Is a Navien NCB-240E overkill?

Discussion in 'Tankless Water Heater Forum' started by GeraldM, Oct 16, 2017.

  1. GeraldM

    GeraldM New Member

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    Location:
    Beaverton, Oregon
    Hello there, this is my first post to this forum and am hoping for some common sense advice on what to use as a source for space heating and domestic hot water.

    I live in Oregon in the Willamette Valley and have a 17 year old timber frame with in-floor radiant heat (tubes embedded in concrete) heating approximately 1600 sf. Per the original heat calculations performed by HeatLink in 2000, the total heat loss is ~22,000 Btu/hr. There are 3 showers, but never used simultaneously by us.

    For the last 17 years, a low-efficiency Bradford-White water heater provided DHW and space heating via a separate coil. The radiant system has 6 zones of varying lengths, zone valve motors on each zone, a primary circulation pump and a secondary circulation pump that brings hot water from the tank to an injection valve (that only opens when the load side temperature drops below the prescribed supply temp as determined by the Tekmar 31320).

    I now need to replace the water heater and am looking for an efficient option. I've considered the HTP Versa-Hydro (although finding someone in my area to install it isn't easy), a Navien NCB-240e, or possibly a separate tankless unit for DHW and a condensing water heater for the radiant side. I've read some of the other forums indicating the NCB-240e may be overkill for the radiant side, but not for the DHW. Irrespective of which option I choose, I'm confused how any of them will integrate with the existing electronics (or do I throw it all away), and how temperature is regulated on a tankless system if it's feeding directly into the heat side.

    I apologize for so many questions at one time. I'm hoping someone can help me make some sense of this. HeatLink customer service tried but doesn't address the "input" side of the system, and the companies who have given me quotes aren't able to answer my questions about the existing system.
     
  2. Dana

    Dana In the trades

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    The minimum modulated output of the NCB-240E is about 17,000 BTU/hr which barely be below your design heat load, which means it's not really going to be modulating in space heating mode. That's one definition of "overkill". Ideally you'd want the boiler's min-fire output to be less than half your design heat load, then set up the outdoor reset curve and pumping rates so that it fires nearly continuously during the heating season for the highest comfort, fewest number of ignition cycles, and highest possible efficiency. If it's not going to modulate you might as well go with a tank type water heater, which is inherently self buffering.

    HTP's Phoenix Light Duty can probably be a near-drop-in replacement to a system previously serving by a standard water heater such as your old B-W. If your's was the Combi 1 or Combi 2, with an internal heat exchanger isolating the heating water from the potable you'll have to add an external plate type heat exchanger, and a bronze-impeller pump on the potable water side of the heat exchanger that runs whenever the heating system pump runs. The Phoenix Light Duty has a modulating burner that will run longer burn cycles than B-W Combi but has somewhat higher maximum fire output.

    A crude schematic of what it would look like is this:

    [​IMG]

    The grey rectangle in the middle llabeled "fphe" is the heat exchanger. Everything currently on the heating system side of your B-W Combi would be the stuff on the right side of that heat exchanger.

    The yellow rectangle labeled "Tank" would be the Phoenix Light Duty.

    The only necessary change to the heating system controls- would be to wire up the potable-side pump so that it runs whenever the heating system's circulation pump is running. The relay that currently turns on your heating system pump would now also energize the necessary corrosion-resistant pump on the tank side of the heat exchanger.

    The venting for the condensing water would be plastic, just as with the power-vented B-W Combi heaters.

    The Phoenix Light Duty isn't cheap, but it's all-stainless, and should outlast the previous unit by 2x. It's only "Light Duty" in comparison with other commercial water heaters with much bigger burners (burners much bigger than you really need.)
     
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  4. GeraldM

    GeraldM New Member

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    Oct 16, 2017
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    Beaverton, Oregon
    Dana,

    This is exactly the common sense answer I was hoping to receive. I apologize for not replying sooner, I guess I didn't expect an answer so quickly. A couple follow on questions.
    1. I'm familiar with FPHEs, just not how to size one. Is there a resource that would help in that regard?
    2. It seems nearly impossible to find HTP products here in Oregon, but I've read there is a Westinghouse heater that is identical to the Phoenix Light Duty and can be purchased at Home Depot. Do you know if these products are in fact identical? Do you know who provides the warranty?

    Thanks again for your response, this will help make the decision much easier.
     
  5. Dana

    Dana In the trades

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    I believe Westinhouse is just re-labeling HTP's products (both water heaters and boilers.) The level of support from either vendor will vary by location. (I'm less than 50 miles from HTP's home office, you're something like 2900 miles away. :) ) The warranty terms are similar, but it's good to ask Home Depot who is on the hook for warranty support (HD, Westinghouse, or HTP) if buying through them.

    Carlson has a handy web-calculator for sizing heat exchangers. Oversizing them lowers the temperature delta and pressure drop, but it doesn't take anything huge to deliver 20,000 BTU/hr at a < 5F delta-T.
     
  6. GeraldM

    GeraldM New Member

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    Dana,
    Thank you for the confirmation about Westinghouse and the advice not to go too big. Given there's little price difference between a 60,000 Btu rated FPHE and a 150,000 one I would have just gone bigger. You've been so generous with your time and advice I feel guilty asking one more question but this has been nagging me. Why does the pump on the potable side need to run whenever the pump on the heat runs? My current setup works that way and I always thought it to be a waste of heat and electricity. Why not kick on the potable pump when there's a call for heat (the Tekmar has a boiler enable switch that could possibly serve this purpose)? Because the injection valve is a wax style by the time it opened hot water would be delivered from the tank ensuring there wasn't a cold sandwich. But this is all from someone who really doesn't understand how this all works.

    Thanks again for your help.
    Gerald
     
  7. Dana

    Dana In the trades

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    The heat exchanger needs to have flow on both sides for it to transfer heat from the water inside the tank into the system side water. If only the heating system pump was running the heat exchanger will rapidly drop to the temperature of the water in the heating system. With both sides running the heat exchanger is constantly being heated by the hotter water from the tank, and that heat is being carried away to the radiation, returning much cooler system water back to the heat exchanger.

    If the heating system side runs constantly even without a call for heat it's worth installing an ECM drive "smart" pump in place of whatever is currently there, and it'll use a lot less power. Those were ridiculously expensive 17 years ago, but on the order of ~$200 now. Different version can be operated under constant power, constant flow, constant pressure, proportional pressure, or delta-T control, and will adjust the power up/down based on what you set it to, using the minimum power necessary to meet the programmed function. You'll have to figure out which modes are most appropriate for your system, and find a pump that covers the pump-curve range of what is currently being used. But any pump that's running a 100% duty cycle most of the heating system will pay for an ECM drive pump well within it's lifecycle.

    Operating the potable side pump with the boiler-enable output of the Tekmar will probably work too. Whether it's worth using an ECM drive pump there depends a bit on the anticipated duty cycle. You can probably keep using what's already there for the potable side pump and monitor it's cycles before biting the bullet on an ECM drive version. There are a number of smarter lower power pumps being installed in potable hot water recirculation systems that might work, as well as stainless or bronze impeller versions of typical hydronic heating circulation pumps.

    Using a 150K heat exchanger will result in a lower delta-T, but if you only need 100F water to meet your design-day load it won't gain you much in efficiency, since even it it needed a 20F delta you'd still be at condensing temperatures.
     
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  8. GeraldM

    GeraldM New Member

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    Beaverton, Oregon
    Dana,
    Ended up with the HTP Phoenix Light Duty (disguised as a Westinghouse WGR050NG076). I also opted to simplify the radiant side of the equation by installing a Taco X-pump block. I got the Westinghouse through a local hardware store (that's how they're being marketed), and had it installed on November 17. Out of the box it didn't work. 10 days later, a new gas valve and many hundred more dollars spent I still have no hot water. I'm about to crate it up, return it and replace it. But with what? And how much longer do I go without hot water?

    Is it worth trying to get the thing working? There now seems to be debate about whether the stated 150' total equivalent length of 2" PVC for intake/exhaust air is valid--I've got 80, max 90. Also, the installer used CSST gas line and while there's good pressure at the appliance (6.8" W.C.), could this be the culprit?

    Do you have any recommendations for another appliance that can perform the function that might be more readily supported?
    Regards,
    Grungy in Oregon
     
  9. Dana

    Dana In the trades

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    I'm surprised at the d.o.a. condition of the Westinghouse badged Light Duty !?! Distributor product support is everything in situations like this!

    The AO Smith Vertex condensing water heaters have side-ports for space heating applications, and will fill the bill. It's usually quite a bit cheaper than an HTP Phoenix LD, but it's a conventional glass lined tank, not stainless, and won't last as long, but it should run about as long as your Bradford White did.
     
  10. GeraldM

    GeraldM New Member

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    Beaverton, Oregon
    Dana,
    As always thank you for the quick reply. I'm giving the Westinghouse one more try. I looked into the A.O. Smith Vertex and it costs about $100 less than the Westinghouse (or at least what I paid). I've got a technician coming again on Monday to hopefully fine tune the burner. While that's yet another expense it will pale in comparison to taking out the Westinghouse and putting in something else. If this doesn't work, at least I have another option to consider. Now on to the next problem, the Taco XPB variable speed pump doesn't seem to be working.
    Gerald
     
  11. GeraldM

    GeraldM New Member

    Joined:
    Oct 16, 2017
    Location:
    Beaverton, Oregon
    Final installment to this long saga. I finally found a plumber with the right equipment and the understanding of how to use it. But even after he was able to fine tune the DWH (CO values of 1 and 4, low flame and high flame respectively, and CO2 values of 9) it struggled to start cold. Only by disconnecting the internal corrugated tube and letting it run using the air in the room would it work. All this led us to believe the 2" vent pipe was choking the intake. Even though Westinghouse says you can run up to 150' equivalent length (total, intake and exhaust), and I had less than 90', I paid to have the venting completely redone in 3", the DWH re-tuned, and alas it wouldn't cold start. The technician then adjusted the ignitor angle by a fraction of a degree and finally it runs quietly, efficiently (although not quite as high as I had hoped), and on outside air.

    With hot water working, we turned our attention to the Taco X-pump block (XPB) that still only operated in 'manual' mode. I established the pumps were wired backward (system pump run as a variable and variable speed pump as a constant speed). Ordered a replacement only to get another unit wired exactly the same way. In the end I re-wired the pumps myself and I now have consistent, uniform radiant floor heat. What has happened to quality control? Three units and all three are defective? The chances of that happening should be astronomical, yet they did. With all of the problems resolved for the immediate future, I hope to not have to deal with this system for many years to come.

    Thanks Dana for responding to my plead for help. I hope these units run for a very long time.
    Gerald
     
  12. Dana

    Dana In the trades

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    Sure they have quality control- all three pumps were wired the same, right? :rolleyes:

    I hope this setup runs forever and eventually proves it was actually worth the patience it took to get it up and running! (Murhpy's Law, I guess!?)
     
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