7 hydraulic problems that get blamed on the BMS
When a heating or cooling loop won't behave, the cause is often in the pipework rather than the controls. Seven common culprits and how to prove each one.
“The BMS isn’t working.” It’s the most common call a controls engineer gets, and quite often the controls are doing exactly what they’ve been told to. The real problem is in the water. Being able to prove that quickly saves you hours of retuning loops that were never the issue, and it gets the right trade out to fix it.
These are the seven hydraulic faults that most often get pinned on the BMS.
1. A 3-port valve piped the wrong way round
You’ll see the temperature move the opposite way to what the valve command says, or the loop drive itself to 0% or 100% and stay there.
3-port valves have a common port, usually marked AB, and two others. Swap them over, or fit a mixing valve where a diverting arrangement was intended, and the valve behaves backwards or unpredictably.
To prove it, put the valve in hand, drive it to 0% and then 100%, and watch the temperatures either side. Then compare the port markings with the schematic. Try not to “fix” it by reversing the loop action in software, unless the client knowingly accepts a non-standard installation. It’ll confuse every engineer who comes after you.
2. An oversized valve
This one shows up as a loop that hunts at low load but behaves perfectly at full load. Nearly all the control happens in the first few per cent of valve travel.
The cause is poor valve authority. The valve’s pressure drop is tiny compared with the rest of the circuit, so it has no real say until it’s almost shut. Trend valve position against the controlled temperature. If going from 0% to 15% gives you most of the heat output, that’s your answer. There’s a full worked example in sizing control valves.
3. Not enough differential pressure
Coils at the far end of the system can’t reach setpoint even with their valves wide open, while everything near the pump is fine.
The pump might be running too slowly, the DP sensor might be in the wrong place, a strainer could be blocked, or the system may never have been properly balanced. Measure the differential pressure across the struggling coil and valve, and compare it with the design. Check the strainers and isolation valves on that branch. If the pump runs on DP control, find out where its sensor actually is and what setpoint it’s working to.
4. Flow imbalance at the low-loss header
The boilers are making 75 °C, the secondary circuits are only seeing 60 °C, and the BMS reports it can’t reach setpoint. So it opens the valves further, which makes things worse.
A low-loss header separates the primary (boiler) side from the secondary (system) side. If the secondary circuits pull more flow than the primary pumps deliver, cool return water flows back up through the header and mixes into the secondary flow.
Compare the primary flow temperature with the secondary flow temperature at the header, especially at high load. A big drop between them means the secondary side is out-pulling the primary. The fix is mostly hydraulic (pump speeds, how many boilers and pumps are running, balancing), though the BMS sequencing may also need to bring extra primary capacity on sooner.
5. Water flowing backwards through idle plant
A boiler or chiller that isn’t running has warm (or cool) pipework. Header temperatures are lower than they should be. Standby pumps are spinning backwards.
That’s water short-circuiting through equipment that’s switched off, usually because a non-return valve has failed or there’s no isolation on idle plant. It dilutes the output of whatever is running. Check the flow and return pipes on idle units, and find out whether there are isolation valves the BMS is supposed to be closing.
6. Air, dirt and closed valves
One coil or radiator circuit never gets hot. You might hear gurgling, or see a pump showing high speed with very little flow.
It’s often an air lock at a high point, a blocked strainer, or an isolation valve someone left shut after maintenance. These are simple faults and they’re common, which is exactly why they’re so annoying when they turn out to be the cause after an afternoon spent on the controls. Check vents, strainers and valves on the affected branch first.
7. A sensor that isn’t where you think it is
The BMS says the circuit’s at setpoint, but the space or coil clearly isn’t. Or the reading creeps along far too slowly.
Look for immersion sensors in dead legs or on the wrong side of a mixing point, pockets with no thermal paste, or a “flow” sensor that’s actually on a branch. Check the physical location against the schematic and compare the reading with a contact thermometer on nearby pipework. There’s more on this in the guide to BMS sensors.
A quick way to work out where the fault is
When a loop’s misbehaving, go through it in this order:
- Look at the trend. What are the setpoint, the measured value and the output doing?
- Check the valve or damper is actually moving, ideally by watching it.
- Put it in hand at a few positions and see whether the temperature changes.
- Check there’s flow and pressure: pump status and speed, DP, and temperatures either side of the coil.
- Check the sensor against a reference reading.
If the valve moves and the temperature doesn’t follow, it’s almost always hydraulic or mechanical. Write it up with the trend and a couple of photos. That’s what turns “the BMS isn’t working” into a properly scoped remedial job for the right trade, and it makes you look good in front of the client.