Navien NFC "Supply/Return Temperature" vs. "SYSTEM Supply/Return Temperature"

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thornx

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Hello,

Under 'Status Information' my NFC-175 displays a "5.Supply Temp", and a "6.Return Temp". Then further down it shows "7. Sys Supply Temp" and "8.Sys Return Temp" I understand the difference between Supply and Return Temperatures, but was curious about "System" temps....

I spoke with Navien support, and they said that 5 and 6 are from thermistors inside the machine. The 'system' temps (7 & 8) are the external sensors attached to the supply and return pipes respectively.

However, my Supply Temp (5) shows 134F, while the Sys Supply Temp (7) shows 123F. The return side is similar with Return Temp (6) showing 131F, and Sys Return Temp (8) showing 124F.

These numbers seem very odd to me. First, I would assume that both Supply temps and should be very similar if not exactly the same, as the sensors are not far from one another. Same with the Return side. Second, my Delta T is only a few degrees (which is another issue) but the thermistors show a DT of 3 degrees, while the return shows a Delta T of +1 degree - which is also odd.

Of course Navien Tech just said call a plumber.....

Any insights from you guys?
 

Fitter30

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Not knowing how your system is piped if its primary/ secondary loops with back to back tees feeding the secondary. For example primary loop boiler pump flowing 5 gpm and the secondary system pump flowing 15 gpm. At the first tee 5 gpm flowing into boiler 10 gpm flowing to the second tee 5 gpm coming from the boiler being added to the 10 gpm making the 15 gpm loop. If system sensors are close to the tees temps could be close to the boiler sensors. If your boiler has this fitting half way down the page its piped primary secondary.
 
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thornx

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Yes, the navien manifold is installed like below:
1646431536843.png
 
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