Navien NCB-240 for this house? (photos)

[vanagon]

We have a small (<2000sq ft) home. It's a duplex so one wall is to another unit. All but the third floor is heated by radiators; the third floor is a separate electric system.


We are considering the Navien NCB-240 combination boiler / hot water heater. We are not worried at all about the hot water side of this, as the small pipes in our house mean that we already only take one shower at a time. (Taking an unlimited number of showers back to back would be a nice advantage of the system!).


I've spoken with an installer who is quoting us 8k for the install. They install these units frequently (this is what they like to install the most, from my impression). The guy who was here to actually do the estimate even has a Navien tankless water heater (but not a combi) in his own house. He proudly showed me photos of several installations he had done. He seemed to know the answers to basic questions (how large the gas line has to be, how long the exhaust line can be run at 2" pvc, etc.)

My concern is two things: first, is the NCB-240 definitely big enough from the standpoint of the boiler? The boiler it is replacing is a 165,000/132,000 boiler. The installer did some quick calculations based on the size of our house and says that was oversized, and that the NCB-240 would be big enough. I suspect he may be correct, but I should emphasize that our 90-year-old house is not insulated at all. (Some attic insulation was installed 5 years ago by the previous owner but it doesn't seem like they did a great job. This is something we are planning to correct.)

We do have all new windows though.

My second question is whether we have enough radiators to expect to achieve condensation temperatures for most of the season. We live in Pittsburgh. I would use an outdoor reset sensor.

I'm attaching photos of the radiators. They are all under metal covers (non-negotiable because of small children and lead paint.) It is all 1 zone. (There is also a short 6' section of baseboard in a bathroom, which I haven't included here.)

Any thoughts would be greatly appreciated.

 

[vanagon]

One more thought:

I was thinking of doing the following to test my radiators:
1) Crank our thermostat up 5 degrees so that our 165k/132k boiler goes full on.
2) Wait 15 minutes (but with the temperature still rising) and measure the temp out and temp in.

The idea would be that this would give me a sense of the temperature delta achievable under maximum load for our current, oversized boiler. Is this what I should be doing? Right now we're having unusually warm weather, but it seems like this test should work anyways, right?

 

[Sponsor]

 

[Dana]

Is it BIG enough? HECK yeah! In fact even the CH180ASME is plenty of combi-boiler if you're only taking one shower at time

That combi-thingy is probably 5-6x oversized for your actual heat load (with the windows closed, that is. ) Even a free-standing 2000' two story single-family house with NO wall insulation and only R13 in the attic would have a heat load in the ~50,000 BTU/hr range @ +7F (Pittsburgh's 99% outside design temp.) If the house is reasonably tight, has at least some storm windows or double-pane replacement windows, and has some wall-insulation you're probably looking a a whole house load of no more than 30-35,000 BTU/hr. Since it's a duplex, if the basement is sealed and insulated could even be under 25,000 BTU/hr.

Your old boiler and radiators are probably sized according to the post-Spanish Influenza Epidemic standards, which required a big enough boiler to heat the house with all of the windows open (!) so that the rooms could be over-ventilated even in winter to reduce transmission of disease (or at least that was the theory). Those radiators are so oversized for the load with the windows closed that people often painted them with a low-E silvery or bronze paint to lower the radiated heat output, and installed covers over them (like yours), to reduce convective heat transfer and still people got roasted out.

If you have some mid-winter gas bills with exact quantities and meter reading dates and a ZIP code (for better weather data) it's possible to put an upper bound on the actual heat load using the existing boiler as the measuring instrument.

Want to run that exercise?

With a condensing boiler and high-mass oversized radiators you get higher efficiency running a constant temperature, with little or no overnight setbacks, and bump the boiler's temperature to the lowest that actually meets the load, using it's "outdoor reset" function. The lower the water temp, the less heat you get out of the radiators. If you removed the radiator covers you'd be able to run lower temperatures and get even higher efficiency. It's not clear if yours were painted low-E silver or if that's white paint, but if silver you'd be able to get ~20% more heat out of them at any given temp by painting them any non-metallic color.

The covers don't really protect much from lead paint- the real solution is to have them stripped & repainted, or replaced with new radiators. As long as the paint is in good shape the hazard isn't huge, and can be reduced by encapsulating them- painting them with standard interior latex wall paint. At the modest temperatures you'll likely be running with the condensing boiler any wall paint would tolerate the heat. I doubt it would take more than 125F average water temp to heat the place without overnight setbacks.

 

[vanagon]

Thanks for the great information!!! This is all good to know. We generally have the windows closed in the winter

We've had another estimate now from a second installer. They quoted 7000 instead of 8000 for the job, and suggested the NCB-210 instead of the NCB-240. They said that even the NCB-180 would be plenty for the heat load (which they determined by going around and counting radiator coils, etc), but like to err on the side of having plenty of tap water heating capacity, which I think is a good idea for us, since as the kids grow, our showering habits could change for all I know.

As for analyzing our past usage, here's our gas usage history:

And historical data for our zip (15217) is here:
http://www.wunderground.com/history...reqdb.zip=15217&reqdb.magic=1&reqdb.wmo=99999

(I would ignore the usage data from the most recent month above, since our December was extremely unusual this year.) We really have no wall insulation anywhere, although we have some attic insulation (and probably, we are putting more in). The radiator paint is white. The metallic look in those photos is from the metallic insulation I put behind the radiators to reduce loss to the walls.


Now here are two more questions:
1) this less expensive installer has installed lots and lots of Navien tankless hot water systems, but no Navien boilers or combi's. Is this a problem? He is a certified Navien technician and seemed to know his stuff. For example, he was very clear about the role of the outdoor reset sensor in keeping the return temps as low as possible to maximize condensing efficiency.

2) One option the more expensive installer discussed was using a Delta-T pump to maximize condensing efficiency under variable load (for example, when we turn down the radiators upstairs). Any thoughts on this idea? Or maybe this is a good compromise: https://www.taco-hvac.com/products/variable_speed_products/viridian_vr1816/index.html

Thanks again for all the help!!!!

 

[Leon82]

The delta t pump is a different part number I don't recall off the top of my head. That 1816 pump is variable speed and also will maintain 3 steps of head feet or pressure if I recall.

 

[Dana]

Navien combi-heaters allow you to program a maximum fraction of the total burner capacity that's devoted to the heating loop, down to as low as 30%. The CH180ASME is good for at least 135,000 BTU/hr out in condensing mode, and 30% of that is 40,500 BTU/hr, which is about your actual heat load. That would leave 95,000 BTU/hr of burner output for domestic hot water even during a call for heat, which is enough to run even a high-flow 2.5gpm shower 24/365 (even with 33F incoming water.)

The fact that this year's December was unusually warm, it was still a consistent heating load month, with no days averaging over 65F, but there's better data points to work with:

During the 2 month span from January 3rd through March 4th 2015, it looks like you used about 62 MCF (x 1.028=) ~64 MMBTU (64 million BTU). The nearest weather station with complete data for that span on degreedays.net was KPAPITTS83, on Bevington Road, Pittsburgh, just a bit east of you. If we add up the HDD from 4 January through 4 March it comes to 2488 HDD.

64 MMBTU into an 80% burner like yours puts 0.8 x 64= 51.2 MMBTU into the house- the rest went up the flue. So the house went through...

51.2 MMBTU/2488 HHD= 20,579 BTU per heating degree-day, which in a 24 hour day would be...

20,579 BTU/24= 857 BTU per degree-hour.

The 99% outside design temp for Pittsburgh is +7F, which is (65F - 7F=) 58 F heating degrees below the presumptive 65F heating/cooling balance point. So that's an implied heat load of:

58F x 857 BTU per degree-hour= 49, 706 BTU/hr.

And that's an upper bound.

Odds are the oversized ancient beast isn't really delivering the 80% steady state efficiency indicated by the 165K-in/132K-out any more, and you probably used some amount of gas for hot water heating, etc. Reality is probably more like 65-70% as-used efficiency and a true heat load of about 40-45000 BTU/hr, that also includes massive standby & distribution losses to the basement that you won't have with the Navien since you'll be running it at low temp, and won't have much in the way of idling losses. Call it 40K, give or take.

OK, so the CH210 is really "only" 4x oversized for your load, not 5-6x!

Going with the delta-T pump is the right way to go on this, since it uses about 1/10th the electrical power of crummy dumb 1-speed or switchable 3- speed, and you can use the delta-T to optimize the operating efficiency of the condensing boiler. With a heat load of ~40K, high-mass radiation, and a delta-T pump you should be able to set up the CH210 (or CH180) to modulate with nearly continuous burns throughout mid winter.

A 40K load is on the high side- air-sealing the place is job-1 including sealing up the old flue ports , air-sealing the chimney chase (with sheet metal air barriers caulked with fire-rated goop at both the attic floor and the basement ceiling), the band joist & foundation sill, and every electrical & plumbing penetration of the attic floor/upper-floor ceiling. Depending on the window flashing/roof overhangs and the wall stackup it's usually pretty safe to blow them full of cellulose, which is very air-retardent at any density, or dense pack cellulose to 3lbs+ or fiberglass to 1.8lbs +. It's also going to be worth insulating the foundation, but it's next in line after air sealing and insulating the upper floors. You should be able to cost-effectively get the load down to under 30,000 BTU/hr with judicious upgrades.

 

[vanagon]

Ok thanks again!!

I am feeling more confident about this option now.

Last question: the more expensive of the two installers was pushing the fact he installs some sort of maget filter on the boiler loop (presumably something like this: http://www.boilermag.com/products/domestic/ )

Is this a good idea on a cast iron system with all the ferrous sludge circulating around through the navien, or is this pointless?

 

[Dana]

With a bunch of antique cast iron rads and iron distribution plumbing the filter is worthwhile. There are system water treatment chemicals to purge a bunch of the crud and prevent/limit further corrosion that may also be worthwhile to use in preparation for installing a mod-con too, but I'm not in a position to recommend any one over another.

It sounds like your more expensive contractor is more experienced with hydronic heating systems than the other one. There are a couple models of Viridian pump that could work, depending on the total system flow requirement you may also be able to use the Taco BumbleBee. If the folks who recommended the smart pump probably know how to spec a pump correctly, something that could be hit or miss with somebody who installs tankless hot water heaters all day & night.

 

[Jac04]

Navien combi-heaters allow you to program a maximum fraction of the total burner capacity that's devoted to the heating loop, down to as low as 30%. The CH180ASME is good for at least 135,000 BTU/hr out in condensing mode, and 30% of that is 40,500 BTU/hr, which is about your actual heat load. That would leave 95,000 BTU/hr of burner output for domestic hot water even during a call for heat, which is enough to run even a high-flow 2.5gpm shower 24/365 (even with 33F incoming water.)

I don't think you can still buy a new CH180ASME (unless you find leftover stock), as they have been replaced by the NCB units. The CH units have DHW priority and bypass the heating loop when making DHW.

It looks like even a NCB-150E would do the job nicely.

 

[Dana]

jac04 - you're RIGHT! I don't know how I had read that as CH-240ASME rather than NCB-240E (too many similar Navien threads going on right now, mayhaps?)

The NCB series are somewhat nicer, and can modulate down a bit further, and they priortize the hot water delivery when needed. The NCB-180E may be a better choice than the NCB-150E if there are multiple hot water taps running at the same time, but the -150 definitely has the horsepower to do it all if the biggest fastest draw is a shower. If you're filling the kitchen sink with hot water at a high rate while someone's in the shower in January there's a good chance you'll hear about with the 150E, but not with the 180E. Prior to fixing up the insulation deficiencies the 150's output could be marginal at sub-zero temps during a Polar Vortex week too.

The 240E is overkill and only modulates down to 18K- in/17K- out space heating mode, which is almost half the current design day heat load. The 180E modulates down to 14K-in/13K out, or about 1/3 the design heat load, which means it can be set up to run nearly continuously all winter long if you tweak it in correctly with an outdoor reset curve, and would still have decent amount of modulation even after insulation & air sealing improvements.

 

[vanagon]

I agree the more expensive guy seems to have a lot more experience with these, so I'm going with him.

There is one lingering area where I am a little bit concerned.

The current plan is to use a Taco VT2218 Delta T pump on the secondary loop. (The NCB has it's own pump for the primary loop.)

What I'm confused about is the following:
Suppose we have the Delta-T pump set to 20 or 25 or whatever, and we use an aggressive setting on the outdoor reset curve to drop the output temperatures down to 120 or 130 or so when it's warmer outside.

Isn't there a danger that at these low temperatures, maintaining the temperature differential will be harder, causing the Delta-T pump to slow down to a rate which is slower than the boiler pump rate, and then causing the boiler to recycle heated water and kill it's efficiency in this range?

This problem I'm imagining would seem to apply to any modern condensing boiler when used in a primary/secondary loop setup with a Delta-T pump on the secondary loop. I haven't seen this problem described anywhere, so maybe it is not the problem I'm imagining it is??

 

[Leon82]

I have the same pump on my secondary loop and it runs basicly at or near the min speed when the water temp is low consuming 8 watts. My weathers been mild so I never saw much over 12 to 14 delta t on the coldest night since starting my new boiler.

 

[Leon82]

But with radiators you will have better performance at low temps than me

 

[vanagon]

Yeah I think I will likely see condensing performance quite often.

What I'm wondering is whether I'm going to decide that the Delta-T pump is doing more harm than good at low temperatures, and end up using it in a constant-speed mode.

When you see your 12-14 degree difference and the pump is at minimum speed, are you recycling water in your primary loop? (Is the water into your boiler warmer than the water on the "in" side of the secondary line?)

 

[Leon82]

I have fin tube and with the factory pump I was getting 4 delta t at the boiler.

 

[vanagon]

I have fin tube and with the factory pump I was getting 4 delta t at the boiler.

What about now with the Delta-T pump? You see a 12-14 degree delta t on both the primary and secondary loops? Or are the delta-t's the same on both loops?

 

[Leon82]

I have the 1816 on the primary at min speed I'm getting 11 delta at the boiler with 116 degree water and the zones are about 8 delta.

But you said you have radiators so your experience will be different

 

[vanagon]

Hmm interesting. I think that's no so bad (well below condensing temperatures anyways.)

 

[Leon82]

Yes plenty of condensent.

 

[Dana]

Hi delta-Ts are hard to maintain at very low boiler output temps. Most low-temp systems run 5-10F delta-Ts on the radiation when the temps are well into the condensing zone. It's all a matter of just how quickly the radiation can emit that heat into the house and the pumping rate- it takes a LOT of radiation to put much heat into a 70F house with 85F water, and you WON'T be seeing a 14F delta-T on the radiation.

The primary difference you'll see with radiators vs. baseboard at low temp is that the amount of heat the radiator is emitting drops very linearly with average water temp even in the low 90sF whereas low-rise 8-10" fin-tube baseboard output begins to fall off a cliff below 110-115F or so, and is very non-linear. Taller convectors (18" or taller) will still have predictable output at low temp though.

The thermal mass of high volume radiators will also keep the burn cycles long even when the heat load on the house is lower than the min-fire output of the boiler.