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Best Location For Temp Sensor On Boiler?

I bought a controller based off the advice I received on this forum so I can control when the pumps turn on that bring water from my wood boiler to my oil boiler. 1 pump brings the water from the wood boiler, and the other is the wrap-around that circulates the water through the heat exchanger and the oil boiler. Both pumps are controlled by a Johnson A419.

I have the controller set to circulate the hot water until it reaches a set temp, then shut off and stay off until it reaches the differential temp. My question is, where is the best location to put the sensor probe that the A419 uses? I currently have it mounted on the return line pipe at the heat exchanger so that it senses when the return temp reaches my high setpoint.

I don't think this is the best spot, and perhaps I need to mount it on the oil boiler itself? If so, where on the oil boiler should I mount it? Thanks!

Comments

  • warno
    warno Member Posts: 229
    edited September 2016
    I would think that yours should on the "oil boiler to HX line" if your oil boiler is up to temp it would be running your "high setpoint" water down that line and kick off the pump.
  • LegendsCreek
    LegendsCreek Member Posts: 65
    So you think it should go on the return line that sends the "warm" water back from the oil boiler, to the HX?
  • LegendsCreek
    LegendsCreek Member Posts: 65
    Wait, I think you meant it should be on the hot supply line right? So when it reaches its high setpoint on that supply it will kick off.
  • warno
    warno Member Posts: 229
    If I remember correctly, you are heating the oil from the other boiler through a plate heat exchanger?

    If that's correct, my thinking is, put your sensor on the line FROM the oil boiler TO the heat exchanger. Basically if the oil boiler is up to temp it would be sending "high setpoint" water down that line so your pump will shut off because the oil boiler is satisfied.
  • LegendsCreek
    LegendsCreek Member Posts: 65
    We are hoping to use no oil at all. (we have the oil boiler aquastat set to emergency temps only so it will only fire if something goes wrong with the wood boiler).

    The wood boiler will be doing all the heating and the A419 controller is to only turn on pumps when the oil boiler water temp reaches the high setting, then shuts off until differential is met.
  • warno
    warno Member Posts: 229
    I guess I'm confused or missing something. What are you doing with the pumps again? Are you heating the oil boiler water through the plate HX to maintain its temp?
  • LegendsCreek
    LegendsCreek Member Posts: 65
    Yes, I'm using the wood boiler to keep the oil boiler temp where I want it, via the plate HX. Sorry I should have described it better.
  • warno
    warno Member Posts: 229
    edited September 2016
    Then I would put the sensor on the line running TO the hx. So if the oil boiler has reached high setpoint it will shut down your pump because it senses the hot water from that boiler flowing to the HX. I may be wrong but that's what I'm thinking would work.
  • LegendsCreek
    LegendsCreek Member Posts: 65
    Thanks a lot for the advice. I am going to try it out. I need to find the sweet spot on the oil boiler to put the sensor so that it reads as close as possible to the actual internal temp of the oil boiler. This way I can avoid firing the boiler with oil at all unless the wood boiler gets too low.
  • warno
    warno Member Posts: 229
    Did you check out that attachment in my last post? That's how I would do it.
  • hot_rod
    hot_rod Member Posts: 22,110
    If the oil boiler is back up to the wood, why not put a control on the wood boiler that senses low temperature and fires the oil boiler and pump?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • AK907_2
    AK907_2 Member Posts: 9
    Mind if I jump into your conversation with my own, similar, dilemma? Can you recommend what aquastat I need to start my oil boiler on a decreasing temp using the boiler's 24v wiring? The boiler has its own circulator, shut-offs, etc. I'd like the wood gasifier to be my primary heat source with the oil boiler kicking in when needed (or if I'm not adding wood). The aqustat I installed is close/open switch on increasing temp. I didn't notice until after install. See attached drawing. You'll probably had more Q's before answering...
  • hot_rod
    hot_rod Member Posts: 22,110
    Most all of those electronic temperature controls have the option for NO or NC, normally open or normally closed.

    NO means the contacts close (make) on temperature rise
    NC means they open on temperature rise

    So it can be used either way. Some have adjustable differential, and other features.

    I think you want the sensor on the wood boiler, just have the sensor touching the metal on top of the boiler with insulation over it. Many wood boilers have strap on sensors for their control, touching the metal with some fiberglass insulation will work fine. They weld a metal tab on the top of the boiler jacket and slide the sensors under that.

    Leave the sensor lead long so you can experiment with different locations until you get the results you want,

    A well is a nice way to install a sensor, but not always doable. Same thing in a well the water heats the brass well and transfers to the sensor inside. The sensor doesn't know or care if it is in a well or surface mounted to the boiler steel, really.

    You have some other logic to decide. Does the primary loop pump run constantly?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • LegendsCreek
    LegendsCreek Member Posts: 65
    Warno, I put the sensor where you told me to based on the diagram you attached, and it is working great but I have a question.

    Now that everything is where it needs to be, the pumps are turning on when it reached my low setting. Right now I have the low set to 150, with a 20 degree differential so it turns the pumps off at 170 and they won't turn back on until the probe reads 150 again.

    I noticed this morning when the probe reads 150, and turns on the pumps, it starts pulling warm water from the oil boiler and the temp quickly dropped from 150 to 130. So basically the water in the boiler was only 130. My concern is that the oil boiler aquastat won't let me go lower than I think 130 as the hi. So I am worried that it will fire oil because its not reading the same temp as the probe.

  • warno
    warno Member Posts: 229
    You will have to put the sensor on the oil boiler then that's the only way you will get a more accurate reading.
    LegendsCreek
  • SWEI
    SWEI Member Posts: 7,356
    Two temp sensors with a ΔT controller would be ideal.
  • hot_rod
    hot_rod Member Posts: 22,110
    SWEI said:

    Two temp sensors with a ΔT controller would be ideal.


    Yep a solar differential control, not much more $$ than that limited stepping control. A model with two outputs would let you control the whole enchilada.



    That may have been suggested in another thread :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream