Navien Combi NCB-240/130H LP Usage

huntsaver

I’ve had my boiler running for a few months now and it seems to be using more LP than I anticipated. It only runs 2-3 hours per day and maybe 4 on the coldest days it has seen 10-15 deg F. Typical cycle time is 30-45 minutes.  It usually runs between 30%-60% capacity. I burned 100 gallons in the month of March with 61 hours of runtime according to my NaviLink.  40x60 barn well insulated with CC spray foam and high end doors.  It heats nice and stays warm but I really just want to know If the usage seems too high.  No domestic hooked up yet.  I’ve tried low mass and high mass setting curves.  .02 gal flow all the way to .08.  I have 9 runs that are 280-285ft in length. 12” apart and I have 2” formular 250 and vapor barrier.  Remote sensor is installed also. Any other info that might help just ask.   Thank you!!

huntsaver

Contractor did set the fuel pressure with a manometer but we have not checked combustion analysis. 

Hot_water_fan

@huntsaver - what temps are you using? Assuming an average March temp of 30 degrees, 100 gallons x 91,500btu/gallon x 95% efficiency = 8,692,000 btu output. Across 31 days averaging 30 degrees, that’s about 8000 Btus/ HDD, so a heat load of about 20kBtu at 5 degrees out if you’re heating to 65 degrees inside. That’s an output you should be able to meet with an average water temperature around 75. This combi is pretty oversized for the load, but most are. Keep it at a low fire, you should have longer cycles than what you’re experiencing.

huntsaver

I’ve got the stat set at 56 degrees. Using the low mass curve and I have the remote sensor enabled so I can’t set the water temp.  I’m pretty sure low mass curve max temp is 120F.  If I have longer cycles won’t it burn even more LP?

huntsaver

NaviLink says March monthly gas usage was 1999ft3 whatever that means 🤷

Hot_water_fan

I’ve got the stat set at 56 degrees. Using the low mass curve and I have the remote sensor enabled so I can’t set the water temp.  I’m pretty sure low mass curve max temp is 120F.  If I have longer cycles won’t it burn even more LP?

Nope, the opposite. 13,000 btus x 5 hours = 1 hour x 65,000 btus, except the lower fire gets more condensing for the same return water temperature. You want the high mass curve, or a custom curve with the lowest water temperature it allows, especially for that indoor set point. Maybe worth looking at the boost function as well, try disabling that. You have a lot of boiler for this heat load, so rein it in some. 

huntsaver

I would love to keep it 60-62 but turned it down to try and save some LP.  I have yet to see the boiler Ramp down below 30%. I’ll have to read up on trying some custom curves.   Any advice on a good flow on the manifold?   If I open them up to .08 gpm the boiler ramps up.  Boiler says supply 118 deg.  Return 78 deg.  Actual gauge temps on manifolds are 118 and 62 return right now. 

huntsaver

Hot_water_fan

A higher flow will lead to a more consistent floor temperature from beginning to end of loop, which may or may not be desirable. It will not impact fuel usage as long as the return temps are about the same. I’d set the max fire to 30%, that’s probably more than enough for a well insulated place like this. If it’s not, adjust it up. The concept is: high runtime with low temperatures. It’ll limit idle loss and maximize condensing. Supply temp of 118 is way too high. 

huntsaver

Set to high mass curve.   Max fire 30%.  It’s now 14.5% output supply 72 deg. Return 64.  I’ll see what it doesn’t.  Thank you!!

huntsaver

Oops.  I forgot to wait for ramp up.  It went right to 30%.  Using 62496btu.  Supply 80F  Return 63F

Hot_water_fan

You can set the low fire timer higher- it’ll start at low fire and stay there for a designated time before ramping up. 

huntsaver

Thanks I had it set to 5 minutes is all. I will increase that. It ran for 1.5 hours and temp raised 2F in the building. System status showed it at 30% this entire time I assume because it was trying to get the water to the set High Mass Curve temp of 120 degrees.

Hot_water_fan

Excellent - the boiler won't run 24/7 except on the coldest days because it's so oversized but should run 12+ hours per day I'd expect.

huntsaver

12+ hours at 30% would be chewing a ton of LP right? Navilink showed using 62k BTU during this burn?
Am I correct thinking my next step would be to create a custom outdoor reset curve with lower temps or just disable outdoor reset all together and set my temp down to around 80F? Currently the High Mass Radiant curve is ramping temps up or trying to and staying at my 30% max fire limiter. It would be nice to see if I can get it running at low fire for less BTU's to simply maintain supply temps around 80F.

Hot_water_fan

It shouldn't be at 30% almost ever. Most time should be at minimum of 10%, so using a custom curve might be best. Common sense tells you that running longer uses more gas: but common sense is wrong!!! The whole point of modulation is to run longer at lower. If your maximum heat loss is around 20kBtu on the coldest day, an average day might be 10k, which is right around minimum output for this boiler.

huntsaver

It was my highest usage day yet unfortunately.  7 hours runtime 156ft3 gas usage. Something has to be wrong with my settings.  I disabled outdoor reset tonight and SH Temp to 80.  Max Capacity is 29%.  Unit wants to run 27% 60k btu to heat supply to 80F. Tried playing with custom curve also with all settings on minimums.  

Hot_water_fan

29% should be ~38kBtu, not 60kBtu right?

huntsaver

I would have thought the same thing. I just opened NaviLink at work and it's currently running.
Current Gas Usage: 58,329.6 BTU
Heating Supply Temp: 82.0F
Heating Return Temp: 68.0F
Current H/C 28% (I'm unsure what this is)
I'm not sure how it can allow this type of Gas Usage either when I have Max Capacity set to 29%

Hot_water_fan

I'd check the space heating min capacity too, make sure that's at 10%.

huntsaver

Yes I cranked min capacity as low as it can go.. Strange item that might be concerning. Under status information / Model it shows the unit as a NCB-250/150H. The unit is an NCB240/130H. Perhaps they use the same controller board and this has no bearing, but the high usage makes me wonder. I also tried turning down the Supply MIN Set-Point as low as it can go along with Supply MAX set-point as low is it can go then selected the Custom Reset Curve vs. the High Mass Curve. Still wants to ramp up and stay around 30%

Jamie Hall

Um... may I go back to square one here? What is your building heat loss at ... what... 56 degrees inside did you say? I'd be freezing at that temperature inside, but whatever. What you need to do is determine that building heat loss. Once you have determined that, you will have some way to check on whether your gas usage is reasonable, too high, or surprisingly low.

Remember that the largest influence on your fuel usage is the building heat loss. Tweaking and adjusting the boiler controls may make as much as a 10% difference on fuel usage (unless the boiler burner is badly out of adjustment -- you did have it set and calibrated with the required combustion measuring equipment, didn't you? If not, why not?). Judging from your numbers, however, you have the return set about as low as could possibly attained, so you are getting as much condensation as possible -- so the boiler is running at as high an efficiency as it can -- again, however, provided that the burner has been properly adjusted. Can't emphasize that enough.

Hot_water_fan

@Jamie Hall seems like the heat loss is in the 25-35kBtu range based on 100 gallons in March, which seems totally reasonable for a structure that size kept at that temperature. The improvements from here are marginal, but this boiler should be running lower and longer. 

huntsaver

I will have to start researching how to calculate my heat loss unless you have pointers on the best way to achieve this data. The boiler was NOT set with a combustion measurement only a manometer on the gas valve. I would have no concern about my efficiency if the boiler was running A LOT, but it really didn't. 61 hours of run-time in March burning on average 1.64 gallons of LP per hour. Even now after trying to get it turned down for longer run-times it still seems to be chewing the LP and running at high capacity even to get water to 80F.

Hot_water_fan

I think the overall consumption is reasonable. What's important to realize is that yes, 100 gallons/61hours = 1.64 but running longer (and at a lower temperature) might only drop from 100 gallons to 90 or so. That said, a mod-cod shouldn't run only 2 hours per day in March. That's a waste of a boiler.

huntsaver

I agree and would love to allow it to run longer if I can get it to actually modulate down. Running 10-12 hours per day burning 60,000 BTU's would force me to start burning wood

Jamie Hall

Are we, perhaps, expecting miracles here? Let's consider this. I agree with @Hot_water_fan that the heat loss probably isn't out of line -- although an actual calculation (Manual J, which was probably never done) wouldn't hurt. I also agree that the boiler could be running lower and longer. But there seems to be an expectation that if only we could get the boiler to run lower but longer we would save a lot of gas.

Sorry. It doesn't work that way. I've said it before, I'll say it again: It takes a certain number of BTUs of energy to maintain a structure at an average given temperature. There are a certain number of BTUs in every gallon of fuel. So long as you don't have the run times so far out of whack that your temperature in the structure is varying widely -- and 5 degrees or so isn't that widely -- it's going to take the same number of gallons of fuel whether it runs hard for 10 minutes and stops for 50 as if it ran feebly for the entire hour.

There are marginal improvements in efficiency -- and thus marginal reductions in fuel use -- for longer runs at less power. In the case here, they are going to VERY marginal, as the boiler is already running in the condensing range.

It would appear that no one has actually taken the time to properly adjust the burner, however. Doing that could make a much more significant improvement in fuel efficiency; I have seen a boiler go from 60% to 85% with correct adjustment. I'm not saying that will happen in this case -- it's remotely possible that the burner adjustment is correct as is. Remotely.

Hot_water_fan

In general, this is pretty low usage for a structure that size! I shudder to think what your usage would be without the insulation.

huntsaver

Cut my flow back to .2gpm on the manifold runs.  Temps went up and capacity went down.  Boiler says it’s flowing 5gpm now. It seems happier. 2-3 hour fires around 20% and it’s 32F outdoor temps today. Found my LoopCAD design drawings that stated .27GPM is what it wanted for my setup. 

ppettersen

We have a Navien combi also for a small 1100 square foot single family home. Made mistake of initially not insulating on the basement side of PEX so far too little of that heat was radiating upward (now solved with stapled foil 1-2 inches below PEX sealed at edges using hi temp foil tape followed by R15 Rockwool). Glad your setup is now working well. Really pleased with Navien…100 percent better/cleaner than previous oil burner with forced hot air.