Air conditioning kw/btu sizing

Ive been in the game for many years. Im quite interested in how people are sizing up the capacity of the units. I seem to go to many houses and find the systems under capacity. I know some people size by the meter square and i know alot of domestic air conditioning guys guess.

http://www.calculator.net/btu-calcul...heat&x=51&y=11

I have come across this site and it seems to be most practical as it accounts ambient temp but not other factors. Then there are some sites which are useless and hard to understand.

I quote for the air conditioning unit to be able cool and heat the whole house with out zoning of half the house or undersizing units. I always calc the size and go the next size up.

There was a way at tafe to size up houses but cant seem to get hold of it. Your comments are appreciated.

I usually measure air flow at the plenum for sizing.

350 to 450 cfm per ton.

So if the real air flow is 800 cfm then it would be able to support a 2 ton system, then once I have the real air flow I then look at over all size of the place, then find a good size unit that fits in the middle of the ranges available.

I've never seen a calculator that can account for all the many variables involved.

Occupancy, size, external heat load, internal heat load, humidity, insulation, occupant expectations. Many of those variables are regional or cultural and ultimately makes sizing a system as much an art rather as pure science.

Two identical homes in the same neighborhood, same orientation, same wall and roof colors but different occupants can call for vastly different systems.

Makes the high efficiency systems attractive. Size the unit based on physical conditions and tailor the system's behavior to occupancy requirements. If needed sizing splits available equipment, upsize the hardware and let the controls fine tune the delivery. All you've got to do is "sell" the much higher cost equipment in the first place (not always an easy thing!)

If the cost of premium equipment becomes an issue, down sizing the split is likely preferred. Will take marginally longer to obtain set point, but usually with a greater perceived comfort.

I'm in Florida, very south eastern United States. High ambient temps much of the year, very high relative humidity. 12000 btu/h per 4000 ft³ of conditioned space for a residence is the rule of thumb, but the aforementioned variables can throw those numbers way out the window.
As noted earlier, you've got to get all the sizing right. Ducting size and layout as well as system capacity. It's all got to match and then meet the users expectation. Won't say it's black magic, probably not VooDoo but science really won't cover it very well either.

We use the Mistral software (provided by HRP or Dean & Wood for free), which has served us well over the last 5 or 6 years.

I'm often submitting quotes, only to be beaten by a cheaper quote that are only cheaper because the equipment is smaller; I’m convinced some companies just take the room size, and don’t consider heat sources such as people and computers.

Last week I was carrying out some maintenance at a local school, and it's IT room with about 20 computers had a 3.5kw Daikin; it works fine in the morning, when nobody is in it and the computers are off!

Sizing a house heat load is not my field but Kitchen heat loads would have to be the worst.
There's not too many customers in my district who would be willing to A/C the entire house, let alone fork out Hundreds of $ per billing period running them. Electricity prices have jumped some 30+ % in the last few years. (Queensland.)

A plain glass window exposed to direct sunlight in Summer has the potential of generating 2 KW of heat per square Meter. Assuming full sunlight exposure for 2 hours.:)

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Last week I was carrying out some maintenance at a local school, and it's IT room with about 20 computers had a 3.5kw Daikin; it works fine in the morning, when nobody is in it and the computers are off!

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Hi Ritic
Just to play devils advocate on your statement and to defend the AC installer who probably had to work to a budget.

If you consider that maybe the AC was installed just to remove the additional heat load into the space from the 20 computers then the AC is correctly sized?
From the sound of it you are considering a sealed room with high solar gain. I would guess that there are other factors to this classroom involved.

The classroom would have been designed for the occupancy and adequate ventilation, mechanical or natural.
Schools policies are that AC is the last resort for design criteria and recommendations are that the temperature should not be above 28 DegC for more than 120 hours / annum of occupied time. Should not exceed Max 32 DegC Maximum. With options such as shading windows, insulating and natural ventilation to be considered first.

The 120 Hours basically covers our summer period of two weeks in the UK ;)

Well written "hookster."
I had to endure 12 years in public and private Schools...with no A/c's to ease the hot and humid ambient temperatures. Just Ceiling fans circulating a saturated mixture of sweat and Body odor.:off topic:

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Originally Posted by hookster

Hi Ritic
Just to play devils advocate on your statement and to defend the AC installer who probably had to work to a budget.

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I very much doubt this. I've quoted for schools in the past, and they always require 3 quotes, as this is the law for public organisations. They’ve never given me a budget to work to, but have always picked the cheapest, regardless of the size or make of the equipment.

I understand where you are coming from with the rest of your post, although I personally think 3.5kw is too small, even if you’re only talking about additional heat load, but it is a good point you’re making. However when have you quoted, or have been asked to quote for a system to “remove the extra heat load”! Personally, I will always quote for the room at hand.

BSRIA Rules of Thumb 2011 gives cooling loads for different buildings in w/m2, I can upload it on here if anyone is interested. Historically I've found them to be fairly accurate for most applications.

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Originally Posted by Dazza_95

BSRIA Rules of Thumb 2011 gives cooling loads for different buildings in w/m2, I can upload it on here if anyone is interested. Historically I've found them to be fairly accurate for most applications.

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Hi,

i would be interested to see this please. how does it work with he number of people in a room eg 30 students in a class room/

Rule of thumb I used when at a wholesaler is 100W per person, 150W per computer then 110W/m2. That's for Sunny England. Not sure how Australia fit into it as got it's hotter parts.
More info here: http://www.engineeringtoolbox.com/me...ons-d_706.html

Bit of a minefield!

Rule of Thumb was never intended to be a heat load calculation tool, but used to check whether your heat load calcs were in the right area.

125w/m2 will nearly always oversize your system in the UK

You guys are nearly half the size of what we use in South Africa. Rule of thumb 500btu(150w) per sqm, On the coast we go up to 700btu (205w). I've never seen anything smaller than a 24000btu in a computer classroom. I would have some annoyed customers if I sized like this. Surely even where you have low ambient temps the units would have to have sufficient heating capacity?

We normally use the equation Length(m) X Width(m) X height (m) X500.

This comes from the AIRAH Handbook for the Aussie market which might be of use:
Note - It does state they are for estimates only and a proper survey and heat load calculation should be done.
Attachment 14244

This is often seen as a black art. The simple fact is, the machine has to have the power to be able to remove the heat added to a building or zone. So if the heat loads are added up, the machine needs to be at least that size.

There are software packages that exist to do this, but some of them require you to be a Harvard graduate to be able drive them. And then there are technologies such as heat recovery that many packages won't account for. I ended up writing my own spreadsheet to account for all of these based on AIRAH (Australian Institute of Refrigeration, Airconditioining and Heating) heatload guidelines.

Remember, if you are in a high humidity environment, that rules of thumb can lead to a poor choice of machine size since, air volumes need to be lower and coil sizes larger, to cope with the humidity. This can be done by oversizing the machine and running it on low fan speed.

This creates the problem of selling to the client a larger (and more expensive) machine and trying to compete with the partially knowledgeable, electrical goods wholesaler showroom salesman or electrical contractor with just enough knowledge to make him dangerous, who are trying to pass off a smaller unit to make the sale on price.

It is up to us to educate the client that this is a bad idea. Our sales conversation with them needs to be one of not price, but one of skill, knowledge and quality installation. They need to understand that they are buying from a guru and not someone with a cereal box certificate. This equipment may be more expensive but has been sized by someone who really knows what they are doing. It's great if they want to buy a smaller cheaper unit, but don't expect it to work when the chips are down and the next heatwave is in full swing. Ask them how small they would like to go? Why stop with only one size smaller? Let's go down several sizes.

Granted, this is not easy but it can be done and is regularly done. It may mean we have to tune up our sales and technical skills a bit so that we can do this effectively.