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A couple of things about this boiler that I experienced from a recent install:
1. The unit performed very well and the control system was impressive. It's well designed and responds very quickly to give priority to domestic. It has some nice built-in features, such as an automatic fill that is controlled by the unit according to pressure. I'm not talking about a PRV, but electronic control.
2. The only hold back that I discovered was that the on- board circ. was limited to 5 GPM. I found this out by calculations from the connected load ( a new AHU) as well as in a training class later. The Navien rep confirmed that the circ was limited to 5 GPM. The thing that is somewhat mis-leading is that the unit has a 199k btu input, thus leading one to believe that you would have about 183k btu output. When I questioned the rep about this he said that if I increased the Delta T in the system, then I could get more btu's out of the boiler for space heating. I assured him that I knew the universal hydronics formula and application of it very well, but it would take over a 70* Delta T to get all the horsepower available from the boiler. His response was that we need to adjust the Delta T if we needed more btu's. I think he missed the fact that when using this for a replacement we're stuck with what the installed radiation was designed to, which is usually a 20 or 30* Delta T.
This issue isn't limited to Navien. The on-board pump in any combi has to operate within the parameters that the domestic side dictates. Talking to my Bosch rep got essentially the same acknowledgement. Their on-board circ is 3 speed, 6 GPM max.
The manufacturers don't seem too eager to put the curve for their on-board pumps in their literature anywhere. Therefore, if you're gonna use a combi (any brand I guess), be advised of this limitation in space heating. I'm posting this because I see a lot of folks are considering these. Know their limitations. With 5 to 6 GPM available for space heating, and a 20 to 30* Delta T, you're only going to get 50k to 90k btu's for space heating. This of course assumes the normal 180* design water temp. Lower design water temps will obviously allow for a wider Delta T on the boiler and more heat will be available then.
Other than this, I was well impressed with the Navien and think it definitely has a place in the mod/con market with its low-end price tag. It was really a good fit for replacing a Polaris in confined quarters.
You can choose to do what you want, but you cannot choose the consequences.