System Performance Boils Down To Location, Location, Location
By MIKE MILLERHydronics
Points to be mindful of when installing and troubleshooting snow melt control.
Winter is here and as the snow starts to pile up, customers are looking to their snow melt systems to get the job done with no shovels and/or salt involved. Unfortunately, the systems do not always perform as expected and controls sometimes end up being the fall guys for poor operation. Those same controls, which are applied to help run systems more efficiently and automatically, need to be installed correctly and with some thought. In my experience this is not always the case. The following pointers highlight some of the more common installation errors and offer solutions to performance problems.
One of the most fragile components, which is prone to failure if it is not installed properly, is the snow and ice sensor. The control system requires it to function flawlessly. The sensor should be installed right in the heated driveway or pad at a minimum of one foot from the exterior of that slab. It also needs to be flush with the finished surface to prevent standing water on top of it. Failure to do so may cause the system to remain in melting mode, even though the surface of the driveway may be free of ice or snow.
Another, and very often overlooked part, is the installation of the sensor. While the circuitry inside it is typically protected by having its housing filled with an epoxy to give it water resistance, proper placement and care of the sensor’s socket are important. Most tradesmen will agree that if you give it time, water will find its way into anything. The sensors are installed outside where rain will hit the surface throughout the year. We also know water will be in and around the sensor’s location several times throughout a year since the control systems job is to melt snow. It is recommended that adequate water runoff from inside the socket is provided, or that water is prevented altogether from accumulating inside the socket. Drainage can be achieved by simply drilling a hole through the slab in the bottom of the socket to allow any water to drain into the ground.
Some manufacturers ship their snow/ice sensor with some putty (or duct seal). Its intended job is to fill out the socket entirely to displace any air gap and therefore any possible place for water to accumulate within the sensor. Making sure the entire socket is filled with the putty will greatly extend the life expectancy of this sensor. Failure to do so will allow water to eventually get into the underside of the snow/ice sensor, even through the epoxy sealed underside. Once the outdoor temperature drops below freezing, the damage will be done.
While it is very hard to see with the naked eye, when water has done damage to a snow/ice sensor you will find the epoxy underneath the sensor to be slightly raised over the brass housing. More often than not, you will also notice standing water or even ice in the bottom of the socket as soon as you remove the sensor from its socket to inspect it. Typically, when a snow/ice sensor fails, it is not due to poor quality of the sensor, but more commonly a failure to install it correctly.
Equally important is the placement of the sensor relative to the tubing installed in the slab. It needs to be centred right in the middle of two pipes. If the slab temperature sensor is not integral to the snow/ice sensor, it needs to be installed about one inch below the finished surface. Failure to do so may have the slab temperature sensor too close to any of the piping. It could pick up warmth from the water passing through the tubing and not the actual slab temperature. The end result could be inaccurate slab surface temperature read-out and premature disabling of the snow melting system.
Equally as important is the placement of the outdoor air sensor. This is not only to true in snow melting applications, but any application or control system that uses the outdoor sensor for guidance. The outdoor sensor must be installed on the north side of a building, where sunlight cannot interfere with its reading. It should be protected from precipitation, so that water cannot get inside of it, as most outdoor sensors are thermistor-based and the temperature correlated based on a resistive circuit. It must be shielded from all sorts of temperature influxes, so that it can read nothing but the true outdoor air temperature.
In snow melting systems, one of the outdoor sensor’s primary functions, but not the only one, is to determine whether the moisture on top of a snow/ice sensor is water or snow/ice. It can also allow the system to be enabled and disabled based on warm weather and cold weather cut out limits.
SENSORS IN CONFLICT
Not too long ago, I was called to a job where the primary complaint was that the snow melting system would function for only a very short period of time, before shutting down again. While there can be many reasons why this happens (from poorly placed water temperature sensors to conflict in system settings and even short circuits disabling the controllers altogether), in this case, I found the outdoor sensor installed too close to the boiler plant’s exhaust venting.
Every time the system went into melting mode and the boiler fired, the controller was placed into a warm weather shut down state. You may ask who would do such a thing? Not to pass blame to anybody in particular, in this case an electrician was asked to run the outdoor sensor out to the north facing wall and place it just below the roof’s overhang. The electrician was not too familiar with the controls sequence of operations or this sensor’s real purpose, he figured the hole already cut to the outside for the venting would also serve as a great spot to route the outdoor sensor wiring through. He then placed the outdoor sensors within three feet of where the venting exhausted. The easy fix there was to move the outdoor sensor further away.
Another very common observation I have made over the years is the wiring itself. Most control terminal strips are suitable for 18 or 14 AWG cable, depending on the type of connection it is for. 24Vac wiring terminals typically are designed for 18 AWG wiring and 120Vac for 14 AWG. If the wiring used is thicker, you may want to consider joining this larger wire underneath the terminal and only feed the appropriate wire size into the terminal itself. Also observed frequently is the amount of bare wire that is inside of the terminal strip. Too often I find that wire may not be stripped back far enough, preventing proper conductivity with the controls terminal. I have found that keeping at least five millimetres of bare wire often suffices.
I realize that the examples here are just a few things to look for when a snow melt system is operating short of its expectations or components are failing prematurely. They do summarize the majority of challenges I have come across apart from actual component failures, which represent the minority of systems with performance issues. <>