Design inputs
System
Tap a pump for its curve, or the separator or a manifold for its cut sheet.
Equipment
Checks
Manifolds and pumps
Pump picks are in the columns on the right. Tap a pump for its curve with the duty point.
Pump picks: the smallest model in each line with at least 5 % head margin, running under 85 % of its maximum flow. Pump curves are approximate (est. = estimated); confirm in Taco's or Grundfos's selection software.
Heat load and tubing
Fluid volume
How this is calculated
- Delivery checks: the heat the slab can deliver upward to a melting surface at 33 °F is a row of tubes 2.5″ deep in concrete (k 1.0), using the conduction shape factor of a row of cylinders below a plane surface, 2π / ln[(2s / πD) sinh(2πd / s)], in series with the PEX wall (k 0.24), at the mean fluid temperature. The Designer also flags insulation below R-5 (the CSA B214 minimum under snow-melt areas) or losses over 15 %, boilers that do not cover the load, manifolds or boilers with no listed pump for the flow and head, and supply lines over 8 ft/s.
- Load: area × heat output at the surface, plus back and edge losses. Back (downward) loss: heat flows from the tubes (mean fluid temperature Tm) through 2″ of concrete below the tubes (R 0.17), the insulation, a 6″ gravel base (R 0.63) and into the ground, which starts at 32 °F (frozen). The ground is treated as a semi-infinite solid warmed over a 24 h melting event: its time-averaged resistance is √(π α t) / (2 k) = R 0.77 for moist soil (k = 0.8 BTU/h·ft·°F, α = 0.02 ft²/h). Edge loss: the exposed slab edge, 5″ deep, loses heat to outdoor air at the design temperature through 4″ of concrete from the last tube (R 0.33), the edge insulation when the edges are insulated (same R as under the slab) and the outside air film (R 0.17); the edge length assumes a rectangle three times as long as it is wide, 4.62 × √area. The boiler output covers the total.
- Tubing: area × 12 / spacing, plus 5 % for leaders. Loops: total tubing divided into the fewest equal loops not longer than the maximum. Manifolds: Rhella XFM with up to 16 loops each, loops split evenly.
- Flow: load / (density × specific heat × 60 × ΔT) for the chosen propylene glycol mix at the mean fluid temperature (glycol properties approximated from ASHRAE data).
- Manifold and loop head: the XFM simulator's network model (2-1/2″ headers, Cv 6 per loop, PEX friction by Churchill, optional TacoSetter 223.7566.519 at Kvs 4.76 fully open). Supply lines: PEX CTS SDR 9 or Type M copper per manifold, twice the one-way length plus 8 velocity heads of fittings.
- Separator: one manifold (one secondary circuit) gets a Rhella AIR air and dirt separator; two or more get a Rhella LLH hydraulic separator. Each is sized so the design flow is at or under its 4 ft/s connection flow, with pressure drop from the Rhella CFD results.
- Boilers: NTI TFTN085–850 or FTVN085–199, matched to the load by their rated heating capacity (output) from NTI's submittals, not their input: a TFTN725 burns 725 MBH and delivers 705 MBH. Auto takes the fewest boilers whose combined output covers the load. Boiler head loss from NTI's TFTN table (FTVN uses the same heat exchanger family, assumed equal). Primary head adds 3 ft for boiler piping (NTI's 50 ft equivalent allowance) and the LLH primary side.
- Expansion: (density at 60 °F / density at the supply temperature − 1) × system volume. Tank: final pressure = (fill + 14.7) × tank volume / (tank volume − expansion) − 14.7, with the precharge set to the fill pressure. Calefactio HGT/HGTV tanks have a 100 % acceptance factor; Flexcon HTX/SXHT acceptance from Flexcon's sizing guide.
- Volume: tubing and leaders, XFM headers, supply and return lines, separator, boiler water content (NTI), plus 1 gal of boiler piping per boiler.