sizing ac equipment - David Broome

Unknown

and heat gain is a skill not a rule of thumb folks. Each house or business should be sized using the manaul j. Your requirments vary from region to region. Some regions require a 25 degree spread, where as other only require a 15 degree spread. You have to include the people load, internal heat laod, heat gain thru doors,windows and floors. THERE IS NO RULE OF THUMB FOR SIZING AIR CONDITOINING PROPERLY. I was recently called in by a medical group to look at their building that they could not keep cool. They were only 39 tons short on keeping the building at 70 degrees when it was 95 outside, but the engineer speced a 20 degree split and the owners signed off on it. Saved them $ 100,000.00 on the upfront cost but couldn't keep their building below 80 degrees last week. I have put a 2 1/2 ton unit with a beefed up airhandler in a 2400 square foot house last year and just last week put in a 2 1/2 ton unit in a 1300 square foot house. If you don't do a load caculation on each application; you are just guessing! It is just as important as when you are doing a heat loss for a boiler. It is true when replacing an outdoor unit without replacing the indoor coil you will not get the proper seer rating. Now some ac units, mostly 4 and 5 tons, unless you run 1" suction and use a tx valve you will not achieve a 12 seer rating. Now the ARI rating folks only check a unit for the first 90 seconds of run time, not alot of checking is there folks? Now on to scroll compressors, you can use the scroll to pump the freon back in to the outdoor unit ( and they really don't like that ), but if you pump the freon into the airhandler and lineset; you will do damage to the compressor! And now to my last point, I will not even think about answering a ho question about a quote! And any one that does, without seeing the job firsthand; well you know what they say is only fools rush in where angels fear to tread. Just some food for thought.

Peace Be With You

David C. Broome

don_9

David

> and heat gain is a skill not a rule of thumb

> folks. Each house or business should be sized

> using the manaul j. Your requirments vary from

> region to region. Some regions require a 25

> degree spread, where as other only require a 15

> degree spread. You have to include the people

> load, internal heat laod, heat gain thru

> doors,windows and floors. THERE IS NO RULE OF

> THUMB FOR SIZING AIR CONDITOINING PROPERLY. I was

> recently called in by a medical group to look at

> their building that they could not keep cool.

> They were only 39 tons short on keeping the

> building at 70 degrees when it was 95 outside,

> but the engineer speced a 20 degree split and the

> owners signed off on it. Saved them $ 100,000.00

> on the upfront cost but couldn't keep their

> building below 80 degrees last week. I have put

> a 2 1/2 ton unit with a beefed up airhandler in a

> 2400 square foot house last year and just last

> week put in a 2 1/2 ton unit in a 1300 square

> foot house. If you don't do a load caculation on

> each application; you are just guessing! It is

> just as important as when you are doing a heat

> loss for a boiler. It is true when replacing an

> outdoor unit without replacing the indoor coil

> you will not get the proper seer rating. Now some

> ac units, mostly 4 and 5 tons, unless you run 1"

> suction and use a tx valve you will not achieve a

> 12 seer rating. Now the ARI rating folks only

> check a unit for the first 90 seconds of run

> time, not alot of checking is there folks? Now

> on to scroll compressors, you can use the scroll

> to pump the freon back in to the outdoor unit (

> and they really don't like that ), but if you

> pump the freon into the airhandler and lineset;

> you will do damage to the compressor! And now to

> my last point, I will not even think about

> answering a ho question about a quote! And any

> one that does, without seeing the job firsthand;

> well you know what they say is only fools rush in

> where angels fear to tread. Just some food for

> thought.

>

> Peace Be With You

>

> David C. Broome

don_9

David

I think we all gone down that road before,Rule of thumb that is.It like setting on a deer stand all day downing
a six pack of bud,then mr bucks show you fire in you miss the target. One must do a load calulation that the only way to know what size of equipment or duct that one would need.
you need to know how much is senible load in what left for latent.This is truly where a lot of guys go wrong.what if my house calls for 48000 btu,44000 is senible and 4000 latent,If i look at my performace data I can see that ari rated 4 ton unit only give me 35250 senible at 10 seer.
not enough capacity. I find i have to go to a58000btu 10seer to give me a 44,400 senible capacity,now I got the right equipment.but now i also have a 5 ton blower to add to the mix,ever wonder why force air has gotten a bad rep.
oh about that deer ,I much rather kill a couple more buds then i would that deer.

Bob Bona_4

Amen to that

it is amazing how contractors create extra work by oversizing ductwork/equipment. The load calcs almost never cease to surprise me with the results!

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

Andy_3

I find more often that people over size the equipment and undersize the duct work. And then wonder why they have rooms that are to hot and cold. Or say that the system is very noisy. I have heard it said that I would rather have the cheapest peice of equipment installed correctly than the best installed incorrectly. Just last winter I changed a furnace and a.c. out for a couple and changed 60% of the duct work because it was undersized, and they called me back excited that the heating bill dropped over 50%. Computerized heat loss/gain is the only way to go.

Boilerpro

Mass and length of occupancy effect on A/C sizing

A couple of very often missed factors are the mass of the structure and contents, and the length of time of occupancy. While its been over ten years since my last mech. eng class, and I am rusty with these cooling calcs, I find that spaces like church sanctuaries nearly always have way too much cooling capacity. IE. My former church with 24inch stone walls, reinfoced concrete suspended floor and a seating capcity of about 300 needed only 6 tons of cooling for our northern IL summers. The "expert" (the wholesaler) one of the forced air contractors brought in looked at the space and said 15 tons without any load calc, another contractor said at least 10, maybe 12. Thank goodness there are some retired engineers (from other fields)with influence that knew bigger is not better for A/C. Right now sitting in my 2800 sq ft 1906 frame with 700 SQ ft of windows and staying cool with one 1 ton window unit and its about 95F high humidity outside. (Have another 1/2 tonner for when there are consecutive really hot days). BTW, also have been in contract with head of maintenance for several medical facilities in the area and most apparently have way too big of equipment and have constant humidity problems.


Boilerpro

GW

ManualJ

David and others,

What is a good fairly priced Maunual J program?

What if the trunk lines are too small, can you simply inform the HO that you can only give them x tons unless they want to re do the ducts (just an example... curious). Are there draw backs besides the obvious (the house won't cool to desired temp on the hot days)?

Can you swap out an old condenser with a 10 seer without too many issues?

Do you ever take manometer readings in a swap-out job to see exactly how good or bad the air is moving through the ducts? Do you ever take that water column reading and motor amp draw to see how much air you can ACTUALLY move? I studied a little bit of duct balancing with NCI.

Thanks, I'm trying to pick up this AC stuff, just got my EPA.

Gary

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

Mike T., Swampeast MO

Also...

Window coverings, screens, etc. Some people (like me) like to leave windows as exposed as possible--others drape layer upon layer of drapery. It makes a BIG difference.

I guess it's my Swampeast Missouri experience, but I size A/C as if there is a BBQ party on design day with all windows uncovered. (BBQ 'cause if I added a full kitchen load, it would be astronomical.)

Consumer Reports has EXCEPTIONAL guidelines on sizing A/C. Their worksheet is nearly identical to HVAC-Calc yet it REQUIRES that you add a reasonable amount of added load for occupation, lighting, etc., etc., etc.

Just my (supposedly power conscious) computer raises the temp of my office 1/2 - 3/4 degree.

don_9

U tube

Gary the only time I use it is to check pressure switch and the performace of a draft induce motor on a furnace. as
far as useing it for checking pressure drop on a duct system no need.Much like a steam boiler if there a design or field problem it will call out to you, and if that the case you have to breakout the duct calculator and measuring tape any way and go over the entire system.Around here 75%
of houses have a poorly design distribution.The key for any mechanical system is in it distribution.Air like water take it path of least resistance.With out proper fittings Turning vanes and balancing damper and the knowledge to calulate
over all pressure drop, well force air will always be frown
upon.Hey dan can you write a book on the lost art of air we sure can use it here in the south. What is it that you say
I cant here you my ac just kick in.

Unknown

This content has been removed.

Jeff_3

HVAC-Calc

6 months ago I asked a buddy of mine who I know downloaded and tried every program known to man, "What's the best program and what's a good price?". He said there was nothing even close to HVAC-Calc for ease of use for a dummy like me and the support is great. So I plunked down my $389 bucks. He was right! I think the web sight is http://www.hvac-calc.com . I haven't even had to use the support yet, and man that's saying something!

Jeff