...design load is less than 30,000 BTU...", why are you designing it to kick out any more than that? At 45K you're pushing up to the 2x oversizing range. Better to set it up with 30K of output at the design-day water temp requirement. It'll run higher than 30K when the slab is colder but modulate down as the slab's temp picks up. Tankless beasties are very tolerant of high delta-Ts, since they're designed for ~80F deltas in normal DHW operation.

for $800, by the time you drop your pump size down (presuming the navien, like other tanklesses, require higher head pumps) and account for energy usage savings, plus the inevitable performance degradation of the navien, you would almost undoubtedly be equal or ahead going with a boiler in very short order.

heat exchange equipment is roughly equal to the cost of an indirect fired water heater or perhaps a tankless dedicated for DHW.

There isn't a good argument for the tankless as a heating appliance in any application that is properly conisdered. Either stick with cheaper tank heaters for low loads or use mod/cons for heating. there is no useful "in between".

by the way, the threshold for "payback calcs" has as more to do with degree days as it does design day heat load. If you're talking celsius, meaning you're in canada, the degree day situation on a 30k heat load would cost justify a mod/con simply on efficiency improvement in a heating application in a reasonable period of time. Unless your fuel is REAL cheap.

Ya,

What Rob said...ehehehee.

Hi Rob, our location is 4,600 °F-day, total annual consumption should be in the range of 60MBtu.


Dana, my concern with dropping the flow rate for a higher delta T is that the end of the loops may not have warm enough water to heat evenly. With it being 1/2" pex in the concrete slab, we will be moving about 0.7gpm through each loop.

Dropping that in half doesn't seem like a good idea, but what have you found in your experiance with low flow loops?

where are you??? I don't see any degree day counts like that in Canada. Are you a transplant?

with degree days that low I would concur with your original tank heater plan. while its EF may be low as has been discussed many times on this forum the efficiency is much higher in heating apps, closer to 80%.
Trim hardware would be smaller and cheaper and less energy consumption. Much less maintenance concern. 10%-15% efficiency in your situation would get eaten up in service pretty easy.

regarding loop flow rates, we design loops for 20 dt standard. can go higher for non-residential situations.

Temperature gradient and feel have as much to do with loop pattern and length as with Delta T. You can't figure this all to the degree you think you need, over the internet.

It is fun exercise for us, but of limited value to you. Your material vendor or certified designer should provide this minute detail.

Rob has a point, but you have to decide what your priorities and budget are before you start.

Total cost, ROI, GREEN heating, control accuracy, system reliability, cost of maintenance...?

Hey, not all of Canada is frozen tundra, eh! I hope I converted it right, we are 2550°C-day, which should equal 4600°F-day?

The tank style heater still makes a lot of sense, but with a requirement for direct vent there is little differance between a DV tank and the Navien unit, which is why I'm asking the question.

It takes a long time to pay back on an $800 upgrade to the Munchkin at our consumption rate...


BTW, this is a fairly small R-30 ICF house from basement to ceiling, with HRV and R-20 under the monoslab. The whole structure is fully encased in foam.