DIY Chatroom Home Improvement Forum

DIY Chatroom Home Improvement Forum (
-   HVAC (
-   -   Load Calculation ? (

RM575 06-22-2007 11:26 PM

Load Calculation ?
I purchased a 2 month license for a computer heat loss calculation and I'm not sure if I've done this correctly. I looked at my home appraisal report and noted my square footage of living space is listed as 1875 (two story home). However, according to the recently purchased load calc program it indicates only 1613. I was very careful measuring the home, but I do have some vaulted ceilings downstairs and may have some slight inaccuracies in my measurement. I'm wondering if my home appraisal report included my attached garage in their calc. I didn't include that in the load calc since it is unheated.
According to the load calc,
Sensible Gain is 19,983
Latent Gain is 920
Total Heat Gain is 20,903
Total Heat Loss is 28,187
It appears the load calc program is recommending a 1.5 ton AC. My home is in California, Bay Area, where summer averages 80 and winter is mild 40's - 60's.

I went to the local plumbing supply to look into a furnace and ac. The salesman asked square footage and recommended a 80k input / 75k output 91% furnace, Fraser-Johnston and a 3 ton condenser, also Fraser-Johnston. I showed him the printout from my load calc and he still recommended the above units. He said the 1.5 ton would be too small for my home.

I checked a few other websites that offer a calculation based on square footage and their numbers are different:
47,850 (min)?
57,750 (max)?
I'm not sure what they mean by min and max.

Any advice on the size of the condenser and furnace would be helpful. I know I can call a pro to come in and do a calculation. But, I should be able to do a reasonably close calculation myself.

Also, I found a website that sells Rheem to DIY's. So, I'm considering that manufacturer.

bigMikeB 06-23-2007 07:40 AM

What your supplier said makes sense to me.

RM575 06-23-2007 11:46 AM

Thanks for the reply. I guess this is why doing a Manual J calculation is not very popular, even with those in the trade. I was very careful when I measured my walls, floors, ceilings, doors and windows. I re-measured my home and verified all of my entries. But, still the same result.

I hear so much about making sure the furnace and ac are sized properly for your home. So, I made that effort and then some. But, after reading so many posts on various forums, I realize there is no real precision about sizing this equipment. It seems that sizing equipment is more intuitive instead and it is better left to those in the trade. I think there are so many variables that influence the calculation, which may render it more of a calculated guess.

bigMikeB 06-23-2007 07:05 PM

Years ago I started out with ACCA manual J and all on a program called write J. They calculations always seemed to end up just short of what the ball park figures were in the instruction manual that came with the software. I have been sizing by my own methods since and have done many installs and have yet to have a complaint from a customer.
When I have any doubts, I talk to my suppliers.

bigMikeB 06-23-2007 07:07 PM

Also, IBR makes a simple sheet to calculate heat loss for hydronics, if you use that sheet it will work for any heating equipment and you will generally find that the heat gain is half of heat loss and can size the a/c accordingly.

pjpjpjpj 07-02-2007 12:00 PM

First of all, don't ever trust the square footage that a realtor tells you. Goodness knows how they come up with those numbers... :huh:

Residential heat load calcs always seem to come out low because they assume that the house is better-built than it really is (this is not a slight to home contractors reading this - just saying that typical home construction is not as "tight" as commercial buildings). So when you put in your wall insulation info, your window info, etc., the calc programs typically don't account for leakage around doors and windows, for leakage between siding and brick where the gaps are not all sealed (for example), and also for things like range hoods, dryers, and toilet exhaust fans which are wasting conditioned air and creating a negative space that pulls in outside air. They also don't account for the fact that, in residential applications, window shades/drapes are often wide open, whereas in commercial applications, if a window has (for example) mini-blinds, they are typically always closed.

Those sorts of things are somewhat "abstract" ideas in residential calcs. In commercial calcs, they are compensated because the outside air is typically brought in through the AC units (intentionally, code-required), and also because leakage and such ("infiltration") is part of the calculations. Also, commercial buildings are typically better built, because you have architects and engineers snooping around and making sure it's done right. :)

Also, if you do an actual calc for residential, you can typically double (or at least 1.5X) the load for the second floor, since homes typically have an open stairwell and the heat rises, plus your attic likely gets heated up and becomes a heat source, convecting heat through the second floor ceiling (despite our best efforts to insulate it, if you have 130F air sitting in an attic all day, the heat WILL seep through).

Anyway, to simplify it (maybe too much), residential AC is typically about 600 sq ft per ton. So if your house is 1875 sq ft., then you would need just over 3 tons of AC. If it is 1615, you would need around 2.7 tons (which rounds up to 3). And gas furnaces are never sized exactly for the space loads - they come in set sizes, so you will just get the next biggest size above what you need (which in some cases is just the smallest unit they make).

RM575 07-02-2007 11:30 PM

Thanks for the reply. I've checked a few more sources and it seems everyone is recommending a 3 ton AC. I think you are right about home appraisal square footage. My home appraisal report actually list 1835 sq ft and my HVAC calc comes in at a little over 1600. So, I'm going to err on the side of caution and say 1700-1750 sq ft. Most of the supply houses are suggesting up to an 80k btu furnace.

As a DIY, I'm very capable as I have worked with gas fired appliances before and I've worked with high and low voltage electrical. But, most of the equipment offered to DIY's seems to be low end equipment. Rheem was the only manufacturer I found to be of reputable quality offered online. Does anyone know of any other brands besides Goodman, Fraser-Johnston, Evcon, or Coleman. I'm now debating whether I want to do this myself or hire it out to the pros. I'd like to have the choice of some good quality equipment without settling for something too basic.

MechanicalDVR 07-03-2007 10:48 PM

Sorry to tell you but there really aren't too many manufacturers of residential equipment, even though they makes several lines.

Carrier= Payne, Bryant

Icp= Comfortmaker, Arocaire & NCP

York = Luxaire, Coleman, Fraser-Johnson

Rheem = Ruud

Heil = Quaker, Tempstar, Magic Chef

Amana= Whirlpool

American Standard = Trane

It all just depends on the latest merger.

Despite recent changes I'd still avoid Goodman

dmaceld 07-03-2007 10:58 PM

Don't be to quick to dismiss the HVAC Calc results. I used it a couple of years ago for the house I owned at that time in Louisiana. It said I needed about a 2 ton a/c. The old one was almost 3.5 ton. I finally opted to go with a 3 ton unit with a 2.5 ton evaporator for humidity removal. I did this for the same reasons you are seeing expressed here, the numbers just didn't "feel right." It turned out they were a lot closer to right than wrong. Temps used for calcs was 95F with 80% RH outside and 74 & 50 inside. On the hottest summer day, when by calcs the 2 ton unit would have been running 100% of the time, the 3 ton unit ran only about 60%, maybe as much as 80% of the time. That showed that the 3 ton was larger than the ideal size. Double check your entries for window data, wall construction, air leaks, etc., and make sure you have it all correct. Run several calculations changing some of the parameters. That will give you a good feel for what you may really need, like accommodating large groups if that's common in your house, or different indoor and outdoor temps. Compare the heat load and cooling load numbers and see if they make sense in relation to each other. If the cooling differential, out to in, temp is only 20F, and the heating is 50F, then the heat load should be somewhere around 2.5 x the cooling load.

Is humidity an issue where you live? If so, make absolutely sure you do not oversize the a/c. With a/c in high humidity you will be better off to undersize the system rather than oversize. Also, consider using an evaporator smaller than the condenser. That helps in humidity removal by running colder.

RM575 07-04-2007 12:37 PM

Thanks for the replies. I'm still debating whether or not I want to install my own equipment for hire it out. The reason I'm at a crossroad is the quality of the equipment available to the DIY'er.

Also, in my area the humidity is not a major problem. I just checked the weather data for my area and it lists humidity at 48% with an outside temp at 76f, at 10:30am.

I'm going to recheck all of my calculations, just to make sure.

Does anyone know about Fraser-Johnston equipment? I know they are manufactured by York, and it's most likely their low end equipment. They gave me some part numbers:

GY9S080b12DH1 Fraser-Johnston(80K, 91%, downflow furnace)

Aspen 3 ton coil 17" w/TXV (FJ34TCC17)

3 ton condensor (FJ3T13SC)

I'm not sure why they are specifying the Aspen coil and not something made by York or Fraser-Johnston. I checked out Aspen coils in other posts and have not heard any favorable comments about them.

All times are GMT -5. The time now is 03:48 PM.

Copyright 2003-2014 Escalate Media LP. All Rights Reserved