HVAC advice for my newbuild (30'x58')

[grkmec]

Need some advice from the HVAC brain trust. Here is my situation:

I am kicking off construction of my new 30' x 58' detached garage (around 1,800 sq ft including bump out). Gable end on 30' and it will have (2) 10' doors. The place will be 2'x6' construction and ceiling with be ~ 13.5' with a raised tie truss - trusses will be 16" OC. The walls will get 3" of closed spray foam (~R20) and I plan to fill the top chord of the truss (2x8) with 7.5" of close cell foam (~R50). There will be no ridge / sofit vent. The rear of the garage will be a gym / play / hang out area. So I am trying to figure out best way to heat and cool. Live in CT.

Here are my preferences / considerations:

- I don't want anything that will be loud. So no modine heater. I have one in my attached garage and that thing is way too loud

- I prefer to avoid having HVAC equipment taking up valuable floor space. I also don't want to build an HVAC / mechanicals closet.

- I would like something that can be controlled via wifi through my phone with an app

- I would like to incorporate some kind of air exchange system (ERV) so I can smoke an occasional cigar during the winter months

- When its cold out, say 20 degrees or lower, I don't want my system to struggle to heat the place to 65. Likewise, if I have to open the garage doors for 5 minutes, I want the system to not take forever to heat the place back up.


So I have gone back and forth with what is best solution. I am considering the following:

1) Go with mini-splits. Given the size of the place, I suspect I would need (2) 30-36k wall mounted units. Would there be any way for these to be linked together or would need separate thermostats?

2) Go with a 5 ton packaged unit that runs on gas + electric. Equipment sits outside and I run ducts up in truss area with registers in ceiling. Downside is that I have ugly looking ducts run up the rear gable of the garage

3) Go with a high efficiency hot air furnace that is like 120-140k BTU that is multi-stage, then hang air handler from ceiling, and have 4-5 ton AC compressor outside.

Anything else to consider? Any advice on what I should do ?

 

[HotRodBoater]

Any particular reason radiant floor heat isn't on your list?

Sent from my SM-G955U using Tapatalk

 

[justinjoyal]

I would consider a radiant setup for heat.

Mini-splits for cooling and the in-betweens when radiant doesnt shine.

You need a load calc if you're gonna do it right.

 

[grkmec]

I am open to idea of radiant heat... have some questions:

1) What is it going to cost to do 1,800 sq feet?

2) What can I expect my ambient temperature to be inside when its 20 degrees outside? Is 65 degrees possible?

3) If the garage doors are open for 5 minutes and its cold out, how quickly will garage get back up to temperature?

4) If about 600 sq feet of the garage floor is covered by 1/2" rubber gym flooring, will radiant heat be able to work? Or will the rubber floor act as an insulation barrier preventing the heat from heating 1/3 of my garage

5) I would still need AC solution. So I still need upwards of 4 tons of cooling capacity. So if radiant is $10 / ft, then I could be looking at close to $30k in expenses going this route ?

 

[Fix Until Broke]

1) are you DIY or paying to have it done?

2) Absolutely - no problem

3) Very quickly with floor heat - 5-10 min max

4) Yes, will be "slower" but still functional

5) A single forced air A/C unit is likely least expensive, however a couple mini-splits will be the quietest solution.

 

[justinjoyal]

How did you figure the 4-ton of cooling ?

 

[grkmec]

How did you figure the 4-ton of cooling ?

Rule of thumb is 500 sq per 1 ton. I have 1,800 / 500 = 3.6 tons. But I have 13 foot ceilings which would argue for more tonnage. On other hand I will have closed cell spray foam and no windows one of the 58' sides. So this argues between 4-5 tons

I also used this website:
https://www.highseer.com/pages/hvac-load-calculator

Inputs: Region 3, 1,800sqft, 13' ceiling, better than avg insulation, average sun, average windows, average window design, no sunroom, 2 occupants or less, no kitchen so extra devices.

Recommended Capacity = 48,000 BTU on calculated cooling load of 58,500 BTU

So that's 4 tons recommended on nearly 5 ton calculated cooling load

Given all the info I have provided, do you arrive at a different conclusion ?

 

[justinjoyal]

Rules of thumbs are obsolete and often provide mediocre results.

If you want the best possible results you need a load calc.

500 sq ft per ton is way too much cooling. I would need over 5 tons of cooling in my house according to that. Guess what, I got 2 tons and everything’s great. [emoji16]

If you dont know how to perform a load calc I could possibly help you with that.

Needed:

Floorplan , type of doors/windows and their size and number and on which side of the building they are, zipcode/town, desired indoor cooling temp.. front door orientation as well.

Also is it built on a uninsulated slab?

 

[Jackfre]

I was the Fujitsu Rep in CT for 14 years and over the years became very comfortable with 600-700 sq ft/ton...but Justin is correct. "Rule of Thumb" is exactly that, especially on a new build. Keep in mind that the cool air will settle and the temp at the ceiling will be higher. It is difficult to guess size as your use of the area will drive much of that. will you divide off the gym area? In CT your biggest issue will be dehumidification.
Radiant is great heat, the best perhaps, but you pay for it on the installation as well as operationally. I am in the business and can do all the work and buy everything wholesale and chose not to do radiant on my shop. I am very much in the minority here and if you choose to go that way there are many radiant guys in your area. Pick carefully.
I would go with two mini-splits properly positioned. Pay attention to your orientation of the buildings glazing. Western and NW windows drive the cooling load. In addition to the mini-splits I would install a Rinnai EX38 gas fired direct vent wall furnace. They are quiet and excellent. I also represented Rinnai for 27 years and there are in excess of 250,000 of those units installed in New England. They are excellent and quiet. All of these units can be controlled by wi-fi.

 

[grkmec]

View media item 83087
Floor Plan is wide open. (2) 10' wide x 9' garage doors on the North end. No windows on the east and south side. On the west side will have 6' wide double french doors (8ft tall - probably anderson 400 series). Then will have (2) double hung windows to the left and right of french door (3' wide x 6' tall total window size flanking the double french doors). So the west elevation is 58' long.

zip code = 06902

desired indoor cooling temp = 70

Slab will have either 2" of foamular 250 or 2-3" of high density closed cell foam (eg. 3lb foam). Slab will be 6" thick.

 

[justinjoyal]

I’ll try to run a calc when I get home tonight.

 

[nsula_country]

Using that same calculator says I need 128k btu cooling for shop in my signature... I have 60k btu, but haven't tested it yet. But 68k btu of electric heat works great!

CT

 

[theoldwizard1]

- I would like to incorporate some kind of air exchange system (ERV) so I can smoke an occasional cigar during the winter months

This is likely going to be your BIGGEST issue, I have never seen any kind of HVAC with a built in air exchanger, they are always "add-on" and the good ones are not cheap !

 

[theoldwizard1]

I am open to idea of radiant heat... have some questions:

2) What can I expect my ambient temperature to be inside when its 20 degrees outside? Is 65 degrees possible?

No issue assuming you have properly sized boiler.

3) If the garage doors are open for 5 minutes and its cold out, how quickly will garage get back up to temperature?

Radiant is NOT know for quick recovery. Large open space will require circulation fans. Adding another heat source, like a mini-split can help this a lot.

4) If about 600 sq feet of the garage floor is covered by 1/2" rubber gym flooring, will radiant heat be able to work? Or will the rubber floor act as an insulation barrier preventing the heat from heating 1/3 of my garage.

I don't think this will be a problem, but those mats will be warm !

5) I would still need AC solution. So I still need upwards of 4 tons of cooling capacity. So if radiant is $10 / ft, then I could be looking at close to $30k in expenses going this route ?

I am a big fan of mini-split heat pumps, but his is "buyer beware" territory ! Some can not produce heat when the outside temp gets below about 20F. Good one work well to below ZERO !

When it comes to sizing, you need some one who really knows what they are doing. A build your size is going to require 4 maybe even 6 air handlers.
Large mini-splits, especially the one capable of driving more than 1 air handler, are less efficient than smaller one. Smaller ones can be "slaved" together or left to operate independently.


Mini-split heat pumps are BY FAR the most cost efftive heating and cooling system to operate, but they are not cheap to install ! They work best from about 0F to 90F ambient. Remember, their job is to "move" heat from one place to another, not "make" heat.

 

[finn]

Our brand new Mitsubishi hyper heat works great at five above, but it’s pathetic below zero.

 

[justinjoyal]

image pour forum

 

[grkmec]

A unit like this is rated down to -15...

http://www.fujitsugeneral.com/us/products/split/wall/asu30rlxeh.html

or this down to -13

https://www.acwholesalers.com/Mitsubishi-PUZ-HA30NHA5-PKA-A30KA7/p88244.html

 

[grkmec]

justinjoyal - thank you. What would you recommend then?

 

[justinjoyal]

Budget? Filtration needs?

Will you be opening the garage doors often?

 

[grkmec]

Budget? Filtration needs?

Will you be opening the garage doors often?

Would love to stay under $20k installed or less. Anything that will filter occasional cigar smoke would be a big plus but not required. Garage doors will be opened infrequently and for short times only.

 

[justinjoyal]

If you dont plan on doing heavy (wood, metal, etc.) work in there and don't need the better filtration of a central ducted system, I would go ductless.

3 x 9k units on the east wall, evenly spaced across the wall lenght. If you want to get closer to the heating load, you could do 2 x 9k + 1 x 18k. Either way, you don't have to use all three at onces so if the cooling or heating load is reduced you use only one or two heads, but for the colder times of the year you would have more heat available from your mini-splits before you would have to use an electric or gaz fired heater.

The load could be handled by fewer indoor units, but given the large open space, you would get better comfort on heavy-load days by using at least 2, ideally 3 units.

If you want the better units, you will have to go with separate "single-room" systems. If not, "multi-room" systems work just fine. They are a bit less efficient though, they don't work as good in colder temperatures and in the event that the outdoor unit would break down, all your indoor units are useless.

You could add a simple HRV or ERV setup (not familiar with CT weather.) to that for some fresh air.

 

[grkmec]

image pour forum

So I got a proposal today from my HVAC guy. He also did a load analysis and said heating load was 42k BTU and cooling was 16k.

He recommend (2) 2-ton (24k) Fujitsu units (AOU24RLXFWH / ASURLF) and he said the following:

• UNIT IS RATED TO HEAT DOWN TO -15 DEGREES
• @-5 DEGREES OUTDOOR EACH UNIT WILL PRODUCE 22.5 K BTU’S W/ 70 DEGREE INDOOR TEMP
• @+5 DEGREES OUTDOOR EACH UNIT WILL PRODUCE 25 K BTU’S W/ 70 DEGREE INDOOR TEMP

Total cost of install w/ wifi is $9,870

Option to go to (2) 30k units (AOU30RLXEH / ASU30RLE)
• UNIT IS RATED TO HEAT DOWN TO -15 DEGREES
• @-5 DEGREES OUTDOOR EACH UNIT WILL PRODUCE 27K BTU’S W/ 70 DEGREE INDOOR TEMP
• @+5 DEGREES OUTDOOR EACH UNIT WILL PRODUCE 32K BTU’S W/ 70 DEGREE INDOOR TEMP

Total cost of install w/ wifi is $11,660

So +$1,790 to go to 30k units. Is this a waste of money or worth it to heat up faster in the winter ?

I was also quoted a Fantech SER 150 ERV for $3,470 installed. I am thinking of upping it to SER 200 unit as there could be some occassional cigar smoking

 

[justinjoyal]

If heat is what you want, you can oversize. However, if cooling is the primary need, oversizing is not good.

One 18k unit would suffice for your cooling needs, a 24k unit (I supose the plan was to run only one of the two in cooling mode) is already oversized but being in a garage it’s not really an issue IMO, other than paying more for BTUs you don’t need...

What will be the backup heat source ?

 

[grkmec]

If heat is what you want, you can oversize. However, if cooling is the primary need, oversizing is not good.

One 18k unit would suffice for your cooling needs, a 24k unit (I supose the plan was to run only one of the two in cooling mode) is already oversized but being in a garage it’s not really an issue IMO, other than paying more for BTUs you don’t need...

What will be the backup heat source ?

Yea, the plan was to run just 1 unit during the summer for AC. No back-up heat source other than having two mini-splits. So if 1 goes down I have the other. And if my electric goes down, I have a 45kw whole house generator.

 

[justinjoyal]

Yea, the plan was to run just 1 unit during the summer for AC. No back-up heat source other than having two mini-splits. So if 1 goes down I have the other. And if my electric goes down, I have a 45kw whole house generator.

2 x 24 + backup heat wouldve been the best, however I guess it’s very unlikely that both your minis would fail at the same time. If one was to fail, 1 x 24k will be enough to keep anything from freezing while you get the other one fixed.

I’d do 2 24k.

 

[Aerospace Eng]

Our brand new Mitsubishi hyper heat works great at five above, but it’s pathetic below zero.

I have Mitsubishi hyper-heats in some of the hangars I have built. There is a switch that you can set for extra heating when it is cold.

Mine have no problem keeping the hangars warm (60F) when it is -10 at night.

 

[dcg9381]

Is the switch triggering some sort of aux heat strip? Many ductless (heat pump) systems really have rapid decreases in heating capacity once below freezing...

 

[Aerospace Eng]

Is the switch triggering some sort of aux heat strip? Many ductless (heat pump) systems really have rapid decreases in heating capacity once below freezing...

No. I do not have any aux heating. It is just a dip switch that tells the microcontroller to run the compressor and outdoor unit fan differently. These are variable speed compressor units, which helps the overall efficiency a lot.

I have become a big believer in the modern variable compressor heat pumps. Some people that built similar hangars went with propane fired boilers and radiant heat. My heating bills are about 1/3 of theirs, and, as a bonus, my hangars are air conditioned and the humidity stays low in the summer which keeps corrosion down. Of course, they didn't insulate under the slab properly (they used 3/4" bubble insulation, rather than 4" of foam they should have), and didn't get the tubes at the proper height in the slab. A few have added heat pumps to supplement the radiant in the winter and give ac in the summer, but that just makes any ROI even worse.

The most efficient would be a ground source heat pump to a radiant system, and I considered it when I built my hangars, but the cost of the wells was prohibitive. When the energy savings takes 20+ years to recover the first cost difference, it isn't worth it.

The fundamental drawback to radiant is the long lead time with temperature changes (days). Basically you have to turn the system on in October (depending on climate) and keep it on until April (depending on climate). It is really good for keeping a space at a constant temperature, and at recovery when you open the door, but if you aren't in the space all the time, even though the system is more efficient, the total energy used is more than a less efficient system.

With a ductless heat pump, I can keep my hangars at 55F unless I am there, and have it warm up to between 60F and 65F in about 30 minutes, which is fine for working in.

 

[dcg9381]

A unit like this is rated down to -15...

I looked at an operator manual for one of those units, ((OM)AOU9-12RL20:

Outdoor temp range: "15 to 75 °F (-10 to 24 °C)"

The Daikin manuals are different, they show ability to produce heat at outdoor temp. That unit may be able to operate down to 15F, but what you're not shown is that the heating BTU capacity drops radically.... So if you have a 24k unit, it may be producing 70% less capacity than it does at 40 degrees F.

If I was in a northern climate, I'd be wary about one of these as the only heat source.

 

[dcg9381]

No. I do not have any aux heating. It is just a dip switch that tells the microcontroller to run the compressor and outdoor unit fan differently. These are variable speed compressor units, which helps the overall efficiency a lot.

With a ductless heat pump, I can keep my hangars at 55F unless I am there, and have it warm up to between 60F and 65F in about 30 minutes, which is fine for working in.

That's great. Remind me what your geography is and how cold it gets there? I'm an airport guy too... But down here in Texas, insulated hangers don't freeze inside, so heating really isn't an issue.

My experience with residential heat pumps below freezing - they have a massive capacity loss. Usually not an issue here. I'd just be wary of a heat-pump-only system with no aux heat if I was anywhere north of Kansas.

Your real-world experience using these units helps..