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hydronic fan coil GPM vs EWT

Darrin Member Posts: 1
I am installing a SpacePak Hydronic 5 ton fan coil unit (WCSP-4860G) for heating and cooling. The manufacturer curves show that at least 10gpm @140F (48F cooling) EWT is required to get the necessary 90000 BTU/H of heat (43000 BTU/H cooling). I used the Taco Radiant Made Easy Application Guide to size a circulator. All literature I find indicates that I need a minimum of 1" copper or 1-1/4" heat PEX or 1" PEX-AL-PEX (PAP) to achieve 10gpm with reasonable friction loss (largely due to the higher velocity required in smaller diameters). The high velocity will also make a noisier system with

faster wear. Please correct me if I am wrong.

My plumbing supply guy, who is a Taco dealer (reseller?) thinks that I should not need the large pipe diameter. He has many years of field experience, but I think the SpacePak has a very large delta-T and requires higher flow than he is used to. The SpacePak curve is clear about the required flow. I computed around 19 feet of friction head loss if I use 1" heat PEX (assuming 30% glycol @140F, equivalent pipe length of 127.2 feet, 10gpm). This does not include the coil drop of 6psi or the higher density when using chilled water.

My heat loss(gain) calculations indicate I need 90000 BTU/H heat (43000 BTU/H

cooling). This corresponds to 10 gallons per minute at 140F EWT (48F cooling). Another possibility is to increase (decrease) the entering water temperature. If I go to 160F (45F) I can decrease flow to around 8gpm. At 180F (42F) I may get down to around 6gpm. I currently have a 145MBH boiler that will supply this zone and a smaller one plus domestic hot water. The manifolds are 1-14" copper and black iron.

My assumption is that it would be more efficient to exchange as much heat as possible through the fan coil at the highest flow practical rather than use over heated (chilled) water at a lower rate. I have not tested this assumption. Also, I want a lower power consumption/less expensive circulator. That means large pipe diameter.

My feeling is 1" copper or 1" PAP will handle the zone runs comfortably. What do you think?

Thank you.


  • Zman
    Zman Member Posts: 7,114
    Condensing boiler?

    If you have a condensing boiler 140 is a good idea. With a conventional boiler , I would run it hotter.

    Taco's chart assumes 4 ft/sec. This is quite conservative. Copper is generally good to 5.5 and pex can go higher.

    I would be comfortable with the pap or copper at one inch. I would uses pex 1" if the the run is not too long.

    Your thinking on the circ power use is right on. You can size the system using 1". I f after you calculate your head loss, your circ is to big, recalc it with the next pipe size up and see if it make a difference.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
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