Detailed load calculator (attic storage)

[67King]

A few years ago, I had found a load calculator that was very detailed, but now all I'm finding are more generic calculators. Basically square footage, number of occupants, and climate zone.

I have a 1000SF storage area above one of hte garages and the kitchen. Had is sprayed with foam insulation. Had electrical, condensate drain, and refrigerant lines run in the event I decided to run a mini-split, so it should be plug and play. In the winter, it is incredibly good. Say upper 50's when it is in the lower 20's outside. That's sufficient. I'm afraid the Summer may get too hot based on some days in the upper 80's so far. I don't need "room temperature" cooling or heating. Just want to keep it say mid 50's to mid 80's. This is for storage, not living. Ceilings are 8' in the middle, and the roof is pitched, so down to 0' at the sides, making the average 4'.

May end up wanting to do the workshop, too. It is "walk out basement" grade, so below ground level on one end, at ground level on the other. 12' ceilings, about 950SF. Same general kind of thing, I don't need it to be room temperature, but I'd like some level of climate control.

If anyone has any links, I would greatly appreciate them. I'm looking at a few options, but am afraid I'd be oversizing the system. For teh attic, I'd be inclined to go wtih a 12,000BTU system. Most rules of thumb would have it about 50% larger assuming 8' ceilings and living space for 2. But I'd really like to try to check the math.

 

[Overboost44]

I went to https://www.avenir-online.com/AvenirWeb/product/heatcad and watched about 30 videos so that I could learn loopcad before going with the free trial. You could choose just get the free trial and start working on the load calc, or watch and learn a bit.

I also tried this one out. https://www.loadcalc.net/

ETA: I actually went back and found that it was load calc where I ran a Manual J and a Manual S.

 

[Buitantruo]

You can get closer to a real load estimate by breaking the space into beam spans, joist size, spacing, and tributary width, then plugging those into span tables from your local code book. I keep notes in a simple sheet and even park time stamps with https://www.gigacalculator.com/converters/time-zone-converter.php so I remember which revision matches which inspection. For a 1000 sqft attic, the key is confirming the joists were actually rated for storage, not just ceiling load.

 

[67King]

I’ve had better luck checking the joist size, span, and spacing, then matching them to standard load tables. It gives a clearer idea of what the attic floor can safely hold.

Thanks, but I'm looking at HVAC load, not structural.

 

[BurtEggley]

Is the attic ventilated where warm air and humidity can enter thru vents?

 

[67King]

Is the attic ventilated where warm air and humidity can enter thru vents?

No, it is insulated with the open cell spray foam full thickness of the rafters, which are 2x10. From what I understand, foam isn't really rated the same way fiberglass is, the R value is estimated to be about 50. It is MUCH cooler than a typical attic with ventilation. Just would like it a little bit cooler.

 

[BurtEggley]

most likely the calculator will assume the usual 72F to 78F set temp. You may have to guess, unless a neighbor has the exact same set up. We have a 1400 sq ft house. On a 105F degree day in the California Central Valley, a 2 1/2 ton Carrier R22 AC could just barely keep up in the afternoon. It might run for hours at 75 - 76F. A 3 ton will handle it just fine. My guess is that you need 1-1/2 to 2 tons to meet the temperatures you want. You can always set the temperature as high as you want in summer and as low as you want in winter. You can also set the temperature spread so that it only comes on when the temperature is say 5F higher or lower than you want. If you undersize by too much then you lose control if the unit can't keep up.

 

[67King]

most likely the calculator will assume the usual 72F to 78F set temp......... My guess is that you need 1-1/2 to 2 tons to meet the temperatures you want. You can always set the temperature as high as you want in summer and as low as you want in winter. You can also set the temperature spread so that it only comes on when the temperature is say 5F higher or lower than you want. If you undersize by too much then you lose control if the unit can't keep up.

Yes, most of the rules of thumb would say the same size you came up with. I was looking at 1 ton, most were saying 1.5 ton. But the reason I'm questioning this is that they all look at square footage, none of them look at volume. My 1000SF room has the same volume of air as a 500SF room.

Problem is that I can't find a calculator anywhere. The one I had found a few years ago allowed square footage, climate zone, insulation, and setpoint. I don't want to oversize, because the systems don't run long that way, which doesn't remove humidity, and can lead to mold. I would prefer to undersize than oversize because of that.

Also I have never seen a thermostat that will have so much swing? I can set a "heat to" temperature and a "cool to" temperature, and have them be several degrees apart, but I've not seen one that will let me set the tempertaure to come on at, let's say 83 degrees, but then cool down to 77. The one mini-split we have uses a conventional thermostat, rather than a built in one. If there is a system or a thermostat that does that, could you let me know what it is?

 

[BurtEggley]

prices on units are going to be influenced by the most common sized units. If a person were to look for say a .64 ton or say .78 ton unit then they might exist but the lack of numbers sold of those unit custom sizes would drive the price way up. Whereas a 1.5 or 2 ton unit might be so common the price is lower because there is so much competition. Keep that in mind as you try to determine what you need. Just using this as an example, you might spend a lot of time trying to get the math perfect, only to find that the next size up is so common it costs a lot less than the perfect one.

 

[67King]

prices on units are going to be influenced by the most common sized units. If a person were to look for say a .64 ton or say .78 ton unit then they might exist but the lack of numbers sold of those unit custom sizes would drive the price way up. Whereas a 1.5 or 2 ton unit might be so common the price is lower because there is so much competition. Keep that in mind as you try to determine what you need. Just using this as an example, you might spend a lot of time trying to get the math perfect, only to find that the next size up is so common it costs a lot less than the perfect one.

Looking at mostly NOS R410a stock, since I have a 25# cylinder of it

 

[BurtEggley]

Looking at mostly NOS R410a stock, since I have a 25# cylinder of it

how does that affect the difference between a 1 ton and 1 .5 ton unit?

 

[67King]

how does that affect the difference between a 1 ton and 1 .5 ton unit?

Any R410a is no longer legal to manufacture so any units are just existing inventory

 

[BurtEggley]

that facy just makes it all the more important to choose what is available now rather than trying to be perfect in the tonage you need. If you get a deal on a 1.5 ton vs a 1 ton consider grabbing it. The worst that can happen is you set the thermostat higher, and the temperature spread before it comes on wider.