In last month’s article on snowmelt systems, we stopped after talking about on/off lighted pilot switch type controls. This month let’s look at four more ways to control snowmelt systems.
Twist timer operation: In this scenario, the system is manually operated by the occupant. A spring-wound timer, however, limits the run time of the system. These timers typically have a 12-hour maximum setting. The system user simply twists the timer to whatever number of hours he feels is needed to do an effective job of snowmelt. When the timer gets to zero, the system shuts off.
I have seen the function of these timers defeated by folding a matchbook cover and keeping the timer from counting down. Another disadvantage is that this system also doesn’t monitor slab moisture or slab temperature conditions. It can result in over or under run of the snowmelt system, which can cause either energy waste or an under-performing snowmelt system, neither of which is desirable.
One major disadvantage to all of the manually operated on/off systems is that they work reactively as opposed to working proactively. If a substantial amount of snow has already fallen when the snowmelt system is turned on, the system may experience what is known as snow bridging. This is situation where the area of snow contact to the slab is melted, resulting in an insulating air gap between the warm surface and the snow. The result is that unless you are willing to tromp around and get the snow back into contact with the warm surface, the snow will take a lot longer to melt and the system will have to operate for a much longer period of time than with other control schemes.
If this simple control system is installed, it is always recommended that the installer tell the homeowner that if he knows that there is an imminent snow storm, that it is recommended that he pre-warms the slab so that it can work proactively as opposed to working reactively. The snowmelt system will perform much better if this information is relayed to the end user.
Partial idle systems: Under this scenario, the snowmelt slab is maintained in a partial idle condition that is below the targeted snow melting condition, say around 20°F to 25°F, but warmer than the ambient condition. When snowmelt is required, the slab is raised to its snowmelt target temperature, typically 35°F to 40°F.
While this system is more expensive to operate on an annual basis than a cold start system, its reaction time is quicker because the mass of the system is partially charged, and it will melt snow more quickly.
This type of control scheme is typically part of an internal control logic of a substantially more sophisticated control system that incorporates sensors in the slab, as well as air temperature monitoring systems. With this particular system, once idle temperature conditions have been met, system pumps shut down and boiler is disabled. If the slab is exposed to a cloudless night sky, the thermal energy sucked off the slab by night sky re-radiation can be substantial and can result in an excessively high utility bill.
Hot idle system: In this scenario, the snowmelt slab is kept at a steady state operating temperature based on the ambient temperature. Typically, if the outside temperature falls below 40°F, the slab is maintained at 40°F regardless of whether snow is falling or not.
This type of system, while maintaining a dry slab, can also have high operating costs. In the case where the heat source is waste heat from some sort of industrial process, it is not an issue. If there is a meter spinning somewhere based on the system operation, however, it can be one of the most expensive systems to operate.
Again, if the slab is exposed to a cloudless night sky, the thermal energy being sucked off the slab by night sky re-radiation can be substantial and can result in a high utility bill.
Partial idle systems with continuous circulation: In this situation, the distribution circulators are run continuously whenever the outside air falls below 40°F. The control logic monitors the supply versus return differential. Whenever the differential begins to increase, which will happen if there is snowfall, it ramps up the return water temperature to a predetermined temperature to melt snow. As the differential in temperature begins to narrow, indicating snowfall has ended, the return water reference temperature is allowed to fall back to idle conditions.
While this system may be more expensive to operate than a cold start system, its reaction time is quicker and the maximum slab temperatures are maintained in a controlled fashion. This logic is a good choice for retrofit considerations where it is not feasible to install a slab sensor to monitor slab conditions.
Tune in next month as we continue to delve into the control, design, maintenance and operation of snowmelt systems.
Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.