Mini Split Question - Two 9000 or one 18000 BTU air handler?

[Onewolf]

Hello,

We are having a detached garage with a "workshop" room built this summer. The workshop room will be insulated (R19 walls, R38 ceiling) and the room is 28x32x10. Three of the walls are exterior and there are two windows facing east but they will be blocked out and insulated inside (they exist only for HOA architecture reasons). We live in east central Florida (ie HOT and HUMID)

The workshop room will contain a model train layout and there will be approx 50-60 fluorescent light fixtures in the room.

Assume it will take approx 18000 BTU to keep this room cool in the heat of summer. My question is whether it would be more effective to have two 9000 BTU air handlers or a single 18000 BTU air handler to cool this room?

Thanks.

Floorplan of the detached garage/workshop

 

[Jackfre]

I prefer multiple evaps. That said, in an open floor plan such as this you could handle it with a single unit. Compare pricing for multi vs single. Compare efficiency as well. It would be best if going with a single unit that it have a 4 way air sweep. Most of the smaller units come with a vertical air sweep. What you will want to look for the the horizontal air sweep. That will give you the best performance with a single. Looks like you have a real nice train room.

 

[Onewolf]

Thanks for the reply.

I'm definitely leaning towards two evaps because I think I would get much better distribution of airflow. The center platform area is actually an enclosed area up to the ceiling.

Unfortunately basements are almost impossible to find in central florida because the water table is so high, so my 'basement empire' has to be built in a detached garage.

 

[joed2323]

Onewolf- i like the size of your garage, wow thats a good sized garage you have
check your pm

 

[fireant911]

I can share my experience with you next Tuesday as we have an 18K outdoor condenser unit with two 9K wall-mounted indoor units. The 'plumbing' and electrical are done and the HVAC guys will be back to do the final install early next week. I struggled over one unit versus two for a while. Jackfre wrote earlier about the advantages of two, substantiated his answer with logic, and it made good sense to me.

On a tangential note, I am now thoroughly convinced that there is NO universal answer for anything to do with mini-splits... sizing of the units, using a multitude of different calculators, revealed a HUGE different in BTU ratings (but this is not your question) and was a troubling issue. I will report my observations on Tuesday.

 

[fireant911]

Our units are installed and powered! WOW - the two Fujitsu 9K wall mounted units do an incredibly quick cool down of the office above our garage. The space is long and narrow (40' X 10') which is part of the reason we opted for two units rather than one. The flow of air from either side is quite noticeable why standing in the middle of the room. It is a muggy 88 degrees here during this initial test - the inside temperature dropped quickly.

 

[pseudorealityx]

With 50-60 light fixtures in the space, I would make sure you double check your heating calcs.

Also, you many want to think about doing something with that enclosed space in the middle. It's not going to get any airflow with 2 wall units. A simple answer would be to provide a small bathroom type fan just to provide circulation. Basically have it pull air from that space, and blow it to the other side of enclosure/wall. You could probably hide the terminate with some groundwork/paint for your layout.

 

[Ruthless53]

Can you not go with LED to avoid the heat from the florescent lights?? If you need 1 18k btu then go with 2 10k btu units. There is nothing worse than being in a hot humid room in our climate come august! I think of spend my last dollar to make sure it would get cool and stay cool! Biggest money I will have spent converting my garage will most likely be the a/c. Well I'm a big boy too so that may be my personal feelings

 

[F16CrewChief]

Ive been dreaming of wanting to do a layout myself. Have you built a smaller one before? Thats a really nice layout. I always want a roundtable on mine.

Sent from my SAMSUNG-SGH-I497 using Tapatalk

 

[Onewolf]

With 50-60 light fixtures in the space, I would make sure you double check your heating calcs.

Also, you many want to think about doing something with that enclosed space in the middle. It's not going to get any airflow with 2 wall units. A simple answer would be to provide a small bathroom type fan just to provide circulation. Basically have it pull air from that space, and blow it to the other side of enclosure/wall. You could probably hide the terminate with some groundwork/paint for your layout.

Yes, the center platform area is problematical for a 2 zone ductless system. My theory was one of the air handlers would be mounted such that it blows about half it's output into the center platform and the other half to the left of the center platform down into the bottom right corner of the room. I'm working with an HVAC guy to see about maybe doing a three zone, or just reverting to a ducted system. I think 18,000 BTU is enough cooling for that room if we can figure out how to get air distributed. Like you said, perhaps an exhaust fan in the center platform could pull air from the center area into the area of the aisle on the top side of the plan.

Can you not go with LED to avoid the heat from the florescent lights?? If you need 1 18k btu then go with 2 10k btu units. There is nothing worse than being in a hot humid room in our climate come august! I think of spend my last dollar to make sure it would get cool and stay cool! Biggest money I will have spent converting my garage will most likely be the a/c. Well I'm a big boy too so that may be my personal feelings

I have investigated LED fixtures and strip/tape. They don't put out enough LUMENS to be cost effective. LEDS would cost 3X fluorescent lights ($6000 vs $2000) And they don't have 6500K color temp. And their CRI is terrible.

The majority of the time only the ten ceiling fluorescent light fixtures will be on. I've done the heat load calculation several times and even with 2000 watts of lighting (1000 watts = 3413 BTU) it appears that 18,000 BTU should be enough.

It will be interesting to see what the HVAC guy says. He said that their load calculation software does not have a specific load type for fluorescent light fixtures.

Ive been dreaming of wanting to do a layout myself. Have you built a smaller one before? Thats a really nice layout. I always want a roundtable on mine.

Sent from my SAMSUNG-SGH-I497 using Tapatalk

I currently have a 11x10 layout in a spare bedroom. Here is the current layout plan overlaid on the new layout design:

 

[Onewolf]

Another (stupid) question about ductless systems. Do any of the ductless air handler units have output vents that move back/forth to spread the airflow around better?

 

[pseudorealityx]

FWIW, my HVAC load calc software is better than your contractor's.

1) What's the average wattage per fixture?
2) Besides the lighting, do you have any computers, or electronics, or anything else in there that will produce heat?
3) What temperature would you like to maintain in that space?


Based on my assumptions based on this thread, I calc'd it out at over 18k BTU. But answering the above questions would clarify...

 

[Onewolf]

1) 21watts
2) Several small transformers that don't produce much heat.
3) 74 (cooling) 68 (heating)

 

[fireant911]

Another (stupid) question about ductless systems. Do any of the ductless air handler units have output vents that move back/forth to spread the airflow around better?

Look at the PDF below (page 5 shows the swing feature). Additionally, look at page 16 as the graph depicts various distances of the fan distributions at different settings). This information covers the Fujitsu 9K, 12K, and 15K wall mounted models.

https://www.acwholesalers.com/hvac/pdf/fujitsu/fujitsu-5-tm.pdf

 

[pseudorealityx]

1) 21watts
2) Several small transformers that don't produce much heat.
3) 74 (cooling) 68 (heating)

Perfect. 18k is more than enough, bordering on too much.

I still like the idea of putting 2 evaps on either side, and then a small quiet exhaust fan in the center area to pull air into and then out of that space. ~75 cfm would be perfect, and it would be nearly silent.

 

[Onewolf]

Perfect. 18k is more than enough, bordering on too much.

I still like the idea of putting 2 evaps on either side, and then a small quiet exhaust fan in the center area to pull air into and then out of that space. ~75 cfm would be perfect, and it would be nearly silent.

Thanks for all the advice.

I really would like to stick with a multi-zone ductless split system because many times I will be working only in the smaller workshop room and it would be nice to be able to cool just the small room vs the much larger train layout room. I happen to already have a spare Panasonic FV-08VQ5 WhisperCeiling 80 CFM Ceiling Mounted Fan from another project I could use to vent the center platform area.