Oversize or the right size?

[3rdgendslmech]

Going to be pulling the trigger on a mini split hopefully this week. 768 sq ft pole barn with R19 in the walls. Ceiling is open as of right now but going to work on metal and insulation this fall/early winter.
I'm not out there a whole lot, maybe at times 2-3 hours 2-3 times during the week, and 4-6 hours on weekends when I have a project to do.
That being said, im stuck between a 18K unit and a 24K unit.
Should I get the bigger one to get things cooled down/warmed a little quicker, or just get the 18K and let it work. For winter it would be kinda nice to keep the temps at 45 to keep paints and other stuff from freezing up and getting ruined. I know its been talked about before how its sometimes better to get a bigger unit if you arent going to be out there a lot to get things warm/cool faster.

Also, I'm think of going with that Advantage series from MR.Cool. I'm a diesel/heavy equipment mechanic. I've already got a Vacuum pump. The only thing I would really need to buy is a can of 410a, separate set of gauges, and a micron gauge.

 

[Jim greengo]

How high is the ceiling,how many heads on system?

 

[pattenp]

Are you going to a bunch of automotive r410a cans? You need to be certified to buy larger size containers of refrigerant.

 

[Perry H]

Most of the units I have seen are precharged with 410. You have to vacuum down the lines (and pressure test ahead of that if you are so inclined). Then you open the valves and release the 410 into the rest of the system.

Some of the units (MrCool DIY) have the lineset charged as well, but that's an extra $700 and you can buy quite the vacuum pump for $700.

 

[3rdgendslmech]

How high is the ceiling,how many heads on system?

10 foot walls and I'm just going with 1 head. Looked at a 2 head set up but $$$$$

Are you going to a bunch of automotive r410a cans? You need to be certified to buy larger size containers of refrigerant.

I've got 608 Cert card. I've also got a friend that does HVAC so depending on if he's available, and what kind of deal he wants he might do the leak check/start up .

Most of the units I have seen are precharged with 410. You have to vacuum down the lines (and pressure test ahead of that if you are so inclined). Then you open the valves and release the 410 into the rest of the system.

Some of the units (MrCool DIY) have the lineset charged as well, but that's an extra $700 and you can buy quite the vacuum pump for $700.

Exactly! After making my choice I said hell, I'm halfway there between the vacuum pump and gauges....I can probably get the meter, and freon for around 300. We have nitrogen bottles at work that we use on spreader bars to unload cargo from ships. Used to use air but found that in the winter the latching devices would freeze up slowing down operations.

 

[3rdgendslmech]

EDIT: I've got a 609 card, not a 608. But I've got a guy

 

[Supertech]

Not to **** in, but i strongly suggest pressure testing with dry nitrogen at 400 psi for an hour or so. Then pull it down to 500 microns and see if it holds it. Those flares are bad about leaking.

The unit should be charged for 15 or 20 foot lineset. Read the directions and it will tell you how mucb more to add per foot of lineset over the set amount of lineset. Oh yeah youre gonna need a 5/16 adapter to get 1/4 inch hose hooked up. Low side only.

Oh and id go with the 2 ton.

 

[Supertech]

Oh and its a good idea to put refrigerant oil all over the threads and the flares and the back of the flare to lube the flare nut. Nylog works better but i figure u probably got some pag oil laying around

 

[yeldogt]

I think the way you plan to use it you may be better off with the 24 .. you are going to need some reserve.

Also -- most will not allow you to run that low inside temp.

I would make sure to insulate well and plan on higher temps.

 

[75gmck25]

For house A/C units they usually recommend sizing more toward the low side of the calculation, since overcapacity in cooling causes short cycling and high humidity when it cools too quickly. Comfort is a mix of temp and humidity, and you want the A/C to run often enough to keep humidity down in the summer. However 18k vs 24k is a big difference in capacity, so I don't know the best choice for your application.

Bruce

 

[Notgrownup]

I got the 18k for my 24x24x10 fully insulated and it cools easily....I leave it on 80 and cool it down fast when I have to be in there...

 

[LS6 Tommy]

I'm think of going with that Advantage series from MR.Cool. I'm a diesel/heavy equipment mechanic. I've already got a Vacuum pump. The only thing I would really need to buy is a can of 410a, separate set of gauges, and a micron gauge.

If the Advantage Series has an inverter driven compressor, go with the bigger unit. You'll also need dehydrated nitrogen to leak check before you evacuate.

Tommy

 

[yeldogt]

I got the 18k for my 24x24x10 fully insulated and it cools easily....I leave it on 80 and cool it down fast when I have to be in there...

The OP's is a little bigger and it sounds like he is going to have a wider swing

 

[yeldogt]

For house A/C units they usually recommend sizing more toward the low side of the calculation, since overcapacity in cooling causes short cycling and high humidity when it cools too quickly. Comfort is a mix of temp and humidity, and you want the A/C to run often enough to keep humidity down in the summer. However 18k vs 24k is a big difference in capacity, so I don't know the best choice for your application.

Bruce

This is very true ... depending on climate with a heat pump .... you may have to size for heat side.

With a home unit where you are going to be maintaining temps -- you want to be on the low side of AC as you state.

The OP is not going to maintain close temps ..... so he needs something with excess capacity.

The goal with mini splits is to get them to run in the upper 3rd of the range most of the time for your space and climate. That's when they are most efficient.

 

[bazar01]

Get the 24k inverter unit. It will adjust capacity based on heat load/gain.

 

[3rdgendslmech]

This is very true ... depending on climate with a heat pump .... you may have to size for heat side.

With a home unit where you are going to be maintaining temps -- you want to be on the low side of AC as you state.

The OP is not going to maintain close temps ..... so he needs something with excess capacity.

The goal with mini splits is to get them to run in the upper 3rd of the range most of the time for your space and climate. That's when they are most efficient.

This I understand, my house was mix matched with a 2ton compressor and a 2.5 ton air handler. 2 years ago we replaced both, matched at 2 ton and more efficient units. House feels better as well as the electric bill both seasons .

Anyhow yeah, I'm not going to maintain any set temp. Highest cooling temp in the summer to keep the humidity down, and I thought, but don't hold me to it that the lowest heat setting was 45°.

 

[u3b3rg33k]

generally speaking, i've seen more turndown on single head units than multi head units (my guess is this is for oil flow). so IMNSHO, a little oversizing on a single head unit is OK, so long as it is a modulating unit. on/off style (they do make "mini" split units that don't modulate!) you should not oversize.

 

[Jim greengo]

This I understand, my house was mix matched with a 2ton compressor and a 2.5 ton air handler. 2 years ago we replaced both, matched at 2 ton and more efficient units. House feels better as well as the electric bill both seasons .

Anyhow yeah, I'm not going to maintain any set temp. Highest cooling temp in the summer to keep the humidity down, and I thought, but don't hold me to it that the lowest heat setting was 45°.

Nothing wrong with an oversized coil,ups the efficiency a little.

 

[MattT]

and I thought, but don't hold me to it that the lowest heat setting was 45°.

It would be unusual for the normal thermostat range to go down to 45*. Some units do have a "vacation" or "freeze protection" mode that'll keep the temp around 45*.

 

[outdoorspace]

I got basically the same unit as the Advantage, but the brand was Ecox, $1100 to my door for the 24k inverter unit. They are both made by Midea.

I'd definitely recommend you go with the 24k. It will work much better in the winter. The lowest setting is 63.

 

[TRWham]

Nothing wrong with an oversized coil,ups the efficiency a little.

All else equal, an oversize evap will have a negative impact on dehumidification. You can mitigate that somewhat by slowing the blower, but it will still raise the SHR by raising the evap temp.

 

[Jim greengo]

All else equal, an oversize evap will have a negative impact on dehumidification. You can mitigate that somewhat by slowing the blower, but it will still raise the SHR by raising the evap temp.

What's the differance between a 10 seer and a 13 seer A coil for a 2 ton unit? Surface area.
What's the differance between a 2t and a2.5 A coil with same seer rating? Surface area.

 

[3rdgendslmech]

From what I understand it's an inverter unit. So once the target temp is close to being met it'll ramp down slowly, and once the target temp is met it will barely spin the fan at all

 

[Jackfre]

To much mixing of metaphors in this post. It is a mini split discussion. I’d suggest the unitary discussion go to a thread of it own. It’s good info and questions, but I’m to easily confused. To the OP, get a wireless compatible unit. That way you can call ahead to get the unit going.

 

[LS6 Tommy]

Get the 24k inverter unit. It will adjust capacity based on heat load/gain.

My point exactly.

 

[LS6 Tommy]

Nothing wrong with an oversized coil,ups the efficiency a little.

All else equal, an oversize evap will have a negative impact on dehumidification. You can mitigate that somewhat by slowing the blower, but it will still raise the SHR by raising the evap temp.

X2.

Tommy

 

[yeldogt]

It's typically done to hit SEER mark. Larger coil = more transfer. The larger coil also allows for less static pressure = less motor run cost. Both can increase efficiency.

The issue can be coil temps w/o a VS blower to keep the coil at the proper temps to remove humidity.

None of this has anything to do with the OP ..... he is talking about overall capacity (I know you know that)

The reverse is also true ... Carrier makes a special model of the 5 speed unit. It's the 3 ton system in a 4 ton condenser case with the 4 ton coil. 37k BTU .. 3T inside coil

 

[TRWham]

What's the differance between a 10 seer and a 13 seer A coil for a 2 ton unit? Surface area.
What's the differance between a 2t and a2.5 A coil with same seer rating? Surface area.

That is one way to get SEER, and was a common solution in the early days of higher efficiency (>10 SEER) units, but it still reduces latent cooling. Larger condensers, ECMs, TXVs and any number of other things increase efficiency without a performance trade-off. Jackfre is right, though- this is really OT for this thread.

 

[Jim greengo]

I was just answering the other post about mis matched system.

 

[dcg9381]

Not to **** in, but i strongly suggest pressure testing with dry nitrogen at 400 psi for an hour or so. Then pull it down to 500 microns and see if it holds it. Those flares are bad about leaking.

The unit should be charged for 15 or 20 foot lineset. Read the directions and it will tell you how mucb more to add per foot of lineset over the set amount of lineset. Oh yeah youre gonna need a 5/16 adapter to get 1/4 inch hose hooked up. Low side only.

Oh and id go with the 2 ton.

Agree on all fronts. Having just had one leak, make sure you do the testing for 24 hours (maybe not to 400psi).

Daikins are pre-charged if you use a standard lineset.
Adapter is needed.
Beyond standard length line sets, you'll need a scale, R410a, and a gauge set. You're gonna need the gauge set anyway...

 

[3rdgendslmech]

Agree on all fronts. Having just had one leak, make sure you do the testing for 24 hours (maybe not to 400psi).

Daikins are pre-charged if you use a standard lineset.
Adapter is needed.
Beyond standard length line sets, you'll need a scale, R410a, and a gauge set. You're gonna need the gauge set anyway...

I've got all that, Minus the 410A at this moment. I do have a question though since I dont fool around a lot with the actual refrigerant end of things. Lets say I hook all the lines up ( Advantage series come with 16' pre cut and flared lines) the only hook up I have to do is at the condenser unit outside. Do all that, pull a vacuum down to 29-30 inHg and it holds for say 40 minutes. Obviously the next step is to charge with Nitrogen to 300 psi. Check for leaks under pressure with soapy water. Everything looks good 24 hours later except for maybe 1-2 psi loss due to temp change. So next is you vacuum out the nitrogen and pull a vacuum again.
Only asking because when I'm working on trucks and heavy equipment that's generally what we do. Well evacuate, see how much we pulled out. Pull a vacuum, see if we can detect a leak. If I do I have 2 desiccant filters and pressurize with 80-90 psi of air to find a leak. ( Before anybody starts criticizing me, I've been doing it this way for 20 years and have yet to see a failure due to air checking) (but I wouldn't use air for home HVAC ) Fix the leak, vacuum once down to 30 or more, if it holds good then I'll run the vacuum pump for 20 minutes or so for extra measure before I recharge.

 

[PoorOwner]

^^ at the last vacuum you need a micron gauge to 500 mircon or less. when you isolate the pump, the micron should not rise quickly to 4 or 5 digits figure within 5-10 minutes


btw i have 18k in my garage similar size it is the right size, two walls not insulated and 3 passive vents.

 

[Probedude]

FWIW, pretty sure you don't need a license to buy 410a, yet.

Yep, here's the info. https://www.epatest.com/faq/R410A/#G3

Are you required to have a license or to be certified to handle and purchase R-410A?
You are required to have an EPA Section 608 Type II or Universal certification license to handle R-410A but no license is legally necessary for purchase. A refrigeration supply house may have its own rules regarding who they are willing to sell R-410A to (they will typically require Section 608 Type II even though the EPA doesn’t require it for safety and liability reasons).

 

[LS6 Tommy]

I've got all that, Minus the 410A at this moment. I do have a question though since I dont fool around a lot with the actual refrigerant end of things. Lets say I hook all the lines up ( Advantage series come with 16' pre cut and flared lines) the only hook up I have to do is at the condenser unit outside. Do all that, pull a vacuum down to 29-30 inHg and it holds for say 40 minutes. Obviously the next step is to charge with Nitrogen to 300 psi. Check for leaks under pressure with soapy water. Everything looks good 24 hours later except for maybe 1-2 psi loss due to temp change. So next is you vacuum out the nitrogen and pull a vacuum again.
Only asking because when I'm working on trucks and heavy equipment that's generally what we do. Well evacuate, see how much we pulled out. Pull a vacuum, see if we can detect a leak. If I do I have 2 desiccant filters and pressurize with 80-90 psi of air to find a leak. ( Before anybody starts criticizing me, I've been doing it this way for 20 years and have yet to see a failure due to air checking) (but I wouldn't use air for home HVAC ) Fix the leak, vacuum once down to 30 or more, if it holds good then I'll run the vacuum pump for 20 minutes or so for extra measure before I recharge.

Your sequence is a little off. You should perform the leak check with nitrogen before you evacuate.

Tommy

 

[PoorOwner]

Vacuuming before nitrogen test is not a bad idea. Not to see if it holds vacuum, but it gets the atmospheric air out so we don’t have to compress the atmospheric air into 300 psi.

 

[LS6 Tommy]

Vacuuming before nitrogen test is not a bad idea. Not to see if it holds vacuum, but it gets the atmospheric air out so we don’t have to compress the atmospheric air into 300 psi.

Or you can save a little time and just purge the system with nitrogen before pressurizing it.

Tommy

 

[MattT]

Vacuuming before nitrogen test is not a bad idea. Not to see if it holds vacuum, but it gets the atmospheric air out so we don’t have to compress the atmospheric air into 300 psi.

Then the pressure test is also the first flush of your triple evac

Though it does depend on how long you plan to pressure test. If you're doing 24 hr probably not worth hooking the pump up twice.

 

[3rdgendslmech]

Then the pressure test is also the first flush of your triple evac

Though it does depend on how long you plan to pressure test. If you're doing 24 hr probably not worth hooking the pump up twice.

Makes sense! Leave your service valves shut, charge with nitrogen, then the next day evac/vacuum down to 500 microns if everything goes well