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Correct temperature and delta for radiant heating

Warm_guy1234 Member Posts: 1
edited May 2020 in Radiant Heating
Hi everyone,
I have a pretty simple installation consisting in heater with two circulators installed in series and a manifold with 8 loops that heats a slab in the main floor of the house.

I was wondering what would be the correct speed setup for the circulator pumps.
Pumps are installed in the primary and only circuit from the boiler to the manifold and are two Grundfos 15-60 identical operating in series.

Both operates in the 3rd speed and that gives an inbound temp of 96F and outbound of 116F. The boiler operates limited at 113F, however there's a misreading or difference where each thermometer measures the water but usually the system runs well despite the differences.

I was wondering if its convenient to reduce the 2nd speed in both pumps and increase the gap between inbound- outbound temperatures, in which case the Delta Temp would be around 98F Inbound -120F Outbound.

Considering that the heater is non-modulant and provides always the same amount of BTU's (always working in the minimum setup) and the flow in each loop is about 0.75 Gallons Per Min at 3rd speed what would be the recommendation ?

2nd speed at 98F-120F
3rd speed at 96F-116F ?



  • Jamie Hall
    Jamie Hall Member Posts: 22,196
    How big is a box? There is no one correct answer. What you need is the correct temperature and flow to the floor to give the heat delivery you need in the space. A higher flow rate will give you a smaller delta T, and a lower one a higher delta T. Depending on the arrangement of the tubing, the variation in temperature across the floor may or may not be significant. All else equal, a higher input temperature -- but I'd not want a floor much over 120 if that -- will give more heat in the space.

    That said, I hope that that boiler is made to operate in condensing mode, because if not it won't be long before you have to replace it.

    And I hope it's a boiler, not a water heater. Water heaters aren't meant to be used as heating system boilers.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • GroundUp
    GroundUp Member Posts: 1,772
    For only 8 loops, assuming 1/2" pex at 300ft loops, you probably don't need the second pump at all. That .75 GPM with the two resulting deltas makes 60k and 66k, which would make 25 btu per sq ft or more, again assuming 300ft loops. Is this slab uninsulated or the building poorly insulated? If I were a betting man, I'd say one pump could safely be removed altogether and the supply temp reduced but that will depend on the boiler's firing rate and loop lengths to possibly make the best of a bad install. What model boiler is being used?