bidding out/designing central AC

SteamedInWharton · July 2022

I'd like to get traditional central AC or some kind of multi split or ducted split for my house. It's about 1625 sq. ft. conditioned space above grade, single story ranch, accessible attic.

Why? I'm running 4 window units: one in each of the three bedrooms, plus a 10k BTU unit in my living room. The bedrooms are small (~130 sq. ft. ) and even 5000 BTU window units are a bit large for them. They're noisy, it's hard to hear the TV over them. The 10k unit is so big it sits in the window year round. I'd like to cool my combined kitchen/dining area but that would probably need a 15k unit.

Lastly, my 3rd bedroom (office) is essentially an interior room: its windows open into my sun porch. Exhausting the AC into there makes the sun porch even hotter so I can't use it for its intended purpose nor can I open an interior door to cool the house. This is the primary driver for an upgrade.

This room would be ideal for a mini-split but only a 4000 or 5000 BTU model. They don't make them that small. They do make small cassettes that can be connected to a multi. I don't object doing a 12k multi and putting three heads in the bedrooms and maybe doing a separate system BUT my understanding is that by the time all this is done it might cost as much as doing a proper, traditional central air.

Goals:

one or two zones
a compressor that doesn't stick out too much and block my side walkway. A rectangular compressor or two smaller compressors would be fine.
Nice HEPA filter and UV bulbs in the ductwork. I have mold issues (hopefully mitigated by upcoming basement work)
interface to my existing Honeywell thermostat and EIM (if two zone, only one zone needs to connect)

Heating? Not needed. I have single pipe steam on natural gas. I won't install a furnace for scorched air. If a system includes heat pump features at minimal cost, I could see using them and letting steam be the backup if the cost-savings are there (not sure if steam is suitable as backup heat, the cycles are rather long).

Timing? Not a rush so could be done during slow season.

My concerns: I see all this info about "Manuals" for heat loss calculations, duct sizing, system sizing, etc. I also know that as Dan says "contractors can't do math." Do I need math to actually be done? I can imagine most contractors would come in, eyeball it, apply their rules of thumb, and submit a bid. Is that good enough? Is it worth hiring someone to do heat loss calculations, system sizing, design, prepare plans, etc. and then have contractors bid according to the design?

And if Morris County, NJ is in your service area, feel free to DM.

GGross · July 2022

You can get mini split heads for multi-head units that are rated at 7k btu cooling. With inverter driven mini splits that rated capacity is when they are locked at 60hz, in actual operation they modulate for quite a range. Personally I see many oversized mini split installs, likely because they modulate and a salesmen will think "better safe than sorry"

Anyway the bottom line is that to do this right you need to get a proper cooling load calculated for the spaces you are going to be cooling, and have the most appropriate sized equipment installed to do that job. A good contractor will offer this to you.

for your goals

1) no problem
2) most of them are "skinny"
3) cassettes and wall units don't have ductwork, they do make small fan coil units that will have ductwork attached, and many mini split manufacturers offer full air handlers. there are products that are "in room UV and hepa filters" I actually own one made by field controls, you can use something like that to clean the air if you go with ductless options
4) Don't ever use a 3rd party stat on a mini split, they do not function correctly generally speaking and you will not be getting what you paid for. This was news to me very recently and made me question why it is even an option. The answer is because homeowners like the look of their thermostat.

EDIT: to add, the cost of install is generally near the cost of central A/C depending on many factors, mini splits are not a "cheap" product

Hot_water_fan · July 2022

If you can, use the attic to add central ductwork. It'll better match the loads of the individual rooms, easier to service the filter, quieter, etc. A ducted minisplit would be an easy option.

SteamedInWharton · July 2022

4) Don't ever use a 3rd party stat on a mini split, they do not function correctly generally speaking and you will not be getting what you paid for. This was news to me very recently and made me question why it is even an option. The answer is because homeowners like the look of their thermostat.

EDIT: to add, the cost of install is generally near the cost of central A/C depending on many factors, mini splits are not a "cheap" product

Existing T-stat would be more for a true central system. And yes I agree that splits seem cheaper but can be more expensive then traditional. I was documenting my thought process of "a split would be nice, but now I can consider traditional." I'm not tied to a particular solution.

clammy · July 2022

Have a competent Hvac contractor give you a quote on a single zone central unit . Mini splits are nice but I see them as a disposal system after there life span is done 10 to 15 year span max u replace it all . I ve yet to hear of anyone replace a compressor in a unit rather just replace the unit . Having zones is nice but I find I de rather see one set of line sets w covers on the side of a house then multi line set and covers all over the place .
Up sides to central only one indoor unit to be service and maintained simple and easy . If you want to get fancy then get a Bosch investor outdoor condensing unit and a ecm variable speed air handler a good hepa filtration system and of course properly sized duct work which of course is sealed ,externally insulated r8 and use all r 8 flex including return duct . Spend the money and make sure decent supply registers and wide bar filter grills not garbage stamped faced grills . Depending on your layout I find for even temperature though out the home it’s well advised to have some returns installed in bedrooms . Being everyone lives w there bedroom doors closed it helps w door closed issues and even temps .
All said it’s a tilted scale onto which is better ,again personally a central air condition system is that total comfort while mini splits are great for impossible areas to get duct work but as a whole house install ,get ready to sit down I ve found its usually cheaper to do central then a 3 or 4 head mini split when all I say said and done .
Also all mini split system are not the same ,a high seer invertor heat pump in the 18 seer and above range is $ and it’s not central . Chose wisely every one likes to not waste money and hope to be comfortable without any issues .
Also remember not all Hvac contractor and installers are the same make sure your comparing apples to apples and as for mini splits Mitsubishi hyper heat units that’s about it ,you get what you pay for and there’s a reason some only use Mitz and other use what ever . And please remember that not all contractors are the same and price and quality always go hand and hand and please make sure any attic duct work is insulated in real duct wrap not garbage bubble wrap ,it s cheap and the sign of seer laziness plus it does not work and it’s rating are bull .
Peace and good luck clammy

Hot_water_fan · July 2022

@clammy I hate the term minisplit! It’s confusing and means too many things to too many different people. I like ducted vs ductless but even that’s insufficient when systems can have both.

SteamedInWharton · July 2022

Peace and good luck clammy

@clammy Mahwah, huh? Are you strictly steam/hydronic or do you do A/C? Would you like to do an estimate after your busy season ends?
-SteamedInWharton (Yes, I'm in Wharton, Morris County)

P.S. I do need a steam guy to do an annual on my boiler, even if you don't do A/C.

clammy · July 2022

I mostly do Hvac work which including retro fit air conditioning installs , boiler installs and re pairs and re dos on just about any thing heating or cooling related ,I don’t do any new construction work ,not that I’m not capable just ain’t my thing and I don’t deal w general contractors . You can e mail me at rdeuza@aol.com peace and good luck clammy

bidding out/designing central AC

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