Thoughts on Navien NCB 240

wchiodetti

I was hoping to get some perspective on the performance of the Navien NCB 240 system. I am a homeowner in Massachusetts looking to replace an old Hydrotherm gas boiler and standalone hot water tank. The NCB 240 looks like a great fit, but it's been hard to find any reviews on its performance. Hoping to get some perspective on whether this system can truly heat a 2,000 sq ft house in a New England winter while running two hot showers and a dishwasher at once as their marketing suggests. (Note: We are a family of four.)

I'd be grateful for any feedback on the product's performance and total cost of ownership.

Thank you,
Bill

jonny88

Don't think it will keep up with your water demand.Personally I would go with a smaller boiler for heating and a indirect for dhw.I would also suggest you take a look at some HTP products which may better suit your needs.

njtommy

I have a Navien NCB240 in my own home. For two heating seasons now with out a single problem with the unit or its performance. The unit can not heat the house and deliver DHW at the same time. It's only able to do one at a time. As far as delivering DHW. I'm not sure about two showers it depends on flow rate of the shower head and temp rise of the water. I've only ran the dishwasher, and a shower at the same time with no problems.

Ironman

There's more to consider than just the btu rating for your space heating. The onboard circ is limited to about 5 gpm. If you have high temp emitters (such as fin tube baseboard) that translates to 50k btus that the circulator can transfer to your heating system. The burner can produce more, but that's all that the circ can get out to the house because it's sized for domestic water heating which has a much higher delta T.

njtommy

Yes to get full Btus out of the combi for heating you must run a 40 degree Delta T or even a bit higher. I'm not sure if they made any changes to the unit for the new NCB- E series.

wchiodetti

Thank you, guys. That's really helpful feedback. I appreciate the time and perspective. This is the first boiler purchase I've made so I'm still getting up to speed, but even as a layperson, that point around the 5 gpm capacity of the onboard circ is what had me wondering if the NCB combi was a good fit for us. It sounds like there may be some better options so I'll keep researching and having conversations (and I'd welcome any additional thoughts on equipment.) Thanks again!

-Bill

njtommy

Getting away from the combi boiler. I really like the Lochinvar KHN floor standing boilers. They have a low turn down ratio so you don't have a problem with short cycling and they work great paired to a indirect tank for DHW.

HDE

I am sure that the NCB-240 has an AHRI rating of 120,000 BTU's input, 112,000 Heating capacity with a net IBR water rating of 97,000 at an AFUE of 95. You cannot fudge this as it is tested and proven to meet the performance criteria that all boilers are rated.

Mentioning the once very popular Triangle Tube boiler for instance, even those 110,000 BTU units with a Taco 007 or Grundfos UP15-58 circulator as recommended or shipped have well below 10 GPM boiler flow, so how could they put out over 100,000 BTU's right?

There remains a mystification by many heating contractors on how modulating/condensing boilers achieve their net outputs.
I understand where this confusion is derived from, the basic hydronic heating formula of 10,000 BTU's per 1 GPM. True this is how many on/off boilers would function with a 20-degree DT operating aquastat.

But you have to remember that BTU's and the ratings are based on per hour, BTUH (BTU's per Hour). So you must take into account that modulation and continued burn can potentially and often will fill those gaps of the time a typical on/off boiler is off during its differential swing period.

It’s true that during operation, recovery may be slower with condensing boilers, but as heat emitters saturate, reducing output, the return temps rise and the DT closes. And this heating process is part of the benefit of a high efficiency boiler potentially staying in condensing mode longer and avoiding swings and overheating. As the return temps rise, boiler modulates down, the constant heat with longer potential circulation fills the gap.

This is important to note because constant flow rate of the boiler is often confused with actual output which it is not. However, the flow rate and burn cycle modulation can and often does have an effect on maximum achievable system mixed water temperature. This is what we should really be paying attention to, the transfer of heat. Achieving maximum desired set point temperature at the given or set boiler flow rate. This should be a concern especially with condensing boilers that don’t match the flow rates of the boilers with the design temp flow rates of the heat emitters such as 20 degrees on baseboard. There are many of those type of boilers out there and you may not even be aware of it. Remember the munchkins? You really think you had 14 GPM on a 140,000 BTU boiler with a 3/8” ID heat exchanger?

The end result often with lower flow boilers is understanding your boiler set point requirement relationship in regards to boiler flow rate in conjunction to system flowrate and heat emitter temp requirement. You do know that boiler DT does not have to match desired system DT right? That is important to get clear, as I believe that is where the largest confusion lies. That’s where primary/secondary piping and system temp mix takes over. It’s not uncommon to need to set the boiler temp higher, especially when boiler flowrate is less than system flowrate that require higher temps. This of course is where some installers can get into trouble without verified system designs that include system water temps at coldest design degree day.

I would be remiss in not mentioning without a primary/secondary piping configuration, direct connection for example, then yes, the boiler available BTU's would be limited by boiler/system combined flow rate.

Depending on heat emitter requirements, the condensing boiler may need to be set at 195 degrees to get baseboard to 180 degrees. For example, if you had a 100,000 BTU requirement, had a system flow of 10 GPM and a 120,000 BTU boiler with 6 GPM:
Mixed stream formula

195-degree boiler setting x 6 GPM boiler flow rate + 180-degree system temp requirement x 10 GPM system flow rate / 6 GPM boiler + 10 GPM System = 185

185 degrees would be the maximum temperature at 10 GPM the system could reach with a 195-degree boiler setting at 6 GPM boiler flow.

As you can see in this example it meets and exceeds the system temp requirements
Don’t focus so much necessary on the boiler flow rate, its only one part of the total equation.