Help me to choose the right mini split!

Growroom Design
#1

I need a mini split for keeping the temps in my rooms right.

I have two sealed rooms (flower and motherplants), total 16.7 x 12.2 x 8.5 feet (1731 cubicfeet) (5.10m x 3.70m x 2.6m) which are side by side and have between air circulation by a big fan and ducting.

The stuff I need an AC for:

4x 600/750 Watt double ended HID
1x 200 to 400 HID light for vegetation
1x 750 watt dehumidifier
1x water cooled propane co2 generator (I could place the water reservoir outside of the room)
1 x 100 Watt clone light
some small pumps and fans

So in total around 4250 Watts (maybe a little bit extra for some pumps and fans) if running flowerlights at 750 Watts. Mostly I will run them at 600Watts. That would be 600Watts less and result in total 3650Watts.

It would be good if the unit can heat also.

How many tons should I get? I did read that it is better to split the load on two small units, but I don’t think I will do that.

I need the unit to be quiet, because it is not common here to have ACs. It will be behind the building at a wall which is you can’t see from the walking paths. Are stronger units louder or more silent?

When is an unit too big? Can you get it too big? (Maybe if the unit cycles too often?)

How reliable are the different brands?

I did read that there are units where you don’t need an HVAC guy because the lines are preloaded or whatever. Is this usually the case? Probably will need to do everything myself.

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#2

See if you can use this site, or another like it to calculate your needs.

#3

I imagine your 4 x 750 are in flower; 3 tons in that room should be sufficient. For veg, you likely won’t need much, if anything. Is it in a basement? How are your outdoor temps? If anything, a 12000 btu portable would be sufficient.

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#4

The two rooms are connected and there is ducting which is circulating the air. So one head unit will serve the two rooms. The 4x750 Watts are flower. I wonder if I can choose an too high BTU rating? I did read that if you choose too high BTU rating, then you waste energy and it could cause problems concerning humidity. How real is that? Does it make a difference if you are using an extra dehumidifier, concerning possible humidity problems. Because I have an extra dehumidifier.

#5

In another forum I did find an instruction calculation and based on this I calculated the following and ended up with a BTU rating between 32000 BTU and 37300 BTU.

Here is my calculation:

1. Floor Area of Room

Room Area BTU = L x W x 40 ( H = 8foot + 5btu per foot after that)

Room size: 16.7 x 12.2 x 8.5 feet

Calculation: 16.7 x 12.2 x 42.5btu = 8658.95 btu

If you have a wall that is facing the sun add in for the extra heat
Sun facing wall BTU = L x H x 40

(I have a room in a room. I think I will add one sun facing wall.)
16.7 x 8.5 x 40 btu = 5678 btu

2.Equipment

This is trickier to calculate than you might think. The wattage on equipment is the maximum power consumption rating, the actual power consumed may be less. However it is safer to overestimate the wattage than underestimate it.

Equipment BTU = Total wattage for equipment x 3.5

750 Watt dehumidifier = 2625 btu
30 Watt Pump mother plants = 105 btu
30 Watt Pump return pump to main reservoir = 105 btu
100 Watt propane co2 generator pump = 350 btu
100 Watt heated clone dome = 350 btu
300 Watt recirculation fan = 1050 btu
small fans???

Ballast in room BTU = ballast wattage/2 x 3.5
4x((750/2) x 3.5) = 5.250 btu
(400/2)x3.5 = 700 btu

3.Lighting

Lighting BTU = Total wattage for all lighting x 4

4x750 Watt for flower lights
100 Watt lightning for dome
400 Watt lights for mother plants
= 3500 Watts total

3500x4 = 12000 btu

4.Occupants

Total Occupant BTU = Number of occupants x 400
1x400= 400 btu

5. Total BTU: 37271.95

I was thinking about removing some BTUs

  • remove Total Occupant BTU = Number of occupants x 400 = 400 btu
    Total BTU: 36871.95

  • if flower lights only run at 600 Watts, remove 4x150Watts, 2400 btu
    Total BTU: 34471.95

  • remove wall that is facing the sun 5678 btu (my room is in a room that is pretty good insulated)
    Total BTU: 28793.95

Maybe it is not the best idea to remove BTUs for the flower lights.So I add the 2400 BTUs back.
Total BTU 31193.95

So finally I need something around 32000 to 37300 BTU.

#6

You can definitely oversize but I don’t think 2 tons will be enough if you are not air cooling your lights. Only issue with oversizing is short cycling. I have the problem sometimes in my room if I don’t get the air thrown enough from the head unit (my last run my plants got wayyyyy too tall for my room). This certainly wastes electricity. In terms of humidity, I can kind of see what you’re saying, but you should have a dehumidifier running anyway. There have been times where I’ve been able to get away with only running dehu with lights off - my old 10kw room was closed off so my 5 ton ran virtually 12 hours - but in my current room my dehu kicks on with lights on every half hour or so for a few minutes.

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#7

Judging by your calculations a 3 ton would be perfect (36000 BTU). Stay away from Aura, they are garbage.

#8

What do you think about buying two smaller units and using them at the same time?

e.g. two times 1.5 ton units

I did hear that if one breaks, then you still can run the system on low lights.

Maybe it would be energy efficient too?

#9

Redundancy is nice for sure. In my experience, sometimes having too many moving parts can cause issues though. In the dispensary I used to work in, our 36 light rooms had 3 x 5 tons apiece, each with its own separate thermostat. We found that some units were being overworked while others barely turned on. In your situation, it may make sense. My AC guy is a phone call away and is usually here within the hour, so I am just rocking a 2 ton on 6 x 315w CMH, with plenty to add 2 more.

#10

Do you think I shoot lower for 32000 or higher for 36000 btu?

Did they have in the dispensary Aura?

Which brands are best?

#11

Fujitsu or Mitsubishi is only way to go. With installation, probably looking at 4-5k for a 3 ton.

I would definitely go with 36000 in your case. Why not? Make sure you get a high SEER rating as this has a lot to do with the efficiency of the unit.

My 2 ton Fujitsu is virtually silent.

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#12

Ever heard of Hantech?

Are the SEER and EER ratings reliable or do the manufacturers just make wild claims?

How often are there repairs necessary at your mini split?

#13

Actually I am based in europe. So I wonder it there are other reliable brands. Maybe I can save some bucks but still get quality. Because money with a 3 ton unit is now a bit short… Also it almost looks as if it is cheaper to buy two smaller units than a 3 ton…

#14

It seems like there is a problem with the proprietary considering installing outside units. It is because of concerns of privacy because of outside visibity. I am not sure if it can be resolved by me insisting on installing a mini split.

So I am looking for solutions to still run the room in summer with co2.

My two romms (flower and mother plants) are sheetrock rooms in a big room which is good insulated and this bigger room has a rectangle opening (a bit smaller than a window) for air intake/exhaust. At this rectangle opening was a. big old fan installed before. We removed that one to close it.

Then this bigger room has some sort of big air chimney which is connected to outside over the roof. It seems it is part of an air circulation system. Actually it might have been part of some sort of older AC system.

Is there a possibility to use this air window and the air chimey to install some sort of AC which could be placed inside, so the concerns of the proprietary would be satisfied and I could still cool these rooms in a satisfied way?

Actually it is quite an important issue.

#15

ok. the proprietary doesn’t allow anything to be installed outside. But I could do inside something. But somehow this might get a bit tricky. I was thinking about a custom solution. Actually I was thinking to put a outside condenser unit of a mini split system into an insulated box inside the building and ventilate this box with outside air.

Here in detail my approach:

This insulated box would have two fans, which are each matched with the cfm rating of the condenser unit and pull air into and out of the box (from and to outside of the building).

I was looking at some of the mini splits and it seems that you can find outside units from mitsubishi which move around 2000 cfm.

I was thinking about using two fans (ruck etamaster 355 Ruck 3320mÂł/h) which are each able to move around 2000 cfm.

Each fan uses at maximum load around 250 Watts of power. Considering a mini split has a COP rating of over 1, I think that even with the extra energy use of the fans I would come cheaper (energy wise) than anything else what would be possible in my place.

Can I build in another room an insulated box which contains the condenser unit and this box is connected to ducting which inputs air (from outside of building) and ducting which outputs air (also to outside of the building)? I know that the condenser needs big amounts of air, so I would install two fans which are each able to move around 2000 cfm of air. I would install one fan for pulling air from outside, pushing the air into the box and then another one pulling the air out of the box and pushing them outside. The fans were chosen under the assumption that most mitsubishi condenser units (in the range of power that I need) are able to move around 2000 cfm max each.

If I could use such a design, I would need to add 500 Watts total for the two fans. I think like that I would still be more energy efficient than other solutions. Maybe only one fan would also be sufficient, then I would only need 250 Watts extra. I am only able to use some indoor solutions and unfortunately geothermal pumps or swamp cooler are not possible. I know it is a bit problematic to find such device if you need a 2 to 3 ton unit.

So what do you know, would such a setup work? I have not enough knowledge and experience in HVAC, that is why I need to ask a pro.

So far I have some concerns:

  • Are the outdoor condenser units working with variable fan speeds, and is it a problem if my inline fans blow directly onto the condenser unit? Maybe the airstream of the inline fans, messes with the heating or cooling process of the condenser unit. Also has fast moving air some sort of own effect in temperature (e.g. cooling through windchill)? If it would be the case that direct air has some detrimental effect on efficiency of the unit, I probably could use some sort of wind baffle like it is offered by Mitsubishi (https://www.pinterest.de/pin/469500329886926899). What do you think?

  • I live in a climate where in winter we can get temperatures as cold as -5 Fahrenheit. It happens rarely but it can happen. That situation means that I would need a unit which would work also in these kind of temperatures. Mitsubishi offers Hyper Heating units which work in these temperature ranges. Unfortunately it seems that the Mitsubishi 2 to 3 ton units which support Hyper Heating are almost only twin systems (have two fans), which would mean a combined airflow of 3600cfm. To provide 3600cfm of air with my inline fans to the insulated box where the condenser unit would be in, I would need to buy much stronger and much more expensive inline fans. So I only know the Mitsubishi units so far, but maybe somebody of you know the product range of the other quality brands. Does somebody know a unit which is capable to work in low temperature settings and has a cfm rating of around 2000cfm? Other option would be if somebody knows strong inline or other fans which would be able to do 3600cfm and are actually not that expensive.

  • also what do you think, would I need two or one of these inline fans? I mean suffice how much would such a condenser unit have postive influence on the static pressure. Would the outdoor unit be a resistance or an noticeable aid in pulling air for one fan in such a setup?

So that is the situation. Maybe somebody can answer some of my questions. Thank you very much!

#16

Hey. What happened with you AC situation?

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#17

no mini split. couldnt install any on the outside. I did build a system which I can run in the winter sealed with CO2 and when it is getting too hot I can switch it to outside air (intake and exhaust connected to outside air). But I still need to see how good that will work. But so far it seems good.

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