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# Fluctuations in Room air temperature, how to avoid this?

Dear All,

I have modeled a radiant panel system and simulating for few days of winter. I have fluctuations in my room air temperature. Is this what we expect in radiant heating system meaning sharp increase in indoor air temperature and then decrease or something can be wrong? I have no internal loads (people, lighting, equipment) in this zone. Your help will be appreciated. Here is the graph for indoor air temperature, the heating set-points are 12:00 degrees till 7:00 and 17 degrees till 17:00.

Below is my ZoneHVAC:LowTemperatureRadiant:VariableFlow object.

ZoneHVAC:LowTemperatureRadiant:VariableFlow,
E.Block%down:0.E.03,               !- Zone Name
Roof.E.03,            !- Surface Name or Radiant Surface Group Name
0.013,                   !- Hydronic Tubing Inside Diameter {m}
autosize,                !- Hydronic Tubing Length {m}
MeanAirTemperature,      !- Temperature Control Type
autosize,                !- Maximum Hot Water Flow {m3/s}
0.E.03 Radiant Radiant Water Inlet Node,  !- Heating Water Inlet Node Name
0.E.03 Radiant Radiant Water Outlet Node,  !- Heating Water Outlet Node Name
2.0,                        !- Heating Control Throttling Range {deltaC}
Heating Setpoints, !- Heating Control Temperature Schedule Name
,                            ! - Maximum cold water flow (m3/s)
,                            ! - Cooling water inlet node
,                            ! - Cooling water inlet node
,                            ! - Cooling water outlet node
,                            ! - Cooling control throttling range (C)
,                            ! - Cooling high control temperature schedule
,                            ! - Condensation control type
,                            ! - Condensation control dewpoint offset
,                            ! - Number of circuits
;                            ! - Circuit length (m)

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Here's how I would troubleshoot this:

1. Check the values in the schedule "Heating Setpoints." According to the EnergyPlus Documentation for ZoneHVAC:LowTemperatureRadiant:VariableFlow, this type of system isn't controlled via the normal thermostat, but rather the heating schedule defined in the object itself (see below). This is different than most every other type of system. Double check that the values in "Heating Setpoints" are correct.
2. Check the autosized Maximum Hot Water flow rate in the EnergyPlus html output file to see if the system is being sized reasonably. If this value is too low, the system may be running, but not have enough capacity to meet setpoint. This probably means that some design/sizing values are incorrect.
3. Graph the flow rate through the object, zone mean air temp, and the heating setpoint schedule all on the same graph. If zone mean air temp is below the heating setpoint schedule and you see no flow through the system in response, there is probably an error in the system controls.
4. If the flow goes up when the zone mean air temperature drops below the heating setpoint, graph the temperature of the water. If the water temperature is too low, check the sizing values for the plant (probably in the sizing:plant object), and the operation values for the plant (most likely a setpointmanager on the supply outlet node of the plant). If the sizing and operation values for the plant are not synchronized, you can end up with a plant that is designed for one temperature, but then asked to operate at a different temperature, which it sometimes can't hit.
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Thanks @aparker and sorry for late reply as I was away. It looks like the problem was the dimension of the radiant panels were wrong. I was handed over the wrong panel's length. I will give it a try again.

( 2015-02-20 08:41:51 -0500 )edit