Combi-Boiler NCB 240 (Navien)

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Plumber'sSon

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I am looking to replace an old boiler.
A contractor suggested me to install a combination of a boiler and tankless water heater. He works with Navien equipment.
He explained that if I replace just a boiler I would need to change the venting of the water heater as the boiler is a direct venting device and now both the boiler and the water heater vent through same B Type Vent

Any opinions on combi boilers and particularly on Navien?

Thanks
 

Dana

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Combi boilers have their place, but without more particulars about the fuel use, your system radiation size/type and the number/size of each zone, there's no telling if YOUR place is one of those places.
 

Plumber'sSon

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Natural Gas; Forced Water, baseboard; ~1500 sf main level - 1st zone; ~550 sf Basement - 2nd zone
 

Dana

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How many feet of baseboard are there on each zone? It matters- here's why:

The minimum fire output of the NCB240 is something like 17,000 BTU/hr, so for zone radiation output to balance with the boiler output it needs to be able to emit something like 17,000 BTU/hr. The temperature of the water makes a huge difference in the amount of heat emitted. If the boiler is putting out signficantly more heat than the radiation can emit, it rises in temperature until it either balances or it hit's the programmed high-limit, losing efficiency to either non-condensing temperatures or rapid cycling (or both.)

In order to get ANY condensing efficiency out of the NCB240 the water returning from radiation has to be below 125F, (average water temp of maybe 130F) and to break into the mid-90s for efficiency it needs to be below 115F, which means an average water temp of something like 120F. Fin tube basboard typically delivers 200-225 BTU/hr per foot of length @ AWT= 120F, , 250-300BTU/hr per foot @ 130F AWT, so to balance against 17,000 BTU/hr you'll need something like 70-80' of fin tube per zone for it to run in condensing mode without short-cycling. You can cheat that a bit if the thing can be programmed for a significant temperature difference between high/low limit (I haven't read the manual to see what the options are, but you should before accepting that as a solution.)

So, do at least the napkin-math on the radiation & output. If it's possible to add either radiation or enough thermal mass that the minimum burn times are at least 3 minutes at min-fire you'll do OK. (I'll save the thermal mass napkin-math for later, and only if you think you're close enough on the amount of radiation at least for the upstairs zone.)

The total design heat load matters too. If you have gas-use history with the old boiler we can use fuel-use against heating degree days to determine a fairly firm upper bound on the heat load. A mid-winter gas bill with both the fuel use and the EXACT meter reading dates it covers, a zip code (for weather & outside design temp data), and the input BTU and D.O.E. output BTU of the old boiler would be enough. If you used auxilliary heating (wood stoves, etc.) or were off on vacation for the gas billing period, pick a different bill.
 

Plumber'sSon

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105 '' fin tube upstairs and 55 '' in the basement. I think the pipe is 3 / 4 ''
The guy told the NCB 240 is modular and can go as low as 18,000 BTU
I have not done calculation on my own. I think you are better at it.

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Plumber'sSon

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It is ' (feet) of course, rather than '' (inches) in the post above: 105 ' and 55 '
 

Dana

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The 18,000 BTU/hr is an input number. With 18,000 BTU/hr input assuming the return water is cool enough to deliver 95% efficiency the output is more like 17,000 BTU/hr. If you have to run it at a higher temp to eliminate short-cycling the efficiency & output BTUs will be lower.

With 105' on the first floor all as one zone it should be fine, since 17,000/105= 162 BTU/ft, which means it still balances at ~110F AWT, a temp at which it WILL deliver ~95% combustion efficiency.

It's probably worth adding another 10-30' of fin-tube to the basement zone if you have room. At 55' you only have 17,000/55= 309 BTU/ft which would require ~140F AWT to balance, and it would be prone to short-cycling if you ran it at condensing temperatures. But at 75-85' you'd be at 2oo-225 BTU/ft, which means you CAN run at temps cool enough to condense. There may be some fine tuning if you allow the output temp to run as low as 120F, but at 125F out/115F return it'll probably be just fine if you have at least 75' of fin tube on the zone.
 

Dana

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BTW: With fin-tube baseboard as the heat emitters you'll need to set the lowest temp boiler output to about 120F anyway, since below that the output response is very non-linear. That should set you up for ~105-115F return water, which is well into the condensing zone, delivering mid to high-90s for combustion efficiency.
 

Plumber'sSon

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Hi Dana,

Thanks a lot for your input.
I need some time to digest the info.
The boiler has been installed. If it does not deliver what was promised I will use your data when asking for fine tuning.
 

Dana

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Please report back how it's working out, even if it DOES deliver, eh?
 

Plumber'sSon

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Hi Dana,

Reporting:

This Monday was pretty cold day here. The temperature in the house did not rise above 65 F while it was set for 68 F. The schedule was to keep 59 F until 5:00 PM and 68 F until 10:00 PM.
At 7:10 PM the temperature was 62 F, at 8:30 PM - 64 F and at 10:00 PM - 65 F.
I was watching the boiler getting on and off without reaching the target temperature.

I called the guy who installed the boiler and next night he spent more than two hours trying to tune the boiler.
First, he changed the internal switch for CPVC exhaust pipe rather than PVC. He explained that the hot exhaust was triggering boiler's premature shut off. Well, 5 minutes later it became obvious it was not the case.
To make the long story short: he was experimenting with pump speed, supply and return temperature settings, bringing them up and down in various combinations.
During his experiments I was touching the pipes in radiators. They would get extremely hot one moment and couple minutes later I was able to keep my hand for quite long time. Probably, the temperature was at those times around 135 F.
The guy told me that the water circulates too fast not releasing enough heat through fins, which makes return temperature very close to supply one. (Probably, this was turning the burner off). Finally, he set pump's speed to lowest possible and tried to close the valve (for return, I guess) to make the circulation even slower. He set the supply temperature to 170 F and the return temperature to 148 F. As a result we could get 68 F in the house.

Next two days it was pretty warm outside, so temperature easily reached 68 F. Today I came home at 5:45 PM. The schedule was to run 60 F until 5:30 PM and get 68 F after. It took two and a half hours to reach 65 F. And I think the boiler was short cycling again. By 9:00 PM we got 68 F.

So, the main question now not how get the boiler to work at highest possible efficiency but rather how to reach the setting in reasonable time.
I feel that if the boiler did not shut off so often it would have warmed the house faster. (I might be wrong here).

Can you give an advise?

Do you need more experiment data?

Thanks
 

Dana

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If you want the thing to run at highest efficiency you have to dial down the temps. When it's running at highest efficiency it should be able to keep up, but won't be able "...to reach the setting in reasonable time". That means you DON'T use temperature set-backs, but rather just keep the house at constant temperature.

The efficiency you lose by not setting back is more than made up in raw combustion efficiency, by being able to run in condensing mode all of the time. With 170F out, 148F back it's a reasonable delta-T (22F), but zero condensing efficiency- it's probably getting about 86% efficiency in that mode. Running it cooler the delta-T will shrink a bit, but once the return water drops under ~125F the efficiency will soar with every degree cooler (until the return water temps are under 110F.
 

Jac04

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Any updates on this?

Dana helped me set up my Navien CH-180 ASME. If you want to really learn something about how to set up one of these condensing boilers, pay careful attention to everything Dana writes. It is fantastic to have someone like him that will spend the time to explain everything in detail. In my case (and it sounds like yours as well) the installer had no idea how to set the unit to achieve maximum efficiency. They set mine to 160F supply and walked away.

I have a similar set-up to yours baseboard-wise. I'm running at 130F supply temp with minimal set-back at night (only enough to keep everyone from feeling too warm at night). The only reason I am not set lower on supply temp is response time to my nightime setback. In the morning, even with the 130F supply temp, it still takes a little while (like 1+ hours) to go from 67F to 70F, especially when it is very cold outside.

My 2nd zone is small as well. Because my 2nd zone is so small and it was going to be a real PITA to add more baseboard, I found my best way to run is with my 2nd zone valve always open. This way I get nice long burn times, lowest possible return temps, and it just seems to work out for me.

One other thing to add: On my CH-180 I am able to set the maximum firing rate for heating. If I leave it at 100%, it seems to fire hard, hit the supply temp too quick and overshoot it, which causes it to stop firing until it gets back down to the low set point. I now have mine set to 35% max firing. It is still able to hit supply temp in a reasonable amount of time, and it usually maintains 128-132F supply temp. You may want to try adjusting the max firing rate if possible to see if it helps.
 
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Plumber'sSon

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I absolutely agree, Dana is fantastic.
I have been totally swamped with the kitchen remodeling last couple months, so have not spent much time with the Navien. I really need to tune it up. Hope to resume my boiler related activities soon.
Thanks jac04 and Dana
 

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Hi:
Based on my plumber recommendation I bought a Navien NCB-240 just 2 months back. After the installation it was working for only 20 days and then we started having problem every day. Hot water will stop without any warning in-between our shower and heating. My plumber added some pressure regulator valve, etc in the hope of resolving this problem but we are still suffering from this unit. Our 8 years old boiler and heater were working without any major problem for the whole 8 years but after spending $4000 for this unit I think we bought a junk. When my contractor contacted the technical service they diagnosed the unit for almost 30 min and tentatively found that some internal unit is not working as expected. They are going to mail some parts and we see what would be the turn out of this. I am still wondering a highly priced brand new unit that is giving this much problems. Last one month alone I got a water bill that is almost 3 to 4 times when compared to my old lower efficiency boiler and heater, because of water wastage and waiting for the hot water to show up from the unit. I want to return the unit to the store I bought but unable to do so because of the 30 days return period limitation. I strongly recommend that Please do not buy this product and do not waste your $4000 like me.
 

Dana

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Any time you buy modulating combustion appliances it's important to have good local product support. Navien offers good value for the type of equipment, but seem to have higher manufacturing defect rates than some (no data, just WAG), and local distributor support & training isn't always there.

For big-burner units like the NCB-240 installer error is more common than the equipment failure, particularly regarding fuel-line sizing issues. It sounds as if this installer didn't assess the fuel capacity requirements ahead of time and tried to fix it with a regulator swap. It may or may not need fuel line upgrading. In warmer weather when incoming water temps are higher and it isn't sharing a space heating load an undersized fuel line might not have much in the way of symptoms, but as the necessary firing rates climb, so do the pressure fluctuations and shortfall events.

This isn't the first tankless-combi nightmare to show up, and it's often easier to blame the equipment than the installer, but that blame isn't always correctly placed.
 

Jac04

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As Dana said, these units are very sensitive to improper installation. I know first hand. I fought fuel supply issues with mine that were the sole result of the installer not knowing what they were doing. Now, with a properly sized propane supply system, things are good. Inadequate fuel flow will usually result in a severe rumbling when the unit fires hard (like on a call for hot water).

My bet is that your water flow sensor is fouled. This is another fairly common problem with this unit. Luckily they are fairly inexpensive and easy to replace. Mine has hung up a couple of times, but only after disturbing the DHW piping and potentially getting debris in the piping. Both times I simply valved-out the unit from the DHW and removed/cleaned the inlet screen, which also allows some water to backflow through the flow sensor & get rid of any debris.
 

BCDave

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My info: Navien 240 combo unit. 3 zones. Basement has 23' of fin. Upstairs has 23' of fin & the bedroom has 6'.
After about 3 weeks, we started getting Error 12. It happened once a day for about a week then it was 3-4 times a day and now it won't stay on for 15 minutes. We thought it was a grounding issue, like it says in the manual, but I ruled that out by turning on the tap to hot for 20 minutes and it worked fine.
I tried changing up the unit to run on Return temp! But it won't work. The flame will kick on but then shut right off again after about 2-3 seconds. We thought maybe high temp return setting was holding us out, so I changed that from 145 to 155. Still nothing.
From what I've been reading.. I need about 100 more feet of fin but I dnt have the room.
So! I'll try setting it at 120°F instead of 140°F and see what happens.
Any Ideas?
 

Dana

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Error 12 isn't always a grounding error- it could also be gas pressure problem or a failing or crudded-up flame sensor. Based on the symptom of shutting down after 2-3 seconds my guess would be the flame sensor. If the flame sensor has developed a varnish or carbon-coating you might be able to get it working by polishing it up with a plastic scouring pad. (Avoid using sandpaper or steel wool on it except as the last Hail Mary if the crud is just too hardened in place.)

17,000 BTU/hr into 6' of baseboard is going to short-cycle the thing like CRAZY at any water temperature! With your stubby chopped up radiation it'll see more burn cycles in the first week than a most units see in a year, which puts a lot of wear & tear on the equipment. Even if you get it working again the prospects for efficiency & equipment longevity aren't very good unless you install a decent sized buffer tank &/or dramatically more radiation.
 

BCDave

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Thx for the reply Dana!
Did my "Professional" put in the wrong size unit? Maybe?

I was thinking that maybe if we put a bunch of tubing on the Return side and put it up in the rafters of my basement, kinda like floor heating, that could bring down my Return temps..?

I'll check that sensor tonight.
 
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