Navien Combi Unit with Nest E Thermostat Troubleshooting Advice

MikeC_3

This is a more a "why is this post" than troubleshooting an issue because I seem to have figured it out.

I have a 4 zone hot water heat system in my four story 3-unit multi-family building.

I replaced my traditional gas boiler with a Navien Combi tankless unit this winter.

The 3rd and 4th floor rental units have Honeywell T6 programmable thermostats and they seem to be working fine.

My duplex has Nest E learning thermostats (2 wire). They worked great with the old boiler. I programmed each with the typical setbacks for the zone that I wouldn't be in at certain times. I turned off all the smart/learning features because my schedule was so erratic that it became an issue. I basically used them for the remote feature for times when I wasn't going to be in the building and wanted to put them on hold, or to heat the apartment prior to returning. For each zone, following the setback, the old boiler would fire until it reached the programmed temperature (maybe even overshot it a degree or two) and then maintained that temperature.

They seemed to work fine earlier in the winter with the newly installed unit until the intense cold set in this week. Both zones struggled to reach the programmed higher temperature following the setback which was only about 4 degrees different at most. The apartment was noticeably colder than the programmed temperature.

I replaced the Nest on the upper bedroom floor with a Honeywell analog thermostat that I had. I still woke up to a temperature around 66, when it was set to about 70 degrees. Increasing the temperature did nothing to alleviate this.

I then reinstalled the Nest thermostat, erased all the programming and set both of them to a constant 70 degrees. The apartment (both zones) heated to the called for temperature and has remained there consistently for over 2 days. It doesn't even fluctuate a degree like it did previously with my old traditional boiler. I believe that this is why people make the "upgrade."

My next step was to install T6's in my unit but now that doesn't seem necessary.

Any idea why this current configuration works?

Thanks!

M

hot_rod

is the Navian running on an outdoor reset curve?

MikeC_3

I don't believe so but I am not sure. I need to read up on how to check that.

The temperature is set to 180 degrees though and it is hitting that mark.

EdTheHeaterMan

I believe that the Navian boiler's unique modulating feature, that your previous boilers did not have, makes it more likely to have a much closer temperature swing. The Outdoor reset feature will make that temperature swing even smaller. That is what you are experiencing with the new boiler. You said it yourself, that there can be overshoot in some cases with the old boiler. That is due to the thermostat differential of as much as 2° between a call for heat and the call for heat being satisfied. Some analogue thermostats can have that much difference in the switching action.

Add to that the fact that the older boiler has more water content. When the burner stops adding heat to the boiler water, that boiler water does not get cold instantly. There may be “gravity flow” of residual boiler heat into the radiators that will continue to increase the room temperature for a few minutes after the burners stop.  That will add to the temperature swing to the point that you can actually feel the change.  

With the modulating gas input combined with outdoor reset, you may not get the same amount of temperature swing that you are used to.  Sometimes the ODR setting may even end up falling 0.5° short of the thermostat setting.  You think it is the thermostat keeping the temperature at 68° but the ODR is actually not allowing enough heat to actually get to that temperature.  This can be verified with the Nest App by looking at the amount of time the thermostat is  actually calling for heat.  

If the room temperature is enough to satisfy the thermostat, then your boiler temperature and small water content will not allow the 2° or 3° overshoot since the amount of extra energy stored in that water is reduced by both the volume and temperature.

You mentioned that the boiler reached the 180° high limit temperature.  That is possible on ODR when the outdoor temperature is very low.  If you are on ODR then that high limit will not be reached when the outdoor temperature is milder.  

ODR and set back are not compatible technologies.  You need higher water temperature to recover from a temperature setback.  ODR prevents higher water temperatures under most operating circumstances.  So pick one or the other, don’t use both at the same time.  

GGross

If a system can maintain temperature, but struggles to recover from a lower temp to that same room temperature at the same outdoor temp that would imply that not enough BTU/hr are being delivered to the space to recover from the setback. whether that is due to ODR (sounds like you are just set to 180) or because of flowrates, emitters, or appliance heating capacity. Or a combination of multiple things.

hot_rod

Many of the mod con controls have a boost function. Basically if the boiler is not keeping up, based on the ODR setting, then to boiler will start increasing the SWT. You need to get into your control and see what settings are have been enabled.

The manual should indicate what the factory default settings are. Play around with the control to customize it to your exact needs.

Generally they have a one push function that takes the control back to factory defaults if you get balled up in the customizing of the parameters.

MikeC_3

Thank you all for the input.

As I mentioned, the house is now at a perfect consistent 70 degrees throughout. With the old boiler, there were periods where I would be chilly and waiting for the next call for heat.

I wasn't intending on teaching myself about another type of system but I downloaded the manual and will give it a thorough read through !

I believe @EdTheHeaterMan hit the nail on the head, explaining what the issues are that I tried my best to describe in layman's terms.

Every issue with this system, my previous one and my mother's steam one seems to be solved by simplifying the controls.