Mini-split sizing

[DuckRyder]

OK, so we have a basement ranch one system heats and cools the entire house. The basement is being made into a mother-in-law suite. The back porch has been turned into a sun room, it has 6" stud walls with insulation and is comprised mostly of 5x5 windows a glass french door and a glass standard door.

Need to give "auxiliary" control the the main room and heat and cool the sunroom. the basement is ~800 sqft and the sunroom 300 sqft.

I am getting fairly consistent recommendations the 800 sqft between 18000 and 24000 BTU but HVAC contractors ar all over the place on the sunroom from a low of 9000 BTU to a high of 24000BTU.... All claim to have figged the heat load and so forth... one wants to put a 3.5 ton unit with 2 24000BTU indoor units, the rest are at 3 tons with the afore mentioned various sized indoor units.

This huge variance is confusing, can anyone shed any light on what is going on here?

 

[Rand]

Where is scrubby city at?

24000btu a/c for an 800sq ft basement? that is nuts.. the sunroom will vary wildly based on their sun load calculation.

 

[PimTac]

Where is Scruffy City? General region.

I am wondering if the sunroom needs a unit? Heat in the winter, yes but if summers are hot then seal it off.

 

[DuckRyder]

Scruffy City is Tennessee.

The sunroom will have animals so will need to be heated and cooled. Forgot to mention the windows are and doors are quality low e and double pane with argon of course.

 

[Chris Meutsch]

I sell refrigeration for a living; we use a proprietary heatloss calculation that takes into account insulation R-values, amounts of glass, type of glass, etc.
I would imagine some of them are "guessing" based on the amount of windows in the sun room. I'd want to see the actual calculations if I were you.

 

[brianl703]

Mini-splits can modulate down to match the cooling or heating load, and if you are using one for heating, it might make sense to make it larger than what the cooling load is so that it has more heating capacity. They don't have aux heat coils.

They are also not as efficient when running at full output.

 

[Rand]

Sounds like you need separate units on separate thermostats.
Doubt the basement needs much if any cooling. are you going to shut off your current basement vents from main system?
if you shut off the current basement vents will your Main furnace AC ice up from backpressure?
you might end up heating and cooling the basement at the same time.

Lots of issues that need consideration.
What type of main furnace do you have? Heat pump should work pretty good in TN.

 

[Wolf359]

I sell refrigeration for a living; we use a proprietary heatloss calculation that takes into account insulation R-values, amounts of glass, type of glass, etc.
I would imagine some of them are "guessing" based on the amount of windows in the sun room. I'd want to see the actual calculations if I were you.

You mean you do something in addition to a Manual J calculation? My guess is that many plumbers and contractors don't really do a Manual J calculation and just wing it on the btu estimate and just go on the high side so you always end up with more than you need.

HVAC Load Calculation - Maunualj - Whole House Loadcalc

ManualJ load calculator, HVAC calculation, Whole house, - www.loadcalc.net

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HVAC Load Calculator - Manual J Calculation | ServiceTitan

Our free, easy-to-use HVAC load calculator will help you determine the necessary thermal output capacity (in BTUs) of any residential building.

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Perform online ACCA Approved load calculations in minutes. Supports block load or room-by-room. 100% cloud based, can be accessed anywhere, anytime, from any device.

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[Chris Meutsch]

You mean you do something in addition to a Manual J calculation? My guess is that many plumbers and contractors don't really do a Manual J calculation and just wing it on the btu estimate and just go on the high side so you always end up with more than you need.

HVAC Load Calculation - Maunualj - Whole House Loadcalc

ManualJ load calculator, HVAC calculation, Whole house, - www.loadcalc.net

www.loadcalc.net

HVAC Load Calculator - Manual J Calculation | ServiceTitan

Our free, easy-to-use HVAC load calculator will help you determine the necessary thermal output capacity (in BTUs) of any residential building.

www.servicetitan.com

Cool Calc Manual J Software | Create a Free Account | Only $3/Report

Perform online ACCA Approved load calculations in minutes. Supports block load or room-by-room. 100% cloud based, can be accessed anywhere, anytime, from any device.

www.coolcalc.com

That's correct.......just sizing this type of equipment based on BTU's and square footage can certainly lead to undersizing, OR oversizing, which then leads to short-cycling and is bad for the equipment.

 

[DuckRyder]

So the service titan calculator comes up with:

RECOMMENDED EQUIPMENT CAPACITY

9,000-12,000 BTU / 0.75-1 Tons
Calculated Cooling Load: 11,500 BTU / 0.958 Tons

For the sunroom and:

RECOMMENDED EQUIPMENT CAPACITY

18,000-24,000 BTU / 1.5-2 Tons
Calculated Cooling Load: 22,300 BTU / 1.858 Tons

For the basement room

hmm.

 

[Gladtojoin88]

One by one is much more efficient if you can swing them . One outdoor unit to one indoor head. Also id go large in sunroom intentionally due to heat load. As far as basement I'd go small intentionally for dehumidification . I'd want it to run nonstop in ac to keep it nice and dry . So if you they call for an 18k 1.5 ton I'd go 12k 1 ton intentionally in basement and largerer than rated in sunroom due to latent heat load no matter how good windows are sun is sun id go top or higher size wise . And also Mitsubishi has been topnotch for us and user friendly for cleaning blower wheels which is a twice a year project if you want airflow and clean air !

 

[PandaBear]

So the service titan calculator comes up with:

RECOMMENDED EQUIPMENT CAPACITY

9,000-12,000 BTU / 0.75-1 Tons
Calculated Cooling Load: 11,500 BTU / 0.958 Tons

For the sunroom and:

RECOMMENDED EQUIPMENT CAPACITY

18,000-24,000 BTU / 1.5-2 Tons
Calculated Cooling Load: 22,300 BTU / 1.858 Tons

For the basement room

hmm.

Are you sure they are not flipped? Basement most likely need much less cooling than a sunroom.

 

[DuckRyder]

I have one dealer saying to do two, but have not gotten his quotes yet. this of course increases cost by adding more high voltage wiring, disconnects ETC.

2 of them are trying to sell Mitsubishi, 1 Gree, 1 Dakin. The one selling the gree would prefer Dakin but says he cannot get one, the other guy says he can get one in a week.

The Mitsubishi apparently has an installation issue and will have to use a ceiling cassette because of clearance above the windows, Dakin and Gree both dealers say it is no problem under repeated questioning.

This is a basement Ranch, i know I've sort of described it before but the basement living area is buried on 2 sides one side is the wall that opens to the garage (double layer x code drywall and 4" rock wool insulation, solid steel door) the 2 buried walls and the back wall are concrete block. there are 2 andersen sliders that go into the new sunroom. I'm not sure how to rate this insulation, i put average.

The sunroom is below a upper screened porch. There is a deck all the way across the back and the rear wall faces South / very slightly SW. The sunroom is 6" stud walls with 4 5x5 and 1 4x5 windows and will have 6" insulation, but there is only 24" of it, the area above the windows is all header. 2 of these 5x5 are pretty well below ground and look at a retaining wall.

So far i liked the guy selling the Dakin best, but he is also quoting the smallest units. He was the least salesmany...

 

[RhondaHonda]

I, like others don’t think you need much at all for the basement. I’d undersize for humidity reduction. The sunroom sounds like it would need more based on S/SW exposure. I considered something similar for a basement garage but it stays between 55 and 75 year round here without any heat or AC so I haven’t pulled the trigger. All I do is run a standalone dehumidifier in the summer to keep the humidity down. Good luck.

 

[Hillbilly Deluxe]

With a sunroom, you really need the Solar Heat Gain Coefficient number for the glass and the exposure (north, south, east, west). These two piece of info are serious with the heat gain calc for this. Did any of the contractors get the SHGC to perform their calc? If no, then they guessed and there's your issue with the BTUs needed being all over the place.

Here's my opinion-

Daikin is the best. Bar none. They have a different variable refrigerant metering technology. Their equipment is better. They didn't play catchup to the mini-split game. They aren't cheap.

You're better to over-size the equipment a little if possible. I'm not talking put a 4 ton in when the calc says 3.1 tons. Go with 3 in that case. But if it's calling for 3.4 tons or higher, then go with a 4 ton. The reason why, especially with a Daikin, is the unit will only run as much as needed. I'm not saying it will "run" less time, just less capacity. The system is likely to run 18+ hours a day but the capacity might be 15-80%....not 0 or 100%.

 

[Rob_Roy]

So far i liked the guy selling the Dakin best, but he is also quoting the smallest units. He was the least salesmany...

He may be the only one that is actually doing a load calc. Ask him what he used for indoor and outdoor design temperatures and make sure it lines up with how you live, and your typical weather.

e.g. If he assumed 78F indoors with an outdoor temperature of 95F, and you expect the house to be 70F when it is 100F outside, you will be disappointed with the performance.

 

[Jetronic]

the outdoor temp of a basement is ground temps, those won't be 100F

 

[Hillbilly Deluxe]

the outdoor temp of a basement is ground temps, those won't be 100F

LOL


This is why people who don’t have HVAC design experience need to just watch and learn

 

[Rob_Roy]

The numbers I provided were not intended to be specific to his basement, just an example - but your point is taken. The load for the basement will be small compared to an above ground space of the same size.

LOL


This is why people who don’t have HVAC design experience need to just watch and learn

Unfortunately there are plenty of the contractors installing minisplits fall into that bucket.

As for one brand vs. the other, I have a 24k BTU Daikin unit. Size aligned well with manual-J load calc and we have been very impressed with the performance.

 

[Jetronic]

Moisture will be the main issue in my experience, the basement I have swings between 12C (with no heater on, oil furnace is placed there) on the coldest days to 18C in summer. But humidity is high in summer. If there's more hot air coming in, the moisture issue will be worse and temps could go up a bit.