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help with flow rates in floor heating and pex design

livinglife Member Posts: 2
Hello everyone.
So many great posts and responders.
Having read many of them, people say the building was piped wrong. MY WORST NIGHTMARE!
I am building a new house.

4 foot frost wall (nova scotia, Canada) the interior of the frost wall will have 2 inch XPS R10 foam from the top of the frost wall down 3 feet. slab on grade ( 4 inches of gravel,3 inches of XPS R15, 6 mil vapor barrier, then 4 inch slab. 1/2 inch pex tied to 6 inch grid mesh and held up with chairs to center of the slab.

Exterior walls are 2x6 (R20 fiberglass with 1 inch XPS on exterior of the sheathing to create a thermal break Total of R 25 in walls) R50 atticat blown in ceiling. The windows are all casement. Steel exterior doors with 1/2 glass in all the doors. I will be using an electric thermo 2000 Combomax boiler. (Domestic and radiant) 1/2 inch pex for radiant.

I will attach the drawing. Its all 1 floor. (Inlaw suite for my Dad)

If anyone would please help with the design it would be greatly appreciated. Any suggestions would also be appreciated with insulation. Nothing is built yet, so now is the time for changes.

Thank you
Have a great day!


  • Ironman
    Ironman Member Posts: 7,337
    The first step is to do an accurate Manual J heat loss calculation. Everything is built on that.

    How can you size a pipe if you don’t know how many btus it has to carry?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • livinglife
    livinglife Member Posts: 2
    Thanks for the advise.
    Im not sure. Thats why I asked for suggestions and help. As I mentioned, nothing is done, so at this point anything can be changed/added.
  • hot_rod
    hot_rod Member Posts: 21,797
    You need to start with a room by room heat load
    If you want to tackle that you can do a long hand form, or look into a software.
    This is a program I use, try a free demo at

    A paper version in the Uponor CDAM manual
    free download at the Uponor website

    this manual takes you through all the steps, heatloss, sizing tube layout, piping,  pump choice, etc
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Teemok
    Teemok Member, Email Confirmation Posts: 472
    Generally a 1/2" loop under 300' flows 0.5-1.25GPM. 0.75 is a typical. How many loops you have is a layout question and that is influenced by how much heat the structure needs. So heat loss calculations are in order. It's way better to have it designed than guess and be wrong. 6" bend radius is the max. 6" tube spacing makes for good btu output but some of your spaces might over shoot if you do 6" tube spacing for all areas. Let a designer with a computer do it.