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Calculating Cooling Capacity Cheat Sheet (DRAFT) by [deleted]

This is a draft cheat sheet. It is a work in progress and is not finished yet.


This is where the rubber meets the road. Capacity planning is part mathem­atical calcul­ations: adding up all power which will be consumed then calcul­ating what kind of cooling will be needed to remove the heat waste, and is also part art: guessing what the facility will require in the future as technology and requir­ements change. Since the “art” part of planning can get a little wonky, let’s start with the math.

The best place to start when calcul­ating required cooling is power draw. With very few exceptions (like POE) all power consumed by IT equipment is turned into heat. Since the electrical signals coming into and leaving the datacenter are neglig­ible, and IT equipment doesn’t output any kind of mechanical work, it is safe to assume that all power is being turned into waste heat. This means if you know how much power each device will consume, then you know how much heat it will produce. Face it, your datacenter is nothing but a big, expensive heater.


Figuring out your total wattage (power) draw can typically be done by looking at the capacity of your UPS system. If your UPS can only output 10,000 Watts/VA (there is a differ­ence, but suffice it to say: it is negligible for IT equipm­ent), then you can start at 10,000 watts (10kW) for your cooling calcul­ation. We will use this as a starting point in the example calcul­ation

UPS Max Output = 10kW

Expected UPS utiliz­ation: 50%, or 5kW of draw by equipment

After determ­ining we only expect to draw 5kW of power, we can add in some additional metrics:

(1) UPS Ineffi­cie­ncies: (.04 × UPS Max Watt Rating) + (.05 × expected wattage draw)
(2) Power Distri­bution: (.01 × UPS Max Watt Rating) + (.02 × expected wattage draw)
(3) Lighting: (22 Watts × Floor Area in m²) or (2 Watts × Floor Area in ft²)
(4) People: (100 Watts × max number of people)

Saving you the long form math, consid­ering 100ft² of space and no people, we end up with 6050 Watts of heat produc­tion.

Cooling Power Requir­ements

The “Art”

Now we get into the “art” and the more elusive metrics. It is obvious that much of the heat gained or lost by a facility is due to the enviro­nment around it. Depending on the insulation in the external walls and the climate, this factor can be orders of magnitude higher or lower. Since this factor is, by itself, very complex and tricky, we will leave it out of the discussion for now. It is also important to note that if your datacenter is an air condit­ioned room inside of an already climate controlled building (like a data room in an office building), then you can, for the most part, ignore this metric.

The other evasive metric is utiliz­ation change or “room for growth”. We scoped a 10kW UPS but only expect to use 50% of that max capacity. If we scope a cooling system which perfectly matches the expected 6050 watts of heat produc­tion, then we have no room for growth in regards to cooling. We can scope a cooling system which matches the UPS, but that can get expensive. This metric varies widely depending on the particular instal­lation. My advice is to calculate the cooling needed for initial expected consum­ption, then calculate the cooling for the max capacity of the power systems, and pick a size in the middle based on your best guess of required headroom.

Converting Units

The cooling cycleNow we have determined we need to account for 8000 Watts of heat production (6050 + some growing room), we can convert this to a number which is meaningful to a HVAC engineer. Using the conversion tables above, we can determine we need 26296 BTUs or 2.272 tons of refrig­eration capacity. Since refrig­eration units come in integer sizes of tons, or factors of 12,000BTU, we have to round up to a 3 ton or 36000 BTU unit. And there you have it.