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Navien Combi opinions wanted

SaraJ Member Posts: 1
Looking for some opinions from people with experience with Navien Combi units. Buying a Vermont weekend retreat that needs serious rehabilitation. Replacing ALL mechanicals. Converting from oil, forced hot air to propane radiant heat. No A/C. Considering a Navien Combi unit as it appears it may be able to provide for all our heat and hot water needs. Are we crazy? Concerned water pressure may not be dependable as water comes from an old pump system on mountain that supplies about 50 homes. Please be kind. I am not a plumber, just a mom who needs to understand everything before spending money on it. Thanks all!!! Sara


  • Ironman
    Ironman Member Posts: 7,334
    See attached link

    Here's a link to a recent thread that I posted on this:

    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Paul48
    Paul48 Member Posts: 4,469

    You'll find the people here a very kind and helpful. Coming here proves you are a very smart mom. It is very important to match the size of the boiler to the home. Too big a boiler means you are wasting money. All homes lose heat at different rates, based on construction and insulation. There are free programs available to help you calculate the heat loss of your weekend retreat. Slant-Fin probably offers the easiest program. Maybe one of the other guys can post a link to the download.
  • BayAreaDIYer
    BayAreaDIYer Member Posts: 5
    Similar Question

    Hi - I have a similar question and did read the other post. I'm trying to do as much research as I can to make an informed decision. Our design temps are mild (low in winter of 35F) and from what our architect has calculated our heating requirement will easily fall below 30,000BTU for a 950sqft new construction addition. DHW is low (3 shower bathrooms - no soaking tubs) The radiant person suggested the Navian, but we are also considering retrofitting and additional 500sft with radiant once our cash flow recovers. Just wondering if the Navian would still be a good fit as the heating requirement may go up. Thoughts?
  • kcopp
    kcopp Member Posts: 4,382
    Not recommended....

    Too many complaints and no real time here on the market to see if they have any longevity.
  • HDE
    HDE Member Posts: 225

    Too many complaints where?
  • SWEI
    SWEI Member Posts: 7,356
    low loads

    There is literally nothing available in a gas mod/con that will properly handle that small a load.  You're going to be short cycling most of the season.  If you're doing solar thermal (or plan to add it soon) and have a big storage tank, you should be able to make a 60-80k gas mod/con behave well.

    A heat pump (either ground- or air-source) would work well -- but in a climate with no real cooling load, the ROI is not particularly attractive.

    Believe it or not, the most affordable currently available solution for a structure with such a low loss is probably a modulating electric boiler with built-in ORC.  The Thermolec Mini http://www.thermolec.com/en/productView.aspx?type=product&id=62 will cost about 1/3rd of what a gas mod/con does and the install is dead simple.  If you have decent passive solar gain, the annual operating cost will be quite low.
  • BayAreaDIYer
    BayAreaDIYer Member Posts: 5
    Low Loads - what to do?

    This is exactly the problem. I can't seem to find a good heat source solution for a low heat requirement. Maybe we just use a DHW heater with a heat exchanger to keep potable water separate. Then the question is which DHW unit? Any advice is greatly appreciated.
  • TonyS
    TonyS Member Posts: 849
    i have installed

    about a dozen or so Navien water heaters so far. The oldest being the one in my own home, going on 4 years. They have had a few problems and growing pains...flow switches, boards, bad plastic fittings. But the company has made good and solved these problems. I worked on one the other day,. an early A model that was leaking at one of the early grey 90s. I called Navien gave them the serial number and model and they air mailed, overnight a complete rebuild kit, board fittings, water control,hoses, flow switch.

    They are a cinch to work on, everything is connected with quick clips and o rings.

    The company is here for the long haul and is ready to make good on their product.

    That being said, I have sold a few combi units recently but have not installed them yet.

    the Ch180 will modulate down to 17000 btus and give you around 60000 on the heating side and 150000 on the domestic side, it comes complete with oudoor reset and since it was designed to deal with the inconsistencies of domestic water, it should have no problem handling a close loop heating system. I dont know what your electric prices are but here we pay about 15 cents a kilowatt hr, 3410 btus...about 23000 btus for a buck. That same buck gets you almost a 100000 btus of natural gas. Its not even a contest!

    In my opinion, if your mechanic is a good hydronics man, I think the Navien, properly installed would be a good choice.
  • HDE
    HDE Member Posts: 225
    edited April 2012

    60,000 BTU's max if direct connected to a heat emitter, 150,000 less comb efficiency loss is its full potential if piped primary/secondary.

    Think of it like a munchkin type boiler if piped properly primary/secondary
  • TonyS
    TonyS Member Posts: 849

    I was under the impression that is was only possible to get6 to 7 gpm through the exchanger even when piped primary secondary and increasing pump size wasnt an option due to the specific pump provided by Navien?

    What is your experience with that unit?
  • SWEI
    SWEI Member Posts: 7,356

    Using a DHW heater (tank or tankless) for space heat will not get the job done well, will not be covered by a warranty, and will leave you and your family open to Legionella infection.  Really.

    You do have a few viable options:

    Add a buffer tank large enough to allow a 60k-80k mod/con to operate effectively.  Coincidentally, the tank will make a great solar thermal storage system.  The outdoor reset control (mixing valve or injection pump) will work with both as well.  Depending on the amount of sun your location receives, a non-modulating boiler can make more sense in this configuration (if it's only meeting a small percentage of the annual load.)

    A heat pump will be relatively expensive to install, but can provide cooling if that is needed.

    The aforementioned small electric boiler.  Truly inexpensive to install and sized for the load.  If you get most of your heat from wood or solar (whether passive or active) or the property is only occasionally occupied, it can be far less expensive than you think.

    Combining loads with another building or wing can often make sense.  A well-insulated 500-800 sq ft cottage may not pencil out on its own, but a small group of them can effectively utilize a mod/con and the cost per unit be quite attractive.
  • bld999
    bld999 Member Posts: 47
    dhw tank


    You responded to Bay areaDIY:

    "Using a DHW heater (tank or tankless) for space heat will not get the

    job done well, will not be covered by a warranty, and will leave you and

    your family open to Legionella infection."

    He was suggesting an hx to separate dwh from heat. I the case he is refering to, wouldn't a conventional tank be ok if he was willing to ignore the warranty? If he later adds 500sf as he mentions, then a better heat plant might be substituted.
  • SWEI
    SWEI Member Posts: 7,356
    yes, but

    The complexity and cost of getting a system like that to work properly (not condensing the heat source plus outdoor reset) will make you wish you'd just done it right in the first place.

    The scope of work here is unclear -- BayAreaDIYer mentions a 950 sq ft addition plus another potential 500 sq ft future addition.  3 bathrooms for 950 sq ft seems like overkill, but maybe they also serve the original building?  It really would be best to look at the entire project.  I'm guessing from the handle that there is no engineer or contractor involved yet -- might be time to bring someone in.
  • BayAreaDIYer
    BayAreaDIYer Member Posts: 5
    Just trying to Plan

    I am not really a DIYer, just heavily involved and wanting to understand what to ask and to plan for. I've had a couple contractors look at the project with different opinions. Radiant is not too common around here so I rely on forums.

    The house is 2200sqft. 500sqft will be added to an existing room that is approx. 450sqft. making a 950sqft fully remodeled space with cathedral ceilings. It's a kitchen/family room with very small guest bed/bath. We want radiant for this newly remodeled space. We are also considering putting radiant in 500sqft of another common living space in the old part of the house. The rest of the house (bedrooms and 2baths) will remain on gas furnace.

    So as I understand it, if we were to only need a heat source for the newly remodeled space we could

    1. Use a DHW heater with a heat exchanger to separate potable water

    2. Use a boiler in a closed loop system.

    3. Use a boiler with an integrated DHW tank (like the Triangle Tube Challenger)

    4. Use a boiler with an indirect hot water tank.

    Of the 3 contractors I've asked, I've had three different opinions.

    We don't think we want a tankless system because that would require us to install a new gas line. Trying to do this on a tight budget.
  • Zman
    Zman Member Posts: 7,542
    In the long run...

    In the long run I think a durable proven down fire style mod con boiler with an indirect tank is your best option. My favorite would be the Triangle tube solo 60 with a heatflo 40 indirect. The boiler will modulate down to something like 12,000 BTU output, and given the boiler water volume and the fairly high water volume of the typical radiant system I think short cycling will not be an issue. The boiler will also automatically increase it's differential temp to further protect itself from cycling.

    The industry has seen it's share of "Dogs". Stick with something with a long track record
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • HDE
    HDE Member Posts: 225
    Navien CH Combi

    That's what primary/secondary is all about. When flow rates don't or can't be matched, you pipe in primary/secondary, heat is transferred between tees.

    We have countless Navien CH-240's serving 125,000 + BTU heatloss homes and systems.
  • TonyS
    TonyS Member Posts: 849

    I understand primary secondary piping, The Navien pump is used for the boiler pump and only has to contend with the pressure drop of the heat exchanger and not the entire system. I find it difficult to believe that the Navien pump is moving 12 and a half GPM through the exchanger...that is what would be needed at a 20 degree delta t.

    I usually have no control over the delta t because I am retrofitting to an existing system,

    I often hear from otherwise knowledgeable people in the field that I can increase btus by widening the delta t, when I ask how to do this they will reply by slowing down the flow.

    It seems they forget that flow is also one of the components of the formula. Slowing down one to increase the other adds nothing.

    So no matter what type of piping I do, the basic math remains the same...I have to move x number of pounds of water at x amount of delta t to get x amount of btus.

    So if you can tell me how many gallons of water per minute you are running through that exchanger and at what delta t, I can tell you exactly how many btus you are moving.
  • HDE
    HDE Member Posts: 225
    edited April 2012
    Don't think 20 DT

    The Navien like many other condensing boilers does not operate on a 20 DT. It modulates and stays fired up to and at set point temp, not cycling on 20 DT.

    What you have been taught on 20 DT for radiation or standard operating controls on boilers does not apply. Thus 1 GPM at 20 DT = 10,000 BTU's does not apply here.

    Look up Siggys explanation on "Mixed Stream Formula" to learn how boiler primary heat at a different or lower flow is transferred to a secondary at a higher flow extracting the heat and driving the primary return to a lower temp resulting in higher boiler DT operation.

    So lets pretend under your conditions the Navien is flowing 6 GPM, and you have a zone that requires 125,000 BTU's, thus you designed the radiation at 12.5 GPM, 180 supply water required, for your DT you selected 20. on a cold day and at room temp, by theory, water is at or near room temp right, or slightly higher? Lets use 80

    Mixed Stream Formula:

    (7 GPM x 180 degrees Navien) + (12.5 GPM x 80 degrees secondary return) = 2,260 / 19.5 (7 + 12.5 combined flows) = 115

    115 degrees is the mixed water return temp to boiler

    180 supply - 115 return = 65 DT

    7 GPM through Navien X 65DT x 8.34 x 60 = 227,682 Potential BTU Firing rate when secondary 12.5 GPM pump activates Navien Circulator at 7 GPM. This proves the Navien would be at full fire till warmer water returns. We all know warmer water returns as emitters become saturated with heat. As warmer water returns DT decreases, boilers modulate down.

    This is simple formulas to understand how Primary/Secondary piping transfers heat that I'm surprised how many don't know and misunderstand. IBR taught 20 DT for radiation too well I guess.
  • TonyS
    TonyS Member Posts: 849
    I understand on startup

    you may have 65 degree delta t but lets say we have an existing home with 4- 60 ft baseboard loops. Each loops is 30000 btus at an average temp of 170 for a total of 120000 btus on a design day. 180 degree water in 160 degree water out, this is how the system was designed and must operate for each room to receive the proper amount of btus on a design day. It doesnt matter what boiler I use or what type of piping. The radiation Must be receiving 180 degree water at 12 GPM and returning at 160 degrees.

    Therefore after the system comes up to temp on a design day and that is what we must design for! The water being pulled into the Navien boiler loop will be 160 degrees and in order for the Navien to continue to satisfy the radiation on a design day it must provide 180 degree water with a flow of 12 GPM. We cannot change this without adding more baseboard to each consecutive room so that each convector continues to satisfy each room as the water becomes cooler and the delta t gets further apart. Then and only then could we increase output from the boiler with a wider delta t without any ill effects on our system. The Navien on this system, receiving 160 degree water back from the loops and only being able to move 6 GPM over the exchanger with a supply temp of 180 will only provide 60000 btus.
  • HDE
    HDE Member Posts: 225
    edited April 2012
    Still not following

    I dont think your following how boilers work.

    For example, whats the BTU firing rate of a cast iron boiler set at 180 degrees supply with a 20 DT aquastat controller with 161 degree return water?

    0 BTU's of course, burners off, so I ask how did it heat the house, one must understand Reynolds Law.

    The Navien CH-240 for instance, with 161 degree mixed water return temp would be a 19DT x 500 x 7 = 66,500 BTU's, certainly adding more heat than a on/off boiler cycling on a 20 DT aquastat.

    "There is simply no way around these mathematical consequences of thermodynamics, and that’s a good thing. Learn it, trust it and then use it to answer many of your hydronic design questions." - John Siegenthaler
  • TonyS
    TonyS Member Posts: 849
    So how is 65,500

    going to heat a home that requires 120000 btus. The structure is losing 120000 btus an hour to the outside, only more insulation will change this fact. The baseboard heat is preinstalled long before I get there and was designed to satisfy the heat load of each room on that design day with an entering water temp of 180 and an exit temp of 160.

    In order for the loops to evenly heat and obtain their proper inlet and outlet temp, the flow needs to be 3 GPM per loop x 4. So where the 4 loops drop into a common return before the closely spaced T's for the Navien boiler loop the water temp is 160 degrees.

    The primary loop has 12 GMP flowing through it and the Navien has the ability to pull 6GPM at 160 degrees over its exchanger, the other 6 gpm flows through the T's and is remixed with the 6 gallons coming out of the Navien at 180 degrees, so we have 6 gpm at 160 degrees mixing with 6 gpm at 180 degrees, so we now have a supply temp of 170 going out to our convection and each time the water runs through the system it will continue to drop 10 degrees until the 60000 btus reaches equilibrium with the outside temp somewhere below 50 degrees.

    Thats how I see it. But who knows, I could be wrong

    Now would be a good time for MARK EATHERTON to chime in, if you would.
  • HDE
    HDE Member Posts: 225
    I give up, you will learn or experience it someday

    Ok nevermind, you keep reversing the logic rather than follow the concept. Don't worry your not alone. Many new designers make the same mistake. And yes my other fellow expert designers can concur.
  • HDE
    HDE Member Posts: 225
    edited April 2012
    Please read fully

    How do you heat a house with 0 Btus when burner is off?

    65,000 is a modulated burn to get water from 160 to 180

    At that point your raising water 20 degrees not 120 degrees.

    You remember that it takes 1 BTU to raise 1 pound of water 1 degree right?
  • TonyS
    TonyS Member Posts: 849

    The structure is losing 120000 btus an hour on a design day. We dont just heat it once and its over. We must inject 120000 btus every hour possibly for weeks.

    In order to do this we must supply the proper temperature water and the proper flow rate.

    The radiation in this particular structure requires 180 degree water because of the amount of baseboard previously installed and the last piece of baseboard on each loop requires a temperature above 160 degrees because that is the designed temperature and flow of that baseboard. I dont care how you heat your water, I dont care if you use your stove top and a funnel, the water must enter 180 and exit 160 in order for that baseboard to heat the house on a design day. If you  slow the flow the temperature of the water exiting the baseboard would be to low and consequently the room would not be getting the proper amount of btus. So can we at least agree on the fact that this house sitting in design outdoor temps for weeks on end must continue receiving a flow of water through its baseboard thats starts at a temperature of 180 degrees and exits at at 160 degrees in order for the house to maintain 70 degree inside temp?

    If we can agree on that then we must also be able to agree that the return water coming back to the common return before the boiler is 160 degrees and it must remain at 160 degrees for weeks on end as long as the heat loss of the structure is 120000 btus an HOUR.

    SO THEREFORE WE MUST TAKE THIS 160 DEGREE WATER AT 12 GPM AND HEAT IT BACK UP TO 180 IF WE ARE GOING TO HEAT THIS STRUCTURE. and we must do this continuously for maybe weeks on end to maintain our indoor temp of 70 degrees.

    In order to heat 12gallons of water per minute 20 degrees..which is 12 x 8.31 x 60=5983.2 x 20(our difference between 160 and 180)==119664 or 120000btus.

    It doesn't matter if we go strait through a boiler or pipe it primary secondary...the system has already given us a given and that is 160 degree water in and 180 out.

    The only thing we have control of anymore is flow through the boiler and that is limited to 6gpm because of the proprietary pump, unlike a challenger that we could hang a larger pump on.

    This is not that difficult!
  • TonyS
    TonyS Member Posts: 849
    Ironmans link

    after poking around on this thread a while, I tapped the link Ironman had pointed Sara to.

    He went through this same thing with you (HDE) on a prior Thread.

    I think it might be a good time to get your group of Experts together and rediscover a few basics.

    And since you like reading Seganthaler, look for one he wrote a few years ago, it was called ...The btus wont jump off the train, this article will help you understand about delta t and flow.

    Good luck with those jobs you installed!
  • SWEI
    SWEI Member Posts: 7,356
    thread drift

    I fear we may have overwhelmed poor SaraJ on this one.
  • BayAreaDIYer
    BayAreaDIYer Member Posts: 5
    Overwhelmed BayAreaDIYer too!

    Just wanted some ideas of equipment on the options of 1, 2, 3, 4.... :-)
  • SWEI
    SWEI Member Posts: 7,356
    edited May 2012
    One more time

    1. Use a DHW heater with a heat exchanger to separate potable water.

    Assuming you need both DHW and space heat, this will require two pumps, a heat exchanger, a motorized mixing valve, and an outdoor reset controller.  And the DHW heater will almost certainly die an early death.

    2. Use a boiler in a closed loop system.

    Whatever you use, the heating should be in a closed loop system.  Assuming the boiler modulates, has built-in ORC, and can be directly pumped (has a low head loss across its HX)  this is by far the simplest way to get the job done.  If you're only heating 500 sq ft of radiant floor, your pump can probably run on 10 Watts (really - check out the little ecocirc e3 and compare power use with a Taco 003.)  The big one here is to make sure the boiler is not oversized.  From my perspective, that means a small electric boiler is your only real option.  It also happens to be the cheapest option here by quite a bit.

    3. Use a boiler with an integrated DHW tank (like the Triangle Tube Challenger)

    Assuming this would replace your existing DHW heater?  The Challenger is tankless.  You're still going to be short-cycling if it only heats this little space.  The Prestige Excellence (which has an integrated DHW tank) is even more oversized for this application.

    4. Use a boiler with an indirect hot water tank.

    Could work if you get an indirect that's big enough to buffer the boiler, preventing it from short cycling on your miniscule heating load.  It's going to cost at least 6x what the little electric wall mount boiler I suggested will, and it's going to need about 25x the space.

    Good luck with the addition.
  • JohnHenry_2
    JohnHenry_2 Member Posts: 70
    High pressure drop HXs

    It seems to me what Tony is trying to get at is these high pressure drop condensing units are just plain unsuitable for high temperature heat emitters such as baseboards, unless the home is WAY over radiated.

    I understand P/S plumbing is a must with these units, but I don't think that's the issue. The issue seems to me to be about the relatively low flow through the HX coupled with heat emitters that need high temp water. If you have 160* water coming back to the boiler and only 6gpm flow, you simply can't add more than ~60K btu/hr to that flow, which of course won't keep a house warm that's losing 125K btu/hr. If the baseboards need 12 gpm, then the hot water from the boiler will be diluted with cooler return water, possibly giving the baseboards lower water temperature than is needed.

    I'd be willing to bet that the 125K heat loss homes you put the 240's in have either large radiators or in floor radiant heat that requires water at or less than 140*.

    If you've gotten 125K btu/hr out of one of these units in a situation where there are high temp (180*) emitters, I'd love to hear how you did it. I just plain can't get there from here.

    That said, I absolutely love my Navien CH-210 hooked up to my large cast iron radiators. I figure I saved at least $3,000 on my heating bill this past winter over the 80 year old, converted to oil coal boiler I had last heating season.
    The bitterness of poor quality remains long after the sweetness of low price is forgotten.
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