Putting VRF and heat pumps on the BMS

VRF and heat pumps are on a huge share of UK commercial jobs now, from small office fit-outs to big refurbishments replacing gas boilers. Both come with clever controls of their own, and getting them to play nicely with a BMS causes no end of confusion. Here’s how integration usually works, and where to draw the line between the two systems.

A quick word on VRF

A variable refrigerant flow system has outdoor units connected by refrigerant pipework to lots of indoor units: cassettes, ducted units, wall-mounted units. There’s no water anywhere in the distribution.

Heat pump VRF systems can heat or cool, but every indoor unit on the system has to be in the same mode at once. Heat recovery VRF can heat some zones while cooling others, moving heat between them.

All the refrigerant control is handled by the manufacturer’s own controls: compressor speed, expansion valves, defrost and so on. That isn’t something a BMS should ever try to recreate.

Getting VRF onto the BMS

VRF systems talk to their indoor units and controllers over the manufacturer’s own bus. To get that data onto a BMS, you add a gateway that translates it into BACnet or Modbus.

For each indoor unit, you’d typically get:

read and write: on/off, mode, setpoint and fan speed
read only: room or return air temperature
alarms: a filter alarm (which you can usually reset) and a fault status with an error code

On top of that there are system-wide points like outdoor unit status, and an emergency stop input for the fire alarm.

A few things are worth knowing before you quote:

Gateways are often licensed per indoor unit. Get the count right at the start.
Polling is slow on big systems. With hundreds of indoor units, each point can take a while to refresh, so don’t build fast control loops on gateway data.
Check the point list for the exact VRF model before telling the client what they’ll see on the BMS.
Translate the error codes into proper alarm text. Otherwise the front end fills up with numbers nobody understands.

What the BMS should actually do

The split that tends to work best is for the BMS to handle time schedules, holiday modes, setpoint limits (19–24 °C, say), fire alarm shutdown, global off, monitoring, alarms and energy reporting. The VRF controls keep everything to do with the refrigerant and the minute-by-minute zone temperature control.

“Last command wins”

Most VRF indoor units take commands from both the local wall controller and the BMS, and whichever spoke last wins. So the BMS sets 21 °C at 8 a.m., someone turns the wall controller down to 17 °C at 9, and the unit sits at 17 until the BMS writes again.

You can lock functions out on the local controllers, have the BMS re-apply its limits periodically, or agree with the client which controller is in charge. Just make it a deliberate decision rather than finding out on site.

Air-to-water heat pumps

Heat pumps that make hot water for radiators, underfloor heating or fan coils are replacing boilers on lots of sites. They connect to a BMS in much the same way as a boiler or chiller: an enable, a flow temperature setpoint and a gateway for status and alarms. The priorities are quite different from boilers, though.

Flow temperature is everything

A heat pump’s efficiency, its COP, drops as the flow temperature goes up. Every unnecessary degree costs money. So weather compensation isn’t a nice-to-have; it’s essential (see how a BMS runs heating plant for how compensation curves work). Run the lowest flow temperature that still keeps the building warm. Many heat pump systems are designed to run far cooler than an old 80/60 °C boiler system.

The other things to plan for

Defrost. In cold, damp weather an air source heat pump periodically defrosts its outdoor coil, briefly pulling heat back from the system. The buffer tank and the BMS strategy need to allow for it rather than treating it as a fault.
Minimum flow. Heat pumps need a decent flow through them, which affects how the pumps, valves and buffer tanks are controlled.
Cascades. Several heat pumps get sequenced and rotated much like boilers.
Hybrid systems. Where heat pumps work alongside gas boilers, the BMS usually brings the boilers in below a set outside temperature, or when the heat pumps can’t hold the flow temperature. Getting that changeover point right makes a real difference to running costs and carbon.

When heating and cooling fight

This is the classic problem on mixed systems. A VRF unit is cooling a room while the perimeter radiators are heating it, each chasing a slightly different setpoint. Both systems think they’re doing a fine job. The energy bill tells a different story.

You can stop it by:

interlocking by zone, so VRF cooling is blocked below a certain outside temperature or while that zone’s heating is on
keeping a clear dead band between the heating and cooling setpoints for the same space
lining up the schedules so both systems follow the same occupancy times
trending both systems together, which makes the fighting obvious straight away

Refrigerant leak detection

Systems with a large refrigerant charge in occupied spaces often have leak detection fitted. Get it onto the BMS as a high-priority alarm, so a leak isn’t only noticed when the cooling stops working.