Energy Efficiency Reduced Cooling Load: By raising the supply air temperature when the cooling demand is lower, the system requires less energy to cool the air, leading to significant energy savings at the chiller or cooling coil level. Fan Energy Savings: When combined with reducing air volume, the strategy lowers the required fan speed, further decreasing energy consumption due to the cubic relationship between fan speed and power usage. There are many other reasons, but energy savings is a key factor.
Does temperature reset consider humidity levels? I can see how it could have a negative impact if the system was sensing and controlling to temperature level only.
Humidity control can be effectively maintained even with chilled water supply temperature (CHWST) reset. The impact of CHWST reset on the dehumidification capability of a cooling coil is quite small as long as the coil is able to maintain the supply air temperature at setpoint. Space humidity is a function of the supply air humidity ratio which in turn is a function of the coil leaving dry-bulb temperature setpoint. Regardless of CHWST, the air leaving a wet cooling coil is nearly saturated; lowering CHWST only slightly reduces supply air humidity ratio. So as long as the supply air temperature can be maintained at the desired setpoint, resetting CHWST will not significantly impact space humidity.
I'd have to do some calculations, but if the general rule is that rooms must he maintained at 21 or 22 °C at 30% RH, which I don't know what it equals in dewpoint or GPP to quantify its absolute moisture contents, cooling outside air that is at 30+ °C with maybe over 40% RH of moisture, down to 13 °C, is very likely going to cause water to condensate and then ensure that the ambient air is conditioned comfortably. [Edit:] The US norm is to maintain an ambient dew point between 4 and 16,5 °C, so roughly put, if your cooling coil cools down the air temperature at 13 °C and is condensates, you could say the air's dew point is 13 °C, which is well inside the prescriber bracket. And if it's too dry, know that most air handler units (AHUs) come with a humidifier.
Funny enough, i know an engineering firme that doesn't use optimal starts, but instead uses night setbacks that progressively creep back to occupied setpoints (based on a schedule). I had the chance to discuss with him once over a teams meeting and he told me that he did this more so to minimise morning peak demands, so that when the occupancy schedule turns on, room controls don't kick in at full demand and cause a spike on the electrical distribution.
Each building is slightly different, so hopefully they have figured out what works best for their building. Hopefully they're using some form of adaptive learning where the system adjusts based on previous performance and external conditions, which makes optimal start a good strategy to save energy. Optimal start strategies avoid having the systems come on at full demand when occupants arrive. There is no benefit to conditioning the space earlier than needed as this can add to an increase in energy consumption. Thanks for the comment.
I also had an engineer recently make me aware of an ASHRAE guideline on duct heating for room controls. Apparently they prescribef that you should heat the air no more than 11 °C above the room temperature, otherwise you risk causing air stratification, resulting in not heating the room efficiently and wasting energy.
excellent explanation
Glad you liked it! Thanks for watching.
I'm new to HVAC DDC controls and your videos are extremely helpful. Thank you.
Glad you like them! Thanks for watching.
Great video, explanations and graphics are excellent.
Thank you
how come the supply air temperature varying for VAV? I thought for VAV temperature is constant but volume modulation occurs.
Energy Efficiency
Reduced Cooling Load: By raising the supply air temperature when the cooling demand is lower, the system requires less energy to cool the air, leading to significant energy savings at the chiller or cooling coil level.
Fan Energy Savings: When combined with reducing air volume, the strategy lowers the required fan speed, further decreasing energy consumption due to the cubic relationship between fan speed and power usage. There are many other reasons, but energy savings is a key factor.
Good explanation
Thanks and welcome
Does temperature reset consider humidity levels? I can see how it could have a negative impact if the system was sensing and controlling to temperature level only.
Humidity control can be effectively maintained even with chilled water supply temperature (CHWST) reset. The impact of CHWST reset on the dehumidification capability of a cooling coil is quite small as long as the coil is able to maintain the supply air temperature at setpoint. Space humidity is a function of the supply air humidity ratio which in turn is a function of the coil leaving dry-bulb temperature setpoint. Regardless of CHWST, the air leaving a wet cooling coil is nearly saturated; lowering CHWST only slightly reduces supply air humidity ratio. So as long as the supply air temperature can be maintained at the desired setpoint, resetting CHWST will not significantly impact space humidity.
I'd have to do some calculations, but if the general rule is that rooms must he maintained at 21 or 22 °C at 30% RH, which I don't know what it equals in dewpoint or GPP to quantify its absolute moisture contents, cooling outside air that is at 30+ °C with maybe over 40% RH of moisture, down to 13 °C, is very likely going to cause water to condensate and then ensure that the ambient air is conditioned comfortably.
[Edit:] The US norm is to maintain an ambient dew point between 4 and 16,5 °C, so roughly put, if your cooling coil cools down the air temperature at 13 °C and is condensates, you could say the air's dew point is 13 °C, which is well inside the prescriber bracket. And if it's too dry, know that most air handler units (AHUs) come with a humidifier.
Funny enough, i know an engineering firme that doesn't use optimal starts, but instead uses night setbacks that progressively creep back to occupied setpoints (based on a schedule). I had the chance to discuss with him once over a teams meeting and he told me that he did this more so to minimise morning peak demands, so that when the occupancy schedule turns on, room controls don't kick in at full demand and cause a spike on the electrical distribution.
Each building is slightly different, so hopefully they have figured out what works best for their building. Hopefully they're using some form of adaptive learning where the system adjusts based on previous performance and external conditions, which makes optimal start a good strategy to save energy. Optimal start strategies avoid having the systems come on at full demand when occupants arrive. There is no benefit to conditioning the space earlier than needed as this can add to an increase in energy consumption. Thanks for the comment.
I also had an engineer recently make me aware of an ASHRAE guideline on duct heating for room controls. Apparently they prescribef that you should heat the air no more than 11 °C above the room temperature, otherwise you risk causing air stratification, resulting in not heating the room efficiently and wasting energy.
There is much wisdom in the ASHRAE Manuals and Standards, as they have invested a lot in providing the industry with the most up to date information.
Thank you 🙏
Your Welcome
🙏
Thanks for watching.