How a BMS runs heating plant: weather compensation, optimum start and boiler sequencing

Heating plant is where a BMS really earns its keep. Done well, it keeps a building comfortable while burning a lot less gas. Done badly, the boilers run flat out, short-cycle all day and heat an empty office from 5 a.m. “just in case”.

Who’s in charge of what

Before getting into strategies, be clear about the split. The boiler’s own controls look after ignition, flame supervision, modulation, high-limit and every other safety function, and the BMS never overrides any of that. The BMS decides when heat’s needed, which boilers run, what flow temperature to ask for, which pumps run and which circuits are enabled.

The usual interface for each boiler is an enable signal plus either a flow temperature setpoint or a modulation demand, sent as 0–10 V or over comms. Find out which the boiler expects. Sending a setpoint to a boiler that’s expecting a modulation signal, or the other way round, is a common reason for heating that never quite behaves.

Weather compensation

When it’s mild outside, a building needs less heat. So there’s no point pushing 80 °C water round the radiators in April. Weather compensation sets the flow temperature from the outside air temperature, following a heating curve. A radiator circuit might look something like this:

Outside	Flow setpoint
−3 °C	75 °C
5 °C	62 °C
10 °C	54 °C
15 °C	45 °C
18 °C and up	Circuit off

Those numbers are only an example. Every building is different, and the curve gets set at commissioning and then tweaked. If people are too warm in mild weather, the bottom end of the curve is too high. If they’re cold during a cold snap, the top end is too low.

Compensation is usually done on a mixing circuit, with a 3-port valve blending flow and return water, while the boilers run slightly hotter than the highest circuit needs.

There’s an efficiency angle too. Condensing boilers only condense, and only hit their best efficiency, when the return water is cool enough, generally below about 55 °C on natural gas. A strategy that keeps flow temperatures needlessly high, or a system with lots of bypassing, keeps the returns warm and throws that efficiency away.

Optimum start and stop

Instead of switching the heating on at a fixed time every morning, optimum start works out how long the building needs to come up to temperature, based on inside and outside temperatures. It then starts the plant just in time for people arriving. On a mild morning that might be half an hour before occupancy. After a cold weekend it could be several hours. The better optimisers learn from previous days and get more accurate over time.

Optimum stop does the reverse, switching the plant off a little before the building empties and letting the thermal mass carry it through to the end of the day.

If you find the optimiser always running at its maximum start time, check the inside temperature sensor and the learning settings. And if someone’s replaced it with a fixed 6 a.m. start, that’s an easy saving waiting to happen.

Running more than one boiler

Most commercial plant rooms have two or more boilers, and the BMS decides how many are running. The usual approach is to stage up when the common flow temperature has been below setpoint by a margin for a set time, or when the calculated load is more than the running boilers can supply. You stage down when the flow temperature is comfortably above setpoint or the load drops. Put proper time delays between stages, or you’ll get boilers coming on and off within minutes of each other.

With modern modulating condensing boilers, running more boilers at low fire is often more efficient than running fewer flat out, because they’re at their best at part load. Check what the manufacturer recommends. Some strategies bring on the next boiler once the running ones pass a set modulation level.

Rotate the lead boiler, weekly or on run hours, so one boiler doesn’t do all the work while the others sit idle. If a boiler locks out, the sequence should skip it, bring on the next one and raise an alarm.

And don’t let water keep flowing through boilers that aren’t firing. It dilutes the flow from the ones that are. If there are motorised isolation valves or individual boiler pumps, the BMS should close or stop them for idle boilers, with a short pump overrun after shutdown to get rid of residual heat.

Interlocks and protection

The essentials in most heating strategies:

• Pump proving before boilers are enabled, via a current switch, DP switch or flow switch.
• Pump overrun after the boilers stop.
• Duty and standby pumps that change over automatically on failure.
• Frost protection that runs the pumps when it drops to a few degrees outside, and fires the boilers if water temperatures approach freezing, whatever the time schedule says.
• Boiler lockout alarms on the BMS, so failures get noticed before the occupants notice them.

Where heating energy usually goes to waste

When a site’s heating bill looks too high, these are the first places to look:

• weather compensation switched off, with circuits sat at a fixed high temperature
• optimum start replaced by a fixed early start
• boilers short-cycling because the staging delays are too short
• heating left enabled all summer because nobody set the summer cut-off
• idle boilers left open to flow
• frost protection running far too often because the outside sensor is in the sun (more on that in BMS sensors)

None of these is exotic. They’re basics that got switched off or overridden at some point and never put back.

Heat pumps

More and more sites are adding or switching to heat pumps. The same principles apply, but flow temperature matters even more, because every extra degree costs heat pump efficiency. There’s more in VRF and heat pumps on the BMS. If the boilers also heat the hot water, read hot water and Legionella before changing any summer shutdown settings.