Navien 240 - house temp

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Dingle

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I live in MA. The NCB 240 has been installed for approx 4 months. House is approx 1800sq ft. The Navien replaced a 50 year old cast iron furnace with gas converter on the front. No issues with the Navien until the cold weather set in. So a few weeks ago when it hit -2f outside the house temp maxed out at 65f. It took 12+ hours for the house temp to slowly rise to 71f as the outside temp reached 10f . It has now warmed up to about 15f but I notice that when the outside temp drops overnight the heating cannot keep up and we wake up to a house a few degrees below the thermostat setting. I keep the thermostat setting at 71f all the time at the moment and dont bother with any schedules as it takes too long to recover. Can someone help explain why this is happening? The old cast iron system took any weather in its stride but the Navien struggles.

thanks
 

Jac04

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Can someone help explain why this is happening?
The heat being put into the house is equal to the heat loss of the house at the temperature you 'max out' at. As the outside temp drops, the heat loss increases to a point at which the boiler can't keep up (or just barely keeps up).

How is the Navien set up now? Are you using supply temperature control? If so, what it the supply temp set at?

My guess is that the old boiler was supplying 160-180F water, and the Navien is set much lower. You were OK up until the really cold weather hit. If that's the case, simply change your supply temp set point. In north central CT, I usually have my Navien set for 125-130F supply, but I have to bump it up a little when the outside temps drop way down.

For example, I was barely able to maintain 68F last night (about 7F outside) at 130F supply, but when the t-stat went up to 70 this morning I couldn't get the house temp to rise at a reasonable rate. So, I bumped supply temp to 145F for a little bit, then changed it back to 130F.

Also make sure that you don't have the firing rate restricted (or, if limited for some reason, make sure it's not set too low). I assume yours is the same as my CH-180, where you can limit the max firing rate when firing for heat.
 
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Dana

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Learn how to program the Navien's outdoor reset curve (IIRC it's called "K-factor" in the manual" to a higher temperature level) to get more output at low temp rather than running it at a fixed temp. It takes a bit of reading and tweaking to get it right, but when you dial it in it'll run 120F or lower most of the time, delivering 95% or better efficiency, but when it hit's negative digits it'll automatically bump up the temp without user intervention.

The fact that it only needs 145F water to cover the load at -2F means your radiation never needed 180F water in the first place, which is a good sign. With that much radiation you probably really WOULD hit the advertized efficiency, once you dial in right K-curve
 
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Jac04

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Learn how to program the Navien's outdoor reset curve (IIRC it's called "K-factor" in the manual" to a higher temperature level) to get more output at low temp rather than running it at a fixed temp.
I found that the selectable Navien curves do not work well for me. I have probably spent 30-40 hours trying different curves, watching operation and tracking burn times. If I could program my own curve I would be all set.
 

jeffsnavien240ncb

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my story sounds similar to yours, except I just installed mine last week. the only way I could get my house warmer than 60, was to slow the flow by gating it down. My basement is much colder since I turned on the ncb 240.
 

Zl700

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But unlike the earlier version CH series combi, the NCB has a custom curve selection, then all four points are completely adjustable to have as flat or as steep of an outdoor curve you want.
Adjustible on the NCB are:
1. Starting water temp
2. Maximum water temp
3. Starting outdoor temp (warm weather shutdown)
4. Coldest Outdoor (where highest water temp hits at that outdoor temp)

You might also consider turning on boost feature (after preset adjustible time boosts setpoint temp 9 degrees to satisfy heat call, handle infiltration, adjust for wrong curve setting)
 
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Jac04

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But unlike the earlier version CH series combi, the NCB has a custom curve selection, then all four points are completely adjustable to have as flat or as steep of an outdoor curve you want.
Adjustible on the NCB are:
1. Starting water temp
2. Maximum water temp
3. Starting outdoor temp (warm weather shutdown)
4. Coldest Outdoor (where highest water temp hits at that outdoor temp)

You might also consider turning on boost feature (after preset adjustible time boosts setpoint temp 9 degrees to satisfy heat call, handle infiltration, adjust for wrong curve setting)

Man, I wish the CH had all that adjustability. Looks like a new NCB-180 is only around $2000, and it should pop right in place of my CH-180. Tempting...
 

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If you dial in the reset curves perfectly you'll save more fuel than doing deep temperature setbacks, since the recovery ramps require higher water temps. Higher water temps==lower efficiency. Any fuel you saved by letting the house run cooler is eaten up by the lower combustion efficiency during the recovery ramp.

The boost feature's 9F step up isn't going to reduce the ramp times appreciably, but will be enough to cover variations in load from wind speed, leaving the front door open for 10 minutes, etc. without losing ground.
 

Beads

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[QUOTE="j... My basement is much colder since I turned on the ncb 240.[/QUOTE]

This highlights an interesting possibility of unintended consequences. If you have an uninsulated floor with the boiler under it and an efficient, cold-start boiler is installed and radiation that is just making it on the design days, you might have a cold house.
 

Beads

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If you dial in the reset curves perfectly you'll save more fuel than doing deep temperature setbacks, since the recovery ramps require higher water temps.

Now that suggests a useful boiler information feature. Give me a boiler information output that predicts what a set-back does for fuel consumption.
 

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[QUOTE="j... My basement is much colder since I turned on the ncb 240.

This highlights an interesting possibility of unintended consequences. If you have an uninsulated floor with the boiler under it and an efficient, cold-start boiler is installed and radiation that is just making it on the design days, you might have a cold house.[/QUOTE]

The solution to a cold floor isn't to insulate the floor from the basement (which isolates the boiler's standby & distribution losses from the conditioned space. The solution is to air seal & insulate the foundation & band joist.

The boiler efficiency is temperature dependent- specifically, the temperature of the returning water entering the boiler's heat exchanger. At 130F return water temps the best you're going to do is 86-87% combustion efficiency. Above that it'll slowly drop to about 85% at some very high return water temp (say 180F). Condensing begins at ~125F return water temp, where condensing boiler will be running 88-90% combustion efficiency. At 110F returning water it'll be in the mid-90s, and below 100F it'll be in the high 90s. If you have to bump up the reset curve by 20-25F to achieve reasonably fast recovery ramps from deep setback, you're burning a lot of fuel at 86-88% combustion efficiency that could have been burned at 95% efficiency or higher just maintaining temp.

The basic curve looks about like this:

Condensing%20Curve.JPG


But the modulation rate of modulating burners also affects the boiler's efficiency, independently of the return water temp. At max fire at 110F return water many/most boilers won't deliver more than 89-90%, but the same boiler at min-fire might be doing 95%+, and in-between it'll be in-between. It will vary from boiler to boiler, but it looks something like this:

boilergraph-1.jpg


To achieve a rapid recovery ramp, the boiler has to be running at or near full-fire, and at a higher temp. Don't expect more than 86-87% combustion efficiency during those ramps, if that's how you're operating it.

If you dial the reset curves to where it just keeps up, it will be modulating almost all the time, and at a lower water temperature. The combined effect is MUCH higher average efficiency. In most cases running deep setbacks will be at-best a wash on fuel use, when averaged over a season, and in the shoulder-seasons when the reset temps are low/very-low, it can even use more fuel than simply keeping the room temps steady.
 

Beads

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It seems like the OP should be able to make the boiler keep the house warm enough or too warm. At 190,000 BTU output in 1800 square feet the capacity should be plenty. It would be nice to know what the output of the old boiler was. OP, please come back! Someone here will help you get your house warmed up for sure.


Risking a thread hijack, my main point was that a cooler basement precipitated by a more efficient boiler could have unintended consequences. In the situation that I am working on right now, I am more concerned with Summertime moisture in the basement than a cooler house in the winter. I’ll just have to see how it plays out and add a dehumidifier if there is a problem.


The solution to the cold basement (or warm basement) problem MIGHT be insulating the basement and bringing it fully into the conditioned space. That would be the case if there are ducts down there. With a boiler, it depends on how you want to use the space. From a pure cost standpoint, it is probably best to insulate the first floor and pipes in the basement and not condition the space. If you want to use the area as marginal or fully living space, insulate it no matter how you condition your house unless you are in a very warm environment and rarely use heat.


I’d love to see some user-usable information about how advantageous or disadvantageous set backs are with mod cons. If boiler manufacturers would incorporate the living space thermostats into their boiler control strategies, set backs should certainly work. In a set-back recovery, the “system” should be able to learn what is needed to recover at particular set-backs and outside temperatures. In turn, the boiler would be run at a rate and recovery start time that puts it in a reasonably efficient mode, not flat-out. I have data from a case study where an (unintentionally) oversized modcon system does run more efficiently with set backs. (Right now, I am doing a little reading that includes some background material about how heat pump mini split setbacks affect energy costs. They run less efficiently at full output, but not because a heat exchanger is saturating which is apparently causing lower efficiency in a mod con at high output. With the mini-splits, it would be nice to set a recovery in a mode that does not put them in an inefficient mode. It should be easily done if the engineers can pay attention to it.)
 

Jac04

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To achieve a rapid recovery ramp, the boiler has to be running at or near full-fire, and at a higher temp. Don't expect more than 86-87% combustion efficiency during those ramps, if that's how you're operating it.
I know we're getting a little off topic, but ...
I'm not seeing this with my Navien CH-180 boiler. I have it set right now at a max firing rate of 30% for heating (I did this last season after seeing the same efficiency curves you posted above). On a really cold morning I will temporarily bump up my supply temp from 130F to 140 - 145F for more rapid recovery and it has no problem rapidly increasing supply temperature. However, I'm talking about going from 68F to 70F t-stat setting - hardly recovery from a deep setback.
 

Dana

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I know we're getting a little off topic, but ...
I'm not seeing this with my Navien CH-180 boiler. I have it set right now at a max firing rate of 30% for heating (I did this last season after seeing the same efficiency curves you posted above). On a really cold morning I will temporarily bump up my supply temp from 130F to 140 - 145F for more rapid recovery and it has no problem rapidly increasing supply temperature. However, I'm talking about going from 68F to 70F t-stat setting - hardly recovery from a deep setback.

The degree of oversizing is going to be pretty ridiculous at max fire of a CH180. It takes 150K-in at max fire, so 30% of that would be 45K. Even at 86% efficiency that would be 38,700 BTU/hr, which is more than the heat load of my house @ 0F.

If you have a 15F delta-T across the boiler, you won't be getting more than 86% efficiency out of it on those recovery ramps, since the return water at 145F out would be 130F, which is above the condensing range. If it keeps up over night with 130F output water at a 15F delta-T the return water would be 115F, and you'd be getting better than 90% efficiency out of instead of 86%. That's a 5% difference in heat output with fuel use. So if the setback was saving you 5% in fuel due to the lower heat loss out of the house at lower indoor temps, you ate up all the savings during the recovery ramp.

But it's the RETURN water temp that's critical, not the output temp. At 145F out with a 40F delta-T it's going to be doing AWESOME, ( but that isn't a typical radiation setup.) At 130F with a 5F delta-T it'll barely hit 90%.

If the radiation is so limited that it needs 150F water to just keep up at very cold temps, the setback approach would save money, since it won't be condensing anyway. That's why you get more heating efficiency out of a mod-con when design condition heat can be delivered with 140F water, which takes quite a bit more radiation. But that's also why outdoor reset is critical, so that you can reap the condensing efficiency advantage whenever conditions allow.

Every house and every system is different- there is no really simple way to model this. But know this: If you have to bump into non-condensing temperature ranges to recover from setback when it would have kept up at a condensing temperature, you've saved effectively nothing (and maybe even lost something) by using the setback strategy.

The mini-split setback efficiency issue has other issues, and they are more pronounced than with mod-cons. At sub +5F outdoor temps you're not going to do much better than a COP of 2 no matter what speed it's running, but at full speed you're looking at 1.6-1.8. At +15F cruising along at part load the COP can be 3-3.5, but at full speed it barely breaks 2. This is way more than a 10% hit in operating efficiency! In an overnight setback situation where you are trying to get the place back up to temp before breakfast, your recovery ramp occurs during the coldest part of the day, and the absolute efficiency is at it's worst. It's hard to come up with reasonable algorithms to adjust the ramps for reasonable recoveries from deep setbacks. Mini-splits also have very little output compared to a boiler, and at lower outdoor temps the capacity is even lower, which makes for very long recovery periods during cold weather. Here again, most of the time limited setbacks during the overnight hours is going to use less power.

The Fujitsu AOU-xxRLS2-H series cold climate mini-splits have an occupancy sensor auto-setback feature that can be programmed on or off, and is marketed as an energy savings feature. But unless they did something really sophisticated to limit modulation levels during the recovery ramp (doubtful) it would increase energy use. Limiting the modulation of the thing during the recovery ramp guarantees lower comfort, since it's triggered by the occupancy sensor. When the room temp is 10F lower than the setpoint you want that heat to come on as fast as possible when you're actually there, not wait an hour or three for it to bump back up. An occupancy sensor isn't like a programmable thermostat, that can anticipate when the place has to be up to temperature, and possibly work some control magic based on indoor & outdoor temps, and the time at which the house needs to be at the setpoint.
 
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Jac04

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If it keeps up over night with 130F output water at a 15F delta-T the return water would be 115F, and you'd be getting better than 90% efficiency out of instead of 86%. That's a 5% difference in heat output with fuel use. So if the setback was saving you 5% in fuel due to the lower heat loss out of the house at lower indoor temps, you ate up all the savings during the recovery ramp.
Understood. Any setback is only used for comfort when sleeping. So, for the past 2 nights I have been keeping the heat up at 68-69 at night, and I even dialed my supply temp back to 125F last night and had no problems keeping the house temp up (even though overnight low was around 20F I believe). So, now the decision - comfort or efficiency? I only use about 500 gallons of propane per year - that's for heat, hot water, grilling, and clothes dryer. Even if I saved 5% of the total, that's only 25 gallons, or roughly $70.
 

Dana

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There's no need to suffer! The boiler is there for your comfort & convenience not conversely!

But if you can heat the house with 125F boiler output at 20F outdoors, you may be able to keep it in the condensing mode even at the 99% outside design condition, even if you bump up the outdoor reset curve from the absolute minimum. But if you used really deep setbacks you'd probably have to crank the curve above the condensing zone.

Propane's condensing zone starts 3-5F higher than natural gas, so it's conceivable you could be at something like 90% efficiency even with 130F return water.

If you don't already have air conditioning and are considering it, note that even at this year's lower propane price and CT's high electricity rates, a better-class ductless mini-split heat pump produces cheaper heat than condensing propane, and insulates you from propane price volatility. (I know folks who were paying $5/gallon for propane in recent years.) For now there is some respite from high heating oil and propane pricing, but at the pace drilling rates in the shale formations are scaling back, it simply can't last. (I give it 3 years at the outside, some oil-biz analysts are saying 10 more months.) The upcharge for a heat pump vs. cooling only mini-split isn't very much, and gives you options.
 

BMWM5

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You can thank the EPA for mandating the settings for the most eco friendly settings as possible. When I first had my unit installed in Connecticut
my house was as cold as an iceberg. I asked the clerk at the plumbing supply house where the Navien 210 was purchased and he said the thing does not get going until the temperature is set at 180. Once I kicked up the temperature to 180 the unit worked fine.
After one year however I noticed water dripping from the pressure valves onto the floor. I have to find out what this is all about. Nice unit. It saves me a ton of money over the old hot water oil system I had.
 

Beads

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Rather than the EPA, I think that BMWM5 can thank the installer for an improper installation or set-up. Calculations could have been done before the Navien was selected to see what water temperature is needed to keep the home warm with existing heat exchangers in the rooms.
 

Dana

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Exactly! What Beads said!

The notion that "...the thing does not get going until the temperature is set at 180..." is a crock of poo. With sufficient radiation the thing would "get going" just fine at temps under 100F (as is often the case with concrete slab radiant floors.) The last and often the most limiting heat exchanger in the system is whatever radiation/other that is the final heat emitter. Most existing systems on oil boiler can still deliver design-day heat at entering water temps well below 180F, but not many can deliver it at 120F. It's up to the installer to figure out the water temps required to get there with the existing radiation.

Even if 180F is necessary to cover the heat load at the 99% outside design temp, with the outdoor reset option installed it'susually possible to set up the reset curve to run in condensing mode much or even most of the time, using less fuel than if set to a fixed 180F output, delivering constant room temps and higher comfort levels. Using a deep overnight setback strategy would require higher water temps to have reasonable recovery ramp periods, chewing up any energy savings from the lower overnight temps in lower combustion efficiency at return water temps above the condensing zone. Even without the ODR option, running it at a fixed temp of 135F or so and not using overnight setbacks will usually get some condensing efficiency out of it, and may come close to covering the 99% design condition load.
 

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Hello, I have a ch 240 with outdoor temp sensor. Normal setup of K factor is 2.6, and it works great when temperatures are below 45For maybe 50F but in a day like today when outside was in the 60F, at 2.6 K setup, the supply temp of the boiler is 63F. I believe something is not set up the right way but what is it? So I just turne the K faCtor up to 4.5 to finally raise temperature in my house to 68F , as it was basically blowing air colder than room temperature air all day while I was at work. I have had this issue since the temp sensor was installed... thanks
 
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