Navien NCB-180 Combi Run-Time

[Mikerf12]

Earlier this year I had a Navien NCB-180 Combi Unit Installed to replace a conventional oil fired boiler. The house is approximately 1500 Square Feet with 1000 Square feet on the first floor and 500 square feet on the second floor. The upstairs zone never runs as the heat from the living room rises and keeps the upper floor which open to the living room heated. For the first heating season and 1/2 of this season the boiler was run at fixed water supply temperature of 167 degrees with a Lux Geo programmable thermostat with a daytime and nighttime setback.

The boiler was essentially running almost the entire time when we were in the home and the setback was off. We also noticed it would take a very long time for the temperature to return from the setback temperature in the morning.

About a week a go I added the Outdoor Reset Module to the system. I adjusted the temperature setbacks on the thermostat to a 2 degree difference at night and when we are at work. The house seems to be heating well, however the the system seems to be running for a very long time. It seems to take at least two hours to climb the two degrees from the night set back to the morning setback.

In looking at the run time reports the thermostat is providing it looks like it is running any where from 10 - 12 hours per day in the three days it has been installed and running on the schedule. On the weekends the temperature is set to a fixed temperature and it runs much longer 18-20 hours. It has has been warm here in Connecticut the past few days.

Below you will find the schedule I have set in the thermostat. The only difference before the outdoor reset was installed was the 66 degress was set to 62.

Monday - Friday
11PM - 5 AM - 66 degrees
5AM - 8AM - 68 Degress - The house doesn't really hit the 68 in this period, as space heating is limited due to two showers and needing DHW, this may need to be removed or adjusted to get warmer earlier.
8AM - 4PM - 66 Degrees
4PM - 11 PM - 68 Degrees

Saturday - Sunday
5AM - 11PM - 68 Degrees
11PM - 5AM - 66 Degress

The question really becomes what is the average run-time a system like this should be running for? It seems like the system is running for quite a while and I don't know if the settings need to be adjusted or outdoor reset curve adjusted.

My plan is to run the system for one month to benchmark the performance, but have been reading a lot of good posts here and figured I would write up a post.

Thanks for the help in advance.

 

[NY_Rob]

Running 18-20hrs/day is excellent for your mod-con.
1 deg rise per hour is reasonable for the low water temps from a mod-con using an ODR sensor... that's why setbacks need to be very mild.

You may want to consider removing the weekday 8AM-4PM 2deg setback and just have it stay at 68F all day- and maybe have it turn down to 66F at 8pm or so for sleeping comfort rather than setting it back to 66F at 11PM.
If you're not hitting 68F by the time you wake up in the morning- back the morning heat call up 30min (to 4:30am in this case) and see if you hit 68F when you get up.... it's trial and error- I had to do the same thing on my system till I found what actually works (4:45AM +2deg morning heat call IIRC).

I don't use any daytime setback on my mod-con, but I do use 2deg nighttime setback from 68F to 66F at 8PM. Luckily this Jan and the previous Jan in our area were quite close temp wise over the month so they made for good comparisons between my old cast iron boiler and the new mod-con. Even with no daytime setback and only 2F nighttime setback my gas consumption is a bit over 30% less with the mod-con. FWIW- with my old cast iron boiler I used 6deg nighttime setback and 4-6deg daytime setback.

 

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[Mikerf12]

Running 18-20hrs/day is excellent for your mod-con.
1 deg rise per hour is reasonable for the low water temps from a mod-con using an ODR sensor... that's why setbacks need to be very mild.

You may want to consider removing the weekday 8AM-4PM 2deg setback and just have it stay at 68F all day- and maybe have it turn down to 66F at 8pm or so for sleeping comfort rather than setting it back to 66F at 11PM.
If you're not hitting 68F by the time you wake up in the morning- back the morning heat call up 30min (to 4:30am in this case) and see if you hit 68F when you get up.... it's trial and error- I had to do the same thing on my system till I found what actually works (4:45AM +2deg morning heat call IIRC).

I don't use any daytime setback on my mod-con, but I do use 2deg nighttime setback from 68F to 66F at 8PM. Luckily this Jan and the previous Jan in our area were quite close temp wise over the month so they made for good comparisons between my old cast iron boiler and the new mod-con. Even with no daytime setback and only 2F nighttime setback my gas consumption is a bit over 30% less with the mod-con. FWIW- with my old cast iron boiler I used 6deg nighttime setback and 4-6deg daytime setback.

Thanks for the input, im used to the oil, standard boiler and running one of those 18-20 hours a day would cost a fortune. I guess in this situation it is a good thing.

 

[NY_Rob]

As long as the mod-con is running at condensing temps (return water below 130F) for most of those 18-20hrs/day you're golden and are pretty much guaranteed to save money over non-condensing atmospheric boilers.

 

[Mikerf12]

The boiler also has a boost functionality that I turned on because i thought i was running for too long. Based on your response above I am going to turn it back off.

 

[NY_Rob]

I've also disabled boost on my mod-con- It kind of defeats the desired goal of having long burns.
I've read the European's don't have the boost feature on their mod-cons, and they invented the mod-con and have been using them for years.

If you get a real nasty short cold/windy day or two that's outside your reset curve- just enable boost again... it's just a quick setting adjustment in the setup menu.
In most cases... once you have your reset curve dialed-in you'll have no need for boost anyway.

 

[Mikerf12]

What is the ultimate goal and process for getting the curve dialed in? Im not sure if the runtime readout on my thermostat is really indicative of how long it is actually running.

Also, I dont know that I can change the curve, ive read through the manual a few times and the only adjustments seem to be for type of heating element, baseboard vs radiator.

 

[NY_Rob]

The goal of using an ODR sensor with a custom reset curve tailored for your home is to use supply water that is just hot enough to keep up with the heatloss of your home at a given outdoor temperature without producing excess heat that causes cycling.
If your home's heatloss is 30K BTU's at 15F outside.. that's what you want your boiler to supply to your radiators, not 40/50/60/80K BTU's.
It's hoped and assumed that your home's heatloss is somewhat linear so that a straight curve (that you can plot with excel) will supply the correct supply water temps at almost all outdoor temps.
If you don't see all the reset curve options in your user manual- download the installer manual where you will see all the custom settings that are available.

 

[Dana]

The boiler also has a boost functionality that I turned on because i thought i was running for too long. Based on your response above I am going to turn it back off.

Yes, turn the boost function off, and dial back the temperatures until it just barely keeps up. A boost function can be useful if you need a deep overnight setback to sleep comfortably (everybody is a bit different on that), but if you can live with a "set & forget" approach to the thermostat, the lower the water temp and the longer the burn cycles the better your average efficiency will be.

Dialing in the reset curve can take a bit of trial & error, and if you set the water temperatures so low for the amount of radiation you have that it can't operate a minimum-fire without cycling on/off you may need to bump it back up a bit. The napkin-math on sizing a modulating condensing boiler for the radiation lives here.

You can also figure out a starting point by estimating the design heat load using the methods outlined here.