How could return water temp be higher than supply

Discussion in 'Boiler Forum' started by mev, Mar 13, 2019.

  1. mev

    mev New Member

    Joined:
    Mar 10, 2019
    Location:
    Billerica, MA
    Just converted to Navien NHB-150 boiler from a 30 year old oil boiler. Still just learning and installer is on vacation.

    I press the display mode button 3 times, until it says info. Press + for the supply temp, 148. Press + again for return and its 149? I know I don't have enough radiation for this zone for high efficiency ( about 25' baseboard and 2 8K btu kickspace heaters (one who's aquastat won't close at this water temperature), but shouldn't I see some type of temperature drop?

    Its a warm day today, in the 40s, house has been at 68 for hours, turned this thermostat to 71 degrees to see what the supply and return temperatures would do. Sometimes the return temp is a few degrees colder than supply, sometimes the same, and some times a degree or 2 above.

    Thanks for any suggetions.
     
  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Were both the pumps & burner actually running when those temperature readings were taken? (I doubt they were.)

    A few minutes after the pumps and burner stop the temperature at the output & input are pretty much the same, due to the high thermal conductivity of the heat exchanger and plumbing.

    If the radiation pumps are running but the burner isn't, due to overshooting it's setpoint, the primary loop temperature difference at the boiler drops to near zero too.

    Since there are separate sensors at the input and output, a difference of 1F in the "wrong" direction is easily attributable to sensor calibration. These aren't precision sensors tracking to 0.01 degrees- even if the actual temperature is identical one sensor could be reading 149F, the other 148F.

    Turning up the thermostat doesn't automatically turn on the burner. If the boiler temperature is already over the boiler's setpoint (either fixed-temp or varying under outdoor reset) it can take awhile for enough heat to be emitted from the system as a whole to get the burner to fire again, even though the pumps are still running. Variations in temperature on different sections on the system plumbing may cause the sensed temperature to rise and fall a bit as the water circulates.

    A picture of the near-boiler plumbing might inspire alternative explanations.

    Is it set up to deliver a fixed output temperature? (If yes, what is that temperature?)

    Is it set up to run under outdoor reset, using the outdoor temperature sensor to set the output temp of the boiler? (If yes, the full programmed setup needs to be figured out.)

    The NHB-150 has a big turn-down ratio and a fairly high high-fire rate (crazy-oversized for the loads of most houses in MA- was there any reason for going that big?). But with a minimum firing rate of 10,000 BTU/hr-in it puts out~8700 BTU/hr at min-fire when the entering water temp is running 150F.

    Most fin tube baseboard would be putting out ~350 BTU/hr per running foot at 150F AWT which means the 25' of baseboard should emit the full 8700 BTU/hr even without the kickspace heaters running, and it shouldn't overshoot, if the boiler's setpoint temp is 150F.

    A kickspace heater rated at 8K at an entering water temp (EWT) of 180F should deliver about 5K @ 150F EWT. Dial that down to 125F for condensing efficiency the baseboard would delivers about 5K, and the kickspace heaters ~3K, so even at condensing temps that zone shouldn't short-cycle the boiler if you can update the controls to get the kickspace heaters run at temps that low. Even with just the 25' of baseboard running you can probably suppress short cycling on that zone by opening up the temperature swing with parameters L & M. (See the parameter settings in the manual.)
     
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  4. mev

    mev New Member

    Joined:
    Mar 10, 2019
    Location:
    Billerica, MA
    First, I appreciate the response and suggestions.

    We used to have a saying for customers: RTFM I glanced thru the manual once, but I really need to read it slowly so I understand the parameters. I would prefer to get things set up correctly for running in condensing mode.

    I will do the test again tomorrow and watch for when the burner kicks in. I can't positively say what it was. I do know that since I turned the thermostat from 68 to 71, it only raised the first floor to 69 after 5 hours. Outside temp is still around 40.

    The outdoor temp sensor had been disconnected because it could not keep the first floor at 68 degrees when the outside temp was 10 degrees at night. I know now that this was not a good test because the heat had been off that day and by the time the boiler was turned on, the house was 55 degrees.

    I wrapped the aquastat with fiberglass and got it to function. Its too old for replacement parts.
     
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    The controller usually has some curves or setpoints to adjust. Without doing that, it may not be adjusting the boiler outlet temp properly to get the temp up in a reasonable amount of time. If it can't raise the home's temperature, it may need a higher output temperature. Depending on the type of radiation you have, their output may not be linear, so raising the supply temp a little may get you a lot more output.
     
  6. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    You REALLY need to RTFM!

    Simply disconnecting the outdoor sensor without changing the settings doesn't give the boiler a defined output temperature! It has to be programmed for a fixed temperature first by disabling the outdoor reset with parameter A, and you then have to pick that temperature. You also need to check the DIP switch settings to see if it's set to control the modulation around the output temperature vs the entering water temperature.

    With parameter A set for outdoor reset, and parameter B just set to 1 it's not surprising that some zone or other might not cut stay up to temperature under cold weather. But setting parameter B to 7 (custom curve) and defining the curve allows you to dial it in for maximum comfort and efficiency.

    With this modulating condensing boiler it's better to dial in the curve, then "set and forget" the thermostats, otherwise the recovery times from setbacks will be long, and requires higher water temperatures (= lower efficiency).

    Most systems can hit better efficiency running it under return water set point control. This is programmed by the settings of positions 1 & 2 of DIP SW2 on the control board, not from the front panel.
     
  7. mev

    mev New Member

    Joined:
    Mar 10, 2019
    Location:
    Billerica, MA
    I did not unplug the outdoor sensor. The installer went thru all the menu button pushes to undo it. I still don't trust myself yet to try things.

    He is coming back next week to set it back up for the outdoor sensor and I'll talk to him more about it and what you and others have said on this site.

    Thanks.
     
  8. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    If it's going to cost you a service call every time you tweak the reset curve it'll be an expensive proposition. Read the manual, have the tech show you how to set up a custom curve, then tweak away, but keep track of the boiler's behavior during milder weather in case you need to adjust it to suppress shorty cycling.
     
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