Navien NCB-210- Water Temp?

[Rich F.]

Question about what my hot water temp should be on this boiler with the following specifics

- 2700sq. ft. house. Two floors. House is located in Long Island, NY.
-House is new construction so insulated very tight across the board, Andersen 400 windows and well insulated Thermatru doors.
Three heating zones. Two managed by Rheem Air Handlers (hot water coil heat) and the third managed with about 40 ft. of baseboard combined. The third baseboard zone is in an apartment. House is a mother/daughter and that's mom's apartment.
-Navien NCB-210E boiler

I'm looking to find out what the hot water for heat should be set at the boiler. Installer is insisting 180 in Jan Feb and maybe 160 rest of year. I'm doubting that, but solely on my gut. I turned boiler down to 165 and house is heating up just fine, but I still think it may be able to be optimized a bit more to keep house at 70 during the day and get the best efficiency and wear and tear out of the boiler.

Based on info above, any thoughts?

 

[Dana]

Without more information on the actual heat load, and the ratings of the coils in the air handlers there is no way to spec a temperature.

A typical 2x4 framed 2700' house on L.I. with storm windows and clear glass double panes will have a total heat load of about 40,000 BTU/hr @ +15F (L.I.'s 99% outside design temperature) if there is no foundation insulation, down to maybe 30,000 BTU/hr if it's fairly tight, with low-E windows and basement insulation to IRC 2012 levels.

Odds are pretty good that the NCB210 is sub-optimally oversized for the space heating load, but not necessarily for the coils in the air handers, which are also most likely way oversized for the actual loads. With actual model numbers and a heat load calc for the zones they are serving we could come up with a reasonable estimate of the water temperature required.

To avoid tepid-air output at the supply registers it usually takes ~125F or higher output temperatures, but 180F is usually a "scorched air" situation to be avoided. You're looking for "warm summer breeze", but water temps that are in the condensing zone (entering water temperature at the boiler of 125F or less.)

The design heat load and short length of the baseboard may ultimately determine what the lowest reasonable temperature is, since it will short cycle if the temps are too low. The minimum input of the NCB210 is 18,000 BTU/hr, which at condensing temps would imply a minimum output of about 17,000 BTU/hr. If you divide 17,000 BTU/hr by the 40' of baseboard you get 425BTU/hr per running foot of baseboard. Typical baseboard needs ~160F entering water temp to deliver that much heat, so if you go much lower than that the NCB21o will begin to cycle on/off when just that zone is calling for heat over an extended period. At 160F it won't really be condensing though, and really only be delivering ~15,500 BTU/hr, so there's room to drop it a bit. You need to get down to 140F or lower to achieve sub 125F EWT and even a minimum 90% condensing efficiency out of the thing.

Try setting it to 140F, and observe it's behavior when just the baseboard zone is calling for heat (turn the other thermostats down 5F, and the baseboard thermostat up by 5F to get a long continuous call for heat.) If there is enough thermal mass in the system that it gets at least 3-minute burns and fewer than 10 burns per hour (under 5 burns is better, but not likely), leave it there. If it keeps up even in winter (odds are that it will), you're done.

It may be possible/desirable to increase the radiation on the baseboard zone so that it can operate in the condensing zone without short cycling. It's almost certain that the air handler zones can run at sufficiently low temps, but model numbers for the air handlers would be useful for making that determination.

 

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[Rich F.]

Thanks, Dana. Ill try to get that info tonight.

 

[Dana]

While gathering information, since you have a heating history on the place, run a fuel-use against heating degree-day load calculation using only mid-winter gas billing periods. (It's simpler than it sounds- takes longer to read that blog than to run the calculation.) If the Navien was the boiler during those periods, for purposes of the load calculation assume ~86% efficiency (about right for operation at 165F or higher.)

 

[Rich F.]

OK, got some info...

First, regarding the house..there is an uninsulated, unheated basement for a portion of the first floor, about half of it. The rest is crawl that is insulated, so maybe split that BTU/hr to 35,000?

Now, on to the Zones...

Zone1 (downstairs main):
Approx 1040 sq ft.
Air Hander: Rheem RHSL-HM4821AA
Fan Coil: ATQ-2-2025

Zone 2 (upstairs):
Approx 1180 sq. ft.
Air Handler: Rheem RHSL-HM3017AA
Fan Coil: ATQ-2-1520

Zone 3 (apartment)
Approx 550 sq ft.
Baseboard, approx. 25' total. NOTE...I messed up the calculation before. Its more like 25' of baseboard, NOT 40.

Now...I ran one more set of numbers..water out and water in temp on all three zones. I dropped the hot water setpoint to 160 and ran all three heating zones independently:

Zone 1: 160 /140
Zone 2: 160/140
Zone 3: 160/155

As you can see, there is little to no water temp drop on the baseboard run. I'm not sure why that is. The fact its short? The baseboard perhaps not run correctly? I have Taco three speed pumps on all zones, all set to "medium." Maybe slowing the pump on the baseboard run will result in more of a temp drop?

Hope this helps.

 

[Dana]

The short baseboard run is probably being over-pumped, resulting in the low delta-T.

At an entering water temp ( EWT )of 180F the ATQ-2-2025 is rated ~90,000-100,000 BTU/hr, the ATQ-2-1520 is good for 65,000-71,000 BTU/hr, both ridiculously oversized for the loads. With an EWT of 140F they'll put out about half that, and would still be ridiculously oversized for the zones. At an EWT of 120F they would put out about 1/3 the 180F rating, which would likely still have WAY more than enough capacity to heat their zones.

Drop all the pumps all to their lowest speed, and observe the boiler's behavior with the boiler set to 125F. The baseboard zone would only be able to emit ~6,500 BTU/hr at that water temp, which may or may not be more than the design load of that zone, but being only about a third of the min-fire output of the boiler it's likely to short-cycle. But measure both the length of the burns and the number of burns. Bump the T-stat enough to give it at least a half hour of measuring time, and turn the T-stats for the air handler zones way down (or off) to ensure that they don't interfere. Be precise on the burner times and intervals to within a second or two, since that will tell us how much you'll have to bump up the temp to avoid abusive short-cycling.

With only 25' of baseboard it's guaranteed to cycling rather than modulating even with it set to 180F, since 25' of baseboard would only be emitting ~13,500-14,000 BTU/hr, and the boiler is dumping 15,500 BTU/hr into the system or 1500-2000 BTU/hr of excess. There isn't much thermal mass in the zone if it's all fin-tube baseboard, and I'm not sure what the inherent differential high/low temp for that boiler is, but it's probably no more than than 10F. Your probably looking at 20-25lbs of water in the zone, and if there's an excess of 2000 BTU/hr going into the zone it's temperature will be slewing something like 1.5 F per minute, delivering 5-6 minute burns when it's set to 180F, which isn't too abusive. When set to 125F there will be on the order of 10,000 BTU/hr of excess heat going into the system at min-fire, and the system temp will be slewing 6-7F per minute, with burn times potentially in the sub-100 second range, but we'll see. Measure it.

 

[Dana]

As a sanity check, build yourself an I=B=R spreadsheet type load calculation for the rooms served by the 25' of fin tube. If it's more than one room, calculate the room loads separately, and note the lengths of baseboard in each room. It doesn't have to be too precise- if it's 2x4 construction call the U-factor of the walls U0.1, if 2x6 call it U0.075. Assume clear glass double-panes or clear storms over single panes are U0.5, as well as any exterior doors. If it's low-E double panes, assume U0.35. Ignore any floor losses, and assume at most U0.05 for the ceiling if it's an insulated attic with at least 8" of fluff. The outside design temp is +15F, the inside 70F, for a 55F difference.

eg: Assume the room has 9' ceilings 25' of 2x4 framed exterior wall length , 25' square feet of exterior door and 40 square feet of clear glass double pane window, with a fully conditioned room above it (no attic), with 8' of baseboard. That's 9x25'= 225' of gross wall area, less 40' of window and 25' of door leaves 160' of wall.

Windows & door losses:

55F x 65 square feet x U0.5= 1788 BTU/hr

Wall losses:

55F x 160' x U0.1= 880 BTU/hr

Total 2668 BTU/hr, and a load/baseboard ratio of 2668/8 = 334 BTU/hr.

That ratio implies you'd need something like 145F water temps to actually deliver the heat with that much baseboard, maybe a bit higher if the place leaks a lot of air.

Run the numbers on the baseboard zone room, see where it roughly ends up. This is a super-crude load calc, but is good enough to get you within 10F of the actual water temp requirements.

Run the fuel-use against heating degree-day calculations to come up with the whole-house heat load too.

 

[Rich F.]

Lots of info here and I have my homework to do. Give me a few days.

In the meantime, I have my boiler now down to 150. Still plenty of nice very warm air out of the vents and of course the unit is much closer to the condensing sweet spot (but not there yet).

I want to run the "tests" you laid out above but its been so warm in NY. It turns cold tonight through sunday, so Ill have time to do some solid testing.

 

[Dana]

Take the plunge, see how warm the air out of the registers is with 125F water. Ideally the exit air at the supply registers would be in the 100-115F range, which is warm enough to not induce a significant wind chill. The boiler shouldn't short-cycle on calls for heat from the air handler zones even with 125F water, and it would be getting 95% combustion efficiency (or better).

Then it's a matter of what to do about that fin-tube zone...

Friday night & Saturday night should be close enough to the 99% outside design temp to tell if the fin tube zone will keep up with the load with system water that cool. Fin tube output gets to be fairly non-linear with temperature once you're below 120F, and the air handler output air gets a bit tepid, so that's really about as low as you'd ever want to go.

If the fin-tube zone short cycles you can think about budgeting for some radiation upgrades to keep that from happening. It's not a cheap fix, but low temp panel radiators have it all over fin-tube baseboards for raw comfort, and provide more thermal mass/water volume to work with. A couple of mid to high end 6' x 24" panel rads would cost about a grand, and would emit over 12,000 BTU/hr with 125F EWT and adds another 20lbs or more of water to the zone to limit burner cycling rates. Three of them could emit the full 17,000 BTU/hr, and the burner won't cycle at all. With economy-priced Myson rads you can get enough panel to balance for under a grand. What's appropriate depends a bit on the existing lengths of baseboard segments, and how much vertical height you have to work with (under windows, etc.).

 

[Rich F.]

I haven't done full testing and calcs yet. But I did drop temp to 145 and the pump speed on the baseboard. I'm getting 1:40 burns at about 3 minute intervals on that zone . My temp drop is a bit better...around 145 out and around 135-140 back in.

The two air handler zones are humming. 145 out, 125 back in, whether they are both running or not.

 

[Dana]

With 125F EWT at the boiler it should be getting about 90%combustion efficiency with at least some noticeable condensation on air handler zone calls. The 20F delta-T on the water temps is pretty good. It'll shrink a bit with lower water temps but it'll still be more than 10F with the boiler output at 125F, delivering return water in the 110-115F range, which should put you in mid-90s combustion efficiency.

It's pretty clear that you'll either need a much higher temp or more radiation to tame the short cycling on the fin tube zone. So it's 100 second of burn out of 180 seconds, a 55% duty cycle, which wouldn't be terrible if the cycles weren't so short. That's ~20 burns per hour, which puts a lot of wear & tear on the ignition components if you run it like that all year, and it takes an efficiency hit from blowing away a heat with every flue purge & ignition cycle. If you dropped the temp to 125F to get it into good condensing efficiency the fin tube will be emitting even less heat, and the burns will drop to about seconds since there will be more excess heat going into the zone, but the intervals are likely to still be in the 3 minute range.

Both the fuel use based whole house heat load calc and an I=B=R type load spreadsheet for the rooms with the baseboard are in order. From there we can figure out the most reasonable options for boosting the radiation output, be it flat panel radiators, thin profile fan coils, doubling or tripling the amount of fin-tube, or something else.

 

[Rich F.]

I made an error in the above calc....its 140 second burns. So two minute, thirty second burns.

Sorry....

 

[Dana]

Yes, you can do it, but it does raise the temp of the return water slightly, though not enough to make a difference here. There's still no way it can run at condensing temps without short-cycling the lifespan & efficiency out of the boiler with the radiation you have. Ideally you would have something like 75' of baseboard with that boiler to be able to emit the whole min-fire output 17KBTU/hr at condensing temps, and you only have a third of that. If you doubled the amount of radiation you still might not have enough, unless it also increased the thermal mass in the zone plumbing considerably.

If you replaced it the 25' of fin tube with ~25' of the ~24" tall Myson T6 series panel radiators (~4" deep from the wall to the front side) you'd be emitting the whole min-fire output at condensing temps and would add something like 100lbs of water thermal mass to the loop. A ~5 footer c 24" runs about $280 , and emits about 3500 BTU/hr (x 5= 17,500 BTU/hr) at an EWT of 125F, AWT of 120F. If that's too tall, 25' of the ~20" tall versions would still be putting out about 15,000 BTU/hr and add a lot of thermal mass. There are shorter versions, but you'd have to increase the total length to eliminate cycling completely. If you dropped to 25' of the ~15" tall versions it would only emit 2/3 of the min-fire output, but would have enough thermal mass to limit the cycling to tolerable levels, even though it would still cycle some during long calls for heat.

To estimate the output at 120F AWT, divide the 180F number by 3. For the thermal mass, multiply the gallons by 8.34lbs/gallon. Cheap fin tube with 3/4" copper runs about 0.025-0.027 gallons per foot, whereas 20-24" T6 series panel radiators run 0.5-0.6 gallons per foot (~20x as much thermal mass for given length). Having 3-4x the output per running foot and ~20x the thermal mass it makes quite a difference in the minimum burn times.

 

[Rich F.]

How about this product to get more out of the baseboard?

http://www.slantfin.com/products/multi-pak-80/

If I'm doing my math right I'm at around 6,600 BTU/hr at 120 AWT. And I don't have to repaint my walls or redo plumbing.

 

[Dana]

With 6600 BTU/hr and no additional thermal math you're still pretty screwed, since you have over 10,000 BTU/hr of excess going into the system.

That's more excess than what you had during the 145F water temp test that short-cycled like crazy. The number you need to hit is 17,000 BTU/hr (or at least 15,000 BTU/hr) unless it has substantially more thermal mass.