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Thermowell on low loss header vs. probe on supply

I'm putting together my radiant system, and curious if there is any advantage to using the (upper) thermowell on the LLH (I have a 1-1/4" Caleffi SEP4 3-in-1 LLH), versus just strapping the mod. con. boiler's supply temp. sensor to the hot pipe leaving the LLH? Certainly it looks slick if it goes to the thermowell but is that really measuring the supply temperature as opposed to a mix of the temperatures in and out of the LLH?



  • hot_rod
    hot_rod Member Posts: 20,826
    If you want an accurate reading of the actual supply to the system, I would install a system supply sensor 12" downstream on the supply to system piping.

    If the LLH is being used as a buffer, a very small one :), the sensor in the well.

    I think that well is useful for a pressure gauge. The temperature at that point will be always changing as flow on either side changes. Zones open and close, maybe the boiler has a V/S circ. So I'm not sure the temperature measurement or sensing there is helpful? It just indicates the mixing inside the chamber at different flows.

    If the primary and secondary flows are always the same, maybe a single zone system, the sensor well may be accurate.

    But with either of the mixed conditions which are most likely on zoned systems, strap the system supply sensor downstream
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream