On-Line Manual J Calculator


  #1  
Old 10-07-02, 05:41 AM
JudyRoyal
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On-Line Manual J Calculator

I found an On-Line Manual J Calculator at:

http://www.hvacsupport.com/jform/


It has a "help" section, and I'm pretty sure I understand what information is required in most areas.

But under "Wall Construction", it says to "Enter the Design Temperature Difference (difference between inside wall temperature and outside wall temperature)"

I have no idea what this means! The same information is required under Floor and Ceiling Construction, too.

Would one of you mind explaining this?

Also, does anyone know if "Manual D" is available on-line, and if so, where?

Your time and attention is really appreciated. Thank you.
 
  #2  
Old 10-07-02, 06:27 AM
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Question Design temp

What outsdie temp do you want the system to run at 100% ??
This depends largely on the area you live.
95 for A/C and 20 For heating works well in Oklahoma.
What is the coldest normal temp you get in your area ??
 
  #3  
Old 10-07-02, 12:28 PM
JudyRoyal
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climate

Our average winter temperatures are around 20-25; average winter low, around 0; and record lows -15 to -20.

Summer average temperatures are around 80; average high around 90; and record highs around 100.

Record highs & lows occur about every 20 years, and we usually only get a week or so of really hot (90 degree) or really cold (0 degrees) in an average year.

If I understand correctly, a furnace capable of heating to 72 degrees when it's 0 outside doesn't operate efficiently at 20-25 degrees. Is this correct?

There are a few baseboard electric heaters in the house. I'm planning on leaving them in. Wouldn't it be more economical to use these as a supplementary heat source for the short periods of extreme cold than to run a furnace that is larger than necessary on a regular basis?

But I still don't understand how to figure the "design temperature difference." The choices on the drop down menu range from 10 degrees to 35 degrees (difference between inside [wall ]temperature and outside [wall] temperature) for walls, ceilings, and floors.

Are we talking about the same thing? Would you mind taking a look at the On-line Calculator, so we know we're talking about the same thing?

Thanks for your help.
 
  #4  
Old 10-07-02, 04:35 PM
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"Design Temperature Difference" is a simple way to determine U-factor or thermal transmittance through a section of wall, floor and/or ceiling.

If the temperature on the walls in a room had a 10 degree difference, it would indicate there is insulation in the walls. A 35 degree difference would indicate no insulation in the walls.

For example take your living room walls. The wall that faces outside and separates the outside of the house from the inside of the house has a temperature of 90 degrees. And the wall opposite this wall that separates the living room from the dining room (inside wall) is 80 degrees, there is a 10 degree difference. It would indicate that there is insulation in the walls and this directly affects the heating/cooling load for the home. This sure beats cutting a hole in the wall to see if there is insulation.

All heat load/gain calculations base their calculation on the coldest day or warmest day of the year in your area. In other words, your system must be sized to heat your home on the coldest day of the year in your area and cool it on the warmest day. What this means is on the days that the temperature is not as cold as the coldest day, your system is oversized. This reduces the efficiency of the system, but not that much. It will make a very big difference if the system is oversized to begin with. Hence the importance of properly sizing your unit.

The accuracy of these on-line calculators are really not that good. Nothing beats an experience heating contractor on-site evaluation. There are so many variables, that even the best program overlook. So an experienced contractor will almost immediately identify and plug in those values, that will produce a better outcome for sizing.
 
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Old 10-07-02, 06:55 PM
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Unhappy IF

This only applied IF your contractor ACTUALY DOES the load calculation.
The reality of it is only the best contractors out there do the load calculations, this is not the norm just yet.
The past 4 homes I have lived in the furnace was over twice the size needed !!
The vast majority of HVAC system are grossly oversized, especially on the heating end.
If your unit is sized correctly it will run at 100% on a design day for the few hours it gets that cold/hot.
There are 0% 5% 10% design days, meaning that the outside tempature will exceed system capacity for that percentage of the time during a typical cooling season.
This means it will be a few degrees too cold or too hot for a few hours a year, if you can put on a blanket when it is 5 degrees outside or turn on a fan when it is 100 outside you will save money on equiptment and operating costs.
 
  #6  
Old 10-10-02, 12:02 PM
JudyRoyal
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...my thoughts

54:

"This only applied IF your contractor ACTUALY DOES the load calculation. The reality of it is only the best contractors out there do the load calculations..."

Exactly what I was thinking! Three different contractors--supposedly good ones--"calculated" what I need in about 10 minutes while sitting in my kitchen. When I told them I was going to be replacing and re-sizing windows, they said it wouldn't make much difference.

From what I've been reading, properly sizing the system is one of the MOST important factors. I've become so disillusioned with contractors that I'm trying to learn how to do it myself! Not that I plan on actually doing the work. I just think it's the only way I can be sure I end up with a system that will operate as efficiently as possible.

About "10096" Is that a system that would maintain desired temperature on coldest days?

"you can put on a blanket when it is 5 degrees outside.." Again, my thoughts... This is a huge old house, and heating bills could be astronomical. Wouldn't it make sense to size the system to run efficiently during average cold, and use a supplemental heat source (either electric or the fireplace) during the few periods when it's extra-ordinarily cold?

Unless I'm really misunderstanding somethings, it sounds like the best option to me.

All you said makes sense. Thanks for answering.
 
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Old 10-10-02, 03:56 PM
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Thumbs up Hvac-calc

For $39.00 you can download hvac-calc and it will give you a very detailed heat loss/gain breakdown for your home.
I had a 3.0 version of it once but lost it in the reformat, and you can't download it anywhere only 4.0 is avalible now.
3.0 worked very well and I am sure 4.0 is even better, I just have a hard time paying $39.00 since I am not in the market for a new HVAC system right now.
If you can live with the unit running wide open on the coldest nights for a few hours you will be more comfortable with a smaller system as well as having lower gas bills.
HVAC-Calc even has all the desing temps listed for each city.
Run the program and use the next size SMALLER than it recommends.
Remember the program calculates LOSS not BTU onput of the new funace.
If you show a LOSS of 40,000 BTU you will need a 50,000 INPUT to be exact on target with an 80% AFUE furnace.
If you had a LOSS of 45,000 BTU you could still use the same furance but you will be a few degrees cool on the "design nights" or will need to get the extra 5,000 BTU from a 1500w space heater
How many sq ft is the house and where is it loacted ??
 
  #8  
Old 10-13-02, 08:59 PM
JudyRoyal
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Information about house

54:

Where do I download this hvac-calc ?

Here's a link to some diagrams & tables that show the information you asked about regarding my house--temperatures, room sizes, etc:

http://www.regardless-shorthairs.com/House/Home.htm

It also shows how the house is divided (you'll see why I think 2 furnaces is a good idea, I think) and where ductwork CAN be run.

I've had 3 more contractors out, and haven't been impressed with any of them. I'm so glad I found this forum... Sooner or later, I'm sure I'll find a good, knowedgable contractor, and thanks to you guys, I'll recognize him when I do!
 
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Old 10-14-02, 06:33 AM
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Cool Link

www.hvaccomputer.com

Great job drafting out the house !!
All you need now is a digital camera

TWO HVAC units is the ONLY way to go for rentals !!
Even if you don't rent it you can shut off the 1/2 you aren't using
 
  #10  
Old 10-14-02, 07:24 AM
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http://www.heatload.com/forcedair/ is a site where you can do your own heat loss calculation.

You can also go to http://www.heatinghelp.com and request a free CD from Slant Fin.

I would recommend going with a hot water system than a hot air system in your situation. Because of the age of the home and the apartment, ductwork require more space to distribute it than copper tubing. Especially when you bring it up to the second floor or the attic. This will bring the cost of installation up.

With forced air systems, a static pressure calculation is done on the ductwork layout. This determines duct sizing for adequate air flow and distribution. Not as elaborate with hot water systems which determine pipe size.

Distribution and stand-by losses are greater in your situation, regardless of delay switches used in energy efficient systems. The greater the load the greater the losses. This is because they are usually expressed in percentages. Because air does not hold heat well and water does, distribution losses can be as high as 40% with forced air. Whereas hot water is around 3 to 5%. It is true that it takes less BTUs to raise the temperature with air than water, however, when you consider the configuration of your home, 2 system are required and the age of the home, the hot water system is going to out perform the hot air system. In both distribution and stand-by losses. The longer the runs, the more losses. Don't confuse this with load.

Hot water system are far more flexible than hot air systems. With hot water it is easier, more available and less expensive to zone, radiant heating, domestic hot water and yes, even have forced hot air. The sites mentioned above do well in explaining this.
 
  #11  
Old 10-14-02, 01:24 PM
JudyRoyal
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Heat Load Calculator

I just used the Heat Load Calculator you suggested, doing each apartment separately.

It recommended a 47,000 BTU furnace for each apt--a far cry from 85,000 BTU furnaces suggested by two local contractors!

I added the calculations to the "house" web page,

http://www.regardless-shorthairs.com/House/Heat.htm

if you wouldn't mind looking at it.

Thank you.
 

Last edited by JudyRoyal; 10-14-02 at 02:51 PM.
 

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