Precision air conditioners vs. split Systems

For data Centers (DATACENTERS) containing more than 5 racks, there are a variety of specialized solutions to provide the necessary microclimate. For Small server rooms (up to 5 racks,<20 кВт рассеивания тепла) использовать подобные решения зачастую слишком дорого. квт="" рассеивания="" тепла)="" использовать="" подобные="" решения="" зачастую="" слишком=""></20 кВт рассеивания тепла) использовать подобные решения зачастую слишком дорого.> IT directors of small and medium business have to independently complete systems of cooling and maintenance of humidity of small server, balancing between price and quality of system. Let's try to compare different solutions for air conditioning and evaluate, based on practical experience, how true the individual claims of specialists and marketers of companies specializing in the delivery and installation of precision systems Conditioning.

Since I am not a specialist in the field of air conditioning, the text is sure to have inconsistencies on the technique. I Very much hope for the help of experts in correction of this material.  Comments, please send to e-mail:

It is Not necessary to send letters with phrases about incorrect comparison, like it is impossible to compare an apple with oranges. My opinion, it is possible to compare because Precision and slabs are purchased for one purpose – maintaining the preset temperature, and ideally the microclimate, in the server room. Undoubtedly, both solutions are not ideal, but they solve the task well, whether it is bad. Therefore It is possible to compare them and it is only speech of marketers about incorrect comparison.

So, what parameters of microclimate should be sustained in server (hardware) premises? In the old SN 512-78 "Technical requirements to buildings and premises for installation of computer equipment" The following parameters of the microclimate of the server are given:

  • The air Temperature in the rooms is 20 ° ± 2 ° с (not more than 25 ° С). The lower bound is Preferable.
  • Relative Humidity-20-70% (not more than 75% in the cold period, in warm for 25 ° C-no more than 65%, for 24 ° C and below-no more than 70%).
  • The Optimal air flow rate is 0.2 m/s (not more than 0.3 m/s for cold, 0.5 m/s for warm periods).
  • The Dusty air of the premises should not exceed: in the server-0.75 mg/m3, with a particle size of no more than 3 microns (ATM. Dust, soot, smoke, spores, asbestos); In the data processing premises-2 mg/m3.
Period of the Year
Air temperature, °c
Relative air humidity,%
Speed of air movement, m/s
Cold and transitional periods of The year (air temperature below + 10°c)
21 ± 2
52 ± 7
Not more than 0.2
Warm period Of The year (external air temperature above + 10°c)
22 ± 2
52 ± 7
Not more than 0.3

Server Room

All the above statements are based on the experience of providing a microclimate in the server area ~ 11 m2. Dimensions of the room 3950 x 2873 x 3500 mm ~ 40 m3. In it the equipment which consumes at calculation on power units of power ~ 14 Kw is placed. Given that the power supplies are designed with ~ 33% Reserve we get that more or less real power consumption is ~ 10 Kw.  You can read More about the energy consumption calculation in the article "Calculating the total electricity demand in data centers" prepared by APC specialists. The Equipment is placed in 3-19 "racks.

At the time of writing the article cooled the premise of two household wall-mounted split air conditioners (though it is correct to call such devices air coolers, because Humidity they maintain not able) McQuay and Pioneer. Brands are not the top in the world of manufacturers of air conditioners, with productivity on a cold on 7 Kw each.  Air conditioners are modified by "winter set" for operation at temperatures of order – 30 oC. Each of the air conditioners has an operating time of more than 3 years without capital repairs, only scheduled maintenance. Usually, for household splits it is close to the limit period of work after which you can expect the air conditioner out of order.

Let's take a Walk on the main advantages of precision air conditioners in comparison with split systems.


The Most important requirement to the system of conditioning of server premises-high reliability. The Precision air conditioner, as the manufacturers claim, has a resource at least 5-6 times greater than that of a domestic air conditioner. However, as if denying the reliability factor, the majority of manufacturers of precision air conditioners give only 1 year warranty on their products. For comparison, the domestic split system of famous brands is given a guarantee of 2-3 years.  Manufacturers of precision air conditioners would make sense to increase the warranty period at least to 3 years, so that this parameter does not lose the split systems. Moreover, if the air conditioners are designed for 24-hour work for 10 years-it means that 3 years they should be guaranteed work without repairs, ie For the manufacturer, the extension of the warranty period has almost nothing to cost. As the manufacturers of precision Technics, as a rule, do not give more than 1 year of the guarantee-the statement about 10-year uninterrupted work can be considered as guile on their part. For Example, I have in the air conditioner APC InRow SC after about 2 years of operation the drainage pump, i.e. The reliability of "iron" is indicative. Only for the departure of the engineer APC rolled out such an account that the desire to use their service was missing. The Cost of a specific drainage pump for such systems is also significantly higher than for widespread splits.

It is Said that precision air conditioners are designed to work 24 hours a day, 7 days a week, 365 days a year, in the amount of 8760 hours per year for 10 years. The Household air conditioner is calculated to work about 1200 hours per year. Therefore, the household air conditioner, in theory, should develop its resource (usually eight years) within one year.

In practice, this assertion is not entirely correct. Domestic split air conditioners of well-known brands are usually practiced in the hardware premises ~ 3 years, after which they have the sense to plan to replace to prevent critical situations. Specially designed for hardware semi-industrial air conditioners of leading brands, such as Daikin series FT 25/35/50/60 or Daikin FAQ (not inverter) adapted by the manufacturer to operate at minus temperatures up to-30 oC, Mitsubishi Heavy, etc. may well work for more than 5 years.

Accuracy of Temperature Maintenance

It is Said that the household air conditioner can maintain the temperature in the room with a precision of no more than 5 degrees, while the precision air conditioner-0.2-1 degree. Let's look at the situation in practice in case of our test server, cooled household split. To do this, we will analyze how the temperature changes from the air intake of the server rack. Measurements were made by the temperature and humidity sensor running in the set with the APC AP9619 UPS Control Board placed on the front of the rack at a height of ~ 1.3 m. The temperature measurement Graph is presented in Fig. It Can be noted that the temperature is almost constant near the value of 20 oC. The corridor of the most frequently appearing values ranges from 19.5 to 21 oC or the temperature variation is only 1.5 oC. Rare short emissions extending the corridor to values 19 – 22 oC or a range of 3 oC can be neglected and say that the household split holds a temperature range of 1.5 in the oC, rather than in 5 oC, as claimed on the sites of suppliers of precision air conditioners. Measurements were made on different days and always the result was unchanged-the spread of about 1.5 oC. Besides, modern Technics (first of all, hard disks, tapes in streamers, accumulators in the UPS) much more loyally refers to temperature changes (not sharp), than old equipment, therefore, I suppose, the single change in 5 oC will be transferred without Consequences.

We got a 1.5 oC spread of temperatures at that. That the temperature sensor in the splits is placed in the body of the air conditioner, and the air conditioner is at the height of the ceiling, which, of course, leads to a deterioration of the air conditioner in temperature. When using specialized remote sensors running in the set with the coordination of air conditioners-IBS, recommended Daikin for the completion of their air conditioners series FT/FAQ, the results should be better. The Wired sensor for the RC can be placed directly at the air intake rack and the air conditioner will react to the temperature change more quickly.

Fan Power

APC InRow SC-row air conditionerIn the room where the household air conditioner is always formed stagnant zones with warm air, and cold air distributed on room unevenly. The Reason of such phenomenon is insignificant performance of the fan of an internal block. For Household premises It is almost imperceptible, while in server it is necessary more even cooling (again for modern equipment-it is not so critical). For the air to be distributed evenly, the precision air conditioner is equipped with a powerful (and) fan (and) – about three times more powerful than that of a domestic air conditioner. For Example, for air conditioner APC InRow SC (ACSC101) in the passport declared air flow of 1200 cubic meters. Feet per minute or 36 m3 per minute (or 2160 m3 per hour). A Similar parameter for the Daikin FT50/R50 air conditioner equivalent in refrigeration capacity is 972 m3/hour (16.2 m3/min), i.e. 2.2 Times less.

For Small server, more specifically for server with heat dissipation of about 2 Kw per m2 of space, the presence of a powerful fan in the conditioner (air cooler) is not so critical, because Having two (three) split air conditioners (100% redundancy is a prerequisite for use of any air conditioners. Because Precision also successfully break down) you can qualitatively disperse cold air on the room.


If the temperature is clear – the hard disks of the servers and storage systems (usually the optimal temperature of HDD-25 ± 5 oC), the batteries of the UPS, and other thin electronics like coolness, then the humidity is not quite obvious. Why should the server maintain humidity close to 50%? The Answer to this question is simple, although in practice for modern equipment with high-quality grounding is not unambiguous.

If the room is too dry (low humidity), then begins to accumulate static electricity, capable of careless handling to disable sensitive electronics. High-Quality grounding of racks and cases of devices reduces such risk, but at extraction of electronic components from equipment danger of damage of electronics by static electricity again increases. In Addition, the lack of moisture contributes to the destruction of lacquer on electronic PCBs, drying insulation of power and switching wiring, which can eventually lead to the failure of electronic components.

If the server humidity is much higher than 50%, then there is a risk of condensation, which contributes to corrosion of conductors and oxidation of contacts, and can cause a short circuit.

In Moscow in the summer in the premises often humidity reaches 80%, and in the winter-15%. For thin equipment, these are bad parameters. The Household air conditioner cannot maintain the necessary humidity. At The Work it drains air (in an internal block the moisture is condensed, taken from a premise and is removed in drainage).

It is Alleged that "Precision air conditioners accurately support the desired humidity by using the built-in humidifier."

The Assertion is interesting, but it concerns only air conditioners working on chilled (chiller) water, or having a separate humidifier with connection to the flow water supply and sewage. For other air conditioners, for example, APC InRow RD working on conventional freon, presence of humidifier-additional (separate and external) option.

How to humidify the air in the server? This is a difficult task. Some points are taken up in the article "Humidification Strategies for Data Centers and Network Rooms".  It is Not enough to put an air humidifier in the server room. It is necessary to minimize the penetration of external air with another level of humidity in the server room. Actually from the server it is necessary to make a hermetic premise that at the minimum expenses to ensure maintenance of humidity at the necessary level.  Ensuring the tightness of the server room is necessary and to ensure efficient operation of gas fire extinguishing systems.

There are a lot of different humidification systems, but not all of them are safe for the equipment installed in the server rooms. The Ideal and most expensive way to support the humidity of air-the use of specialized precision air conditioners, working on the water and in an automatic mode supporting the necessary humidity with minimum deviations. This is the most expensive and surest way to maintain optimal humidity. This option is not very convenient because it requires take stock in the server water, which is not particularly fond of IT-shniki when organizing server. The Use of precision air conditioners on water is probably the best option for cooling the server in terms of energy efficiency, but the organization of such air conditioning will be the most expensive, because Not only that it is necessary to find a place under the overall precision air conditioner and chiller, but it is necessary to provide 100% protection against water leaks.

In small servers It is enough to use simple household and semi-industrial humidifiers. Sometimes for moistening use a usual bucket with water, but the efficiency of this method is extremely low, although "on fish-free and cancer". The Most economical air humidifiers are traditional, for example:

  1. Venta (,
  2. Boneco (,
  3. Carel (
  4. etc.

The Work of traditional air humidifiers is based on the effect of natural cold evaporation. Due to such self-regulating process high efficiency of moistening is achieved. The relative humidity of the air can only reach a certain value for a given temperature (50-60%). At high temperatures the evaporation intensity increases.

The Performance of such humidifiers is determined by the humidity of the indoor air-the higher the humidity, the lower the evaporation rate, but it is rather an advantage than the disadvantage, because the air humidity is "automatically" maintained at the optimum Level.

Together with Moistening The air is purified-the particles weighted in it fall on the surface of walls and floor, as after the wet cleaning.

Air Humidifiers based on other operating principles, such as ultrasonic or steam humidifiers, can significantly humidify the air, bringing humidity to 100%. Therefore such humidifiers should be completed with hygrometers for timely disconnection of the humidifier in order not to increase the air humidity to excessive quantities. If The hygrometer fails, the humidity of the air may exceed the permissible values. In addition, such humidifiers can depend on the quality of water and require for their work pre-prepared water. Otherwise, white sediment may be formed on the surfaces.

Thus, the most energy efficient, reliable and inexpensive way to maintain optimal humidity in the server room-the use of traditional humidifiers. Unfortunately, It is not easy to calculate the optimal performance moisturizer. In the case of traditional humidifiers it is easier to buy with a significant stock, because In this case, the "stock pocket is not pulling". But it is possible and to scale humidifiers of air installing several simultaneously working humidifiers as the measurements of humidity are carrying out.

Undoubtedly, the use of household humidifiers adds a lot of hassle in maintenance, because Water humidifiers should be filled daily. Venta Industrial humidifiers Allow automatic opening of the valve to replenish the tank. But craftsmen can easily remake domestic humidifiers to work from a large capacity with water, or at least from the standard 19 liter bottles, which will last for 2-3 days of operation of the humidifier (depending on the season and the degree of sealing the server from the environment).

Air Dustiness

One of the dangerous enemies of electronic equipment is dust. Dust trapped in devices reading from tape or disc can cause mechanical damage to the media and streamer heads. It accumulates quickly on charged parts of the equipment. At high dustiness The cooling ability decreases, thus parts of the equipment having a layer of dust work at temperatures exceeding the estimated values. Dust settles inside the equipment in places with electrostatic charge and forms conductive bridges, etc. This leads to a reduction in the life of the equipment and premature failure. Therefore, in the server from the air conditioner also requires maximum cleaning of air from dust. The Household air conditioner has a weak fan, which means that it passes through a little air, its filter does not work effectively. The Precision air conditioner, treating 3 times more air, cleans the air several times better than the household air conditioner. In this case, the precision air conditioner can be equipped with very effective filters.

The Assertion is true, although in the case of split systems things are also not so bad. Indeed, the productivity of the air exchange split is about 2-3 times lower than that of the precision air conditioner, but it is necessary to remember that with the correct organization of the server room in it will be 2-3 air humidifier like AOS E2251 with Capacity 250 m³/h x 3 = 750 m ³/h. By Adding here the performance of at least one Daikin FT50/R50 – 972 m3/hour we get the value of 1722 m3/hour, which is already quite close to the benchmark 2160 m3/hour. Considering that for the purposes of redundancy the installation of minimum two conditioners is necessary, we get that the productivity of air exchange will make 2694 m3/hour that more productivity of one (not reserved) air conditioner APC.

Cooling Efficiency

As It is known, in domestic air conditioners from an internal block the moisture is allocated, in it there is a process of condensation as heat exchangers at passage through it freon become very cold (temperature below dew point). The condensation of moisture in the domestic air conditioner leaves 30-40% of useful work. In Precision air conditioners Almost does not stand out, as the temperature of heat exchangers is higher (that is achieved by special measures). As a result, the losses on condensation are very small, 5-15%. Therefore, the efficiency of appreciable cooling in precision air conditioners on 1/3 is higher. That Is why the cooling capacity of the household air conditioners for the server should be about a third higher than the precision air conditioners.

Compare the power consumption of the air conditioner APC InRow RD (ACRD101) and the two condters Daikin FT60.

Daikin FT60 x 2
Cooling Capacity, Kw
6.6 x 2 = 13.2 in
Power Consumption (cooling), Kw
2.39 x 2 = 4.78 in

In Total we get that at the received stock on cold in 36, 2% at two Daikin FT60 power consumption above only on 8.6%. Such insignificant difference is obtained including Because of the significantly lower performance of the fans of domestic air coolers.

Undoubtedly, the real values may differ depending on different conditions, but such a comparison-a reason to think, whether the manufacturers of precision air conditioners, pointing to their significantly more efficient energy consumption.

From the point of view of cooling efficiency, air conditioners on chiller (chilled water) and air conditioners with Free Cooling mode (cooling by using cold outdoor air) are of undoubted interest. It is impossible to put Air conditioners on water in a small room, because The chiller and the air conditioner itself is very dimensional. Moreover, a competent water-saving leak is a difficult technical task. Not all air conditioners are equipped with Free Cooling mode.

Partly the Free Cooling mode can be simulated with the use of exhaust ventilation. When constructing such a system, it is important to ensure that the warm air removed from the room is properly kneaded to the outside air. Properly isolate the warm air at the output of the servers to avoid mixing with the cold air coming to the Vozduhozaboru servers, you can use the equipment Rack Air Removal.  Similar systems are available from different manufacturers. Mixing cold air coming from the street in the cold season with warm air collected from the servers can provide air with the necessary temperature. However, this solution will have problems with ensuring the necessary level of humidity and the provision of gas fire.

Low temperature Operation

Domestic air conditioners Though work in a wide range of temperatures of external air, their possibilities still is not enough. Increasing the ambient temperature results in a sharp loss of efficiency of the external unit, and at some point the external unit is disconnected. This Usually corresponds To the ambient temperature of the order plus 40-45 ° C. In Winter, when using additional devices (so-called "winter Start") domestic air conditioners can work at minimum temperature of external air minus 30 degrees. This is the temperature limit for domestic air conditioners. Precision Air conditioning systems can operate in a temperature range from minus 60 to plus 60 ° C (in some cases the range may be even wider).

One of the main advantages (for premises where there are people) of household split-systems-that in an internal block there is only a fan and the evaporator, and the most noisy equipment-compressor-is put in an external block. But at low temperatures of environment in the compressor the oil accumulates, that leads to reduction of its resource. At very low negative temperatures, the oil may become so thick that the compressor will fail when starting. In Precision Air-conditioning systems, the compressor unit is located in the inner block, not in the outer unit. There is only a heat exchanger and a fan On the street. Therefore, theoretically, the precision air conditioner can work at any frost.

This is a definite advantage of precision air conditioners. However, in the middle zone of temperature more than + 35 ° С and less – 30 ° с is rare, therefore for this climatic zone this advantage is not especially significant.  For all time of operation of split systems I have not encountered significant problems when operating in the heat in summer or cold in winter. Even abnormally hot summer 2010 all the split system (at that time Daikin FT60 + FAQ 100 B) worked fine. But, undoubtedly, the precision air conditioners are more adapted for operation in harsh climatic conditions.


Significant minus of precision air conditioners for cooling of a premise-their considerable dimensions. It is difficult to Place them in a small server. Special models Must be selected:

Other manufacturers of specialized equipment for the conditioning of server rooms also have similar solutions.

Almost all the split systems in terms of dimensions-quite compact equipment.

Installation and Maintenance Costs

Precision air conditioners require careful installation. Companies that deal with precision air conditioners are much smaller than installing and operating slabs. Accordingly, the cost of installation of such equipment is much higher. The Price of spare parts for precision air conditioners is also significantly higher, and because of their lower prevalence, the availability of a spare part is also questionable. Therefore, the repair of the precision air conditioner can be significantly delayed because will have to 2-8 weeks to expect the receipt of necessary spare parts. For Example, we supply a drainage pump for APC InRow SC waited more than 2 months.

Partially delivery time can be reduced when buying a service contract from the manufacturer and then it will keep under you reserve spare parts in the warehouse, but this pleasure is not cheap and makes a significant percentage of the cost of air conditioning.

In the splits, as a rule, all spare parts are in stock, because This equipment is widely distributed. On The other hand, precision air conditioners, according to manufacturers, should be less whimsical in service. In practice in the case of a small server-the periodicity of maintenance is roughly the same, and the cost of maintenance of household air conditioners Daikin much lower service APC. Of Course, for large server slabs are not used.

Remote Control

Remote control of the operation of the air conditioning systems for server rooms is very important because Failure of these systems can lead to disconnection, and in some cases the failure of IT equipment. That is why in specialized solutions for the conditioning of the server issues of monitoring of air conditioners paid close attention. For Example, in the line APC InRow management is organized very competently. Air conditioners can be controlled both by HTTP protocol and by SMTP or Telnet. When the parameters of the air conditioner are released for some limits, the service team is immediately notified to resolve the problem as quickly as possible. The management Interface is similar to the control system implemented on APC UPS control cards, like AP9619.

Domestic even semi-industrial split systems can not boast such convenient and detailed means of monitoring of air conditioning systems. For Air Conditioners Daikin Series Ftxx There is a specially developed air conditioner ACS-M, which provides synchronous operation of 2 (3) air conditioners Daikin. This agreement can be controlled by RS-232 interface, and also has on its board an Ethernet converter-RS-232. Unfortunately, the main function of the SRK-M is To harmonize the work of air conditioners, so the functions of remote control, compared with the management of APC InRow, implemented poorly. There are no HTTP/SNMP/telnet Controls, but some simple air conditioning operations can be performed.

So, in terms of manageability, specialized precision air conditioners for server rooms, undoubtedly, give the handicap to semi-industrial split systems.

Feasibility Study

Compare the cost of ownership of the split system built on the basis of two air conditioners Daikin FT60 synchronized with the help of SRK-M and modified Winter kit with air conditioning APC InRow RD (ACRD101). The air conditioning System is designed for heat dissipation of the equipment 10 Kw.

Daikin FT60 + R60 x 2
Cooling Capacity, Kw
6.6 x 2 = 13.2 in
Power Consumption (cooling), Kw
2.39 x 2 = 4.78 in
Energy Efficiency, cooling capacity/energy consumption
Warranty, Years
Cooling capacity
36 in.
Without stock
66 in.
Without reserve
Estimated service life, Years
Air conditioner Cost, US $
3154×2 = 6308
~ 20000
Cost of the coordination of the SRK-M, $ USA
Adapters in air conditioner KRP4A53, US $
224×2 = 448
Adaptation for the cold, US $
252 х2 = 504
Total, US $:
Comparative value,%
248 in.
Total Cost/Estimated service life, US $:

So, what happens? Even taking into account the fact that the estimated life of the precision air conditioner is twice as much as the split system, the annual cost, in case the split will work for 5 years, is obtained below. If the split will only work for three years, then it really makes sense to think about the acquisition of precision, although the chances that the precision air conditioner will work for 10 years without a single overhaul, too small, still there is too much Mechanical parts. Our experience shows that after 2 years can fail, and the repair in total cost more than half the price of Daikin FT60.

If you consider the moment that the split is guaranteed to work 3 years and additional capital investment will not be, because The warranty period is 3 years, and for precision workers only a year, the investments in precision are becoming unjustified.

I note that this comparison does not take into account the cost of installation work. In The case of precision, it can be very significant.

It is also to be taken into account that in calculations the split system was duplicated by 66%, i.e. At failure of one conditioner the temperature in the server should increase, but still remains at acceptable level. In The case of precision, the air conditioner was considered without duplication. If you count the air conditioning with duplication, the comparison will not be in favor of the precision air conditioner.

In terms of energy efficiency, the air conditioners working on Freon, in terms of physical principles of work, are very similar to the split-system. I did not have to meet Some reasoned advantages in terms of energy saving on such capacities in cold weather. There is No doubt that the large capacity of the economy can reach large quantities, but in the case of small server "smearing" of the cost of electricity in terms of operation (OPEX) is more important than the one-time significant investment in the acquisition Air conditioner (CAPEX).

Again, the cost per year was calculated on the basis of zero value of money, if you consider the present value of money at the time of investment, then a small amount of CAPEX for a short period will be even more profitable than a lump-sum high Investment for a considerable period of time.


Undoubtedly, the choice of equipment to maintain the microclimate-a very crucial moment and everyone is free to choose the strategy that allows you to achieve the most planned goals, budget. In The article an attempt was made to facilitate the choice of optimal solution. Undoubtedly, the article does not purport to be complete and will be adjusted as new information and constructive comments emerge.


  1. Calculation of the total electricity demand in the data center.
  2. Humidification Strategies for Data Centers and Network Rooms
  3. Passport for air conditioners Daikin type EconAir series FT 50/60, well proven to be installed in the server.
  4. Instructions for designing of air-and premises for electronic-computing machines SN 512-78.
  5. Documentation on the Coordination of Work of Air conditioners Daikin-SRK-M.

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