Navien CH-180 victory

[Bathsheba]

So this is the only place on the internet that might appreciate this story, and I wanna tell someone. My CH-180 was installed 7 years ago and it's been nothing but trouble. Lots of ignition problems, never been able to heat the apartment properly when it's below freezing out, and now the repairman says it's just undersized. I don't agree, and I finally decided to read the manual myself.
The supply temp won't go over 160 no matter how I set the controller. Also there's no outdoor sensor installed, so OK, I bought one on Ebay and installed it. This doesn't seem to help: it's 20 degrees out and no matter where I set the K-curve the house is not warm.
I looked over the dip switches 50 times. Then I looked one more time at the set of 6 dips that are supposed to be set at the factory and never changed, and I saw it: the model number switches were set to CH-240! I flipped one to make it CH-180. Now the supply temp is still 156, but it's 19 degrees out and 70 in here.
I don't know exactly what happened here, but I think I fixed it.

 

[NY_Rob]

It's amazing how many bad mod-con installs there are in the field.

Between bad installers and homeowners who don't follow maintenance guidelines it's no wonder mod-cons get a bad rep.

 

[Sponsor]

 

[Samat]

The board that comes with the unit is a 160 degree max. You need to get a 180 degree board from Navien if you want higher temp. But then you need to change to CPVC exhaust plumbing. Very expensive . I switched to the 180 board trying to run it like a regular boiler. No need to do that. Put in a outside sensor and played with the Kfactors and now unit is running great and never goes over 160.

 

[Dana]

Unless yours is the largest and leakiest apartment in Boston there's no way this thing is undersized! At high fire it could heat my antique ~2400' house down to outside temps of about -250F or so! According to The Manual, the minimum input to the CH-180 is 17,000 BTU/hr in, which at 95% efficiency would be 16,000 BTU/hr of output.

And 16,000 BTU/hr is also enough heat to cover the heat load of a typical 800' top floor triple decker at +10F outside, or 1200-1500' of middle-floor triple decker apartment. (And that's at it's MINIMUM firing rate!, not it's maxim. Undersized? Don't think so!)

The radiation might be undersized though, which is a separate set of issues:

* If the radiation can only deliver the heat at the coldest outdoor temps at water temperatures higher than the boiler can put out, it's a problem!

* If the boiler is too big to operate at condensing temperatures without short cycling or to modulate with it sort of misses the point of a modulating condensing boiler.

But with a bit of napkin math we can figure this stuff out. As a start, run a fuel use heat load calculation based on say, 87% efficiency (it won't be doing much better than that if the output was set to over 150F) using exact meter reading dates and usage from last winter's gas bills. (Winter-only billing will give the most accurate number.) Use +12F (Boston's 99% outside design temperature), or +5F (the 99.6th percentile temperature bin)- it'll be close enough.

The napkin-math on optimizing the size of this type of boiler and the issues that come up when there isn't sufficient radiation can be found here.

Cutting to the chase, it takes about 75-80 feet of fin-tube baseboard to balance with 16,000 BTU/hr of output at an average water temp of 120F, where it would be getting 95% efficiency without cycling on/off during a continuous call for heat. How much radiation (of what type) do you have?

With the heat load & radiation information it's possible to estimate the maximum water temperature needed, as well as the water temperature at which there is a danger of efficiency-robbing short-cycling. With those two water temp numbers we should be able to zoom in on optimizing the K-curve.