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Thread: Need Help Tuning Navien CH-180 ASME Mod Con Combi

  1. #16
    DIY Junior Member jac04's Avatar
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    The weather has been warm here, and I definitely found the limitations of the Navien K-factor curves. I woke up this morning to a 66F house, even though the t-stat was set to 68F. It was about 52F outside. The t-stat was calling for heat for several hours, but the boiler never fired, even though the supply temp was down to 68F. I had to bump the K factor up to 2.0 before it would fire.

    Unfortunately, the Navien makes me pick from pre-set K-factors, and I can't set my own curve.

    I'm thinking about maybe changing the setting to a return water setting instead of supply, and just leaving it for the time being. I was going to try 120F return water setting and see what it does. Worth a shot I guess.

    Anyhow, I'll probably be taking a break from tuning this rotten Navien. It's been 3 months since it was installed, and it took about 6-8 weeks for the installer to get it to the point where it seemed to run good. However, I'm still battling a weird vibration/buzzing coming from the main gas valve, which vibrates the whole back of my house. I just got notice from Navien that they will be replacing the entire boiler, so I'll have to start over once the new unit is installed.

    Wish me luck.

  2. #17
    In the trades Dana's Avatar
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    Good luck!

    The most likely reason it doesn't keep up during warmer weather is that fin-tube baseboard output is extremely non-linear with water temp at temps below 110F. This is because the convector height is only ~6" with baseboard, which isn't enough stack-effect to move much air volume when the fin-tube is at bath-water temps.

    Setting it up the curve to work off the return water temp is probably going to fix that issue. But rather than 120F, set it at 100-105F. That is plenty high enough so that the aquastat on the Myson will trip, and under sustained calls for heat you'll still be putting out sufficiently high temps to cover the load while still condensing, assuming it still allows the +15F overshoot while cycling the way it did on your prior curve tuning experiments. If you set it up at 120F it'll only be condensing half the time as it cycles, since anything more than a 5F overshoot would take it over the condensing zone.

    Do you have any beta on why the unit is being recalled/replaced?

  3. #18
    DIY Junior Member jac04's Avatar
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    No, no explanation of why they are replacing the entire unit. In general, Navien has been fairly responsive to my questions, but they don't explain much when it comes to troubleshooting my boiler. I'm an engineer, so I understand how the unit works and I want to know what's going on. I believe that the gas valve buzzing is due solely to gas supply (since we are on the 2nd gas valve and they both make the same noise), so I have no idea why an entire new boiler will solve my issue.

    I tried setting the unit to 120F return temperature with high hopes. The problem is that on a call for heat, the burner fired as hard as it could to try to get 120F return as quickly as possible. I have the max heat input limit set to 50% of maximum, but the supply temp still shot up to about 175F fairly quick, then the return temp would overshoot and the burner would shut down. I might try setting it at 100F and see what happens.

    EDIT: Just tried setting to return temp of 105F:
    Burner fired for less than 2 minutes and shut off due to overshooting the return temperature at 120F.
    Burner off for 4 minutes
    Burner fired for less than 1 minute when it shut off at 120F return
    repeated 4 min off, 1 min on cycle


    Now, Navien just had my contractor replace the PCB board, main power transformer & wiring harness - all in an attempt to solve the gas valve buzzing problem. They never checked & re-set the gas/air pressure settings per the manual - apparently the Navien Tech Line (telephone) told them it wasn't required. Now I'm having intermittent ignition issues and the burner seems to 'hunt' after it modulates down to maintain supply temp. I'm hoping they schedule the boiler replacement soon, because I'm not too confident in the way it's operating now.

    I'm very frustrated at this point. I'm at the point where I'm ready to tell them to take the Navien out and replace it with something else. Only problem is that the Navien CH180ASME seems to be well-suited to my heating requirements, since it modulates down to about 16,000 BTU/hr. It looks like other modulating/condensing/combination boilers don't have turndowns that low. If I were to replace this with something else, do you have any suggestions?
    Last edited by jac04; 12-28-2013 at 09:59 AM.

  4. #19
    In the trades Dana's Avatar
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    It's OK to let it go to high-fire for bringing the return water temp up quickly as long as the burn times are reasonably long. Return water temp is the primary determinant of combustion efficiency. The firing rate is secondary, but still matters, but when the return water temps are well below the setpoint, the cooler return water temp more than offsets the efficiency hit from the higher firing rate.

    It's not OK to have running a dozen 1-minute burns per hour. Sounds like they don't have the same hysteresis designed in when controlling it by return water temp. If you can find a return water temp where it gives you 3+ minute burns and doesn't sound like fighter aircraft on a takeoff roll when cold starting a zone, that would probably be a reasonable operating point.

    If you can set it up with a fixed temp 125-130F output and ignore the outdoor reset it will still deliver condensing efficiency and probably won't short-cycle itself to death, depending on how much built in temperature hysteresis it allows.

    As for alternatives, since it seems you can heat the place even with 130F water, if you have the space for it, a 48 gallon AO Smith Vertex (or a 34 gallon Polaris) HW heater and an isolating plate-type heat exchanger would likely be able to handle your loads and still run in the low-mid 90s for efficiency. This takes a bit of design work to get right, but it's not a rocket science type hydronic design problem. The Polaris controls have a very tight hysteresis, but that can be modified with a fairly easy resistor hack if you are comfortable with a soldering iron and tinkering with electronics. I'm not sure if that's the case with the Vertex. Both water heaters have side ports on the tank specifically designed for combi heating systems. The thermal mass of the water in the tank is an order of magnitude higher than in your fin-tube baseboard system, and even without a 20F+ hysteresis it can get there without short-cycling.

  5. #20
    DIY Junior Member jac04's Avatar
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    Quote Originally Posted by Dana View Post
    If you can set it up with a fixed temp 125-130F output and ignore the outdoor reset it will still deliver condensing efficiency and probably won't short-cycle itself to death, depending on how much built in temperature hysteresis it allows.
    Interesting that you mention this. I have actually had it set up at 130F fixed supply temp for a little while (since 12/23). It seems to work well. Even with the weather being fairly warm, the burn times are still good and the burner isn't shutting down on high limit during calls for heat. It comes up to 130F fairly quick, then modulates to minimum fire, and the temp slowly creeps up to about 135-138F during a 10-12 minute burn (that's with outside temps in the upper 30s / low 40s). I'll get to see how it does in the cold - it's supposed to be in the teens later this week with one night about -5F.

    Now, I just got my brand new replacement boiler installed today, and I'm more frustrated than ever. The new boiler acts exactly like the old boiler - meaning I get a weird buzzing/vibration from the main gas valve at low fire (which is quite a bit of the time), and on a call for heat the burner will sometimes give 5-8 short 1-second 'bursts' of flame at start-up before it finally ignites and stays burning. Both Navien and the installer are stumped. They say they haven't seen this before.

    However, my propane supplier took one look at the piping and said the gas supply piping is undersized. This is based on the max input of 150,000 BTU/hr. What's weird is that the unit seems to have no problem firing at max rate to make DHW. And my gas valve buzzing issue only happens when the burner modulates way down when heating. I would think that if I had a gas supply issue I'd see problems at max fire & ignition when making DHW.

    Anyhow, my money is on how the gas is entering the main gas valve - seems like there is something unusual about the flow pattern entering the main gas valve.

  6. #21
    In the trades Dana's Avatar
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    This is the first mention of propane as the fuel- I had assumed you were on the gas grid. I don't have much to offer on your valve/pressure flucutation enigma, but I DO have a suggestion on a cheaper/better way to heat at least part of the house.

    At CT-style propane & electricity prices heating with a better-class ductless air source heat pump would have about 1/3 the cost of heating with condensing propane. With any reasonably open floor plan you should be able to heat/cool at least one of your zones with a single ductless head (a 1-head mini-split), and you maybe able to do both reasonably with a one-head per floor approach. Though that works better for high-R houses, even if it's only covering your average winter load you'd be able to cut your propane use by more than 3/4, making it self-funding in short years on propane savings. A pretty-good high efficiency 1-ton like the Fujistu AOU 12RLS2 (or 12RLS2-H) would run about $3.5-4K (installed) and can deliver about 17,000BTU/hr @ +5F, more at higher temps. A 1.5 ton Mitsubishi MSZ-FE18NA puts out over 22,000 BTU/hr @ +5F and runs $4-4.5K installed.

    Getting a handle on your room by room and whole house heat load would be necessary to size it correctly and pick a model that works, but it's almost a no-brainer type investment if you intend to stay there more than 3 years. For the broader picture, download and ponderthis short policy piece from last March. If you need or want climate specific performance information I can point you can get pretty deep into the weeds on it on the NEEA field metered test data. In a coastal CT climate they are nearly as efficient as ground source heat pumps if you get a decent one.

  7. #22
    DIY Junior Member jac04's Avatar
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    Thanks Dana. Looks like I'll probably stick with the Navien unit, assuming the buzzing gas valve can be sorted out. I'm 99% sure it is gas supply related. If I have any other propane appliance running (clothes dryer or garage furnace), the boiler has a lot of trouble firing. The plumbers put the boiler last in line on the gas piping - not good. I ran the numbers on the piping, and it is indeed undersized. I talked to the propane company, and they confirmed this as well. What's frustrating is that a while back the propane company actually called the plumbers to tell them that their piping was undersized and would cause trouble. The plumbers simply ignored them. So, think once I get the gas supply taken care of, I'll be in good shape.

    With the supply temperature set at 130F and an overnight low of -5F I have found my limit. I left the t-stat set at a constant 69F all day yesterday & last night. When I woke up, the house was 67F with the heat on non-stop. Once the sun came up, the house warmed right up to 69F.

  8. #23
    In the trades Dana's Avatar
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    It sounds like 130F covers you pretty well for all conditions then, given that the -5F anomaly is well below your 99% design temp. And yes, modulating burners are more sensitive to undersized propane plumbing, so fixing that will probably fix the mis-behavior.

    I wasn't suggesting that you scrap the Navien for mini-splits. I'm suggesting that putting at least one decent sized mini-split would be cost effective in short years heating one large zone, and would provide high efficiency air conditioning too. A mini-split doesn't provide hot water, and is rarely a whole solution for heating in standard construction housing. A good one would use less than half the power of window air conditioners and are much much quieter. If sized for the cooling load it'll likely be big enough to handle your average heating load. While your cooling loads are too low to rationalize the upfront cost of a mini-split just for cooling, your heating fuel costs are very high and volatile, and the payback is short enough that even if you're planning to sell in 3-5 years you will have paid it off in propane savings, and have another selling point for the house. (If my 401K had that kind of internal rate of return I could have retired decades ago! :-) )

  9. #24
    DIY Junior Member MikeG88's Avatar
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    Quote Originally Posted by jac04 View Post
    on a call for heat the burner will sometimes give 5-8 short 1-second 'bursts' of flame at start-up before it finally ignites and stays burning. Both Navien and the installer are stumped. They say they haven't seen this before.
    I have the same problem with a Navien CH-210 (about 1 1/2 years old) Navien has had a local plumber put in a new Air pressure sensor, Install the newer hi temp computer board, and neither or those solved it so now I am waiting for plumber to install new burner rods to see if that fixes it. It is the only appliance on the propane line and pressure has been verified several times. And funny- it doesn't seem to happen when using DHW. I have also been instructed by the dealer the original plumber got it from to turn down the capacity, I currently have it down to 60% (at 100% and 80% it would go through the burner cycle the full 20 times causing an error 12 and shutting down). One of the tech at Navien's support line told me not to worry if it's not going the full 20 and erroring out, they don't seem to know what to do, just keep changing parts till it gets fixed or the homeowner gives up. Any luck in solving it at your end?

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