Hydronic Snowmelt Load and Boiler Sizing (ASHRAE)
The heated-driveway sizing balance: warm the falling snow to 32 F.
Example
You enter
- Snowfall rate, water equiv (in/hr) 0.1
- Design air temperature (°F) 20
- Design wind speed (mph) 10
- Relative humidity (%) 80
- Snow-free area ratio A_r (0 / 0.5 / 1) 0.5
- Heated slab area (ft²) 500
- Back + edge losses (%) 20
You get
- Q (s) 3.12
- Q m 74.6
- Q h 37.9392
- Q e 27.5894
- Design surface flux q_o 110.5 Btu/hr-ft^2
- Boiler (Btu/hr) 66290.6
- Mean fluid temp (rule of thumb) 88 F
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeMelt it (746 Btu/hr-ft^2 per in/hr of water equivalent), and over the snow-free fraction pay the wet surface's wind-driven convective and evaporative losses - q_o = q_s + q_m + A_r(q_h + q_e), the ASHRAE / Chapman steady-state flux. The class is the design decision: the same 20 F, 10 mph storm runs 78 Btu/hr-ft^2 on a residential drive allowed to whiten (A_r = 0), 110 on a commercial walk (0.5), 143 on a must-stay-black hospital ramp (1.0). Boiler = flux x area plus ~20% back loss. A sizing aid for the chosen storm, not an annual energy or a stamped design.
Formula or tableq_o = q_s + q_m + A_r(q_h + q_e), with q_s = 2.6 s (32 - t_a); q_m = 746 s; q_h = 11.4 (0.0201 V + 0.055)(33 - t_a); q_e = 1075.5 (0.0201 V + 0.055)(0.188 - p_av); p_av from the Magnus curve x RH; boiler = q_o x area x (1 + back_loss); t_m ~ 0.5 q_o + 33.
Source and editionASHRAE Handbook (HVAC Applications, Snow Melting and Freeze Protection) steady-state surface flux, with the Chapman (1956) IP component forms as printed in Lund, Pavement Snow Melting (Geo-Heat Center / OSTI), by name.
Free accessThe energy balance is stated in the ASHRAE snow-melting chapter and the Chapman closed forms are reproduced verbatim in Lund's Geo-Heat Center paper (free via OSTI); the Magnus saturation curve and steam-table h_fg are public; the arithmetic is public.
What governsEstimate. AHJ and licensed professional govern.
API field namess_inhr, t_air_f, wind_mph, rh_pct, ar, area_ft2, back_loss_pct, q_s, q_m, q_h, q_e, q_o, boiler_btu_hr, t_m_f
- Energy balance q_o = q_s + q_m + A_r(q_h + q_e) at the 33 F melting filmASHRAE / Chapman via Lund
- Class ratio A_r = 0 / 0.5 / 1.0 for Class I / II / III; the class, not the climate, is the first sizing questionChapman (1957)
- Back loss boiler = q_o x area x (1 + ~20%) for an insulated slab back; exposed backs run higherASHRAE snow-melting chapter