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Supply and Return Water Temperature off DHW

1sttimeposter1sttimeposter Member Posts: 39
Dual temperature guage (generic image attached below) placed on boiler's supply/return pipes shows, during DHW cycle (always on priority), the temperature difference between supply and return water does not exist at all lately. Zone heating, however, seems fine and shows usual temperature differential in supply/return water.

We have a zone valve system for the zones and DHW with one system pump on the boiler

Operationally, while everything seems ok, this's not our usual reading that we can recall. Typically, the difference is around 15F to 20F specially during the 1st minute or two after DHW first calls for heat and gradually narrowing towards end of cycle.

Right now, differential temperature is fairly equal or close within 5F for the most part as if there is no heat transfer/pickup by the HW tank. But yet we get hot water supply as normal with no delay or anything. Are we worried for nothing or something has gone wrong?

Any tip or advice we need to look would be greatly appreciated. Just wanted to educate myself before calling in a pro.

Thank you very much in advance.

Comments

  • hot_rodhot_rod Member Posts: 13,850
    Is the boiler running longer than usual to recover the DHW load? Could be the coils getting scaled inside the tank. What type of tank? How old? Water hardness? Tank temperature? All of those lead to coil scaling. Finned type could sometimes scale within a few years.

    If you drain the tank all the way down to your incoming water temperature, high load, should show the greatest delta, it closes as the tank get to setpoint.

    Any changes in the pump or speed?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • STEVEusaPASTEVEusaPA Member Posts: 4,420
    Are you only relying on the gauge? Have you confirmed temperatures with something else (K-type thermocouple), thermal imaging, or just (carefully and quickly) grabbing each pipe?
    steve
  • hot_rodhot_rod Member Posts: 13,850

    Are you only relying on the gauge? Have you confirmed temperatures with something else (K-type thermocouple), thermal imaging, or just (carefully and quickly) grabbing each pipe?

    Looks like he has an Azel differential thermometer, in the attachment. Those are usually quite accurate.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • 1sttimeposter1sttimeposter Member Posts: 39
    Good eye Bob! it's indeed an Azel dual thermometer unit.

    This is an identical replacement to the old we bought recently. The old isn't displaying no more.

    Like the old, we attach the two probes on the same immediate supply and return sections of the pipe off the boiler and extend the line all the way up to the family room with the actual temperature unit screen fixed on the same wall location.

    We notice the unusual undifferential supply/return water temperature reading on the Azel (only on DHW, floor zones seems fine(at least shows rather big difference as usual)).

    Like I said, the old unit used to read 15 to 20F difference when DHW first call for heat for the 1st minute or two and the gap gradually narrowed as the cycle progressed.

    DHW cycle isn't noticeably taking any longer than before(usually needs 2 cycles to satisfy a call). It takes about the same time to finish a call.

    Boiler has a Grundfos UPS15-58FC 3 speed circulator pump that we never touch. It sets at the highest speed.

    The 12-year old leak-free indirect hot water tank itself (pdf attached) has a single coil heat exchanger with 1” O.D. x 30 ft long high grade AISI 316L stainless steel with its aquastat set at 120F. Not sure if it's finned or not though.

    According to the city's website, our city water hardness is at 17.1 - 18.8mg/L.

    Not sure if the Azel is the culprit which I am not suspecting as Bob points out. But Steve's point is valid and since I have one of this (image attached below) laying around (used before and replaced by this Azel gauge), I am going to test it out and report back to you all who have been kind and helpful to me and my family at desperate time when it comes to trouble-shooting and education on our heating needs.

    Greatly appreciated!


  • hot_rodhot_rod Member Posts: 13,850
    A tank that age, with very hard water, which you have, my guess is the coil is getting limed. Some of the nicer tanks have hand holes to clean and inspect. Without looking into the tank, by removing a connection, hard to know. Low delta like that could be an indicator of constipated best transfer.

    IF you drained the tank you could use one of those tankless water heater cleaner kits to de-lime the outside of the coil.

    I use these kits to delime 3 way thermostatic mix valves.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • NY_RobNY_Rob Member Posts: 1,370
    What are the actual temps you're seeing on the supply and return for the DHW tank?

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