Snow Melting with Air to Water Heat Pumps
Snow Melting with Air to Water Heat Pumps
Snow melting in winter for driveways, flat roofs, gutters and other external surfaces in nordic climate is extremely energy demanding and expensive. This blog offers a technology that can reduce energy consumption related to snow melting and save money.
Introduction
Snow melitng of outdoor surfaces can be found in luxury homes, airports, commercial and condominium buildings, elderly residences and other buildings. Beside Luxury homes, Snow melting can be classified as process heating since energy consumed is not associated with space heating of indoor spaces. Large snow melted areas are usually equiped with embeded hydronic heating pipes in Slabs, Asphalts, Tiles, Stamped Concrete, etc.... Heat Transfer Fluid (HTF), usually propylene glycol and water mixture, is heated via Gas / Propane Boilers and pumped through infloor pipes causing the surface temperature of the heated area to rise above freezing and melting accumulated snow.
In cold winters, such as Canadian Winters, the heating requirements of snow melted areas, can be as high as 50-73 Watts/ft² (170-250 Btu/hr.ft²). The seasonal energy consumption for such as system can be 100-146 KWh/(Year.ft²).
Air to Water Heat Pumps with Enhanced Vapor Injection (EVI) technology, has a Coefficient of performance (COP) higher or equal than 2 at lower winter temperature. Using non conventional ways of producing hot fluid, such as Air to Water Heat Pumps. will reduce operating cost of the entire system by simply taking advantage of the Heat Pump COP, which reduces the amount of energy purchased from utility compagnies.
Most Common Hydronic Snow Melting Details
This method is for designs with heavy vertical load requirements or cost constraints preventing insulation of the entire slab. With only edge insulation, the heated soil beneath the slab acts as a heat sink storing energy and later supports the slab during a sudden air temperature drop requiring additional heat. Response time is quite slow.
This method is for designs with light to moderate vertical load requirements across the slab. With under slab and edge insulation, the heated slab is isolated from high movement of energy from the slab to the surrounding frozen soil. Response time is fairly quick and even faster if the slab is idled.
This method is for designs with light vertical load requirements. With insulation, the heated area is isolated from high movement of energy from the system to the surrounding frozen soil. Response time is fairly quick and even faster if the system is idled.
This method is for designs with light vertical load requirements. With insulation, the heated area is isolated from high movement of energy from the snow and melting system to the surrounding frozen soil. Response time is fairly quick and even faster if the system is idled.
This method is for designs with heavy vertical load. It’s used only when the asphalt installer requires the higher level of compaction in order to move the paving machine up a slope, or if the application will operate above freezing throughout the heating season. The heated soil beneath the system acts as a heat sink storing energy, and later supports the system during a sudden drop in air temperature that requires additional heat.
This is a commercial installation. It is critical to make an effective thermal break between the heated slab and the structural slab below. Otherwise, the rate of heat migration into the structural slab will result in a loss of control over the snow-melt system.
This is a commercial installation. The insulation under the decking is usually sprayed on after construction. It is critical that the resistance value (R-value) of the insulation meets or exceeds the requirement specified in the snow-melt design. In suspended installations, the effect of wind must be considered in the design. Suspended installations require greater amounts of insulation to overcome the impact of moving air. The insulation should be covered to reduce the wind effect. Additionally, some spray-on insulation is vulnerable to ultraviolet (UV) degradation.
N.B:Before designing the snow and ice melting system,
it is important to educate customers about the
system’s capabilities and limitations. Proactive
communication with customers, especially
homeowners, helps manage expectations, limit
frustrations and avoid callbacks because they think
the system isn’t working properly. For example, if
the temperature or wind speed exceeds design
parameters, educated customers will anticipate
some snow or ice accumulation.
Snow and ice melting systems must be designed
to perform to the customer’s needs and
expectations. There are many ways to design a
snow and ice melting system. They vary in the
spacing of tubing, the BTU/h required, how the
area is insulated, the depth of the concrete and
the controls selected to run the system.
Just as it is impossible to predict what the next
snowflake to fall out of the sky will look like, it is
difficult to predict what kind of snow will fall. It
could be a light and fluffy snow shower, a heavy
wind-driven snow, sleet, freezing rain or even an
ice storm. Many factors affect the ability of a
system to melt snow. The most critical are:
• The density of the snow
• The outdoor temperature
• Wind conditions
• Slab surface temperature
Design of Snow Melting System
Snow and ice
melting systems are typically designed to melt
snow at 0°F with a 10-mph wind. Local conditions
may require higher or lower design temperatures. A snow and ice melting
system is often unaffected if the outdoor
temperature drops slightly below design. Wind
speeds greater than design conditions will affect
system performance more adversely. Strong winds
steal heat energy from a slab faster than in calm
conditions. The presence of buildings, landscaping
or even snow fences can reduce the negative effect
of wind on a snow and ice melting slab.
The slab surface
temperature is the result of supply fluid
temperature, flow, tubing on-center distance and
climatic conditions. The below performance charts show the surface temperatures at
varying climatic conditions, on-center distances
and BTU/h/ft2 loads. These performance charts
outline where the BTU/h/ft2 and required supply
fluid temperatures increase or decrease as the
design conditions become more or less severe.
Typically, design surface temperatures vary from
35 to 45°F.
When using Electric / Gas / Propane boiler, HTF Supply temperature isn't an issue since a boiler will consume the same amount of energy whether HTF fluid is supplied at 125, 150 or 175°F for the same thermal energy density expressed in Btu/(hr.ft²).
In snow melting application using a boiler, designer should always seek the maximum allowed piping spacing in order to reduce required PEX pipes length which reduces project initial cost.
Ex: A snow melting project in the city Toronto (ON). To maintain snow melted surface temperature at 38°F, a boiler needs to produce 145 Btu/(hr.ft²) at either 120°F (for 6 inches PEX pipes spacing) or 144°F (for 9 inches PEX pipes spacing). Assuming the snow melted area is 1 000 ft², a boiler needs to produce 145 0000 Btu/hr whether pipes are at 6 or 9 inches center to center.
At 6 inches spacing , a 300 ft PEX roll of ⅝" diameter, will cover 150 ft². so a 1000 ft² surface will require 2000 linear feet of PEX (7 Rolls of 286 Linear Feet of PEX). Changing PEX pipes spacing to 9 inches , will require 1333.33 Linear Feet of PEX (5 Rolls of 267 Linear Feet of PEX). Materials and Installation cost will be less for 9 inches spacing while energy consumption remains similar.
Typical Snow Melting System with Boiler Vs Snow Melting System with Air to Water Heat Pump
A conventional snow melting system is comprised of a Boiler, Primary and Secondary Pumps, Hydraulic Separator and a HTF (Propylene Glycol Mixture) distribution manifolds. Flow distribution to loops is controlled by a snow melting controller getting feedback from Ice and Snow sensors.
When Swaping the conventional boiler with an Air to Water Heat Pump, a buffer tank is required. The buffer tank will fulfill the duty of a hydraulic separator and acts as the thermal energy mass when Heat Pump goes into defrost mode (HP draws Hot HTF from the tank to melt the ice accumulated between the fins of its air cooled condenser).
Unlike in a conventional boiler, Heat Pump modulate its compressor speed to maintain the thermal storage tank at its setpoint. Design HTF setpoint is a parameter that can be entered via Heat Pump control interface (either the physical interface or its WIFI adapter). No etxernal controller is required to turn the Heat Pump ON/OFF.
Energy Savings
Typical Snow Melting System runs 2000 hours per season. Assuming a 1000 ft² drivway that is snow melted with ⅝" diameter PEX pipes installed at 6 inches center to center with a required heating density of 145 Btu/(hr.ft²) and HTF supply temperature of 120°F.
- A Conventional snow melting system with boiler will require 1000 ft² x 145 Btu/(hr.ft²) x 2000 hr / 3411.8 Btu/hr/KW = 84 999.91 KWh/Season.
- Snow melting system with Air to Water Heat pumps instead of a boiler will require: 1000 ft² x 145 Btu/(hr.ft²) x 2000 hr / 3411.8 Btu/hr/KW / (2 to 2.6 COP) = 32 692 to 42 499 KWh/Season.
Using Air to Water Heat pump as a heat source for snow melting system will bring between 50 and 62% of seasonal savings. In place where electricty is not produced by combustible fuels, switching to Air to Water Heat Pump will contribute to the reduction of Green House Gas Emission.
N.BFor 120°F Supply Temperature, the Coefficient of Performance (COP) of our Monoblock Air to Water Heat Pump at 0°F is around 2. The COP of our Monoblock Air to Water Heat Pump at 20°F is around 2.6.
That means that every KWh of electricity taken from the grid will deliver between 2 to 2.6 KWh of thermal energy to the snow melting system.
Producing 145 000 Btu/hr requires 3 x HSS080V2LM Heat Pumps. The 3 HPs will be installed in parallel and will be hooked up to the same thermal storage tank.
HPs have DC inverter compressors, so no need for an external controller for staging the 3 machines.
HSS080V2LM - Monoblock Air to Water Heat Pump Performance
Thanks for this blog post. I recently installed a snowmelt system when replacing our Minnesota concrete driveway, and we have a gas boiler. However, we have been gradually electrifying everything in the house including a geothermal HVAC system. I would really like to have an electric way to heat the system instead of gas.