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heat load calculation-HELP

Hello to all and thanks for any input, it is greatly appreciated. I am trying to keep this as short as I can, really.  I have worked in the HVACR field for almost a decade and have recently started my own company.  Now that I am on my own I  am encountering certain situations that, although I am familiar with the concepts, I do not have a lot of practical experience with them. For example, heat loss/gain calculations. I went to trade school and have read manual J (I think its the abridged version).  I constantly encounter houses where, for example, the top floors where the attic has been finished are too hot or cold.( I understand this could be a ductwork issue I have read manual D too). Many, but not all, of the contractors I speak to say that manual J (I wont say which one) is not accurate or that it undersizes or that it has just never worked for them. I had a situation with a room is a finished attic with one radiator. I calculated BTU of radiator (from lost art of steam heating) and heat loss of room and my calculation was that the radiator was more that sufficient. I dont want to simply oversize, I wantto be able to do a proper calculation, both for myself and for the customer. I am puting is a bid for a furnace replacement and the existing is 120,000 BTU, I would like to be able to know with confidence if it is oversized.

Any advice would be greatly appreciated


  • So,

    what is your question?
    Often wrong, never in doubt.
  • charliechicagocharliechicago Member Posts: 68
    good question

    Alan, thank you for your response.

    I had the same question after I posted;  will anyone actually understand what my question is!!

    Basically I am asking, is the ACCA manual J (or others) accurate. Is there a "learning curve" to it. Are there certain common mistakes that can be avoided. For example, I have read on HVAC forums of homeowners writing is to ask why three different contractors gave them THREE different load calculations and three different equipment sizes. It is not a difficult calculation, I wonder why they came up with different answers.  4+4=8. I am afraid to use it and them find out that the equipment is undersized. Or am I just influenced not to think it is accurate by all the contractors who dont use it and say its no good.

    Again, thanks for your response, Michael
  • icesailoricesailor Member Posts: 7,265

    I don't know what your question is but I find that Manual "J" is for air systems and was designed as such. The trade organization "IBR" developed a program based on their research on hydronic heating and developed instructional programs. They taught thousands and thousands how to calculate heat loss. I bought the books and taught myself. I figured my own heat for years. The first computer heat loss program I bought was an old DOS program that was based on Manual "J". I found it was more cor AC/cooling with heat as an afterthought. The Slant-Fin "Hydronics Explorer" is based straight on the IBR method. They well compensate for the differences in construction and heat loss between floors.

    I had old IBR manuals. The grey one, the yellow one and the orange one. I loaned my yellow one to someone and never got it back. I tried to buy new ones but they have been changed. I heard they are no longer available.

    Heat loss is sort of an acquired experience. You need to know a lot about building construction and how insulation effects heat loss etc.
  • icesailoricesailor Member Posts: 7,265
    You've said:

    It will all depend on what heat loss factors are used and the corrections applied. A lot of old houses were done by the wall method. Go wall to wall on all outside walls.

    The whole concept was FM and only understood by wholesale house salesmen. They sold you the job. They were obsessively afraid of a cold house so they always put plenty of heat in. The stuff I see the airheads do is beyond my comprehension.
  • ChrisChris Member Posts: 3,056
    edited February 2013
    The Math

    Never lies. You have to understand what a heat loss is and it's really simple. Loss through outside panels and loss through infiltration. That's it. Nothing more. Simple formula for figuring the loss through an outside panel.

    Ti-TO/R-Value = Btu/hr sqft

    Ti = Temp Indoor

    To= Temp Outdoor

    R-Value of your panel

    70-0/19 = 3.68 Btu/hr SqFt

    10' Wall

    8' Ceiling

    No windows

    Heat loss is 10x8x3.68 = 294.4 Btu/hr loss through the panel.

    Infiltration is a little tricky and that is really where software helps. I've been using Uponor's ADS program since it was born many years ago. It is free and also will do cooling and radiant.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • charliechicagocharliechicago Member Posts: 68
    manual J

    thanks for the response. much appreciated
  • charliechicagocharliechicago Member Posts: 68
    manual J

    thanks for the response. much appreciated
  • heatpro02920heatpro02920 Member Posts: 991
    A lot can be told by the existing system

    If you have a system there and its 120K BTU boiler, ask if it runs a lot on cold days, and go from there...

    Not saying just throw in whats there by any means, but keep it in mind.... As far as one part of the house being warmer or colder, just make sure you use the same formuler to figure every room...

    I have seen baseboard jobs where guys install 20 feet in a 10x10 office and then 15 feet in a 15X20 foot parlor all on the same zone, then the customer is calling wanting to know why their office is 15* warmer than the rest of the house.... Same thing with bathrooms and duct work, I see 4x12's in these tiny half baths and then 2-4x12's in the living room that is 10 times the size of the bathroom... IMO uniform sizing is very important for comfort...
  • charliechicagocharliechicago Member Posts: 68

    Thanks for the response, i see the logic in what you are saying. Michael.
  • Pughie1Pughie1 Member Posts: 116
    Heat Loss - just a suggestion

    If you are looking for a heat loss method try going to -

    thats John Siegenthalers web site. Go to the download section. There is a free download there called Heat Load Pro Trial Version. I've found it to be a very

    accurate program for calculating heat loss. It even accounts for the effect of studding. If you like it you can purchase the full program.

                                                                                 John Pughe
  • charliechicagocharliechicago Member Posts: 68
    heat loss

    thank you john. what is "studding"?
  • Pughie1Pughie1 Member Posts: 116

    Sorry Charlie,

    I should have described it better. I was referring to a wall - 2x4 or 2x6's on

    16" or 24" centers or ceiling rafters. The program takes into account the

    different "R" values for the "stud"area and the cavity.

                                                     John Pughe
  • charliechicagocharliechicago Member Posts: 68
    i thought

    thats what it might be but was not sure. thanks
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