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Navien Combi Boiler

IronmanIronman Posts: 5,013Member
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.
Bob Boan

You can choose to do what you want, but you cannot choose the consequences.


  • meplumbermeplumber Posts: 678Member
    Thanks Bob.

    I appreciate the report.
  • HDEHDE Posts: 223Member
    edited February 2012

    Many combis such as the Bosch and Navien fire on the same principle as tankless do. Flow rate and temp rise. Since you can't control the flow rate via the internal boiler circ, to achieve max fire rate you must give it the rise required. Since the internal circ wouldn't support a large system at 20 degree diff which could be as high as 19 GPM, you turn to primary/secondary piping.

    Read up on the hydronic principle called mixed stream formula to understand how to apply a 5 GPM boiler to a 20 GPM system with 20 Delta T and hit 199,000 BTU fire rate.

    Siegenthaler and many others have explained this in their books.

    Some of his explanation here

    For example using the formula:

    Boiler flow 150 degree supply at 5GPM pipe primary secondary with secondary circuit, 15 GPM pump, 20 DT at room temp of 70 results in a mixed water temp return to boiler of 90 degrees

    150 - 90 = a 60 degree boiler operating DT

    60 x 500 x 5. = 150,000 BTU fire rate, thus you are not limited to 50-90MBTUH with proper install and piping
  • ZmanZman Posts: 4,868Member
    Thank for the info

    This looks like a nice little unit. I think it would work nicely in many applications. The hot water capacity is robust. At 5 Gpm and 20 delta T it gives you 50,000 BTU for heat.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Paul48Paul48 Posts: 4,492Member

    These manufacturers require that they be plumbed on the secondary side, off the closely spaced tee's. In your example,the supply would be mixed water temperatures. The return would not be.
  • IronmanIronman Posts: 5,013Member
    Mixed Stream Formula


    I understand the formula and it proves my point about the limited capacity of these units when applying them to existing radiation that was designed at 180* with a 20* Delta T.

    For the most part, the unit must be piped P/S. But when you have series looped baseboard that was sized for the figures above (as most were), then you are not going to get anything more than 50k btu's. Even though it's piped as a secondary, the baseboard is still only getting 50k btu's. Attempting to widen the delta T in this scenario would only cause insufficient heating in the last rads in the loop. If there is no cascading average temp between the primary and secondary, and both are operating at approximately the same Delta T, then the GPM would have to be the same in both.

    Again, I think the point that's being missed is that you don't have much room (maybe none) to change the design parameters of an existing system. The return temp to the boiler cannot be any lower than the return from the secondary. You can't run 90* return water from baseboard at design temp.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • JohnHenry_2JohnHenry_2 Posts: 70Member
    My experience

    With the ch-210 has been pretty good as well. I really like the high turn down ratio.

    One thing I'm kind of concerned about is that according to Navien, if you want to use the outdoor reset, you need to use the remote controller as the thermostat and therefore can't use zone control.

    I tried using outdoor reset with a separate thermostat and it worked fine for about a month and then suddenly stopped heating. I unplugged the outdoor temp sensor from the PCB and it started heating again but i had to set the supply temp manually.

    That worked fine for about another month and then the system overshot the set temperature. I turned the t-stat off (verified t-stat was working correctly) but the system kept on heating. So I shut down the heater, mounted the remote controller in place of the t-stat, set it up to control the indoor temp and hooked up the outdoor sensor and it seems to work fine.

    One thing i haven't tried yet is to set it up to run without the remote controller using the dip switches. If i get some extra time i may try that.
    The bitterness of poor quality remains long after the sweetness of low price is forgotten.
  • HDEHDE Posts: 223Member

    Ironman, there are few condensing boilers today that allow enough flow to match system flow. One needs to remove the old IBR taught 1 GPM equals 10,000 btus at 20 DT. Thats not how many of the function such as BAXI, HTP or Navien.

    I've seen countless 100,000 BTU + systems installed including naviens set at 185, delivering 180 to baseboard. In fact over a conventional on/off boiler cycling between 160 & 180, with modulating turndown the average water temps stays higher increasing emitter output.
  • HDEHDE Posts: 223Member
    Big picture

    I guess you should just just know it can be done and the 1000's of jobs with baseboard are working.

    You missing the total picture here, if the water temp is set at 180, the firing rate would at that point be reduced. If the room and system is cold, it will be awhile before the water returns at above 160. Whats the fire rate of a on/off boiler set at 180 with a 20DT control with return water temp of 170? It's 0 because burner is off, the mods just reduce fire as BB is saturated and heat transfer is reduced.
  • 12plumber12plumber Posts: 1Member
    Navien combi question

    I just installed my 1st Navien ch240 combi. I fired it up for the 1st time yesterday and cannot get a 20 degree delta. Not even close. I plumbed it primary/secondary with t's spaced within 4 pipe size diameters. I've installed other high efficiency boilers but never anything like this. I know that my piping is done correctly and my supply is feeding at 170 but my return is coming back at 166. I spoke to 10 different techs at Navien and was told I should install more heating element. I have 2 small loops of baseboard on 2 zones and have absolutely nowhere to add any more element. It's a small condo. My other option I was told was to get a high heat circuit board which will allow the unit to run at 180 as well as losing its efficiency. Just wondering if anyone out there has had this problem?
  • Paul48Paul48 Posts: 4,492Member

    did you come up with for a heat loss?
  • Jean-David BeyerJean-David Beyer Posts: 2,620Member
    Delta Ts?

    I am a homeowner, not a heating professional. I have a mod-con with outdoor reset and two heating zones. I also have an indirect fired domestic hot water heater.

    I can get about 20F delta T when running the indirect.

    The other two zones, the heating zones, get far less than this.

    My large zone is a radiant floor at grade slab. It will sink about 24,000 BTU/hour when it is 0F outside. But it never gets that cold outside. Design temperature is 14F and it only does that for about 5 hours a year. When it is really cold, I sometimes get 10F delta T on that zone. When it is warm out, but with a call for heat, I may get 1F or 2F delta T. When warm out, I supply 75F water to that slab. Since the thermostat for that zone is set at 69F, there is no way I could get even 10F delta T from that zone in warm weather. When it is 14F out, I put about 112F into the slab, and I might get a 10F delta T at that point (I do not remember). It seems to me if I get enough heat and I get a low enough return temperature to assure condensing, I should be OK no matter what the delta T might be.

    Similarly, I have a small zone (baseboard) that needs only about 6500 BTU/hour when it is 0F outside. When it is warm out, I put 110F water in there. When it is very cold outside (6F), I put 135F water in there. This heats just fine. If I were to put even more baseboard up there, I could run even lower temperatures,  Right now, baseboard covers an entire wall in each of the two rooms. Since this condenses all the time now, it would be difficult to justify more baseboard up there. On the other hand, I doubt I have ever seen a delta T of over 6F up there. Here, too, if I get enough heat with low enough return temperatures to assure condensing, why would I need greater delta T?

    As long as the flow is not so high as to cause noise, cavitation, etc. I might save a little electricity to use smaller circulators, especially on the small zone. But I do not think I would save enough to justify the price of the circulators and the labor to change them. If the circulators are too small, I would get uneven heat.
  • IronmanIronman Posts: 5,013Member
    Insufficient Load

    What is the btu output of your emitters? What size circ are you using?

    The boiler has a 5 to 1 turndown. If your connected load is less and / or you're over pumped, then you're going to have a low Delta T.

    Did you use the outdoor sensor and properly setup the reset curve?
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • ZmanZman Posts: 4,868Member
    Are you talking about?

    Are you talking about delta T at the boiler? With primary secondary the delta T at the boiler is not the same as the heat loop. I just walked down to the boiler room. I have one small slab calling. The heating loop delta T was 12 ans the boiler said 2. The boiler loop has 6x the flow of the heating loop. I wonder if someday the boiler will vary the speed of the boiler pump to match the load and optimize efficiency?

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • JohnHenry_2JohnHenry_2 Posts: 70Member
    the Navien ch-240

    actually has a 10/1 turn down ratio as the this unit will fire as low as 20K BTU/hr input. In my search of boilers/water heaters the only one I found with a lower firing rate than that was the Navien CH-180 which is either 15K or 17.5K.
    The bitterness of poor quality remains long after the sweetness of low price is forgotten.
  • JohnHenry_2JohnHenry_2 Posts: 70Member
    Design Question

    This Navien unit has an exhaust temp limit of 149*F. How can the exhaust temp be under 149*F if the return water from the system is 166*F? In a boiler like this, isn't the lowest the exhaust temp can be equal to the return water temperature?

    Are direct vent, mod/con boilers really even suitable for high temperature baseboard heating?
    The bitterness of poor quality remains long after the sweetness of low price is forgotten.
  • IronmanIronman Posts: 5,013Member
    10 to 1

    Thanks, JohnHenry.

    You're absolutely correct. I knew it went down to 20k btu's. My mind just wasn't computing very well after an extremely long day. Might have something to do with age too. :)
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • IronmanIronman Posts: 5,013Member
    Flue Temp

    PVC has been de-rated to 140*. You must use CPVC if the supply temp is set to 160* or higher.

    Flue temps are based on return water temp as that is the closest water to the flue in the heat ex.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • JohnHenry_2JohnHenry_2 Posts: 70Member

    Is that de-rating for solid core as well as cellular core?

    So with a boiler that has a flue temp limit of 149*F probably shouldn't be installed with emitters that need 180*F water at design temp if they can only shed 20*F before sending the water back to the boiler?
    The bitterness of poor quality remains long after the sweetness of low price is forgotten.
  • kcoppkcopp Posts: 3,324Member
    you can't use.....

    foam/ cellular core PVC for venting,  ever.
  • JharrinJharrin Posts: 5Member
    Navien 210

    I just installed a Navien combi boiler, a CH 210. I have a load of 100k btu's. I used primary/secondary piping. There are two zones, both split loop. Can I expect to cover the heating load or Am I in trouble because of the 5 gal flow through the boiler?
  • IronmanIronman Posts: 5,013Member
    edited February 2013
    It Depends

    What is the average water temp required for your emitters at design temp? What's the design Delta T? What type of emitters?

    If you have an existing baseboard system, then it was most likely (99%) designed for 170* average water temp with a 20* Delta T. That's 180* supply, 160* return.

    With 5 gpm. available to move the water through the heat exchanger, that translates to 50k btu's.

    If you have some other type of emitters, or if you can lower the required average water temp or widen the Delta T, then you'll get more btu's.

    There is nothing magical about a Tee or a set of them. They don't create more btu's. As Gil Carlson (the inventor of p/s piping) used to say: "What goes into a Tee, must come out of that Tee". I'll take it a step further and say: "ONLY what goes into a Tee, can come out of that Tee". The logic of this is inescapable. Some have drifted into cloud land and would try to convince us otherwise.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • EastmanEastman Posts: 927Member
    cloud land

    What is the root of all this prim/sec misunderstanding?  I feel like the issue comes up way too much.
  • IronmanIronman Posts: 5,013Member
    What is the root of all this prim/sec misunderstanding?

    I'm not exactly sure in each case. Apparently, Gil had to deal with it in his day though. The quote is related to us by Dan in "P/S Piping Mad Easy". He says Gil would open lectures to engineers by stating it twice. It take two times for engineers to get it.

    To me, it's a matter of someone not letting common sense control their imagination. Theory becomes fact even though it's not provable in the light of fact. Think about it: A set of Tees causing more btu's to magically emerge from them than was put into them???

    Cloud land.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
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