Tie into existing central HVAC or mini-split

[Mesozoic]

I'm at the point of my build where I need to start thinking about the airflow on my main floor, the compressor room, and the office. The garage is attached to the home which has 2 existing 3-ton, 14 SEER HVAC units (one on each end of the house). Since the 130 sqft office within the garage is located very close to one of the existing air handlers, I have been considering the possibility of augmenting said HVAC unit to cool it. Of course, I'm no HVAC expert, so I'm unsure of whether there would be a need to tie into the return for the system as well and whether connecting to the existing system would be worth the effort considering the cost and availability of modern mini-split systems.

A mini-split has been spec'd out to cool the main floor on-demand - so a separate indoor unit could be spec'd to cool the office, but this would certainly require active control of an additional HVAC system in the house. I'm not opposed to it entirely due to the high SEER ratings of mini-splits, but would like to hear some opinions.

 

[Innovate1]

Code doesn't allow HVAC vents through the fire wall between house and garage (there may be exceptions if you employ smoke detectors and automatically closing dampers, etc. as done in some commercial buildings but that isn't practical for residential). Depending on the layout you might be able to make the wall between the office and garage the fire barrier and basically make the office part of the house.

If it's only 130 sq ft you might be able to cut a hole or two to the garage and put in a small fan.

 

[LS6 Tommy]

A garage can't share a unit with living space no matter what you do. Even if you could, if your system is sized properly you really "can't" just add rooms to it.

Tommy

 

[Mesozoic]

I think a new mini-split with multiple zones is going to be the path forward. Regardless, I don't know what's going on with one of my central HVAC systems, but the air handler is a 4 ton unit and the compressor/condenser is a 3 ton unit. This unit is still running R22 as well, while the system on the opposite side of the house is running R410. Both systems sit outside of the house in their own mechanical rooms. The one on the garage end of the home is located within the garage, so I didn't think it would be a difficult tie in.

 

[justinjoyal]

A garage can't share a unit with living space no matter what you do. Even if you could, if your system is sized properly you really "can't" just add rooms to it.

Tommy

6 tons of cooling. The place must be huge if it is indeed sized properly. [emoji28]

 

[Mesozoic]

3300 sqft liveable area and 2400 sqft non-liveable area. The new garage area will be about 1900 sqft, so thinking about mini-split.

 

[Mesozoic]

Ok, so the garage has been permitted and approved for a 3-ton Daikin mini-split unit with 4 ports. Parts for the system can be had for around $4K and I was shocked when I got a quote for the labor to install the system: $6K. Of course, I'm not an HVAC expert, but would that be a reasonable cost for installation in a rough framed structure with 4 wall units and a single outdoor unit? I was thinking a few thousand dollars, but not more than the cost of the system itself.

 

[yeldogt]

Has a load been done? Are you removing one of the systems ....

what size rooms are the heads going into? How many installs has the company done using the multi-head units.

Around me using decent equipment it's 4k per- head .with simple install ... above three it drops a bit per head on the multi. Simple install . My three singles are 16k .. walls are open ... but it's still complex. I did a 3 zone a few years ago that was 14k

 

[Mesozoic]

Has a load been done? Are you removing one of the systems ....

what size rooms are the heads going into? How many installs has the company done using the multi-head units.

Around me using decent equipment it's 4k per- head .with simple install ... above three it drops a bit per head on the multi. Simple install . My three singles are 16k .. walls are open ... but it's still complex. I did a 3 zone a few years ago that was 14k

Yes, a load calculation (Manual J) on the new space results in capacity just shy of a 3-ton. The other two 3-ton systems will remain in place as they are cooling the main house.

The split is 7K + 12K + 12K + 12K via the following parts:

• Outdoor unit: Daikin 4MXS36RMVJU
• Indoor unit (office, 7000 BTU): Daikin CTXS07LVJU
• Indoor unit (main floor, 12000 BTU): Daikin FTXS12LVJU
• Indoor unit (main floor, 12000 BTU): Daikin FTXS12LVJU
• Indoor unit (main floor, 12000 BTU): Daikin FTXS12LVJU

I was hoping I could get the labor done for less than $3K, but my HVAC sub (through my GC) indicated otherwise.

 

[yeldogt]

The multi equipment is more expensive (compressor) -- lots of parts. A good hidden install takes time ... looping lines outside and covering them is much easier. each head needs a proper drain. With new construction it's typical to install drywall behind and set the brackets all up .... then you have to come back later and do the install.

It's not just -- drill a hole and plant the compressor. like you can on the back of a garage

 

[Mesozoic]

I hear that, but my project looks like this right now. So now drywall or anything - this should be the best time to install the condensate, refrigerate, and power lines, right?

 

[yeldogt]

No -- you need all the walls in place and it set ....make sure the plumber have been there.

Goes in around the same as electric .... the mini installers need to place a section of drywall up so it all set ... then they take the head down and protect the line sets.

After the dry wall is finished they can come back and attach the head and get it all going -- so it's unpack and pack ...store .. unpack.

 

[Mesozoic]

Should be finished with framing today, but just need to get the ball rolling with the HVAC installers - lead times are long these days.

 

[yeldogt]

I'm with you on that one.

My electric starts Monday

 

[Mesozoic]

I'm with you on that one.

My electric starts Monday

Nice! Good luck with your electrical! I've got some cameras setup to record some time lapse for me - I will share once it's all framed.