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Radiator Sizing/Water Temperature

Kestrel Member Posts: 102
OK, gang, I'm zero-ing in on the final design, and the help I've gotten here from this wonderful community has been tremendous!

I'm wrestling with the final emitter choices - Runtal v. Buderus v. Dianorm - as well as what size for each particular room.  This leads to a question of water temperature.

I've done a whole-house and room-by-room heat loss calculation.  The plan is for wall-mount radiators on the second floor and basement, and 3/8 Pex tubing in the floor joist spaces of the first floor - with extruded plates (Given some constraints in the joist space, and obstruction by cabinets and such, I may need a couple of radiators on the first floor as well to achieve sufficient heat delivery)

My question - does one pick a design day water temperature arbitrarily,  and from that size the radiators based on the room's heat loss?  If I want to keep as simple as possible and run radiators and floor tubes at the same temp, am I limited by the max temp that should be run under the floor?

I suspect that I need to take into account the water flow rate and deltaT for the various emitters, but where do you start?


  • Gordan
    Gordan Member Posts: 891
    Humble suggestion

    Start with limitations: budget, and your most difficult zone (which will probably be one or all of the under-floor zones.) Design those zones and figure out what the supply temp needs to be. Then design other zones around that supply temp. Then look for ways to, with minimal extra expense, drop the temp a little - upsizing radiators, for instance. It's an iterative process, but at each iteration you start with solving the toughest problem first and then seeing how it affects the less troublesome parts.
  • Kestrel
    Kestrel Member Posts: 102
    Yup, it's the under floor system...

    The main floor is pretty much one big room - kitchen/dining/living, without walls really, except around a small bathroom.

    Problem is, there are two big old bay windows that leak heat - my heat loss for this ~700 sq ft space is about 18000 BTUH at design temp.  Due to cabinets and such, and obstructions under the subfloor in some of the joist spaces, I only have about 500 ft sq total to put in the plates/tubes (Using 3/8 pex and JoistTrak plates)

    I don't think I can get to 36 BTUH/Ft Sq that I would need, so I'm thinking that adding a couple of radiators to provide 8000 BTUH, then I only have to get to 10000 BTUH from the floor - or 20 BTUH/ft sq.

    That said, calculating Qu and Qd, I get 20 and 1.22 BTUH/ft sq (given R 19 in the joist space below the tubes)

    From that I calculated a=1, (Siegenthaller's formula 10.7c, first edition).

    Bear with me!

    I'm calculating for loop lengths of 185ft, and I calculated b at various GPM:

    1 GPM, b=.37

    1.5 GPM, b=.24

    2 GPM, b=.19

    I started with a Tin of 150'F, and using the above 'b' values, I got:

    0.5GPM, T in 150, Tout 107, delta T 43

    1 GPM T in 150, Tout 124, delta T 26

    1.5 GPM, Tin 150, Tout 133, delta T 17

    2 GPM, Tin 150, Tout 136, delta T 14

    What confuses me, is at reasonable GPM (ie < 1), the deltaT is HUGE, and seems too big.  What am I missing here?
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