# HVAC Equipment Sizing VS Load Calculations

| Posted in General Questions on

I’m building a small house in zone 4a.  1008SF, 2×6 Walls with ZipR6, conditioned craw space with ICF walls, R60 Attic with zip for the ceiling air barrier, Alpen R7.7 windows, and close attention to air sealing at every step.  All HVAC Equipment will be in the conditioned crawl space.  I have input this data into CoolCalc and Better Built HVAC Sizing tool as best I can.  They have given me different data, but not extremely.

Cool Calc is giving Heating of 11485BTUH, Total Cooling of 8366BTUH
HVAC Sizing tool is Heating of 8357BTUH, Total Cooling of 10500BTUH
Design Conditions of 5F and 95F.

My main question is how close should I size my equipment to these numbers.  I’m looking at the Fujitsu Ducted units.  The 12k will do 16000 BTUH Heating and 10500BTUH Cooling.  Which by one of the calculators is cutting it really close on cooling.  Meaning that on days that it gets above 95 it might struggle keeping up?

I can go up to the 18K unit which will more than cover my loads.  The specs also show that the 18k and the 12K both have the same minimum BTUH of 3100.  So I’m mainly only saving money by purchasing the 12k.  Unless there is some other downside to the larger unit that I’m missing?  Any advice on the topic would be appreciated.  Thanks

Justin

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### Replies

1. Expert Member
| | #1

I'm looking at the NEEP.org sheet for the 12K ducted Fujitsu here:
https://ashp.neep.org/#!/product/51560/7/25000/95/7500/0///0

At 5F it only produces 10,120 BTU/hr max, which seems a little light.

Note that throughout the temperature range, for heating and cooling, COP is higher at minimum output than at maximum. So I wouldn't be afraid of oversizing a bit.

1. | | #2

I believe you have the 9k model pulled up in that link and I think that’s a bit small. Thanks for showing me another nice site for reference. I will have to search the 12k and 18k versions to see what data they have available. I’ve been leaning towards the larger unit since they both have the same minimum values.

2. Expert Member
| | #3

At the Neep.org site, when you're looking at a unit click on the button that says "Advanced Data -- System Sizing." Put in your zip code and heating and cooling loads.

That will give you a wealth of information.

3. | | #4

Different output from different software is annoying, reason could be software itself, could be user, or could be misunderstanding in the phrase used in certain software. same word in different software could mean different things, very very annoying.

But as for design, for commercial building ASRHAE suggest oversize the heating by 25%, and oversize cooling by 15%, based on 99.6% extreme temp, which should be your design temp. Then size up when you select the system, meaning if your load calc shows 10 MBH, you oversize it to 12.5 MBH, and if the manufacturer provides 12 MBH and 15 MBH, you should go with MBH.

But of course this is for commercial building, if you are willing to do some supplemental heating with other equipment in your own home then it is totally fine, a slightly undersized system won't do you any freeze damage.

1. Expert Member
| | #6

In resi world you should always size for 99%. The rest of the hours left over usually happen in the middle of the night so the heating not keeping up simply does not matter.

The oversizing recommendation is usually the max oversize as man J itself is conservative and there is already some oversizing built into the calculations.

In general you want to right size the unit but if the next size up has similar turndown, you can get a bit of efficiency boost by running the unit at lower power. The one thing you have to watch is low load cycling as this can kill any efficiency improvement from oversizing, generally you don't want the unit to cycle more than 30% of the year.

The NEEP calculator is great for figuring some of these details out and comparing different units.

4. | | #5

Just so you understand the big risk from oversizing is in the cooling mode in muggy climates.

The oversized unit may not have a long enough run time to remove much moisture from the air. You will not feel comfortable at your normal set point so one lowers the set. The classic complaint is the house is cold and clammy.

I like having some strip heat, as sooner or later the HP will go down and having some strip heat means the down time is an annoyance in that the electric meter is spinning faster than it should. Without the strip heat it is a full-on crisis you need someone out today and will the pipes freeze before the overnight package of parts arrives on day 4?

The other thing the strip heat get you is your installers confidence. With the strip heat you can talk him into installing a smaller unit he would refuse without the strips.

Yes, the strip do use more kWh per BTU but it is not as bad is one might imagine.

Consider that if the strips are locked out unless it is very cold outside at that point the HP COP is going to be pretty low 2-2.5 may be lower.

Walta

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