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Dialing In Navien Boiler Settings

Chevra Member Posts: 11
Recently installed a Navien boiler in my shop for the in floor heat. Now I am messing with all the settings trying to get it as efficient as possible. The shop is a late 2000s build and already had the in floor heat with a very inefficient water heater setup. I know sealing the building up better will help me but lets focus on the heat system for now. Maybe there are some in floor heat wizards in this forum. Here is the setup first:
- Navien NHB-55 Condensing Boiler with Outdoor reset control. (Propane)
- Two Grundfos 3 speed pumps. One for boiler and the other for the system
- Manifold has 6 loops total acting as a single zone
- Basic 70 series Emerson thermostat
- Shop is 30x50

Now my first question. I understand that condensing boilers are most efficient when the return temperature is lower, but what is more important, the return temperature itself, or the temperature differential? I would think if my return temperature is to low I run the risk of cold spots in the concrete?

First thing I messed around with was the pump speeds. I tested at all 3 speeds. I get the highest temp differential on slow speed (15-16 degree difference). With the pumps on High or Med the differential was more in the 13-14 degree range. I am getting this differential directly from the boiler, not the manifolds. So technically the return is a mixed temp. After switching to low speed the boiler firing rate went down and it isn't working as hard anymore. From all the research I have done on temp differential I have yet to get a simple answer for what is ideal for a concrete floor. Some sources say 20 degrees and others say 8-10 degrees differential. So far I don't see any reason to take the pumps off low speed.

Now I have been messing with my ODR temp range. Currently it is set for a max supply temp of 46C (115F). An outdoor temp of -15C (5F) or lower defaults to the max supply temp setting. I am consistently getting the 15-16 degree differential at these settings, producing a good amount of condensate so I must be somewhere in the condensing efficiency range. Would it be wise for me to lower this max supply temp even more, to say 42C (108F)? Boiler might run longer but at a lower output, and colder return temps. (I also live in Canada, so it isn't uncommon to see temps in the -30C (-22F) range)

I see that Navien sells temp sensors that attach to the supply and return manifolds, and these temps are turned off on the boiler currently. Would these sensors benefit me much or not really?

Any input will be helpful. Guess I am just not 100% sure what exactly I should be chasing more to increase efficiency. The boiler has many other parameters that can be changed and I have left them mostly at the default settings. I just want to save propane where I can. I will address sealing up the building better during the summer months. The shop feels nice so that isn't a complaint at all. Also my thermostat is pretty basic. I did cut a resistor so it would operate for a heated floor system better, but it is still just an on/off type of thermostat.


  • psb75
    psb75 Member Posts: 834
    Once the slab and indoor climate is at the desired temperature, a 10°F Delta T is best for temperature maintenance. More constant circulation is best--keeping the slab in a 1°F differential range. As far as condensing is concerned, keeping the return water temps under 130°F is all that is important. Should be easy to do this with your system. A thermostat with a slab sensor would be best.
  • hot_rod
    hot_rod Member Posts: 22,157
    Run the boiler at the lowest possible temperature that will get the job done. If there was a heat load and design done on the building it would show supply required. If not, trial and error.

    If there is an outdoor reset function on the control enable and set that. It will modulate the boiler based on loads. Should be able to get well below 100°F on many days
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Chevra
    Chevra Member Posts: 11
    Thanks for the replies. Unfortunately I don't have much documentation on the building other then a rough grid sketch of how the loops are routed. Think I will just keep messing around with the Outdoor Reset temp range. I took some surface temperatures of the slab and it was ranging 2 to 3 degrees higher then the air temperature reading. So no cold spots at least. Will just slowly lower my ODR max temp and see what happens.

    For a thermostat with a slab temp does it only use the slab temp for control? Or is it a mixture of air and slab temp?