Navien NBC 240 combi unit question

Users who are viewing this thread

Rob P.

New Member
Messages
4
Reaction score
0
Points
1
Location
Rhode Island
Hello all,

I have been trolling the forums for a while now and have found some good information but did have a few questions about the Navien NBC 240 as well as the contractor I have right now.

For reference I have answered many of my general questions here:
https://terrylove.com/forums/index.php?threads/combi-boiler-ncb-240-navien.58469/


Some background on my house, which is in Rhode Island. It is currently setup for oil heat with a baseboard system, I will be converting to gas. The house is just short of 2500 sq ft and is two stories. I am not currently in the house so I will use a conservative estimate on baseboard length and say I have 100 ft per floor. Based off the above thread I think the NBC 240 is a good fit but am also weary about the contractor and would like some feedback from the community. They came over and told me they typically deal with Navien and had taken their factory training, I did not find any information that indicated they did not do good work *(reviews BBB rating etc). They asked me the sq footage of the house and how many bathrooms I had (2). Right after I told him he said you want the NBC 240. Grabbed a factory manual flipped through some pages and confirmed his answer.

Does this seem legitimate to you all? I asked about heat loss calculations and baseboard length and they really just told me that they typically do not do those things during the free estimate phase but if I wanted to move forward (not necessarily sign a contract first) they would do a more in depth analysis.

Aside from the contractor notes does the NBC 240 seem like a good unit for my house? Please let me know what other information I would need to supply if what I have is inadequate. This is my first house and I am, naturally, a bit nervous about my decision.

Thank you in advance for you help. It is appreciated.
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
Asking the square footage of the house is only vaguely relevant, since it would take either a fairly large &/or poorly air sealed & insulated to blow over the ~70,000BTU/hr output of even the -180. I've never seen a 2500' house worth living in that had a heat load that high at RI type outside design temps. Any of the NCB series could heat your house (provided you don't sleep with the windows wide open when it's +10F outside :) ), and if you have 100'+ of fin-tube per zone the -240 wont' short cycle.

The issue is what you expect or need for domestic hot water performance. You would still be able to support two mid-winter simultaneous 2.0 gpm showers with the NCB-180 (the smallest in the series) but not much more than that. With a one-bath house the -180 would be fine, unless you had a big soaker tub to fill, but in that case even the -240 might be painfully slow. Even the -240 would still be challenged if you have luxury showers with sidesprays or antique gusher 3-4 gpm shower heads, but it has some margin (not a lot) for two simultaneous 2.5gpm showers. If you only use low-flow showerheads or never run two showers at a time the -180 or -210 would do just as well as the -240.

The actual heat load of a pretty-tight not-super-insulated 2500' house at the typical +10F 99% outside design temps in RI would be between 30,000-40,000 BTU/hr, depending on a number of factors. The min-fire output of the -240 may be more than half your 99% heat load, in which case it'll modulate some in mid-winter (especially this past February), but the -180's lower min-fire output is less than half, more like 1/3 of your peak load, and if you tweak in the reset curves it should modulate most of the time in winter, which is good for both comfort & efficiency. If you can live with the lower potable hot water performance, it's probably the better choice on a comfort basis, but there isn't a HUGE difference within the series- they all have way more space heating capacity than you'll ever use, and the hot water performance is as limited as any residential gas-fired tankless.
 

Rob P.

New Member
Messages
4
Reaction score
0
Points
1
Location
Rhode Island
Dana thank you for your response. I did read about short cycle issues with this unit so this was a concern of mine. I mentioned this to the contractor and he has never had issues and had never heard of the issue until mentioned to him by Navien rep.

I typically do not sleep with my windows open in +10F weather so I think I am safe there. I will confirm that I do in fact have ~ 100ft of tubing per zone but feel this is accurate within a reasonable approximation.

As far as hot water usage. I think it would be rare for us to run both showers simultaneously in mid-winter but we do plan on installing a standard bath sized soaker tub nothing too extravagant. We will likely use a standard shower head, no side sprayers etc. Your comment on filling the tub may be painfully slow confused me a bit. Are you saying that the hot water will be present but the flow will be reduced?

What would be the benefits of using the smaller units outside of cost savings on the unit itself? Is there a benefit to choosing the larger unit, increased domestic hot water capacity etc? This may be a dumb question but I have an Amtrol boilermate now would there be any reason to keep this and run it in tandem with the Navien?

Are there drawbacks to using the 240?

Thank you again for the comments your feedback is appreciated.
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
It's not any tubing in that 100', just the finned tubing. The heat emittance of the distribution plumbing is very low. Measure up the lengths of all the baseboards (the outer housing is fine), and sub-total it by zone. (If it's all one zone, that's fine.)

The tub-filling rate is burner constrained with any tankless. With the NCB-180 you have a max input of 150,000 BTU/hr going into the burner, about 140,000 BTU/hr going into the water. In winter you have 35-40F water coming in and you want 105-110F water coming out of the tap, call it a 70F delta-T. With only 140K /hr going into the water, at a 70F rise that limits the flow rate to 140,000/70= 2000lbs of water per hr, which at 8.34 lbs/gallon= 240 gallons/hr, or 4 gpm for a not super-steamy bath, not counting distribution plumbing heating loss. At 4 gpm a typical 35-38 gallon tub takes about 9-10 minutes to fill.

But that's not to say the -240 is going to rock that tub in a big way, since it's only about 33% more output- call it 7-8 minutes. In the summer it would be more like five minutes with the 240 compared to about seven with the -180. Like I said, the difference in domestic hot water performance is measurable, but not huge. A 55-60 gallon soaker tub would take (seemingly) forever to fill with either.



FWIW: I lived with a ~100,000 BTU/hr output tankless in MA for about 15 years, and it was just fine running a 3gpm gusher shower as long as nobody tried to fill the old school clothes washer when somebody was taking a shower, and filling a standard tub was not painful. I currently heat my house with a combi-system that has a buffer tank and the burner never modulates higher than about 60,000BTU/hr even when all zones are calling for heat AND somebody is taking a shower, but it's being augmented by a drainwater heat exchanger that's returning another 25-30K back into the shower stream- call it 90K total. If you don't think you'll be doubling up on showers and the shower flows are 2gpm or less you'll have no problem at all with the 140K output of the -180. (If you wanted or needed higher showering performance you could also spend about $500-600 and hack in a drainwater heat exchanger too. There's not huge financial payback at current natural gas prices, but it gives you quite a bit of margin for showering flows.)

Bucket-test your shower heads to actually measure the flow: Take a large bucket and a gallon jug, fill the jug and dump it in the bucket, and mark where the 1-gallon level is. Then turn on the shower, and time how many seconds it takes to fill the bucket to the 1-gallon mark. This doesn't need to be super-accurate- round it to the nearest 5 seconds:

If it fills it to your mark in 15 seconds you have a 4 gpm gusher and you'd be wise to change it even with the-240.

If it takes 20 seconds it's a 3 gpm gusher, and you wouldn't be able to run 2 showers even with the -240, but you'd be able to run one shower with any of the series.

If it takes 30 seconds it's 2gpm, and the -180 would be able to handle 2 showers in summer but not during the coldest part of the winter.

If it takes 35 seconds or longer it's 1.7 gpm (which is pretty common even with standard heads, depending on the local water pressure) even the -180 could manage 2 simultaneous showers even in the cold part of the winter.

It's ignition cycles and temperature cycling, not burn time that takes the bigger toll on the boiler itself, and that's the potential drawback to the -240:

The min-fire fuel input of the -240 is 18,000 BTU/hr , so at 95% combustion efficiency that would be about 17,100 BTU/hr out, which is probably about or slightly above your average wintertime heat load (depends a lot on the particulars of the houses.) That means even in the dead of winter it will be cycling on/off half the time.

With the smaller unit it would modulate with much longer burn cycles once you dial in the outdoor reset curves, which is more comfortable, and less wear & tear on the boiler. The min fire input of the -180 is 14,000 BTU/hr which at 95% efficiency is ~ 13,300 BTU/hr out, which is probably quite a bit LESS than your average winter heat load, so if you dial it in correctly it would spend the majority of the time modulating with long burns rather than cycling on/off.

If you keep the Amtrol you'd have shorter tub-filling times that would not change much with the seasons, but it would also make going with a combi-boiler pointless. Odds are pretty good that even a 50K mod-con would cover your heat loads even during the Polar Vortex events. If you have a recent oil bill with a "K-factor" stamped on, or oil usage between two EXACT winter dates it we can make a fairly accurate upper-bound estimate on your actual heat load, which may help with this decision.
 

Rob P.

New Member
Messages
4
Reaction score
0
Points
1
Location
Rhode Island
Wow thank you again for all your help.

Yes I was referring to fin'ed tubing on my 100 ft estimate.

I will absolutely try out the water flow hack you have suggested. We bought the house in a foreclosure and will be remodeling everything including bathrooms and applances so I am sure it will be easy to take note of the flow rates when buying new equipment.

Unfortunately our purchase situation also means we have 0 data on energy consumption.

I figured keeping the Amtrol would defeat the purpose of going tankless but it was worth asking. Perhaps I will hold onto it until next winter just in case I need a backup.

You had mentioned the outdoor reset. I could be mistaken but I thought I remembered the contractor saying they did not use these. Will this be an issue for the longevity of the unit? By the way what is the typical life cycle of one of these?

All in all you have helped me a lot with the information shared. I will be confirming total fun length per zone along with current water flow rates in the bathrooms, even though this will soon change. Assuming my current information is accurate I am more comfortable going with one of the NCB (not NBC as my title says) units.
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
If you acually have 200' of fin-tube and a design heat load of say, 40,000 BTU/hr (possible, for a 2500' house, though 30-35K is more likely), that's 200BTU/hr per foot of fin-tube, which takes an average water temp of ~120F, which is well within the condensing range. In that case you can probably skip the outdoor reset control and do just fine. But if you actually have only 150' you might have to bump the water temp high enough that it'll be above the condensing range to cover the 99% load.

Outdoor reset doesn't have a dramatic affect on longevity, as long as the boiler temps & flows are properly tweaked in for the load and doesn't short-cycle. I have no idea what the typical anticipated lifespan of the Navien is- with maintenance and properly tweaked in I'd be surprised if it didn't last at least 15 years, but if it's short-cycling and overpumped it might not last 5.
 

Rob P.

New Member
Messages
4
Reaction score
0
Points
1
Location
Rhode Island
I measured the upstairs last night and came up with about 110'-115'. I will get measurements for the downstairs tonight but feel it will be close to this. I also have the contractor coming back out soon to take a closer look before I commit to this unit. Your information along with the other post has given me enough background to make at least a partially educated decision. I will follow up after the contractor fine tunes his diagnosis.

Thanks again!
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
At 100'+ of fin tube per zone you should be able to run at condensing temps without short-cycling the -240. The question then becomes whether it's enough that you'd be able to hit 95% efficiency running at a fixed output temperature, foregoing outdoor reset control.

With fin-tube the output at temps below 110F becomes very non-linear making it a bit tough to tweak in outdoor reset curves perfectly, but it's probably still worth setting it up with outdoor reset unless your true 99th percentile heat load is low enough that it can be met with 110F average water temps (say 115F out, 105F return.) That's a cool enough temp to deliver mid-90s efficiency. If it needs an AWT north of 130F at +10F outdoor temps, you DEFINITELY want to use outdoor reset, since that would be above the condensing range, even though the vast majority of the time you'd be able to deliver sufficient heat at low condensing water temperatures.
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks