Does higher temperature setting actually save money?

[crvia]

I have a generic technical question about commercial DDC HVAC and energy savings: Can raising the summer temperature settings on a DDC system actually INCREASE operating costs? In other words, if the discharge air temp is constant at 55 degrees, do you actually need to add more reheat at the VAV boxes and thus burn more energy to keep a higher office temp? Or does the system simply run less for shorter cycle times, and it really does save money to raise the thermostat? (I am a facilities manager at a large company trying to save money by raising thermostats a few degrees. Unfortunately, I'm a civil and not mechanical engineer, so I can't say intelligently whether we'll save money or not. We have many different building types and HVAC systems, so my question is not specific to any particular setup - just asking in broad terms).

[crvia]

It's been a couple of days with no replies -- maybe I'm in the wrong forum? If anyone knows a better place for me to find an answer to my question, please redirect me! Thanks

[jbcrane]

If you have constant discharge air at 55° then yes it will increase your energy cost to raise the thermostat if you are conditioning the air with reheat. It really boils down to what type of system you have. I would think you would do better with some way to stop the constant 55° air than try and reheat it. If you're running a steady supply air of 55° I can see q large amount of money wasted to cool areas that don't need that much constant cooling.

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[Nuclrchiller]

jbcrane, could you please explain what is meant by "55&#176"? I presume "55" refers to 55 degree supply air temperature, but I'm kinda lost on the rest of it. I'm thinking "#176" refers to something about moisture content of that air (pounds mass?), but would like to be better informed. Thanks.

[ascj]

&#176 is the html ascii code for °. Don't think html works in the threads.

[Nuclrchiller]

Thanks Ascj. I've seen that a couple of times since yesterday. So, "html ascii code" is some kinda computer speak? Ignorance is not only bliss, it is sometimes confusing.

[air1]

In broad terms, yes, you will save energy by raising set points in the cooling mode.
If you have Variable Air Volume (VAV) terminal units (TUs) you shouldn't need to reheat during the summer if the VAVs are sized and set up properly. VAV terminal units vary the airflow in cooling to maintain space temperature. If the room gets too cold the air flow will reduce down to a minimum airflow CFM. If the temperature continues to drop the TU will switch to heat and the reheat will come on. But if the TUs are not oversized you shouldn't switch over to heat during the cooling season. If it's warm enough outside the heating water system should be disabled anyways and you wouldn't be using reheat.
If you have constant volume then yes, the air will need to be reheated if the supply air is too cold.
There are many ways to save energy in a DDC controlled system. Raising set points in the spaces, reseting AHU discharge set points based on outside air temperatures, reseting AHU fan speeds based on demand,...etc. I would recommend you hire a consultant to audit your system and help you identify ways to save energy. With DDC controls you have a lot of control over your system and you could potentially save a lot of energy.

[crvia]

Thanks air1 and jbcrane - this is helpful feedback!

[vangoghsear]

If you do not have energy recovery units and use a large amount of outdoor air due to occupancy and it is not used for makeup for processes or other forms of exhaust and is merely relieved, then CO2 sensors can be placed through the building and used with the DDC controls to reduce incoming air to the minimum needed. Reducing incoming air can reduce load at the unit level and is a major way to reduce energy use that is just wasted going once through the unit and out.

Adding CO2 sensors is a relatively cheap way to save a lot of energy.

[simux]

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[jpsmith1cm]

simux

This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

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