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superdave Member Posts: 155
I have a small question on piping from a boiler to Hydro air unit. If I have a wall hung boiler that has about 120,000 BTU with a supply and return 3/4". The hydro  unit is 20' - 25' of total piping both supply and return. Unit has a piping of 3/4". Do I need to up size the piping unit BTU is 82,000 


  • Big Ed_4
    Big Ed_4 Member Posts: 2,783
    The Most

    you can supply with a 3/4" full size pipe is around 48K ,,, But what do you need to heat the space ?
    I have enough experience to know , that I dont know it all
  • superdave
    superdave Member Posts: 155

    About 72,000 BTU for this building. I know the rule for t 3/4" is about 40,000 BTU or 4 GPM. What is my question is why is the boiler 3/4" at 120,000 BTU and Hydro 3/4" at 82,000 so if you have a small run like this does it make any difference up sizing the piping when it is so close.
  • Zman
    Zman Member Posts: 7,569

    What boiler is it?

    3/4" for 120k is odd.

    Is it a tankless water heater?

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Snowmelt
    Snowmelt Member Posts: 1,414
    Hydro coil

    Hydro coil has to be sized from 2 things

    1 gpm

    2 wat temp

    Usually a coil is in between 2.5 and 5 gpm

    And water temp.

    I forgot fan speed also in the equation

    But you want to size water pipe to to the btu's to the load it self.

    The reason why it's done ie you said is probably couldn't get the boiler the exact size so they went one higher, the Reason why only 3/4 pipie is being cheap.

    A few things will happen , but if it keeps up with the demand just let it rip.

    If you find the coil not keeping up then upsize the copper to. 1 inch.

    What kind of delta t do u have.
  • Jason_13
    Jason_13 Member Posts: 304

    Size pipe according to flow requirement of duct coil. The different sizes sometimes is to change the velocity of flow such as in high efficiency boilers. Flow in heating pipes are normally designed at 2-4 ft second and some newer boilers and even indirects are upwards of 5.5 ft second.

    The coil manufcaturer will normally specify multiple flow requirement for multiple btu outputs.Choose the one you need and size the pipe for that.
  • icesailor
    icesailor Member Posts: 7,265
    Boilers and Cabinet heaters:

    Cabinet fan coil heaters like Air Therm all come with 3/4" coils. You might pipe a smaller one with 3/4" copper. If you turn the fan speed from low to high, you might not feel much of a temperature drop from low to high. If you replace the cabinet heater with a larger one that should have been piped 1" from the main to the coil, reducing to 3/4, and you do the same thing with the fan speed switch, the water leaving the coil will be hot with the fan off, but as the speed goes up, the coil outlet will get colder. On high speed, the outlet might be cold.

    Small boilers act the same, But they are always extracting heat from the fuel and the mixed return will get warmer. Its the high flow through the boiler that is important so as to not let the boiler overheat because of low flow.

    Why you need to PUSH cold water into the boiler instead of SUCKING the water OUT of the boiler and running the risk of having local boiling/cavitation inside the boiler.
  • superdave
    superdave Member Posts: 155
    Thanks for all the info

    I always run 1" to the units. But most of the time the boiler is about 100'  of piping combined supply and return between the two of them. It was just a question of would it make a difference seeing the unit is within 25' total run and both have 3/4" piping on supply and return.
  • Jason_13
    Jason_13 Member Posts: 304
    edited March 2014
    Pipe Length vs diameter

    Diameter is more about flow and length is more about resistance. Generally and simply speaking if you need to move 7 GPM in a pipe 4 ft long you need 1 " pipe. If you need to move 7 GPM in a pipe 200' long you need 1" pipe but the added resistance requires a larger pump.

    This example is exaggerated but I think makes the point. 7 GPM with 200 ft pipe you would probably jump to 1-1/4" pipe to use a smaller pump and less chance of hydronic harmonics.

    Pushing or pulling water in a boiler does not change the pressure so no cavitation will exist. If you push into the boiler with a static fill of 12 psi the boiler pressure is 12 psi. If you pull out of the boiler as long as the circulator is after the expansion tank connection the boiler pressure will be 12 psi. The difference is pushing into the boiler the upstairs system pressure will drop and pulling out of the boiler again assuming you are pumping away from the expansion tank will add pressure to the system. You always need at least 4 psi at the high point of the system.