AC Insulated Mini Split Enclosure

[orion0314]

Hello,

I recently purchased and installed several mini split AC units in order to cool our server room, only to discover that these mini splits cease running below 64 degrees F. I would like to build an insulated enclosure around the unit with the proper clearances including an opening on the bottom of the enclosure to allow for air flow. I plan to install a thermostatically controlled fan within the wall of the enclosure in order to evacuate at a given temperature. If necessary, I could also place a small heater that would prevent the temperature from decreasing below 70 degrees. I am suddenly on a tight budget and am unable to purchase new units but am in . Do you have any suggestions? Feel free to tell me that this is a terrible idea and why it shouldn't be done or how it could be done differently. I have explored the option of a low ambient kit, but the unit is an inverter unit that it is not compatible with these kits.

 

[75gmck25]

Is your problem that you want the server room cooler than 64 degrees? I know that cooler is better, but 64 degrees seems adequate for most standard servers.

I also assume that the units quit running only when the server room is cooler than 64 degrees, not when outside ambient is lower than 64 degrees? Most low temp shutoffs I've seen in specs allow the unit to run down to near freezing outside.

Bruce

 

[orion0314]

Hello Bruce,

Thank you for your response. The unit is a 24000 BTU AirCon Blue Series with a Mitsubishi Compressor and the specs list the cooling ambient temperature range for the outdoor unit as 64F-109F, meaning that the unit no longer cools when it is less than 64F outside in order to prevent freezing of the coils. I was confused when I purchased these units because the user manual has a section that states the following: "The operating temperature range (outdoor
temperature) for cooling only unit is 18℃ ~ 43℃ ;for heat pump unit is -7℃ ~ 43℃ ."

I made the mistake of taking this to mean that the model that contains the heat pump, which is the one that I have, has a minimum outdoor operating of -7C or 20F, but in reality it is referring to the temperature at which the actual heating function runs.

Here are links to the AC user manual and spec sheet:

User Manual: http://airconint.com/wiki/index.php?title=File:Blue-Series-UM-1824-F.pdf

Spec Sheet: http://www.airconint.com/wp-content/uploads/Specs/Blue-Series-Specs.pdf

 

[LS6 Tommy]

Normally, for low ambient cooling operation, simple condenser fan cycling will accomplish what you want to do. It runs the fan based on liquid line or discharge line temp. Most every mini split I've ever seen has some version of that. I'm not familiar with your model, but I think 64* is the temp the machine's operating software will not allow cooling, not that it isn't mechanically capable of cooling below that temp. You'd have to build a pretty high tech enclosure to maintain a minimum 64*ambient around the unit and still flow enough ar to properly cool the condenser.

Tommy

 

[pseudorealityx]

How much heat load do you have? What's in the adjacent areas? Is there an exterior exposure? All on grade? Budget? What temperature do you want in the server room?

Lots of options here...

 

[Sokoloff]

I'd look to see if I could use the heat from the server room to heat the rest of the building (assuming it's a mixed-use space, given that you're somewhat home-brewing your CRAC).

Servers will be happy at any temps that you'd be willing to heat your office space to, so if you just exchange air to/from the office space, you will probably be fine when the OAT is under 64.

 

[monkeyspanners]

Wonder if you could fool the outdoor ambient sensor into thinking it is warmer than it is (variable resistor, or maybe even a small light bulb next to it to give it some warmth) and do something with the air flow so it recirculates the warm air back round to the intake side of the condenser. Even just restricting the airflow onto the condenser may be enough, it depends how cold your winters get.

If the winters are really cool, could you just ventilate the server room with outside air?

 

[Jackfre]

I can only speak about the Fujitsu as that was what I was trained on and supported. The factories position was that with the inverter units (variable speed compressor, condenser fan and evaporator fan) there was no way to get into them and fool the programming. When server room problems occurred they said it was misapplication. Server rooms having specific needs beyond what the units were designed for. That said, I had them all over New England and they did well...most of the time. There were those nightmares. Lieberts are the units designed for this application, but at the cost, no wonder folks used mshp's.

As to using outside cold air, the problem is filtration and dehumidification. I think it could be done but would be a maintenance nightmare. If the building happens to be in a pine forest and the pollen is dropping...well, you get the idea.

To the OP, what is Air Con saying?

 

[truckman5000]

mitsubishi p series sized correctly works

 

[pseudorealityx]

I can only speak about the Fujitsu as that was what I was trained on and supported. The factories position was that with the inverter units (variable speed compressor, condenser fan and evaporator fan) there was no way to get into them and fool the programming. When server room problems occurred they said it was misapplication. Server rooms having specific needs beyond what the units were designed for. That said, I had them all over New England and they did well...most of the time. There were those nightmares. Lieberts are the units designed for this application, but at the cost, no wonder folks used mshp's.

As to using outside cold air, the problem is filtration and dehumidification. I think it could be done but would be a maintenance nightmare. If the building happens to be in a pine forest and the pollen is dropping...well, you get the idea.

To the OP, what is Air Con saying?

Anything commercial over 5 tons (if not in FL) needs an economizer anyway per code... so you HAVE to be able to deal with outside air filtration. Easy enough to do with a mixing box and a couple of motorized dampers. You control it via comparative enthalpy and you don't have to worry about dehumidification.

Liebert is basically the "Kleenex" of the industry... lots of other players, but that's the name everyone knows. We've use Liebert and Schneider Electric (APC) on most of our data room/center projects.

The whole 'low temperature' thing is old school. The failure rate of modern equipment isn't anything like it was 15-20 years ago. You can run a data center reasonably higher, and if you can keep that temperature up high enough, you can run dry coils and save yourself a bunch of phase change energy.

 

[Jackfre]

Anything commercial over 5 tons (if not in FL) needs an economizer anyway per code... so you HAVE to be able to deal with outside air filtration. Easy enough to do with a mixing box and a couple of motorized dampers. You control it via comparative enthalpy and you don't have to worry about dehumidification.

Liebert is basically the "Kleenex" of the industry... lots of other players, but that's the name everyone knows. We've use Liebert and Schneider Electric (APC) on most of our data room/center projects.

The whole 'low temperature' thing is old school. The failure rate of modern equipment isn't anything like it was 15-20 years ago. You can run a data center reasonably higher, and if you can keep that temperature up high enough, you can run dry coils and save yourself a bunch of phase change energy.

Thanks, excellent information!

 

[orion0314]

Thank you everyone for your great comments. To clarify, it is the outdoor unit that ceases cooling once the outside ambient temperature dips below 64 degrees. I would prefer to avoid installation of an economizer or open air ventilation at this point, but I may consider it in the future. If I am allowing an opening around the bottom of the enclosure, shouldn't this be enough to allow for fresh intake air? Also, why would the exhaust fan on a thermostat in combination with the heater on a thermostat be insufficient to maintain a safe temperature range? I greatly appreciate your feedback.

 

[pseudorealityx]

Thank you everyone for your great comments. To clarify, it is the outdoor unit that ceases cooling once the outside ambient temperature dips below 64 degrees.

Yes, we get it. You chose the wrong equipment and are now covering your ***.

I would prefer to avoid installation of an economizer or open air ventilation at this point, but I may consider it in the future.

If I am allowing an opening around the bottom of the enclosure, shouldn't this be enough to allow for fresh intake air?

Yes... but that's not the problem.

Also, why would the exhaust fan on a thermostat in combination with the heater on a thermostat be insufficient to maintain a safe temperature range? I greatly appreciate your feedback.

Because you're got a relatively small mostly enclosed area that is going to have a significant head load from the condenser. The amount of heat coming off that condenser is MORE than is being pulled out by the indoor unit in the server room, because it's also the work of the compressor in addition. Your fan/heater would need to be able to cycle very fast, and you'll burn both out in a rather short time.

You're also setting up a maintenance nightmare, since now every unit has 3 more stand-alone variables that you cannot monitor. A heater, a fan, and the enclosure itself. Are you going to put the enclosures on every night and take them off every morning as the temperature increases? What if you get a rain storm coming through and the temperatures fall? What if it's a weekend? What if you want to take a vacation? What if you get sick? What if you have to leave early?

Imagine it's 60 degrees outside in the morning... too cool for the unit to run, but if you put on the enclosure, as it warms up, it will get too hot very quickly as you dump thousands of BTU's in to a small box that's now sitting in 80 degree sunshine.

 

[orion0314]

Yes, we get it. You chose the wrong equipment and are now covering your ***.






Yes... but that's not the problem.



Because you're got a relatively small mostly enclosed area that is going to have a significant head load from the condenser. The amount of heat coming off that condenser is MORE than is being pulled out by the indoor unit in the server room, because it's also the work of the compressor in addition. Your fan/heater would need to be able to cycle very fast, and you'll burn both out in a rather short time.

You're also setting up a maintenance nightmare, since now every unit has 3 more stand-alone variables that you cannot monitor. A heater, a fan, and the enclosure itself. Are you going to put the enclosures on every night and take them off every morning as the temperature increases? What if you get a rain storm coming through and the temperatures fall? What if it's a weekend? What if you want to take a vacation? What if you get sick? What if you have to leave early?

Imagine it's 60 degrees outside in the morning... too cool for the unit to run, but if you put on the enclosure, as it warms up, it will get too hot very quickly as you dump thousands of BTU's in to a small box that's now sitting in 80 degree sunshine.

Thank you for your feedback. Do you have any suggestions for what I should do at this point? Would I be better off selling my units and purchasing the correct ones once I have the funds? I appreciate it.

 

[LS6 Tommy]

A more drastic option would be to move the outside unit into the garage, completely enclose the outlet side of the fan, ducting it in a relatively unrestricted way to the outside of the garage. Of course, you will be using some conditioned air to cool the unit, but assuming the garage stays above 64* when it's less than that outside, it would allow you to run cooling in cooler weather. The flip side is, depending on the design of the unit controls, it may not allow it to run in the heat mode if the ambient air around the unit is too warm.

Tommy

 

[pseudorealityx]

Thank you for your feedback. Do you have any suggestions for what I should do at this point? Would I be better off selling my units and purchasing the correct ones once I have the funds? I appreciate it.

You could start with answering the questions I asked in post #5.

 

[pseudorealityx]

A more drastic option would be to move the outside unit into the garage, completely enclose the outlet side of the fan, ducting it in a relatively unrestricted way to the outside of the garage. Of course, you will be using some conditioned air to cool the unit, but assuming the garage stays above 64* when it's less than that outside, it would allow you to run cooling in cooler weather. The flip side is, depending on the design of the unit controls, it may not allow it to run in the heat mode if the ambient air around the unit is too warm.

Tommy

I'm 'guessing' these are cooling only units. I don't specify heat pump units for server rooms, unless there's something really unique about the situation.

 

[LS6 Tommy]

I'm 'guessing' these are cooling only units. I don't specify heat pump units for server rooms, unless there's something really unique about the situation.

Good point. I was going off the OP's statement about changing over to heat mode at 64*.

Tommy

 

[orion0314]

How much heat load do you have? What's in the adjacent areas? Is there an exterior exposure? All on grade? Budget? What temperature do you want in the server room?

Lots of options here...

My apologies, I overlooked this posting. The heat load is approximately 72000 BTUs, The rooms are located in an old basement area. I would like to maintain temperature of 70 degrees. My budget is fairly low at $1000.

 

[pseudorealityx]

First off, you need more budget... $1k isn't going cover this.

Some more questions...

1) Is 72000 BTU your max or an average capacity? Are you taking into account any diversity?

2) How big of a room is this? You have it setup as hot aisle/cold aisle?

3) Is the basement fully/partially underground? Anything else in this basement? Is it insulated?



Realistically, at 72000 btu, I'm beyond the point of suggesting just a mini-split. Because even with low ambient, if you get harsh winters, you're going to be below that for enough time that you're not going to be able to control your temperatures, and you're beyond the point where you can just use air circulation with an adjoining space to deal with the extra heat.

You need to start looking at true commercial data room equipment.