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# Manual J calculation questions

#41
12-27-14, 06:34 AM
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Being 6", they would each carry about 110 CFM at 0.1" w.c. static. Because of the (3) 90* turns in entrance door duct, it is going to suffer. Air, like most other things, will take the path of least resistance.

#42
12-27-14, 09:20 AM
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How did you calculate these? Do these numbers depend on the blower?

11x7 will handle 360 CFM,
14x8=600 CFM,
21x8=1050 CFM.

All flow rates are at 0.1" water column static. If the round branches are 6" metal, they will carry 110 CFM each at 0.1" w.c.

Considering the below design that means the whole system is undersized:
the top left main duct can supply 600 and it has 10 registers that draw 110 each

the other one (bottom right) provides 360 but it has to fee 4 registers (440)

Am I correct?
Would a better blower improve the situation?

(bigger image here http://i.stack.imgur.com/9Uw2r.png )

#43
12-27-14, 11:20 AM
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The numbers came from a duct calculator slide rule. I'm sure they are available online. I get mine at any of several of my supply houses. They are usually published by a duct or register manufacturer.

Obviously, the branches can't put out more than the trunk can supply. If your trunk does not reduce in size as you take off air, it should in order to maintain a relatively constant velocity. The best solution would be to re-do the trunk.

#44
12-27-14, 11:31 AM
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How about re positioning the basement registers on the basement ceiling as one of the already is installed. Would shortening the basement ducts improve the CFM numbers?

Does your slide line calculate the CFM based on blower capacity?

#45
12-27-14, 01:06 PM
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Don't know about re positioning the registers.
No, the calculator will only do air flow @ various statics or velocities. It can also be used in reverse to determine what size duct is needed for a certain air flow. I just did a quick search for "duct calculator" & found a number of them you can use online for free.

#46
12-27-14, 01:53 PM
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Shouldn't the heat loss on a 1200 sq ft home be more around 30-45 K BTU? And the 45K btu being the high end of a home that is rather drafty...

The current furnace is 90K I believe from the data plate. Would that contribute to the op's overheat condition?

#47
12-27-14, 04:38 PM
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It is actually 76K and that is the highest and the theoretical output.
In reality is should be less than that.
The very precise area is actually 1890sqft of livable space (893 in basement ,997 upstairs) + the furnace room itself

I don't really know how you calculated that 30K-45K BTU, I would be happy to have that loss only.
The main floor has no insulation. Just brick veneer, (plywood?) sheating and studs and of course drywall inside.

#48
12-27-14, 05:11 PM
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If I am to trust these numbers yours are off big time. This table shows 8x14 rectangular section as allowing 1750 , your number for the same section is 600
Can you point me to one of those free calculators that would confirm your number?
Maybe it is the friction loss that is different in your case?

#49
12-27-14, 05:26 PM
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That 30-45K is an average range for a house of that size in Lawrosa's area. Simply a ballpark number.
Something out of that range would make one ask 'why'.

I was using 0.10" w.c. as a friction loss. The online calculator shows 0.78. Every residential duct system I've ever seen designed (other than high velocity) used either 0.1 or 0.08 for design purposes. Total friction loss after A/C coil & elbows will probably actually approach 0.3-0.5.

#50
12-27-14, 05:38 PM
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I don't really know how you calculated that 30K-45K BTU, I would be happy to have that loss only.
The main floor has no insulation. Just brick veneer, (plywood?) sheating and studs and of course drywall inside.
Typically the ballpark calculation is sq ft x 35 btu for not well insulated homes. 25 btu for normally insulated homes.

Ive done many heat loss calcs and that ballpark is 90% of the time very close to the slant fin calculator. That calculator over sizes by 20% there about too.

Since you cant download the slantfin calc on your computer try the smart phone app... Hopefully you have a smart phone...

Please let us know what you get...

Slant/Fin Heat Loss Calculator | Slant/Fin Boilers & Baseboard

#51
12-27-14, 09:49 PM
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I did manage to install Slantfin on my android phone but I wouldn't go again through calculating the heat loss.
I already did it using the Canadian equivalent for that, that is Heat2000. I posted the results below

#52
12-27-14, 10:26 PM
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I did manage to install Slantfin on my android phone but I wouldn't go again through calculating the heat loss.
I already did it using the Canadian equivalent for that, that is Heat2000. I posted the results below
Just my opinion, but if your not going to take the time to use another program to compare how can we help?

Grady gave you great advice but you dont want to change things as suggested...

So what are we to do?

IMO 50 posts here and we are going in circles??? No?

Are you zoning the two floors???

If I put a 50K funace in that home it would be a lot... Just my opinion though...

#53
12-28-14, 07:14 AM
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OK ...I have honestly tried. Just 5 minutes ago. I am trying to calm down. There is a reason why this program is free. After going through all the dialogues for one room it crashed on me and when I restarted the program all the info was gone. I am not going through that again. If you want I can post you here all the info that I used in Hot2000 and ..if you have the PC version or a more stable version try it if you want, or pass me the PC version

I think that I am making progress although you might not be able to see that, it is my personal view and understanding. I currently have two active threads, this one and the other one where I am asking about some calculations.Both provided me with good clues on how to do it.

In the mean time I managed to make sense of the way some of the Equal frictions steps are calculated, here they are

1) Compute the necessary air flow volume (m3/h, cfm) in every room and branch of the system
2) Use 1) to compute the total air volume (m3/h, cfm) in the main system
3) Determine the maximum acceptable airflow velocity in the main duct
4) Determine the major pressure drop in the main duct
5) Use the major pressure drop for the main duct as a constant to determine the duct sizes throughout the distribution system
6) Determine the total resistance in the duct system by multiplying the static resistance with the equivalent length of the longest run
7)Compute balancing dampers

And here are the maths behind this
1) room width x room length x room height
2) Sum(room volumes)
3)comfort systems - air velocity 4 to 7 m/s (13 to 23 ft/s)
industrial systems - air velocity 8 to 12 m/s (26 to 40 ft/s)
high speed systems - air velocity 10 to 18 m/s (33 to 60 ft/s)
4)Sensible heat=Hs=1.09*q*Temp rise
Sensible heat=heat loss calculated with Heat2000 to be ~55K,
Use Daikin DM96VE0603BN 96% efficiency 60K*0.96=57.6K
q=cubic feet per minute=Hs/(1.09*Temp rise)
Temp rise=5F
q=57600/(1.09*5)=10568cfm

All good so far? I hope it is!

No we use this diagram to detemine the static friction loss in our ducts
The diagram is based on standard air 0.075 lb/ft3 in clean round galvanized metal ducts.
Then we use this table to convert from round duct diameter to rectangular size.

So in my case if we use 0.08 friction loss we get ~22-23" main duct size (rounded)
That comes down to this:

Now this is where it becomes particular to my case. I have two main ducts so I will have to repeat the above for each of the two main supply ducts
Here is the CFM distribution based on rooms volumes as percentage of the total volume of the two floors

Main supply duct 7822
Secondary main supply duct 3187
(this one feeds the office, & the master bedroom and the entertainment room that is just below them in basement)

So that gives us this
20 inches rounded
Air flow RecSizes EquivDiam Velocity Friction Loss*
8000* 12 x 30 20.2 3595 0.8
14 x 25 20.2 0.8

14 inches rounded
Air flow RecSizes EquivDiam Velocity Friction Loss*
3000* 12 x 14 14.1 2767 0.75

And I have 8x14 and 8x11 !!!
No wonder why I have low pressure downstairs !

Is it all good so far? I don;t know about you ..but I call that that I am making some progress here and I thank you all for that...

Note: if I consider the above for the furnace that is already in place I think that everything becomes even worse since the current system is trying to deliver 76K BTUh

PS: your ballpark calculation with 35BTU/sqft is not far from my number
I have 1890 sqft of area that has to be heated.
1890x35=66150
I am not zoning the floors, not sure how I can do that with one thermostat, one furnace and no dumpers

It would be really useful for other readers if you change the name of the thread to Manual J and Manual D...

Last edited by PF4DIY; 12-28-14 at 07:41 AM.
#54
12-28-14, 10:13 AM
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PF4,
Everybody here understands you want to do the job right & as cheaply as possible, as does nearly everyone who comes here. I also understand the frustration. Been there, done that as they say. Being completely honest with you, that program from HVACComputer.com is probably the best \$50 you can spend on the project. Not only will it do the room by room Manual J, it also does the Manual D, and you get TECH SUPPORT. I have personally used the program & was very happy with it.
The way I see it, if you want comfort throughout the house, you are going to have to do some duct modifications if it is only dampers in the branch ducts where they take off from the trunk(s).

#55
12-28-14, 03:58 PM
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Yes that program at HVAC computer looks decent but so is Hot2000 (I managed to find a beta version 1 for Hot3000 but I can't see the differences)
I will give it a try, I downloaded the trial version, I am curious to see how far I can get with that, I am not sure what are the restrictions imposed on the trial

Do you see any problem with what I calculated below?

#56
12-28-14, 04:33 PM
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What is this 14 & 20 inch round mess? 14" will carry aprox. 1050 CFM & 20" will carry about 2500.

Don't worry about the volume of each room. When you do a room by room calculation, it will tell you how many btu/hr each room needs & maybe even the CFM. From that, it's easy to determine duct size. If all it gives you is btu/hr it's only a couple of simple steps to determine CFM & thus duct size.

#57
12-28-14, 05:08 PM
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Your 1050 and 2500 are based on 0.1 friction loss.
My numbers are based on 0.8 which is more realistic for a 25 years old (at least!, more like 56 years old) duct system.

#58
12-28-14, 05:21 PM
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I certainly don't think your furnace would be able to move enough air at 0.8. If yours was the data plate I remember looking at, the max was 0.5.

#59
12-28-14, 05:49 PM
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#60
12-28-14, 06:18 PM
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That's the one. Thanks Mike. I wasn't going to dig back up to nearly 60 posts to find it.

#61
12-28-14, 09:57 PM
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Is the external static pressure the same thing as the Friction loss ??

#62
12-29-14, 08:14 AM
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that program from HVACComputer.com is probably the best \$50 you can spend on the project. Not only will it do the room by room Manual J, it also does the Manual D, and you get TECH SUPPORT. I have personally used the program & was very happy with it.
This program does not allow me to add a window to my door. Also there is no way to add user specified values to the components you add.

How do you add something like this?

#63
12-29-14, 11:38 AM
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Here are a couple of other problems:
-the program does not allow me to consider crawl space
-there is no clear delimitation between the area above grade and below grade for a basement
-there is no way to add a wall that is adjacent to the garage
-there is no way to use it with older houses and custom R values (empirically determined or by adding R values for the layers within the wall)

#64
12-29-14, 01:10 PM
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That door, for all practical purposes, is a window.
If you have a problem with or questions about the program, call the vendor.

#65
12-29-14, 03:11 PM
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No, I don't have a problem with the program. I am not just going to pay 50\$ for something worse than a free program which does the same thing or more. Below I was replying to your recommendation to use the program.

BTW my post #53 has so many errors...I wonder why nobody could see them :-)

#66
12-29-14, 03:26 PM
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BTW my post #53 has so many errors...I wonder why nobody could see them :-)

For me its too much information. I like the KISS approach...

Being completely honest with you, that program from HVACComputer.com is probably the best \$50 you can spend on the project. Not only will it do the room by room Manual J, it also does the Manual D, and you get TECH SUPPORT. I have personally used the program & was very happy with it.
The way I see it, if you want comfort throughout the house, you are going to have to do some duct modifications if it is only dampers in the branch ducts where they take off from the trunk(s).

I would spend the \$50 bucks and get the tech support from the makers of the program...

How can you beat that???

#67
12-29-14, 04:31 PM
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