Troubleshooting VT Heating Circuits

Troubleshooting VT Radiator Circuits: A Step-by-Step Guide

 

Heating issues in VT (Variable Temperature) radiator circuits can arise from problems with the BEMS, mechanical equipment, or plumbing systems. This guide provides a systematic process for diagnosing and resolving these issues.

 

Step 1: Boiler Operation

 

Start by confirming that the boiler is providing the necessary heat to the VT circuit.

1. Verify Flow and Return Setpoints:

Check if the boiler is meeting the flow and return temperature setpoints specified by the BEMS. For example, the boiler flow temperature might need to be 70°C while the return is 50°C to provide sufficient heat for the VT circuit.

If Setpoints Are Not Met:

• Investigate whether the BEMS is sending the appropriate signal to the boiler. If the BEMS controller has failed, it may need to be replaced.
• Check the relays in the control panel for faults. A failed relay can prevent the boiler from starting or maintaining operation.
• Inspect the boiler for lockouts or internal faults, such as sensor failures, ignition problems, or pressure issues.

 

2.  Check Boiler Thermostats:

Ensure the boiler thermostats are set above the BEMS flow setpoint. For example, if the BEMS flow setpoint is 70°C, the boiler thermostat should be set at 80°C or higher to prevent underheating. Misaligned setpoints can lead to inefficient operation and inadequate heating for the VT circuit.

3.  Confirm Boiler Pump Operation:

Verify that the boiler’s primary pump is running. Heat cannot be transferred to the VT circuit if the primary pump fails.

• If the pump is not running, check for:
• A BEMS failure to enable the pump.
• Faulty pressure sensors that fail to detect flow, causing the pump to remain off.
• Mechanical issues such as pump cavitation (often caused by low system pressure), impeller blockages, or pump motor failure.
• Redundancy Checks: If the primary pump fails, the standby pump should automatically activate. If it does not, check the BEMS changeover logic, control panel relays, or mechanical faults with the standby pump.

 

Step 2: VT Valve and Actuator

 

The VT valve regulates the temperature of water delivered to the radiator circuit. Issues with the valve or its actuator can disrupt the system’s ability to provide heat effectively.

 

1.  Check Actuator Modulation:

Ensure that the actuator is receiving a proper control signal from the BEMS (typically 0–10VDC, where 1VDC = 10% open). If the actuator is not modulating as expected:

• Test the power supply to the actuator using a multimeter.
• Inspect for valve seizure caused by sludge or sediment in the pipework, which can prevent the valve from modulating correctly.

2.  Ensure Correct Valve Placement:

Confirm that the VT valve is installed before the VT pumps. Incorrect placement can lead to deadheading, a condition where the pump operates against a closed valve. This causes pressure buildup, potentially damaging the pump and reducing system efficiency.

3.  Verify 3-Port Valve Configuration:

For 3-port valves, ensure that Port AB remains open to allow continuous flow, as closing it can cause system blockages, deadheading, or thermal imbalances. Ports A and B can be configured as flow or return paths, but this must follow manufacturer guidelines to maintain system reliability and efficiency.

4.  Check VT Flow Setpoints:

The VT valve should adjust flow temperature based on the outside air temperature (OAT). For example:

• At 0°C OAT, the VT flow should be 80°C to meet higher heating demand.
• At 20°C OAT, the VT flow should drop to 20°C, conserving energy in milder conditions.

These values are calculated based on traditional heat loss principles and radiator system design. The BEMS should modulate the VT valve to maintain these temperatures dynamically.

 

5.  Inspect Space Temperature Sensors:

If space temperature sensors influence the VT flow setpoint, ensure they are accurately placed. Sensors located near heat sources such as printers, portable heaters, or kitchen appliances can produce false readings, leading to overheating or underheating in the space.

 

Step 3: System Losses and Balancing

 

If heat reaches the VT circuit but spaces remain cold, the issue may lie in the distribution system or its configuration.

 

1.  Inspect for System Losses:

• Changes to building layouts, such as additional glazing or reconfigured spaces, can increase heating demand and affect the VT system’s ability to maintain comfort.
• Look for faulty radiators, leaking valves, or missing pipe insulation, particularly in older buildings.
• Aging pipework or valves may degrade over time, reducing overall system performance and requiring increased VT flow temperatures to compensate.

 

2.  Perform a Balancing Test:

• Open the VT valve to manual mode (100% open), enable the VT pumps, and turn the boiler on.
• Measure the VT circuit flow temperature and compare it with the boiler flow temperature. If the temperatures do not match, balancing issues may be causing heat to be diverted to other circuits.
• Measure the VT circuit flow temperature at the penetration point into the actual zone (this maybe in a floor riser or above the ceiling). If the temperature leaving the plantroom ant the penetration point does not match (considering reasonable losses) then there may be a system pressure or flow related issue in the distribution system.

3.  Investigate Pump and Pipe Sizing:

Incorrectly sized pumps or pipes can lead to inadequate flow rates or excessive resistance, causing uneven heating. Consult with a mechanical contractor to evaluate and correct these issues if necessary.

 

Step 4: VT Pump Operation

 

Once the VT valve is confirmed operational, check the VT pumps to ensure they are running and providing adequate flow.

1.  Verify Pump Operation and Speed:

Confirm that the VT pumps are running at the correct speed to circulate heat effectively. Improper speed settings can lead to insufficient or excessive flow, impacting heating performance.

 

2.  Check for Mechanical Issues:

• Look for cavitation, which occurs when the pump operates under low-pressure conditions.
• Inspect for airlocks in the system, which can block flow.
• Check impellers for blockages caused by debris or sediment.

3.  Test Pump Redundancy:

If the primary pump fails, ensure that the standby pump activates as intended. If it does not, review the BEMS changeover logic or check for mechanical faults in the standby pump.

 

Step 5: Local Heating Controls

 

If the boiler, VT valve, and VT pumps are all functioning correctly, the issue may lie in the local heating controls.

1.  Inspect TRVs (Thermostatic Radiator Valves):

• Check individual radiators for blockages or faulty TRV heads.
• Manually adjust TRVs to verify that they open and close correctly.

2.  Check Two-Port Zone Valves:

In some systems, local thermostats enable two-port zone valves to control specific areas, such as meeting rooms or transient spaces. Verify that the thermostats and valves are functioning properly, and ensure that valves are not stuck in the closed or partially open position.

 

Conclusion

 

This systematic troubleshooting process helps identify and resolve VT radiator circuit issues by addressing the root causes, whether within the BEMS, mechanical components, or plumbing systems. Proper maintenance, accurate commissioning, and collaboration with your BEMS provider and mechanical contractor are key to ensuring reliable and efficient operation.