Combi-Boiler NCB 240 (Navien)

[Dana]

There is no boiler that can modulate down enough to support a separate zone with only 6' of baseboard. Individual zones of only 23' are pushing it to run at condensing temperatures. The only solution other than a lot more radiation per zone is thermal mass. So it's not really the size of the unit that's wrong, it's just the wrong TYPE of unit. A high-mass tank-type combi heater would be more appropriate.

 

[Inkiii5]

Let me tell you my story...

So for one, I have radiant heating with pex through the house thats 1200sqf divided into 3 zones, so 2 rooms plus the living room.

Now I ran an open system heating and domestic off of a simple $1k tankless water heater with 3/4 piping that includes the mechanical setup with pumps(boiler loop set-up) ran for 3 years FLAWLESSLY open tap instant 140degree, pump kicks in instant 140deg. Unit still works fine.

So I decided to divide my heating and domestic so heating will be on a closed loop and so I decided to purchase this Combi boiler Navien-180 NCB

Now I have LP, and Navien only sold in NG but they say conversion kit is included - so im thinking this is a scew on screw off type deal NoProblem.
I redesign my heating panel with primary/secondary loop all in 1inch I get my Navien NCB180. Falowing instruction, replaced the gasket for LP
next step - connect a Combustion Analyzer to adjust the CO levels, so i google tool cost $1k forget that option 2 connect a Manometer $200 tool to adjust the levels. Ok fine i bought the manometer the readings jump i dono wtf is going on I call a technician. This guy comes over says ohh youve tempered with the gas valve I need to call Navien and ask how many turns need to be made to be set up for LP. He called got the number turned it was working after 30 min shut off with E012 (flame Failed)

Get this.

I call Navien and say my boiler shuts off with 012 whats wrong - the moron tech on the line says WE CANT RELEASE ANY INFORMATION TO YOU BECAUSE YOUR NOT A LICENSED TECHNICIAN. I said I paid $150 an hour for some guy with a license and all he did was call you and you told him to turn the gas valve X amount of time, do i look like a moron to you ? you could of told me that and I could of made the turns myself, I do know how to count. Sorry I cant help you.....

Then this MORON TECH says get a technician have him call us we need to diagnose the problem, maybe your heating system is designed wrong your pumps are not circulating enough water perhaps we need to go to a 1.25 or even 1.5inch pipe.

I told him are you a F***ing moron ? what does my pipe size, my pump GPM out has anything to do with the boiler shutting off ........ ????????
I ran SAME SYSTEM on 3/4 pipe for 3 years not a single issue. Smaller pipe means LESS GPM with that setup i had 2.3 GPM on the entire 3 loop system if all 3 pumps were active. Now with Primary/Secondary and 1inch all the way I get 4GPM on one loop and could get 2GPM on the other 2 zone cause I installed Alpha pumps and with auto mode they push 1GPM each.

Bottom line is this $150 an hour tech didnt turn the gas valve all the way - so not enough gas was coming in and it was shutting off. I turned it Half a turn myself and works fine now. The point is with this CRAP Navien boiler I CANT GET TECH SUPPORT I NEED TO SPEND $150 an hour and they are always booked and wait 1 week before they show up for Navien to tell them that they need to turn the gas valve a couple of turns left or right.

I am not leasing a boiler where they are paranoid if i do something wrong to it that they require a licensed tech, I bought it its mine I have the right to know this information but apparently I cant.

This moron tech also gives me an example -"what if a part in your car broke do you go buy it yourself and fix it? I SAID YEEEEEEEESSSSSS" he says no you take it to a dealer have them diagnose it buy part and install it for you.

NAVIEN S**KS their TECH S**KS they employee a bunch of idiots and they wont release any information to you the owner you must spend MORE money waste more of your time just to have the gas valve turner 3 clicks or so.........

Now Navien 180 cost $2k, I could just buy another tankless $1k unit and separate my domestic and heating ,instant 140 deg, cheap, runs flawlessly out of the box. But I do have radiant so its suitable for low temp.

 

[Sponsor]

 

[Dana]

Hey, don't candy-coat it for us- TELL IT LIKE IT IS!

Yes, licensed certified techs can be idiots too, and there a limits to how well the manufacturer can control that. That tech sounds pretty worthless other than his ability to squeeze technincal info out of the manufacturer.

But Navien's unwillingness to share technical details over the phone to help homeowners figure it out is pretty standard across the entire HVAC industry. ( The notable exception in the tankless world is Takagi.)

 

[Jac04]

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.
..
Any Ideas?

I think it could still be a gas supply problem. I was getting Error 012 on my CH-180 (only on heating and not on DHW) due to gas supply issues, and it got worse when the outside temps dropped.

 

[BCDave]

Hey. Sorry for the late response.
Jac - I'll see if i can find the equipment to check the gas supply. I'll check the manual for the right psi or kpa or whatever it is before we do the test.

What the plumber ended up doing is installing a AquaStat. Its on the return side, set at about 135. The Supply side is 150. What ends up happening is the unit will turn on for about 3-5 minutes then the aqaustat will hit 135 and shut the boiler down. It turns off for about 3 minutes then fires back up. The return temp will get down to about 128 before the unit fires back up.
So.. we have heat in the house. Is it 95% efficient? NO. ( or whatever Navien is supposed to be )

 

[BCDave]

What I ended up doing, to get some heat in the house, is turning the heat up on the supply side. From 150 to 170 & changing the Aquastat to 150, to keep that 20 degree spread. Now theres heat in the house.. but its still doing the samething. Cycling on and off every 3-5 minutes.
Could there be a bunch of air in the glycol lines & messing it up that way?
Why didnt it work when i tried to run off the return temp Ages ago!? The unit is setup up for that.. I read it.

Let me know what ya think.. I have heat tho..

 

[Dana]

AT an average water temp of 160F you're only getting 450-500 BTU/foot out of the baseboard. Your longest zone is 23' so you're getting something like 10,000-11,500 BTU/hr of heat out of the fin-tube into the rooms, but the Navien is putting at least 17,000 BTU/hr of heat in at it's minimum firing rate. That's more than 5000 BTU/hr of excess heat going in.

You probably don't have more than 25 lbs of water in all the plumbing on a 23' zone, including the boiler heat exchanger water. At 5000 BTU/hr of excess the temperature of the water in the zone rises at 5000/25lbs= 200F per hour, or 3.3F degrees per minute. So in 3 minutes the water rises 10F, in 5 minutes it rises 17 F. The boiler is designed to maintain a temperature within some range around it's programmed point (maybe that range is even programmable- read the manual), but it's not at all surprising that it would be cycling at a high rate. It's probably burning 2-3 minutes on, 1-2 minutes off, repeat, unless both of the 23' zones are calling for heat at once at which point it stretches out a bit, but it'll still be cycling. Ideally you would get the minimum burn times north of 3 minutes/burn, and fewer than 5 burns per hour, but I'm not sure that's possible without either adding thermal mass or adding more radiation (to each zone.)

At 150-170F it's nowhere near the condensing zone, delivering something like 85-86% efficiency (best case.)

 

[Reas]

Hey Dana,

How do the numbers change if you have cast iron and not baseboard? We have 1/2" copper supply lines to our cast iron... Had a guy recommend this system, just want to make sure it's the right fit for our house. Thanks!

 

[Dana]

I've spelled out a lot of it in this recent blog piece but I'll review it specific to your case here.

To analyze the cast iron you need to estimate three numbers first:

1: The total square feet of equivalent direct radiation.

2: The total water mass/volume in the system, including the distribution plumbing and radiators. (For half inch copper it's about 1 gallon (8.34lbs) of water per 100' of pipe. )

3: The thermal mass of the cast iron in the system.

In an ideal system the ratio of the output of the boiler at minimum fire will be less than 50 BTU/hr per square foot EDR. At 70 BTU/hr per foot the thermal mass of the system may keep it from short-cycling and still run at reasonable efficiency but it won't magically turn it into a right-sized boiler that modulates.

You may have to look up the water volumes and shipping weight of new radiators of similar size & shape to figure out the water volumes. The relevant number for both is the weight/mass. The specific heat of water is 1 BTU/lb per degree-F, the specific heat of cast iron is 0.11 BTU/lb per degree-F. So to convert the mass of the iron to water-thermal-mass equivalent, multiply it by 0.11, and add it to the water mass.

As an example case, say you have a zone with a half dozen Sunrad or Burnham Radiant type radiators that add up to 225 running inches, and have 80' of total supply & return plumbing, all half-inch. According to Burnham's specs there is 1 square foot EDR per running inch, and 0.15 gallons per every 2.25" section, which is comparable to the OCS Cast Ray. Shipping weight on the Cast Ray is 5.1lbs per 2.25 section, so we'll use 5lbs/section as iron weight.

Plumbing water mass: 80' of pipe /100' per gallon is 0.8 gallons, x 8.34 = 6.7 lbs.

Radiator water mass: 225"/2.25" per section is 100 sections, x 0.15 gallons/section = 15 gallons, x 8.34lbs/gallon= 125 lbs.

Radiator cast iron thermal mass: 100 sections x 5 lbs/section is 500lbs of radiator, x 0.11 = 55 lbs water-equivalent.

Total thermal mass: 6.7 + 125 + 55= 187 lbs of water-equivalent.

Total sq.ft. EDR: 225" x 1 sqft/inch= 225 square feet EDR.

Ideally the boiler's min-fire output would be under 50 BTU/ft-EDR, which would be 225' x 50= 11,250 BTU/hr, in which case you'd be able to run at mid-90s combustion efficiency continuously, without cycling the boiler on/off. If that's the case the boiler is sized reasonably for the radiation, and you should be able to tweak the system to get it to run high

But let's say it's the NCB 240, which has a min-fire input of 18,000 BTU/hr, and at 95% efficiency would be delivering 0.95 x 18,000= 17,100 BTU/hr. (That would be a ratio of 17,100/225' = 76 BTU/hr per foot EDR.) The rads are only emitting 11,250 BTU/hr which means there is (17,100 - 11,250=) 5850 BTU/hr of excess going into the system. When that happens the temperature will rise until the boiler's internal controls sense that it's over the outdoor reset setpoint and quench the burner, and wait for the rads to emit enough that the temperature drops back into the right range before re-firing the burner.

A typical mod-con will have somewhere between a 5F and 10F difference in temp between the burner-off and refire, and this is where the thermal mass of the system comes into play. It takes 1 BTU to raise the temp of a pound of water 1 degree F, 5 BTU/lb to raise it 5F. So the BTUs to raise the 187lb thermal mass of the system 5 F is then 5F x 187lbs= 935 BTU. At the min-fire excess of 5850 BTU/hr that takes about 935/5850= 0.16 hours, or 9.6 minutes.

That is plenty long, not even close to a short-cycling situation, and would be plenty of radiation + mass to get 95% efficiency out of it. But you can see that as radiation EDR shrinks, the excess BTU is more, and the thermal mass less, and with much less radiator than that the minimum burn times would shrink an order of magnitude. You need burn times of at least 3 minutes and fewer than 5 burns an hour to be in the "reasonble" range. If it looks like 10 burns/hr or more at min-fire it's going to take a toll on equipment longevity and efficiency.

If you run this napkin analysis on your system and it's close, be sure add in the water volume inside the boiler as well.

 

[DanB]

Hi all - have found the advice on this thread and linked pages extremely helpful. I'm looking to do an oil-to-gas at my house (zip 02184) and had a few questions from my napkin math. Trying to decide standard vs. mod-con, have gotten plumbers with different opinions. The house is currently broken into 8 zones, but realistically has 3 zones, an upstairs where we live, an unfinished (but hopefully in the future finished) basement, and a very large garage. We have forced water with most of the house covered by cast iron baseRay baseboards. I will try to track down the current oil bills and snag the boiler info as well, but first wanted to pose the zoning question -

Exact (ish) stats for the 3 zones are:

Zone 1: Upstairs (1180 sq ft, 67' cast iron baserays, 12' fin, 1 cast iron radiator (68 EDR) = 20,000 BTU/h @ 120 F, 152 BTU/F Thermal Mass)
Zone 2: Basement (570 sq ft, 19' cast iron baserays, 9' fin = 5900 BTU/h @ 120 F, 37 BTU/F Thermal Mass)
Zone 3: Garage (910 sq ft, 42' fin, 2 cast iron radiators (120 EDR) = 11600 BTU/h @ 120 F, 35 BTU/F Thermal Mass)

Note, thermal mass calcs include only the radiator iron and water inside the radiator, not the system piping.

A couple questions -

1. By my math, a Navien 240 (as suggested) w/ a 17,100 BTU/H min output would short-cycle the Zone 2 like crazy. One option would be to combine 2&3 for now, and install more cast iron in 2 when we finish it. However, would the higher efficiency just be swamped by keeping Zone 3 (garage) much warmer than we would otherwise?

2. BaseRay's website seems to show 61 BTU/H/sq ft EDR. This is 20% higher than typically quoted here and given the importance of that #, just wanted to see why folks thought there was a discrepancy

3. What range of input/output temps should be run for these calculations? I used a 120 return to generate the above outputs and was considering a 130 output when looking at cycle times and thermal mass.

4. A couple concerns going from 8 zones to 3 (or 2) - 1st, more expense for the initial install. 2nd, We are like the house pretty cold at night, and are relatively tolerant of cold in some of the obscure zones (a front porch, a downstairs bathroom, etc.). Part of me feels like the uniform high efficiency won't be as much of a value-add over just managing 8 zones with thermostats. Thoughts?

Thanks all -

 

[Dana]

The BTU/square foot EDR numbers listed by the manufacturers are under some ideal conditions, where the convecting radiators/baseboards are never behind furniture, with no dust-kittens impeding convection, etc. You can pretty much count on at least 50 BTU/EDR' in real world installations but 61 BTU could still be it's performance in a laboratory or at some AWT other than 120F, or perhaps that's assuming a leaving water temperature/return temp of 120F (= AWT ~ 125F )

The thermal mass numbers are more useful if summed for the entire zone including the thermal mass of the distribution plumbing AND the water volume of the boiler itself.

Given the size of the house t's likely that your whole-house heat load at +12F (or whatever Braintree's 99% temperature bin is) would be on the order of 20-25,000 BTU/hr (assuming the garage is pretty leaky, and the foundation is un-insulated), which by itself makes the Navien a lousy choice, since it'll be cycling rather than modulating at your average mid winter temperature. With your existing zones you'd be fine with mod-con that modulates under 10K, but if you want to keep it micro-zoned you can still do just fine designing it around a condensing tank type hot water heater rather than a modulating combi.

To know what water temps you'll need requires a room by room heat load calculation along with the amount of radiation in each room, but that's not an exercise well suited to design by web forum. Short of a Manual-J, building & maintaining a computer spreadsheet heat load model with room by room and radiation info included using I=B=R methods, can get you reasonably close for figuring out your water temp requirements. But it's still worth running the heat load numbers for the whole house, which can be derived from fuel use. If you have a heating history on the place, running a fuel-use based heat load calculation based on your oil billing (particularly mid to late winter) is still important as a sanity check. If you tend to keep it 60F at night, 60-65F during the day with only a few rooms at 68F for only a few hours, use 55F as the base temp for making the calculations, and only go off the fuel use and boiler efficiency, not the K-factor (which presumes base 65F.)

Tankless combis are generally only a good fit for houses with fairly big heat loads and fairly modest hot water needs, and that's not the likely description of your house. A tank-type condensing water heater big enough to fill your tub will usually have more than enough burner to handle the heating loads of a house your size and simultaneously serve at least of hot water than the typical 40-50 gallon gas-fired standalone.

Don't combine the basement zone with the garage zone unless you intend to air seal and fully insulate the garage to code-min when you finish off the basement.

Before finishing the basement, check back here for strategies on insulating the foundation & band joists. (I've covered the topics multiple times on the remodel forum section on this site, and elsewhere.)

 

[DanB]

Thanks Dana - much appreciated!

 

[DanB]

Thanks again Dana -

A couple quick follow-ups.
1) For the tank-type, do you prefer the Phoenix or the Polaris?
2) I read somewhere that some people combine a tankless with a storage tank, the logic being that if one breaks, they only have to replace one unit vs. effectively both w/ the tank-type condensing heater. Thoughts on this?
3) What are the primary disadvantages for using a tank type heater? Just the upfront $$?

 

[Dana]

1) Phoenix (or Versa)- especially for MA residents (HTP is headquartered in MA, with good local support.) The Polaris can be prone to short-cycling, require more system designer sophistication (&/or a warranty voiding hack to the electronic controls.)

2) Flawed logic, comes with significant system design risk, won't likely hit the same efficiency as a well engineered combi. If you screw up the tankless + storage system design you'll be lucky to get more than 7 years out of it. (For the record, I'm using a tankless as the combi-boiler in my house, but with a "reverse indirect" buffer tank for the domestic hot water, storing heating system water at temp rather than potable water, and only running heating system water through the tankless. The potable hot water is less than 2 gallons inside the heat exchanger coils inside the buffer tank. There are pros & cons to this approach too, but works OK in my all low-temp heat emitter heating system. ) A stainless storage tank or stainless water heater should last pretty much forever (OK, maybe only 25-30 years ) , unlike a glass lined mild-steel tank.

3) Tanks have higher standby losses than boilers (more than made up for by not short cycling). From a total system cost point of view a condensing hot water heater based system is often cheaper than a boiler + indirect.

 

[DanB]

Thanks Dana - super helpful!

 

[DanB]

Getting close on this - down to Versa or Phoenix Light Duty. Having a little trouble picking the 2 apart. Is main difference just the total BTU/hr? The Versa's 130K BTU/hr seems way high, but does have a 10:1 turndown and obviously the thermal mass of the tank itself to prevent short-cycling. The Phoenix Light Duty has 75K BTU/hr, but only turns down to 25K BTU/hr.

Also - if considering solar in the near-ish future, would it make sense to go ahead and get the solar version to make that hook-up easier?

Thanks again!!

 

[Dana]

The Versa is already set up as a combi-boiler with separate potable and heating system connections. The Phoenix LD would require a (small) amount of hydronic heating design work, adding an external heat exchanger to isolate the potable water from the heating system water, etc. .

From a thermal mass point of view even the smallest Phoenix has PLENTY of thermal mass for limiting short cycling at a firing rate of 25,000 BTU/hr= in (x 0.95= ) 23,750 BTU/hr out. The smallest Phoenix has about 417 lbs (50 gallons) of water in it. To raise 417lbs water temperature even five degrees takes 417 x 5= 2085 BTUs. At 23,750 BTU/hr that takes (2085/ 23,750=) 0.088 hours, which is (x 60 minutes/hr= ) 5.28 minutes.

And that's just the water in the tank, there's a bit more water mass in the distribution plumbing and baseboards, and still significant of thermal mass in the iron of those BaseRays. With any of the zones calling for heat the burns will be longer, since some of the heat is being radiated/convected into the house, and there are no conditions under which it would be doing more than 4-5 burns per hour even under worst case conditions. Under most conditions it would be 2, maybe 3 burns/hr during the shoulder seasons, and less when it's actually cold out. It will only kick up into high fire when there is a sustained draw of hot water, such as filling a bathtub or taking a shower.

It's hard to make any kind of financial rationale for solar thermal systems, in the age of cheap natural gas and cheap photovolatic (PV) solar. Roof real-estate is not infinite, and you'll get a better ROI in MA putting PV panels rather than evacuated tube thermal systems, given MA style electric rates, and MA style incentives for solar PV. PV is also much lower maintenance, with a lot less to screw up than thermal solar. Only if you were committed to the solar thermal for some other reasons would it be worth considering the solar versions of the Versa. (A Stiebel-Eltron Accelera 300 heat pump water heater and 500-600 watts of PV panel would likely cover hot water heating loads at a lower net cost of energy than any solar thermal solution, but it would be completely separate from your heating system.)

 

[Rich F.]

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