# Thread: Manual J Calc Question

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I just completed my first try at the HVAC-Calc Residential 4.0 for a new home. My results came out as:
Sensible gain: 53,545
Latent gain: 14,030
Total heat gain:67,575 (5.5 tons)
Total heat loss: 33,464

I have been trying to determine if a zoned system is a good option, and that is one reason why I wanted to do a Manual J calculation, to see if one unit could handle the house. Based on my first try the heat gain is too much for one 5 ton AC unit.

Here's my question (don't laugh too hard):

When the program asks for how many people are in a particular room I estimated maximum head count for each room at any particular time, although it is highly unlikely that that many people would ever be in the house at the same time. For example, I put 5 in the kitchen, 4 in the nook, 10 in the family room, 8 in the dining room, 2 in a hallway, etc.

The total heat gain for the "people" component is 29,150, and that is what prompts the question. 29,150 represents 43% of the total gain, so I'm thinking maybe I should not use the total capacity for each room simultaneously.

If I have mistakenly loaded up each room too high then perhaps the total heat gain may fall into a 5 ton range.

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One other topic relating to the J Calc. I am working with a Trane dealer and was considering the XV90 furnace. The first stage of the smallest XV90 (on the Trane website) is 37K, 2nd stage 56K.

If my J Calc. is correct at total heat loss of 33,464 does that mean I would rarely, if ever, use the 2nd stage on the XV90?

The smallest Trane XV80, on the other hand, shows 31.2K first stage and 48K 2nd stage.

How would the heat loss result affect the choice of furnace? I want a VS.

Thanks

[Edited by nathan9999 on 06-11-2005 at 06:46 PM]

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People input is usually 2 per bedroom,so yes it will reduce the load a lot,by correcting it.

Smaller furnace isa better match,but the 90 is more effiecent,as you likely know.Larger furnace shorter run times,common problem when needing a small furnace.

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I changed the "people" input to 1-5 people per room and it dropped the total heat gain to 55,585. The program indicates 4.5 tons.

Dash, thanks for the advice. I'll show this to my contractor.

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## House S.F. + Windows determines

Originally posted by nathan9999
My results came out as:
Sensible gain: 53,545
Latent gain: 14,030
Total heat gain: 67,575 (5.5 tons)
Total heat loss: 33,464

.. although it is highly unlikely that that many people would ever be in the house at the same time.

The total heat gain for the "people" component is 29,150, and that is what prompts the question.

29,150 represents ___
29,150 BTUh sounds like > ~50 people.
MAX that I use =~ DOZEN.
Something like
Dining 4
Kitchen 1 ..+. = 1.0 appliance ( 1,200 BTU)
Laundry 0 ..+. > 0.5 appliance
Family 4
Bedroom 2
Bedroom 1

S.F. Floor ___
S.F. Glass ___
Wall Height ___

My Standard Selection = ~90% of Total.

6. With such a small heat loss and such a high heat gain wouldn't a heat pump be more in line?

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has ManJ changed? seems like ASHRAE suggests 2 people for M bdrm, one for each other bdrm. However, place the heat load where most people will gather at one time, probably now days the entertainment rm (family rm) -- or split between family & dining.

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MechAcc, electricity rates her around 12-13 cents/kwh, so heat pumps have never made much of an inroad, but I must admit that it has crossed my mind to look into one.

cem-bsee, the programs helps did not make it clear (at least to me) if I should be going with (1) a cumulative maximum number of people in all rooms simultaneously, or (2) just the maximum number of people in the largest room.

So, let's say i speculate a maximum of 25 people in the family room/kitchen/nook at any one time. Would I enter that number and leave out any other rooms?

Thanks

[Edited by nathan9999 on 06-12-2005 at 12:29 AM]

9. 25 people in the familt room? What you having a buck owens reuniun?

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2 per bedroom,and yes for the load put them in the living area.If you entertain a lot maybe a couple extra.Adding to many (10 or more,maybe even 5 extra)will oversized the system.

Party load,use two systems or a two stage/speed compressor.

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Bigbird, I actually know Buck (related by marriage) but I routinely travel with a much cheaper crowd. But we do have family holiday dinners with 20-25 in attendance.

Dash, as of now I'm looking at 2 stage AC unit.

If my calc is correct at 4.5 tons then I would be looking at a 5 ton unit with 3 or 4 zones for 3139 sq ft.

Just as an example of the validity of the posts on this forum on oversizing, consider this:

The architect drew the plans (approved by the county) for 6 tons. The local Carrier dealer said he would not install less than 7 tons. Every other dealer (with the exception of 2) recommended a "so many tons per 400-500 sq ft approach".

So, subject to verification of the calc, the actual load calls for 4.5-5 tons, not 6 and not seven.

Many thanks to the originator and participants in this site.

Without the information on this site I would have accepted a builder's grade system and never even been aware of the options for higher efficiency and comfort.

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how many families have 2 per bdrm?
how many times will someone have a big party?

so, set the temp down/ up 3h before the party. Or, if couple does this often, go with Dash

esp if putting in hi seer --

yes, just put people in 1 or 2 rooms, none in others.

BTW, are you doing houses with theaters? but, probably no more load than my 2 400w cmptrs, + monitor + printer + scanner + ...

[Edited by cem-bsee on 06-12-2005 at 04:09 AM]

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Originally posted by nathan9999
(1) a cumulative maximum number of people in all rooms simultaneously, [Edited by nathan9999 on 06-12-2005 at 12:29 AM]
Why design a system for that "one-a-year party"? Are those 25 people gathering for a mid-afternoon ( exactly at 4 PM - 6 PM peak load) event on a day when it is ~94'F?

Your residential design is using simultaneous that may occur for ~2 Hours per Year. Plus, How much is the infiltration load when 30 people are Opening All the doors! ?

I probably would estimate 32 people total for a 9,000 S.F. residence. TRANE XL19i's are always used for this type of load in my designs.

You may wish to be somewhat more Realistic about developing this Simultaneous Load in any other designs you may perform.

... Do you have room for parking 20 cars?

Another equipment selection combination may be:
2 A/C systems ...
No Zoning
with a 2-ton XL16i bedroom area system
and a 4-ton ( or 5)XL19i common area.

NOTE:
Costs for Zoning may nearly be the same as adding a small system.

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