Navien Combination Hot Water Heater / Boiler

[HankR]

Hello,

We are looking to replace our natural gas; 40-gallon power-vent hot water heater (currently dead) and hot water boiler with a combination tankless unit. We are looking to create some additional space by going this route. It would be in a mud room on the first floor.

We live just south of Boston along the ocean. Our home is less than 900 sq ft. It’s currently heated by forced hot water via ~62’ of fin-tube baseboard. It’s a single zone, 1-bathroom house. Our current boiler states the heating capacity is 42,000 BTU.

We’ve reach out to some local plumbers and they are suggesting the Navien NCB models. The plumber that is certified with this manufacturer is suggesting the NCB-240E while another plumber says that is too big for our house and won’t even quote that size (and is not certified, though licensed and experienced). He’s suggesting the NCB-180E, saying that is more than enough.

As I read through the posts on this forum, I tend to agree with the second plumber. It seems if I do purchase a model that is too big for the house, I could end up causing more long-term issues. I’m looking for any guidance any of you would have on:

Is the Navien manufacturer the system to use?
Would the smaller size be the most appropriate?
Anything else I should be asking or am I missing anything else?
What would be the appropriate cost for this?

Appreciate any feedback you may have.
Thanks,
Hank

 

[Dana]

For a 1 bathroom house the smaller unit is fine.

Before buying any low-mass modulating boiler read and understand this bit o' bloggery. The minimum-fire output of the boiler is more important in many respects than it's max output.

The short version is that at temps low enough for condensing the radiation on each zone must be capable of emitting the full minimum fire output of the boiler. If the boiler short-cycles it will take a significant drop in efficeincy, and puts a lot of wear & tear on the boiler. At 120F average water temp (AWT) the temp needed to edge into the mid-90s on combustion efficiency typical fin tube baseboard emits about 200 BTU/hr per running foot. So with 62' of baseboard on your single zone system it can deliver ~12,400 BTU/hr per hour.

The min-fire output of the NCB-240E and 210E is about 17,000 BTU/hr, so there's going to be almost 5000 BTU/hr of heat going into the system that isn't being emitted into the room. If there's enough water mass in the system it can be tweaked to not short cycle, but it's not ideal.

The min-fire output of the NCB 180E at 95% combustion efficiency is about 13,300 BTU/hr.

With 12,400BTU/hr being emitted that's only 900 BTU/hr (60 BTU per minute) of excess heat going in, so if there's even 20 lbs of water in the system (probably is), the slew rate of temperature rise is about 60/20= 3.3F per minute. The default settings of the NCB is to turn the burner off when the water temp is 5F higher than the boilers setpoint, and won't refire until it drops 5F below the setpoint temp, a 10F difference. So even without tweaking those parameters you'd still get a non-abusive 3 minute burn time out of it.

With the 240E you can change the parameters to increase the burn times, but it would be spending more time at a lower combustion efficiency.

Your heat load is probably pretty low, so it won't be doing days-long continuous modulating burns the way a small mod-con boiler would, but you'd still be well advised to set it up with the outdoor temperature sensor and tweak the outdoor reset curve. This boiler can be set up to operate on return water temperature rather than output temperature, which is usually going to add another percent or two of condensing efficiency with the amount of radiation you're working with.

If you don't know your heat load and have a heating history on this place, run this math, using the DOE (not IBR net water) output/input numbers to determine it's steady state efficiency, and wintertime-only fuel use. (Winter is when the errors from hot water use and solar gains introduce the least amount of error.) With the heat load number we can determine how to optimize the outdoor reset curve on the boiler to achieve maximum efficiency & comfort. The typical installer is going to set-it up with the default settings for baseboard, and that would yield pretty crummy efficiency (and more cycling, less comfort) than what's actually achievable here.

With 150K of domestic hot water output the -180E will be enough to support two simultaneous low flow showers or a single gusher shower (within reason- not a 6 sidespray ultra-luxe type shower) with margin to spare for other uses. At a midwinter 35F-in, 105F out the shower head a 2gpm shower is pulling only 70,000 BTU/hr. Tubs would fill at about 4gpm of 110F water, which is fine- still less than 10 minutes to fill a standard bathtub.

Navien is a first-tier Korean boiler manufacturer that has spent more than a dozen years developing a support network in North America, and is reasonably well supported in MA.

 

[Sponsor]

 

[djdavenport]

Not a technical response--just an observation. We have a 2,100 sq foot house. Design temperature here is about 16 degrees (IIRC). We have a NCB-240E. The heating is high mass radiant floor heating. The boiler most times just loafs along, at minimum firing rate. (We're a family of two, so the DHW side is certainly adequate--another consideration in choosing to go the combi route.) I would think that a 240 in a 900 sq ft house, with that amount of available emitters would be hugely oversized.

As for the brand itself, we're going on year three and its performed flawlessly, once the mistakes of the initial installer were corrected. (I guess the concept of "closely spaced Ts" is subject to some interpretation. 18" doesn't exactly qualify in my opinion.) But the saving are substantial over the cast iron beast that was previously there and we're very happy with the unit.

 

[Dana]

Not a technical response--just an observation. We have a 2,100 sq foot house. Design temperature here is about 16 degrees (IIRC). We have a NCB-240E. The heating is high mass radiant floor heating. The boiler most times just loafs along, at minimum firing rate. (We're a family of two, so the DHW side is certainly adequate--another consideration in choosing to go the combi route.) I would think that a 240 in a 900 sq ft house, with that amount of available emitters would be hugely oversized.

As for the brand itself, we're going on year three and its performed flawlessly, once the mistakes of the initial installer were corrected. (I guess the concept of "closely spaced Ts" is subject to some interpretation. 18" doesn't exactly qualify in my opinion.) But the saving are substantial over the cast iron beast that was previously there and we're very happy with the unit.

On the boiler side, yes, it's ridiculously oversized, but if there were more than one bathroom or a big spa tub to fill the 240E wouldn't be insane for the domestic hot water size.

In general combi boilers are better suited for houses with large heat loads and modest space heating needs. In this case both the hot water and space heating are pretty modest, but with 60' of baseboard it'll work OK with the -180E right out of the box without a lot of tweaks, but the -240E would be PITA to set up properly to work without short cycling.

For space heating comfort a mod-con that could modulate to under 10,000 BTU/hr would be even better, but would usually need an indirect water heater to handle the domestic hot water loads. The NCB-180E isn't a bad compromise, and should provide better comfort than the existing cast iron beast once the reset curve is dialed in.

If it's a well insulated 900' house with a ridiculously low heat load it might be suitable for working with an air-source chiller (reversible heat pump) or even Sanden's heat pump water heater (probably not, but maybe.) It comes down to the actual heat load and the required water temperature for getting that much heat out of 62' of basesboard. Either of those options would be more expensive than the -180E, but would be comparable in operating cost, and lower in lifecycle greenhouse gas emissions, local air pollution, etc. After the Andover/Lawrence Columbia Gas disaster it's not unreasonable to consider getting off the aging local gas grid when replacing the mechanical systems. If it's an open floor plan a mini-split heat pump or two for the living space a heat pump water heater in the basement would be another way to. If there's a full basement for running some ducts, a 1.5 ton mini-ducted Fujitsu would probably handle the load, but until we get the heat load numbers it's hard to make that call.

 

[HankR]

...If you don't know your heat load and have a heating history on this place, run this math, using the DOE (not IBR net water) output/input numbers to determine it's steady state efficiency, and wintertime-only fuel use. ....

Hi Dana - Thank you! This is fascinating. I'm attempting the math you'd suggested and am hung up on one of the steps. When calculating the BTU per degree-day, the example states, "14.56 MMBTU / 937.7 HDD is 15,527 BTU per degree-day." But when I calculate 14.56 / 937.7, I'm getting 0.012 not 15,527. Am I missing a step?

I've gotten this far on the inputs/calcs...

Thanks,
Hank

 

[Dana]

You found an arithemetic-error!?! YOU should have been the editor for that blog! (I'll have to get on the editor for that! )

So you put 8.184 MMBTU in the heating system over the course of ~1003 heating degree-days. That means you use 8.184 MMBTU / 1003HDD = 8110 BTU/HDD, or a rate of 8110/24= 338 BTU per degree-hour for every degree below 65F.

So at your 99% outside design temp of +12F it's 65F -12F = 53F cooler than 65F, for an implied heat load of 55F x 338 BTU per degree-hour= 18,590 BTU/hr @ +12F. At 0F (a temp that happens during extreme Polar Vortex cold snaps in Quincy MA) it's 65F heating degrees, and a load of 65F x 338= 21,970 BTU/hr.

For 62 feet of typical baseboard t emitting 18,590 BTU/hr is ratio of 18,590/62 = 300 BTU/hr per running foot, which takes an average water temperature of about 135F, not hotter. At reasonable pumping rates that would be 140F out, 130F return, which would be just above the condensing zone for natural gas burners, delivering 87-88% combustion efficiency.

When it hit's zero and the heat loss is 21,970 BTU/hr it needs to emit 21,970/62'= 354 BTU per foot, which takes an average water temp of about 145F (150F out, 140F return). So when setting up the condensing curve under supply temperature control you'll want roughly 150F out @ 0F, (or 140F @ 0F return water temp if under return temperature control.) Since the heat load calculation is only a linear approximation it isn't precise to even 2 full significant digits even when adjusting for some of the error factors, but it is at least a reasonable starting point. As the seasons change you may need bump up the water temps if necessary, or if it's still keeping the place warm when it's in the 20s outside, start bumping the water temp down a bit to increase the amount of condensing operation.

With an implied load of 18,590 BTU/hr @ +12F and 21,970 BTU/hr @ 0F it means you in fact could heat this place with a Fujitsu AOU/ARU18RLFCD high efficiency modulating air source heat pump. At +12F it's good for over 20,000 BTU/hr (see page 16) , and the shortfall isn't huge even at 0F (it might drop to the mid-60s indoors on nights when it hits below 0F outside.) Since it can modulate down to 3100 BTU/hr @ +47F (a temp where your house still has a heat load of about 6000 BTU/hr it will run almost continuously, modulating at extremely high efficiency even during the shoulder seasons. Since it involves a bit of design work and duct installation it would be more expensive than ductless mini-splits, but still not outrageous. If you go that route make sure the ducts are installed in the basement and NOT in the attic, above the insulation (which is easier for the installers, but BAD for net efficiency, and wastes a substantial amount of it's output capacity), even if it's more expensive to do it that way. There are probably some ductless solutions that work too, but I'd need a room-by-room load analysis (the amount of baseboard in each room and the layout would be enough), to make reasonable recommendations on that front.

MassSave will give you a decent subsidy check for the Navien solution, but probably not for the Fujitsu, but there may still be 0% /7 year financing available for the Fujitsu solution, and it doesn't solve the hot water issue, which seems to be the pressing. A heat pump water heater in the basement wouldn't get a subsidy since it's replacing a broken gas-burner, but for about $1500 it would get you in hot water in under a day, and it would dehumidify the basement, keeping the stinky-basement smell under control during the humid summer season.

 

[HankR]

Ha! Again, thank you for the calculations and guidance. I think I'll be going the route of the Navien solution. I'm leery of putting anything into our basement due to flooding concerns. In 12 years here, we've had 2 significant floods. Having the utilities on the first floor has been a lifesaver. It has been helping us understand more fully why the first plumber was suggesting the 180 model. Understanding how to set up the condensing curve will be helpful and not something I would have even thought of.

Thank you!

 

[Dana]

Ha! Again, thank you for the calculations and guidance. I think I'll be going the route of the Navien solution. I'm leery of putting anything into our basement due to flooding concerns. In 12 years here, we've had 2 significant floods. Having the utilities on the first floor has been a lifesaver. It has been helping us understand more fully why the first plumber was suggesting the 180 model. Understanding how to set up the condensing curve will be helpful and not something I would have even thought of.

Thank you!

See if you can't find a serial number or date code on the cast iron beastie. If it was installed in 1988 or earlier MassSave will cut you a $3,250 check, a substantial part of the installed cost! If it's newer than that they'll still pay out $1600 on a combi-boiler that's 95% AFUE or better (the NCB-180E tests at 95%.) Be sure to document what equipment got retired (both the water heater & boiler) including model names and serial numbers if you can find them, and fill out the rebate forms even before the combi boiler is installed to make sure you don't miss anything.

Download and read the installation manual before hand too, to make sure your installer doesn't have any stupid-attacks (hey, it happens!), and read up on how to set the heating parameters, and what those parameters mean.

 

[djdavenport]

Stupid-attacks, indeed. Nothing against old school, but the current state-of-the-art equipment compared to the old stuff is the difference between a DC-3 and a Dreamliner. Stick and rudder versus fly by wire. Lots of sensors and processing power, and a fair amount of "dialing things in" to get it right. You still read discouraging reviews about tankless units (or low mass, condensing boilers, et al) being junk, throwing error codes for no reason, not keeping up with the heat load, or just plain breaking down (Yes, I'm talking about you Eternal.) But proper installation and set-up is absolutely essential. In our case, the swap-out was an emergency. Dead of winter. Family member just home from the hospital. Installer who really didn't understand the equipment. Besides the aforementioned widely spaced Ts, he wasn't going to install the Outdoor Reset ("It's just a gimmick.") and proceeded to set the space heat temperature at 160 (for radiant floor heating!!) and left. (And, yes, we DID have heat. Lots of heat. In little bursts). I corrected the primary/secondary loop problem myself, installed the Outdoor Reset and spent a lot of time fiddling with the curve, bumping it a few degrees up and down, looking for the sweet spot, which I eventually found. Now, during the cold seasons, the boiler runs for about 16-18 hours a day, AWT about 115 degrees during the coldest days and the ambient temperature in the house doesn't vary a degree. Really sweet. Hope your experience is similar.

 

[HankR]

Stupid-attacks, indeed. Nothing against old school, but the current state-of-the-art equipment compared to the old stuff is the difference between a DC-3 and a Dreamliner. Stick and rudder versus fly by wire. Lots of sensors and processing power, and a fair amount of "dialing things in" to get it right. You still read discouraging reviews about tankless units (or low mass, condensing boilers, et al) being junk, throwing error codes for no reason, not keeping up with the heat load, or just plain breaking down (Yes, I'm talking about you Eternal.) But proper installation and set-up is absolutely essential. In our case, the swap-out was an emergency. Dead of winter. Family member just home from the hospital. Installer who really didn't understand the equipment. Besides the aforementioned widely spaced Ts, he wasn't going to install the Outdoor Reset ("It's just a gimmick.") and proceeded to set the space heat temperature at 160 (for radiant floor heating!!) and left. (And, yes, we DID have heat. Lots of heat. In little bursts). I corrected the primary/secondary loop problem myself, installed the Outdoor Reset and spent a lot of time fiddling with the curve, bumping it a few degrees up and down, looking for the sweet spot, which I eventually found. Now, during the cold seasons, the boiler runs for about 16-18 hours a day, AWT about 115 degrees during the coldest days and the ambient temperature in the house doesn't vary a degree. Really sweet. Hope your experience is similar.

Thanks for all the responses. I'm reading through the manual as Dana suggested. I am nervous about finding the right installer. I found the plumber that suggested the 240 through the Navien website while the contractor that is saying to go with the smaller unit is not a 'certified' installer. Both are large companies, one on a national level and the other is local and state they are experienced with this. While we're currently without hot water, I do not want to rush into something I'll regret. All the guidance and feedback has been greatly appreciated.

 

[djdavenport]

I think, armed with what you know (and what Dana has shared), I'm not sure I would be too afraid of going with the Navien recommended installer. I think the installation for the various flavors is basically the same. You now know what size would be appropriate, so you can call your own shot on that, without being subject to the "bigger-is-better" up sell. I think using a Navien certified installer might also make it easier if there are warranty issues. Good luck.

 

[Dana]

I'm not sure how important Navien certification is- it's certainly not as important as being competent hydronic heating designer/installer.

The system design, pumping and installation labor would be identical between the 180E and 240E. The 180E is about $400 cheaper than the 240E at internet pricing. If you prefer that the contractor who proposed the 240E do the work you're in a position to negotiate. If you counter-propose doing the 180E for $500 less than whatever was quoted for the 240E on the grounds that you did the napkin-math and decided it was a better fit for the radiation they'll probably go for it.

 

[Jac04]

I'm running the NCB-180E with outdoor reset in my 2100 square foot house just outside of Hartford, CT.

The unit works well, at least for now. Downside: If you like to turn the heat down at night for sleeping comfort, then the modcon boiler has a tough time recovering from a temperature setback, so be aware of that. To get around this, the unit has an adjustable 'boost time' feature. If the t-stat is not satisfied after the set time, then it will raise the supply temp set point by 9 degrees. This is my first season trying to use this boost to recover from a night-time setback. So far, it works well, but I am obviously trading off some efficiency for comfort.

Anyhow, now for the not so good part of my Navien experience. My first unit, a CH-180, lasted 4 years before the heat exchanger failed (the outer casing corroded and holes formed). They replaced the unit with the NCB-180E, but I had to pay the $1000+ install cost, even though Navien was quite aware of the design issues with their heat exchanger. Now my 180E appears to be corroding inside the heat exchanger, again on the flue gas side but this time it looks like it isn't the outer aluminum casing. I'm kind of stuck with this thing since I can't see spending any $ to replace it right now. But, if I had to do it all over again (which I will likely have to in the near future), I definitely would not use a Navien unit. More info: https://terrylove.com/forums/index.php?threads/condensate-trap-sediment.77504/#post-566413

 

[HankR]

I'm running the NCB-180E with outdoor reset in my 2100 square foot house just outside of Hartford, CT.

The unit works well, at least for now. Downside: If you like to turn the heat down at night for sleeping comfort, then the modcon boiler has a tough time recovering from a temperature setback, so be aware of that. To get around this, the unit has an adjustable 'boost time' feature. If the t-stat is not satisfied after the set time, then it will raise the supply temp set point by 9 degrees. This is my first season trying to use this boost to recover from a night-time setback. So far, it works well, but I am obviously trading off some efficiency for comfort.

Anyhow, now for the not so good part of my Navien experience. My first unit, a CH-180, lasted 4 years before the heat exchanger failed (the outer casing corroded and holes formed). They replaced the unit with the NCB-180E, but I had to pay the $1000+ install cost, even though Navien was quite aware of the design issues with their heat exchanger. Now my 180E appears to be corroding inside the heat exchanger, again on the flue gas side but this time it looks like it isn't the outer aluminum casing. I'm kind of stuck with this thing since I can't see spending any $ to replace it right now. But, if I had to do it all over again (which I will likely have to in the near future), I definitely would not use a Navien unit. More info: https://terrylove.com/forums/index.php?threads/condensate-trap-sediment.77504/#post-566413

Thank you for the response. I had planned for the heating to adjust based on whether we are home or not i.e. ~60 when not home or at nights and ~68 when home based on a timed thermostat. It looks like based on your experience I would use the 'boost time' feature. It looks like something that is set by the installer, right? I'll have to research this further...thank you for bringing it up. I had just taken that for granted. Thanks for your post!

 

[Jac04]

Ideally, using the outdoor reset, the unit would fire at the beginning of the season and never shut off until spring. The heat being input by the burner would just equal the heat load of the house. In reality, this doesn't usually happen, but using the outdoor reset allows you to dial-in the lowest temperature required to heat the house without losing ground, and it will result in nice, long, efficient burn times. You will end up with a supply temperature that is above what you actually need for the majority of the time, but you want to make sure that the system can keep the indoor temperature stable even on those cold, cloudy, windy days. Unfortunately, setting up the unit for maximum efficiency doesn't allow the system to respond to significant indoor temperature changes very well.

For example:
I set to 68-69 during the day with a bump up to 70 for breakfast in the morning and when the kids get home from school. Last season, with boost NOT activated, I would only go down to 68 at night. I tried going down to 65-66 for sleeping comfort, but the system just couldn't raise the indoor temp in the morning by more than 1 degree in about 2-3 hours. I then activated the boost and set it to 40 minutes, and set my t-stat to 67 at night. I have my t-stat set to bump the heat from 67 to 70 at 4am. With the boost = 40 mins, by 6am I can usually get to about 68 - 69. Not too bad, but we would still feel a chill on some of those brutally cold mornings, and we were still a little warm at night.

This season, I just changed the boost to 20 minutes with a night setback to 66. My t-stat bumps up the temp from 66 to 70 at 4:20am. It hasn't been too cold, but it is working well, and I'm hitting 70 by 6 in the morning. I'll probably end up somewhere between 20 and 40 minutes after playing with it this season, and maybe a night time setback to 65 for sleeping comfort. It's just a comfort vs. efficiency trade-off that I need to play with.

As far as who sets the boost time, my opinion is to read up on the unit and learn what it does and how to adjust it. I was forced to learn about my first modcon due to an incompetent installer who simply set it to 160F supply and walked away. This forum (especially Dana) helped me understand the unit, how it operates, and how it should be operated for maximum efficiency. On my 180E, I have determined and programmed my own outdoor reset curve, and I obviously set my own boost time. It takes some time to "dial in" the system, seeing how it responds over different outdoor condition, so I doubt the installer will want to do anything other than maybe set one of the pre-programmed outdoor reset curves available from the menu.

 

[HankR]

Ideally, using the outdoor reset, the unit would fire at the beginning of the season and never shut off until spring. The heat being input by the burner would just equal the heat load of the house. In reality, this doesn't usually happen, but using the outdoor reset allows you to dial-in the lowest temperature required to heat the house without losing ground, and it will result in nice, long, efficient burn times. You will end up with a supply temperature that is above what you actually need for the majority of the time, but you want to make sure that the system can keep the indoor temperature stable even on those cold, cloudy, windy days. Unfortunately, setting up the unit for maximum efficiency doesn't allow the system to respond to significant indoor temperature changes very well.

For example:
I set to 68-69 during the day with a bump up to 70 for breakfast in the morning and when the kids get home from school. Last season, with boost NOT activated, I would only go down to 68 at night. I tried going down to 65-66 for sleeping comfort, but the system just couldn't raise the indoor temp in the morning by more than 1 degree in about 2-3 hours. I then activated the boost and set it to 40 minutes, and set my t-stat to 67 at night. I have my t-stat set to bump the heat from 67 to 70 at 4am. With the boost = 40 mins, by 6am I can usually get to about 68 - 69. Not too bad, but we would still feel a chill on some of those brutally cold mornings, and we were still a little warm at night.

This season, I just changed the boost to 20 minutes with a night setback to 66. My t-stat bumps up the temp from 66 to 70 at 4:20am. It hasn't been too cold, but it is working well, and I'm hitting 70 by 6 in the morning. I'll probably end up somewhere between 20 and 40 minutes after playing with it this season, and maybe a night time setback to 65 for sleeping comfort. It's just a comfort vs. efficiency trade-off that I need to play with.

As far as who sets the boost time, my opinion is to read up on the unit and learn what it does and how to adjust it. I was forced to learn about my first modcon due to an incompetent installer who simply set it to 160F supply and walked away. This forum (especially Dana) helped me understand the unit, how it operates, and how it should be operated for maximum efficiency. On my 180E, I have determined and programmed my own outdoor reset curve, and I obviously set my own boost time. It takes some time to "dial in" the system, seeing how it responds over different outdoor condition, so I doubt the installer will want to do anything other than maybe set one of the pre-programmed outdoor reset curves available from the menu.

Thanks again for your responses. I now understand a little better what you're talking about. I'd mentioned the outdoor temp sensor and the installer did say it does have some downsides during fall and spring. I feel like that happens with our current boiler, today. While the thermostat is set to a schedule, some mornings feel like it's too hot while others not warm enough, even though the thermostat shows the same temp. It does sounds like this unit will take a bit more interaction and I have more settings to get familiar with.

Thanks to this forum, in general, I'm feeling more confident with the first installer that had suggested the 180e. They've been responsive in answering my questions, verified they are certified and have quite a bit experience with these installs.

Appreciate your help along with everybody else's involvement.

Thanks,
Hank

 

[Jac04]

While the thermostat is set to a schedule, some mornings feel like it's too hot while others not warm enough, even though the thermostat shows the same temp.

Well, that's because your skin has very high absorptivity and emissivity, so you are very sensitive to the temperature of the things around you. You lose heat easily to cold outside walls, and you gain heat easily from warm outside walls. So, at the same air temperature you tend to feel colder in the winter and hotter in the summer. Human skin is amazing stuff.

 

[Dana]

Good luck then! Let us know how it goes, including the subsidy money payout from MassSave. They usually have annual deadlines and budgets that I believe are in November, so you may have to move fast to get it in this round. The subsidies change from year to year too. We all pay for it on our utility bills in the fine-print "efficiency charges", etc., but it's gratifying to hear when it's being applied in the right places.