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Old 02-03-2011, 10:43 PM   #1
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Sizing a furnace for your home


I need some expert advice here. I keep hearing different things. I wish i could afford to buy manual j but i cant. There are two questions here. 1: how do i calculate how many btu needed for the fau to heat the house. 2: why is it detremental to oversize the fau? I remember in school hearing something to do with humidity but i cant find it in my books. Thanks for the help guys.

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Old 02-04-2011, 01:24 AM   #2
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If you want to do it right you have to go manual J, no way around it. You can get HVAC calc for a couple hundred bucks. If you oversize your equipment you're doing your customer a disservice, its too bad most pieces of equipment are over-sized. The 2 worst things about over-sized equipment in my opinion are; Short cycling- this causes the system to bring the house up to temp too fast resulting in discomfort, more temperature fluctuation and constant starting and stopping which is annoying and burns up more energy. Static Pressure; in most oversize cases the blower size is too big for the ductwork causing the system to run at high static resulting in premature equipment failure. Also, with refrigeration systems oversizing is especially bad and can cause many different problems.

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Old 02-04-2011, 04:14 AM   #3
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Sizing a furnace for your home


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Originally Posted by thehvacguy View Post
I need some expert advice here. I keep hearing different things. I wish i could afford to buy manual j but i cant. There are two questions here.


1: how do i calculate how many btu needed for the fau to heat the house.

Manual J.

You can still buy the 7th version and its not expensive. Also, your local library may have or be willing to get a copy of it in.


2: why is it detremental to oversize the fau? I remember in school hearing something to do with humidity but i cant find it in my books. Thanks for the help guys.
An oversized furnace on undersized duct work will cycle on its high limit. And use more fuel to heat the house then need be. If its duct work was sized large enough for it. It would still tend to short cycle, and often over heat the house.

For an air conditioner, as above, it wouldn't dehumidify. Causing the people to have to set their thermostat lower to try and get the humidity down. Which would of course make the cooling bill higher. And may still not be able to keep the humidity low enough.
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Old 02-04-2011, 09:06 AM   #4
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Sizing a furnace for your home


Oversizing a furnace causes major fuel consumption, short cycling, stress on equipment due to starting and stopping of electrical and pnumatic controls. Ductwork can be corrected by increasing return and supply lines and using the fan speed feed to increase or decrease the cfm and static required. Overall, it is not a good thing. You basically have no choice other than to do a load calculation to determine the btu's needed to heat the required space. This means doing a manual J, gain-loss calculation.

As far as cooling goes, yes it will not remove the humidity creating a false temperature feel. Humidity holds heat thus creating a feels like temp along the same lines as winter in Georgia versus winter in Colorado. Too much humidity will also cause mold, mildew growth and provide an environment for other sicknesses within the space, it will also warp wood such as floors, trim moulding, and structure.

Heating and air conditioning is exactally as it states, Conditioning the air in a confined space. It is controlling the airborn pollutants, humidity, temperature, pressure, and in some cases microbes.

The manual j alone shouldn't cost you all that much. Look up the Ga.Tech book store and see what the price is. I think you may be surprised.
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Old 02-04-2011, 09:59 AM   #5
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If you are interested, you can do all the calc's manually, but you have to do some research for the constants to use in the heat loss and heat gain calcs, but its doable without any fancy programs, excel will do it, just have to do more research, IE, thermal conductivity for building materials, surface area, temperature differencial (ASHREA data for extreems)

Heres a link for some ideas - http://www.vonwentzel.net/HVAC/HVAC-...ors/index.html
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Old 02-04-2011, 09:02 PM   #6
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Ok, from what i remember manual j was $1000 and i tried to download it for free from frostwire and got a virus that killed my computer. I will look on e bay and see. I really appreciate the knowledge guys. Im 3 years away from getting my contractors license and i need to know my stuff.
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Old 02-04-2011, 09:16 PM   #7
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If you look at the link that I provided, I believe you can run a single set of calc's for your house for $49

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Old 02-04-2011, 09:34 PM   #8
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I do a lot of jobs. My house is done already but i need to be able to do load calcs permanently. This is my career. I checked ebay and the acca manual j book is $150 and the software is $350. Guess what im gonna spend some of my tax returns on lol
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Old 02-05-2011, 02:45 AM   #9
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Originally Posted by thehvacguy View Post
I do a lot of jobs. My house is done already but i need to be able to do load calcs permanently. This is my career. I checked ebay and the acca manual j book is $150 and the software is $350. Guess what im gonna spend some of my tax returns on lol
A thousand bucks is a small amount to pay toward your career.

The really good programs will also have modules that you can buy and add on later. And yes, they are a thousand plus. But will also do your duct work sizing and layout.

HVAC CALC is based on version 7, and is less then 500 bucks(the 49 dollar license is for 60 days use). Its easy to use, and fairly quick.

I advise you to get the Manual J book, weather version 7 or 8, only so you can better understand why some of the info is needed, and to help explain to your customers why making envelope improvements is better then just installing larger equipment to keep them warm or cool.
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Old 02-05-2011, 11:15 AM   #10
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I have been trying to read up on HVAC myself since I plan on running new ductwork. I don't understand why everyone keeps saying that you MUST do a Manual J calculation to properly size your furnace. Yes, I fully understand the problems of not properly sizing your furnace but there aren't an infinite number of furnace sizes for the homeowner to choose from. I live in southeast Wisconsin and my house was built in the 1950s. It is approximately 864 sq. ft. It is a 2 bedroom ranch. My furnace is 80% efficient. It is a 60,000 btu unit. The current duct is horrible. All of the duct lines measure approximately 5" x 7" and the supplies are on the inner part of the house. My intention is to run round duct so that the supplies are on the outer side of the house, and to insulate them as well.

This is what I plan on running based on the little knowledge I have gathered on HVAC...

Living Room: 2- 7" lines. This is the longest run from the furnace and will have quite a few elbows as well. Also it is the room with the most heat loss due to how many windows there are. It is approx. 232 sq. ft.

Kitchen: 1- 7" line. It is approximately 154 sq. feet and would be the second longest run and would be a straight shot from the furnace.I figure I don't need an 8" line because it's the kitchen and not much time is spent there.

Both bedrooms would also be a 7" line each. They are about 132 and 108 sq. ft. respectively.

The bathroom is very small but does have an exterior wall so I will put in a 5" line.

I would also like to drop one 7" line into the part of the basement that will be finished.

Using charts I have concluded that with all of these ducts, the total CFM for the house would be 960. The return air ductwork measures about 22" x 9" right where it meets the air filter box.

The walls of the house will all be R-13 as soon as I get the kitchen renovated. The ceiling will also be brought up to today's standards with blown-in insulation.

I will also be putting in dampers where the lines take off to get the airflow to the rooms properly regulated.

So now that you know about the house I guess I have a few questions....what good would a Manual J calculation do for me in determing my duct sizes? The only room I can see being changed is the living room to 8" from 7" due to the number of elbows and length of the run. But doesn't the air velocity slow down when you increase the duct size and wouldn't it impede the ability to heat and cool the room?

Right before my air filter where the duct measures 22" x 9", is that big enough to support my furnace pushing 960 total CFMs? I assume it is because I see that a 22" x 8" line supports 1,000 CFM. Just wanted to make sure that I'm right on this one.

If you took the time to read all of this, thanks. Any input would be appreciated.
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Old 02-05-2011, 11:24 AM   #11
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Manual J calcs are to determine furnace/AC size in BTU's/hr, Manual D is for duct sizing.

Each of which can be done by hand using excel if you understand all the dynamics at work.

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Old 02-05-2011, 08:56 PM   #12
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Sizing a furnace for your home


Manual J would tell you how many CFM each room needs. So that you can determine what size supply/supplies each room needs.

How much air do you "think" your going to get through "each" of those 7" supplies, and how much thruogh the 5"?

What if your house only needs a 40,000 BTU 95% furnace, instead of a 60,000 BTU.
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Old 02-05-2011, 10:11 PM   #13
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Basically i came to this conclusion... Manual j is the scientific way to give your home the proper system. With an improper system it will work but you jeopardise the life of your system. With manual j you get more for your money because you will be more comfortable and you wont need to repalce your furnace for 20-30 years

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