How to- controls question

You need those slab sensor to be the main control if you plan on idling above slab freezing temperature? So you need the two sensors, two 170 control.

Actually all you need is the 150 setpoint controls, save some bucks. Unless you want the WiFi fuction?

You could just let the pumps run. That way any solar gain on the slab gets circulated.

Or the pumps wire to the boiler system pump output, so the pump starts when the boiler calls.

The setpoint controls just call on the boiler. The 170 or 150 needs power from a 24V source, then S1 and com for the sensor. XX goes to the boiler

Did the boilers come with outdoor sensors? If so maybe that function to shut off the boiler above 38° If it has that range of adjustability. If not you need a sensor in the bay to know when it is above 38. Although the air could be 38 and the slab still cold enough to ice over?

I'd just try setting the differential on the 150 on at say 34-35°, Off at 38 or so.

Did you install the 073 sensors in a conduit so it can be replaced? Sensors fail from time to time.

After a review of the Navian boiler WWSD, the lowest setting is 50° which is too high for your purpose

After a review of the Tekmar 654 snow melt control the WWSD feature is able to be set so that when the slab temperature is able to melt snow without heat input, the boilers will not operate. This is better than your suggestion of a random temperature of 36°F. If the slab has retained heat from a warm sunny day and the outdoor temperature drops to 30°F the boiler may not be required. Also of the slab temperature is cold and the outside temperature is above freezing, the rain may actually land on the slab and freeze on contact. This control will eliminate that as a problem.

That's expensive wire, $3.95 / ft. more for the longer version. I wonder how that sensor would hold up in that car wash environment.

Here is the info from:

The Wi-Fi Setpoint Control 170 provides precise temperature control of a one-stage heating or cooling system. When connected to the Internet, the Watts® Home mobile app allows the setpoint control to be controlled remotely. An Away mode is quickly and easily accessed to provide energy savings when the heating or cooling system is unoccupied. The setpoint control can be used to control the heat for an indirect domestic hot water tank, a pool, a hot tub, or small outdoor patio, as well as provide cooling for a wine cellar.

The Snow Melting Control 654 is designed to operate electric or hydronic equipment to melt snow or ice from any surface including driveways, walkways, patios, business entrances, parking ramps, loading docks, hospital entrances, helipads or car wash bays. The surface temperature for snow melting is controlled automatically to reduce operating energy costs. The 654 has an automatic start and stop function when used with the Snow/Ice Sensor 090 or 094. Automatic start with a timed stop is available when used with the Snow Sensor 095. The 654 can operate a dedicated hydronic boiler or a mixing device. Isolation relays are required to operate line voltage pumps. Electric systems require a separate GFCI and electrical relay contactor  

Looking at the two different controls I see that the 170 is basically a WiFi thermostat that you can check the status of an on/off system over the internet, based only on temperature. This will require constant monitoring to do the job you need it to do.  If the outdoor temperature is such that the slab that you do not want ice to accumulate on is at the set point you select, then the 170 will operate the boiler even if there is no need for boiler operation.  It will only operate based on the temperature you select and the sampling of the temperature by whatever sensor you attach to it.

The 654 will measure not only temperature of the slab, but will also make a decision on boiler operation based on the presence of ice or snow when using a 5 wire sensor placed in the wash bay slab. If there is no ice at the selected temperature, then the boiler will not operate.  So, the 090 sensor that does all the savings by keeping the boiler off if there is no ice, does not connect to the 170.    It does connect to the 654.    There is a less expensive 095 aerial control that may or may not work for your application.  The aerial snow sensor will not operate the boiler if there is no precipitation (SNOW). The car wash always has water on the floor during operation even if there is no precipitation otherwise.  

So, I believe the floor sensor will be more accurate. However, if you strategically locate the 095 where the clean rinse water or clean prewash water will make itself known to the 095 then you could go that route.  The aerial sensor is not confined to the concrete slab and is less expensive and you can have an extra on hand if needed.  

You get what you pay for.  If you don't mind constantly checking the system and adjusting the temperature accordingly then go the less expensive route.   If you want to take a vacation (or hospital,wedding, or funeral) from the car wash business in January, then you need the control that you can set and forget.