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How to control DHW temps with a Munchkin T50 boiler?

Hi - I've got the manual for the Munchkin and have read it many times cover-to-cover. I'm clear on how to set the upper and lower temps that control when the boiler fires if a t-stat is calling for it. However I don't see anything that describes how the domestic hot water gets controlled. I do NOT have the Vision system if that matters.

I know the DHW circuit gets "priority" but at least in the user manual there is no mention of how that works or how to control that. Would welcome any pointers or perhaps i need to track down a service manual?


  • pedmec
    pedmec Member Posts: 907
    From my understanding the vision one is programmed into the 925 controller. the 925 is the standard controller for all lines of munchkin. if you use a 10k dhw sensor it will automatically pick up the sensor when connected and start functioning.

    if you use a aquastat its just on off. both connect to the x8-7, x4-2.

    I have a munchkin 199m. Installed in 2005. I have not used the vision one programming as put my indirect hot water tank on the heating manifold. Can't get hot water in warm weather shutdown. I hate the 925 controller as it seemed like a dinosaur even when i put the boiler in 2005. I can never remember the proper procedure to access the parameters. i really love the small footprint.

    I did the schooling for the vision 1 and 2. I have the cds and i even had the cables set up so i could access all the controls on the 925. At the time that was pretty cool. set up my laptop and away i went.

  • HomerJSmith
    HomerJSmith Member Posts: 2,386
    edited November 2022
    Vision 1 sys is an outdoor reset sensor and a indirect tank sensor. Programing the tank sensor is done with the control board by going into the 925 board in the boiler and setting the parameters.

    Since you don't have an indirect sensor, the high limit is controlled by a tank aquastat, therefore, the hi-temp of the tank is controlled by the aquastat and not by the 925 board. The aquastat controls the temp differential, hi-limit, and when to turn on the boiler indirect circuit.

    The aquastat turns the indirect pump on or opens a zone valve and turns the boiler on. The programing of the 925 board determines a lot of the parameters that affect the indirect.

    To control the temps and other settings, you need Vision 1.
  • SENorthEast1895er
    SENorthEast1895er Member Posts: 78
    @HomerJSmith @pedmec am I understanding correctly that I can use the Vision 1 without an indirect circ? So if I just hook up my 10k aquastat to the correct Vision 1 wires (blue wires), the Vision 1 will know to run the primary circ?

    I ask because I installed the vision 1 ODR and then got a really cold shower on the first warm-ish day since.

    And there’s nowhere to install the indirect sensor in my tank (TT Phase III smart); at least, as far as I can tell. I just started looking into this a few hrs ago.
  • HomerJSmith
    HomerJSmith Member Posts: 2,386
    edited December 2023
    My thinking.
    am I understanding correctly that I can use the Vision 1 without an indirect circ?
    No. You need a indirect pump. You have a boiler sys pump that does space heating. I haven't found any where in my manual where a indirect pump is not used with an indirect tank. The WWSD with ODR turns the boiler space heating sys pump off. The aquastat on the indirect tell the boiler to turn on the indirect tank pump and fires the boiler to the programed temp setting and when the aquastat temp setting on the TT is reached it turns the boiler off.
    You have two sys with an indirect tank (Space heating circuit and an DHW circuit). You are asking the space heating circuit to do the job of the DHW with a zone valve (?). Properly wired, the 925 board does all this for you. The mechanical aquastat sets the tank water temp. It's just an on and off switch and will not give you a tank temperature reading on the display board as the Vision1 tank sensor would. The mechanical aquastat works just fine, but not without an indirect circulator with a check valve.
  • pedmec
    pedmec Member Posts: 907
    The other issue with the munchkin 925 control is you have to have a separate loop for the indirect tank if you are going to use the sensor. On a demand call for hot water the indirect circulator must be able to circulate thru the boiler even in warm weather shutdown. If you put your indirect on your heating manifold you will get no hot water while you are in WWSD as the boiler circulator is no longer operable. Plus you need to make sure you are piped as a primary/secondary. page 33 in your manual
  • SENorthEast1895er
    SENorthEast1895er Member Posts: 78
    edited December 2023
    @HomerJSmith @pedmec and I’ll mention @Mad Dog_2 because I saw elsewhere that he’s a Munchin expert.

    edit: just called HTP homeowner tech support and (after being elevated to second level) was told that either placing a circ on the return side of the water heater or tricking my boiler into runnnig the 1 cir we have for both CH or HW would work.

    However, he answered in a split second; certainly not enough to compare piping diagrams. So I wouldn’t mind some more confirmation.

    Yeah, I’m piped in something similar to the “Retrofit piping” listed in my 925 manual. No primary/secondary; a single circ on supply-side for the whole system. just zone valves to control hot water vs central heat.

    What is the reasoning for needing to be piped primary secondary? Perhaps this will make all my other questions moot.

    Perhaps the easiest (not supported) way to do this is install a circulator on the return side of my HW (as I currently have a circ supply side, per retrofit install. And, as far as I can tell, placing circs with flow checks on opposite sides of supply/return fuctions to ensure that only 1 circuit gets flow at a time).

    I read somewhere on here this was to ensure adequate flow through the boiler, which is not as much of a concern for the hot water side given that there isn’t as much flow resistance/restriction (head pressure?) through the indirect tank. This isn’t a huge concern of mine; I assume the Retrofit piping shown in the 925 manual assumes you’re retrofitting to a system with oversized pipes (gravity systems; ours is a direct return), so head pressure isn’t as much of a concern, hence placement on supply side for retrofits.

    Alternatively, I could use the circulator I have as both CH and DHW circulator.

    Thinking about this more, I could also do this with the following (reminder: HTP said this would work, although they said it so fast I have my concerns).

    assume that both CH and DHW leads off of the control board are wired to the same circulator (they shouldn’t fire at the same time, so that shouldn’t be an issue, but I’d but some sort of switch in here to safeguard it).

    1) A 10k aquastat (my TT has a 12k; HTPs recommended is 10k). I don’t think I can use the provided aquastat because it doesn’t have a thermostat connected and goes directly into the board, where resistance is read (I assume, when using an external aquastat, it just looks for a closed circuit, in which case, shouldn’t my 12k work?). I need a thermostat so I can get a

    2) A switch/zone valve control/relay (not sure what the part would be called) that will close my CH valve when my DHW is called.

    possible issues? frequency start/ stop of circulator. 925 control boards default switching time between zones is 30 mins, so it doesn’t seem terrible.

    WWSD wouldn't be an issue, because the Vision 1 would be using 1 circulator as both (it would call the 1 circulator I have to circulate for DHW.

    Zone valve switch prevents overheating of the house and heating of the house in the summer.

    There is no pg. 33 in my vision 1 manual (although, of course, all piping is p/s), and pg. 33 of my 925 manual shows a dual-boiler system.
  • pedmec
    pedmec Member Posts: 907
    me thinks that you're confusing things here or they have changed how vision 1 operates.

    you can't use a 12 k sensor because it will give the wrong reading. the resistance value changes with temperature which is then interpreted by the control. so when a 10 k sensor sees a 10,000 ohm value it knows the temperature of the medium it is measuring is 76 degrees. the 76 degrees is the standard value for all temperature sensors that we use. for example hydrotherm use a 1k sensor in their old control board. if i used a 10 k in there it would read and interpret the wrong temperature.

    the other thing that i don't agree with is the " 1A suggested" piping plan. what stops the flow from going towards the manual (not a good idea) bypass and return to the DHW circulator and back thru the indirect? too prevent this your going to run P1? your running two circulators in series. And I know for a fact that P1 does shut down in warm weather. That's why they recommend on a munchkin the drawing 3A not 1A unless you move your DHW supply before the system circulator