A client of ours is currently going through not only an unseasonably cold snap but some of the coldest weather in the history of the North West, USA. His server room is well designed with cooling for the summer but who would have thought that cooling would fail in the fall and winter months. Yet it is, and with AC down his server room only takes 15 minutes to rise in to the 90s F. So he is left with the choice of securing his server room or cooling his server room by opening the door. It is a no win situation.
So why does this happen? Most A/C units don't work below 20 F (-7 C) and so they kick out. The solution is to change the cooling method from summer to winter. Now it depends on the size of your server room and the type of servers you are running but the following is a couple of rules of thumb that apply to any server room design where the server count is less than ten.
Three methods for Winter Cooling:
Using fans and door vents (Outside Temperature 15 C to -40 C or 60 F to -40 F and lower).
In general you can use the waste heat from your server room to supplement heating your office. This is a very efficient and often involves minimal expense. The method involves using bathroom fan or similar forced are fan to blow air into one of three places. Talk to your HVAC specialist to help you size the size of fan required. In general bathroom fans are designed to turn over air 8 times per hour and you would want about double that to keep the heat in check. Also remember that this is for continuous operation so make sure the solution you use is capable of that type of usage. Furthermore, consider using larger diameter fans for quieter operation. To ensure proper air flow ensure a door or wall vent is installed near the floor and that the fan is installed directly above the server exhaust fans. For hotter rooms consider channeling server exhaust heat. Thermostats may be used to control fan speeds and you should size your fans for warmer outside temperatures since the air flow required to cool will be higher when the outside temperature is higher. As the outside temperature decreases the maximum cooling effect will occur directly offsetting your heating costs.
- General Office Space (Do not place exhaust heat near thermostats)
- Colder rooms or warehouses (Do not place exhaust heat near thermostats)
Cold Air Return for your heating system.
If your servers are producing too much heat you may have to use AC and lower the fans CFM in order to not over heat the other areas of your office. This may mean that the outside temperature at which you use the method must be lower.
Using Outside Air, Fans and Thermostats (Outside Temperature 10 C to -40 C or 50 F to -40 F and lower).
Outside Air has a couple of problems. The first is that the humidity is variable and the second is that the temperature is variable. This requires thermostats to control the intake of outside air. Air that is 100% Humid at 10 C(50 F) is 50% humid at 20 C(68 F). So if it is raining outside at 10 C your air is just barely dry enough to cool your server room by the time it warms up to 68 C by mixing with your hot server air. People often combine this method with AC working in dehumidifying mode to keep moisture in check. As the outside temperature drops the air gets drier until it reaches zero humidity at freezing. Many people only use this method when the temperature outside reaches freezing to alleviate any concerns about humidity. The advantage of this method is that it has unlimited cooling potential so if you have a larger server room with more servers you can keep it cool by increasing the air flow.
When using this method it is important to ensure that cool dry air enters near the server air intakes and that warm air is vented from the server room either outside, into the office or into the cold air return of the building heating system. An HVAC specialist will help with ensuring proper air flow.
Using Server Room Air to warm your AC unit when it drops below -7 C or 20 F
This method is not commonly used since it is often expensive and has limited use. In some circumstances, people have been known to create venting systems which keep the air around the AC unit above 7 C or 20 F. I will not dedicate much time to this and you should only consider it if your HVAC specialist has specific reasons why it is required in your environment. Be sure to cover the other methods first. This method is usually only used when environmental factors such as dust or salt have a negative effect on the server environment.
I hope these methods help you achieve balance and give you some starting points in considering cooling for small server room design. Should you have more questions be sure to leave a comment or contact me directly.
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