Free Hydronic Snow Melt Sizing Tool
Calculate ASHRAE Chapter 51 radiant snow melting heat flux (BTU/hr/ft²), back losses, glycol circulation GPM, and boiler gross kW capacity.
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ASHRAE Chapter 51 Hydronic Snow Melting Fundamentals
Hydronic radiant snow and ice melting systems circulate heated propylene glycol through embedded PEX tubing in concrete or asphalt slabs. Design criteria are governed by ASHRAE Handbook - HVAC Applications, Chapter 51.
Surface Heat Flux Equation
The total heat flux at the slab surface (q_o) (BTU/hr/ft²) is expressed as:
$$q_o = q_s + q_m + A_r cdot (q_h + q_e)$$
- (q_s = ho_w cdot s cdot c_{ice} cdot (32 - t_a)) : Sensible heat warming snow to 32°F.
- (q_m = ho_w cdot s cdot 144) : Latent heat of fusion melting solid snow to water.
- (q_h = (h_c + h_r) cdot (t_s - t_a)) : Convective wind and radiation loss.
- (q_e) : Evaporation of surface water film into blowing wind.
- (A_r) : Free-Area Ratio (0 for Class I, 0.5 for Class II, 1.0 for Class III).
Hydronic Flow & Glycol Derating
Because cold propylene glycol has lower specific heat and higher viscosity than pure water, circulation GPM is calculated with a glycol capacity factor ((sim 0.86) for 50% solution):
$$ ext{GPM} = rac{Q_{total}}{500 cdot Delta T cdot ( ext{Glycol Factor})}$$
Frequently Asked Questions
Why is idling slab temperature recommended?
Warming a frozen slab from 15°F to 34°F takes several hours due to thermal mass. Idling the slab at 33°F-34°F before forecasted snow ensures immediate melting upon impact.
What is the maximum circuit length for 3/4" PEX snow melt tubing?
The recommended limit is 250 to 300 feet per loop to prevent excessive pumping head loss from viscous cold glycol.
Why should automotive antifreeze never be used?
Automotive antifreeze uses toxic ethylene glycol and contains silicates that foul radiant tubing and heat exchangers. Use only inhibited non-toxic propylene glycol.