Manual J Calcs

[away822]

Hello All,

Been lurking around here forever and finally have a question to ask.

I am planning on having a home built and will ask the builder (very nicely) for a copy of the load calcs but something tells me it will be like talking to a brick wall. Either that or I'll get an answer of "yes they are all sized correctly". The home is not a custom home so some of the stuff like the HVAC may be out of my control. But I will ask nonetheless.

My question is in reference to the design temps that are used in the load calcs. Is this something that is standard depending on the area? Is it something that can change depending on use or desired preferences? Can the variables be modified in such a way to actually undersize a system? I know an oversized system is not good but I don't want a system that struggles to keep the house at 83 degrees either. I have literally been there and done that.

I guess it boils down to knowing the system has been designed and sized based on realistic numbers and not according to what the builder is willing to pay for.

Any help would be greatly appreciated.

Andrew Way

[gruvn]

Away,

The first rule of contracting is this

" He who holds the money has the power"

Even if it is already written out of your contract that you won't get a Load Calc done on your home

( actually just not written in)

You still have a couple of options, one and the most reasonable is to just go ask your builder for a copy of the load calc to be written in and provided to you,

( remember if you owe him money he is more likely to be agreeable to this, it all depends who has the most to lose, sad to say that but thats the way that it is)

If he says "no the contract does not provide for it" then ask him for the layout of your system with all unit specs, so that you may take it to an independant contractor and have it evaluated. You will probably have to pay for this service,

option 2 If you wanted to, and are ok with punching numbers into a computer program and doing a small amount of research on the materials that your home is being built with, you could purchase a load calc software that is inexpensive and works very well, it will generate a detailed report for you , then if your report

( the one you made or the one you paid for from a contractor)

does not add up with the equipment or design going in your home, you will at least have good documentation to continue your efforts to get it right with the contractor, my experience has been that when you show solid documentation the contractor will give and do something reasonable.

[dhvac]

red tab top of page says hvac calc

knowledge is power :D

[agb]

The poster also asked about how to select the appropriate design temperatures. While Hvac-calc will supply area specific values, I wonder if they are the proper values to use.

I live in the San Jose, CA area and HVAC-calc has an AC outside design temp of 83. If it never got above 83 outside, I wouldn't need an AC. But sometimes it gets up to 100 and then an AC would be really helpful. The design temps have a big impact on sizing. Are HVAC-Calcs values accurate? Should one designing for the average summer day or for those very hot days when high pressure weather is camped out over the area?

[HVAC Pro]

ASHRAE Handbook of Fundamentals lists San Jose @ 85/34 Summer/Winter. FYI.

[gruvn]

Originally posted by agb
The poster also asked about how to select the appropriate design temperatures. While Hvac-calc will supply area specific values, I wonder if they are the proper values to use.

I live in the San Jose, CA area and HVAC-calc has an AC outside design temp of 83. If it never got above 83 outside, I wouldn't need an AC. But sometimes it gets up to 100 and then an AC would be really helpful. The design temps have a big impact on sizing. Are HVAC-Calcs values accurate? Should one designing for the average summer day or for those very hot days when high pressure weather is camped out over the area?


In most programs that I have used you may enter the design value that you wish to use, the programs default to the MAN-J standard for the area. You the homeowner should determine whether you want to design for a more extreme day. That option is there for you to do that.

[away822]

All,

Sorry for dragging this back to the top but I have been away from my computer for a while.

Thanks to all who replied.

It seems that if the default values are used the calculation will be fairly accurate.

In light of that I will really try and get the actual calculations.

If that falls on deaf ears, I will just pay to get a load calculation done.

And if all else fails I will get the limited use version of HVAC Calc. I have played with the free version before and am somewhat familiar with the parameters needed. About the only thing I don't know for sure is the size of the windows but I can get a rough guesstimate on that when the framing is going up.

Just want a reasonably sized system that can keep the house relatively cool on most days. Not expecting 60 degree indoor temps when it is 110 outside. But then I don't want a system like the last one, that struggles to keep the house at 83 degrees when it is 95 degrees either.

I hope this doesn't sound like I am trying to catch the HVAC contractor doing wrong, I actually blame the builders more than anyone. From what I have seen, they tend to cut too many corners, then drag things out hoping people will just go away, and only after repeated complaints, will they make things right.

Thanks again.