Examples of 'energy efficient' control strategies that waste energy - optimal start warm up

Many people believe that they have energy efficient control strategies, but what they don't realise, is that these control strategies were originally developed for comfort control and don't really save energy. 

Lifecycle Controls Bryce Anderson independent BMS consultant

Here is an example of an 'energy efficient' control strategy that only after you have adapted it does it become an optimised control strategy resulting in real energy savings. This blog discusses warm up but also applies to cool down.    

Optimal Start - Warm Up: 

The purpose of optimal start is to start the AHU as late as possible, reduce fan power, save energy and achieve target temperature at occupancy time. However, most optimal start modules are programmed to achieve comfort conditions at occupancy time, regardless of energy consumption.  

  • The target warm up temperature set point should be 21°C, and the target cooling temperature set point should be 24°C. Many buildings have a target temperature set point of 22.5°C. This means that the AHU's are starting earlier and running fans for longer to warm up the floor 1.5°C more than they need to.

  • Half the strategies that I review still have the AHU's starting up in normal mode when warm up is activated and controlling to a set point reset from the floor demand. The AHU's would start up later if you forced the supply air temperature set point to 30°C, rather than starting up in normal mode.

  • Occasionally I still see warm up being activated with the the VAV's in normal mode. VAV's are traditionally cooling devices, if the floor is cold then the VAV damper controls to its minimum volume position. The AHU's would start up later if you reverse control the VAV damper during warm up so that it delivers full design volume with the AHU's delivering 30°C.

  • Surprisingly, not all optimal start programs are self-adapting. This means that a BMS technician needs to review if the floor reached target temperature at occupancy, and then tune the 'building factor' point if needed. Reaching target temperature too soon is wasted energy on AHU fans running longer than they need to. Reaching target temperature too late is technically a breach of lease conditions. Believe me, almost no one checks this; buildings all over the city could be achieving target temperature half an hour early and no one would know.

  • My favourite, difficult to fully explain, but in nutshell, in Australia, we start the VAV's and AHU's first, then once the AHU heating valve is open more than, say 50%, we initiate the boilers. After all the delays between starting the pumps, the lead boiler starting, staging up and heating the whole buildings volume of water, we finally get heat through the AHU coils. Lets assume this is a 30 minute delay. That is 30 minutes of AHU fans running for nothing, all AHU's, every morning.

Optimal start was originally developed to achieve comfort conditions at occupancy and not to save energy, and to this day we are still wasting fan power on optimal start.  

Provide maximum heating to the floor for the shortest time possible, if you started 18 AHU's across the low, medium and high rise plant rooms, 15 minutes later everyday you would notice the saving the next day.          

Changing the priority of your control strategy from comfort to energy efficiency will then give you an optimised control strategy.