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Discussion Starter · #1 ·
Hello everyone.
Seems like a great forum.

I would like imput on the indoor unit sizing versus the Whole House Manual J load calcs. I've listed the areas, s/f, and the higher of the heating/cooling Manual j calcs.

I had two contractor give me estimates and they have figured (without any calcs) that I need (2) 4 ton systems... hmm. One tried to have me commit on the spot. Didn't understand how they could know without any calcs what I need.

So... I paid for some Manual J calcs and would now like to size the indoor units properly. I plugged in the indoor units .... Size is under what I was told I need but over what I think it should be.

Not sure what the consequences are using too large of a system.
The system I plan on installing does have a variable speed heat pump.

Here are the room sizes.

1st floor kitchen, dining room, toilet. 45o s/f total. (three interconnected rooms)

Manual J calcs: 5586 btu's / 257 cfm.
Figured - 18,000btu slim duct unit, 3 ducts approx. 10 ft each max.
These rooms are all interconnected and would be run through upgraded existing ceiling vents.

1st flr Family Room (276 s/f).
Manual J - 3845, cfm 183.
Figured 12,000 btu wall unit.

1st flr office (188 s/f)
Manual J - 2424 btu's, cfm 134.
Figured 7000 btu wall unit.

1st flr bedroom (130 s/f)
Manual J - 2696 btu's, 125 cfm.
Figured 7000 btu wall unit.

1st flr TV Room/ hallway (373 s/f).
Manual J - 3171 btu's, 178 cfm.
Figured 12000 slim duct, two ducts for main room and hallway. Each duct about 8' long.

2nd floor Master Bedroom/master bath (465 s/f)
Manual J - 3775 btu's, cfm 189.
Figured 12,000 slim duct with two ducts approx. 10 l/f each.

2nd flr Bedroom / bath (442 s/f)
Manual J - 4262, cfm 199.
Figured 12,000 btu slim duct w/two ducts approx. 8' long.


Any comments would be appreciated.
dmyster
 

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Your manual j numbers seem okay, i roughly tallied up around 2300 sq ft and 25k btu/hour.

For a whole house a/c based on what u put i would request a 2.5 ton if central. (maybe 3 if it's split between two 1.5 ton systems for zoning)

A single 4 ton would be grossly oversized in all but the hottest climates (with ducts in attic), never mind two 4 ton units. Evidently a lot of people selling this stuff are lost.

For mini-splits you can safely oversize a bit because most are variable capacity.

Putting a mini in each room could get very expensive; how is the place cooled now?
 

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Discussion Starter · #3 ·
The 1st floor rooms (except the TV Room) are existing. No cooling - just an old old gas furnace that doesn't work. Don't really need it except maybe a few times during the winter - I'm in San Diego.

The 1st floor TV Room and the 2nd floor bedrooms are a two story addition that I am building as an owner/builder. No heating or cooling is installed.
 

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Assuming that the ductwork is decent (hopefully in a basement or crawlspace, not in an attic), it would make more sense to get a central unit for the main floor.

If the furnace has a working blower u can use it for that purpose.

You could also get an air handler installed.

May be a good idea to spend a little more and get a heatpump. Gas furnace is overkill if u switch it on once in a blue moon, climate doesn't really drop to freezing or below -> even if electricity is expensive.

For the addition u could get a separate central system or mini-split. (mini split is better than ducts in a hot attic any day)
 

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Looks like you want to install 4 times as much cooling as the calc says you need.
 

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Discussion Starter · #6 ·
In response to "beenthere" ..... I don't understand why the two contractors want such big systems... I had the calcs done and maybe there is something I'm missing .... I figured a smaller system than they figured but to me it is still way over sized which is why I am asking for opinions from someone that knows. YOur saying it's 4x what I need... Yeh ... again that is why I am asking.

In response to user_12345a.....

The furnace I had was installed in the hallway... which I don't want.
One of the contractors (only one) looked up in the attic and said "boy .. not much room up here" ... he did recommend a garage unit or indoor unit.

I called a third contractor ...... I did not say anything about having calcs ... just had him do his thing.... he didn't take any measurements ... didn't say much either. Said he would email his price two days ago... have not heard from him.

Ugh...

Maybe I should ask this question.... The Manual J calcs should be accurate.
If they say I need 3775 btu's for cooling the Master Bedroom and Bathroom..... both being serviced by (1) slim duct with (2) approx. 8' long ducts... the heat pump would be mounted on the same level .... line set is 35 l/f.... what slim duct size unit would you use?

Dana
 

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No expert on the minis - naturally you would want to match the heat gain closely.

If I was sizing a window unit it would be the minimum, 5000 btu for a 3000 btu cooling load. I don't think the mini-split heads come that small but they modulate down.
 

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You said they wanted to do 8 tons. You listed you want to do 6.5 tons. But your posted calc is for just over 2 tons.

You need to find a way to get to more rooms/areas with smaller units then you are listing.
 

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Discussion Starter · #9 · (Edited)
Yeh... I don't like to say this but maybe the two contractors (actually their estimators) ... don't understand what the equipment's capabilities are.... I don't either but one look at calcs and what they wanted to sell me on made me jump.

I've been looking at the ranges (cfm & btu's) of the indoor units and will probably be downsizing all around to fit the ranges as best as possible and possibly combining adjacent smaller areas.

BIG QUESTION .... (opinion only) - will a 36000 btu heat pump work OK with (2) 7,000 wall units, (1) 9,000 slim duct with (2) short duct runs? That would leave room later on in case I wanted to add a unit in the garage?

Dana
 

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Yeh... I don't like to say this but maybe the two contractors (actually their estimators) ... don't understand what the equipment's capabilities are.... I don't either but one look at calcs and what they wanted to sell me on made me jump.

I've been looking at the ranges (cfm & btu's) of the indoor units and will probably be downsizing all around to fit the ranges as best as possible and possibly combining adjacent smaller areas.

BIG QUESTION .... (opinion only) - will a 36000 btu heat pump work OK with (2) 7,000 wall units, (1) 9,000 slim duct with (2) short duct runs? That would leave room later on in case I wanted to add a unit in the garage?

Dana
Don't know myself. I don't do much with mini splits.
 

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We call this "diversity" you should match the indoor units to the outdoor capacity. Generally, the range is between total indoor unit capacity is 100-150% of outdoor capacity. You would be at 64% leaving you with little room left to modulate down. (as it's already modulating all the time)

Which manufacturer and model line did you want to go with? Each one has different ratings.

Cheers!
 

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Discussion Starter · #12 ·
I am thinking about the Daikin 36000 heat pump (4MXS36NMVJU) with their indoor units but that may change if the contractor I choose says no way.

The Daikin unit will handle up to 48,000 btu's (on the indoor units).

There may be times during the hottest months where I would only have the wall unit (7,000btu's) in the office (175 s/f) running... I wonder if that would work? I may have to have the other adjacent zones running also to meet the outdoor unit's minimum capacity.

I do have solar so I'm not too worried about saving every extra penny...
but on the other hand .... I really don't need the entire house cooled during the day. Does that make sense?
 
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