How do you model a heating only radiant system?

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The ZoneHVAC:LowTemperatureradiant:VaribaleFlow does require three schedules i.e. one for availability, second for heating control temperature schedule and the last one for cooling control schedule. These systems can be modeled using availability and heating control schedules (for heating purposes only) i.e. without defining schedule for cooling control. I just simulated without defining cooling control temperature schedule and it worked fine for me. See below the way I defined the object;

ZoneHVAC:LowTemperatureRadiant:VariableFlow,
Block1:Zone1 Heated Floor,     ! - Component name
On 24/7,                                 ! - Availability schedule
Block1:Zone1,                          ! - Zone name
Block1:Zone1 Heated Floor Radiant Surface List, ! - Radiant surface group name
0.0130,                             ! - Hydronic tubing inside diameter (m)
autosize,                            ! - Hydronic tubing length (m)
MeanAirTemperature,         ! - Temperature control type
autosize,                            ! - Maximum hot water flow (m3/s)
Block1:Zone1 Heated Floor Hot Water Inlet Node,  ! - Heating water inlet node
Block1:Zone1 Heated Floor Hot Water Outlet Node, ! - Heating water outlet node
2.00,                                  ! - Heating control throttling range (C)
Block1:Zone1 Heating Setpoint Schedule,     ! - Heating control temperature schedule
,                                        ! - Maximum cold water flow (m3/s)
,                                         ! - Cooling water inlet node
,                                         ! - Cooling water outlet node
,                                         ! - Cooling control throttling range (C)
,                                         ! - Cooling high control temperature schedule
SimpleOff,                          ! - Condensation control type
1.00,                                  ! - Condensation control dewpoint offset
OnePerSurface,                  ! - Number of circuits
106.700;                            ! - Circuit length (m)

P.S. I did try this using E+ 8.1, 8.2 and 8.3, all worked fine for me. If you are getting some kind of error then please share that. Also, my question was based on using E+ text editor not OpenStudio (as you did not mentioned that you are using OpenStudio) but the idea is same. You just have to use availability and heating control temperature schedule.

OpenStudio

If you are doing it in OpenStudio, then once you have applied the Zone Equipment then you can apply your Cooling and Heating thermostat schedules. I don't know whether in OpenStudio we have to specify both schedules or specifying only one will work as I did in IDF file but you can assign Always Off schedule for Cooling. You can find this schedule under Library --> Schedule Rulesets, all you need to do is Drag and drop. See picture below;