# Manual J vs CoolCalc

| Posted in General Questions on

I’m rebuilding another house and I learned a ton doing my last one about mini split sizing and j-calc’s.

I’ve done a million CoolCalc calculations but this does not seem to take into account mini split positions on the first level and how they effect the room directly above them.

This early 1900’s house  has “heatilator” vents probably 16×30” cut into the ceiling of the living room that are in the floor of the master bedroom.  This made me think about how I sometimes won’t even run my second level mini-split in the shoulder season at my house, because the heat makes it’s way upstairs anyway.

Long winded question being, would a true professional manual J take into account a bedroom being below a living space for heat loss, and while CoolCalc doesn’t know the floor plan, surely it at least takes into consideration what’s on the first vs second level in its numbers?

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### Replies

1. | | #1

I don't believe any manual J accounts for natural convection between floors. The floor or interior wall is considered a surface with no load contribution.

If you look at the total design load for the whole house, all the systems need to meet that load. You may be able to get away with under sizing one system if there is excess capacity on another, but you need to consider the potential for temperature variation.

Personally, I would seal up the vents for privacy and noise considerations if more than one person will live there.

1. | | #2

Thank you. The question was curiosity as much as anything just because I’ve noticed my own mini splits end up doing most of the work on the main level in the winter (and most of the works happens on the second level in the summer).

This new house will just be a couple and a baby. They want to leave the heatilators so they can still burn the stove and get the heat upstairs - but I am considering adding a fire baffle.

1. | | #3

I wonder how effective these "heatilators" even are unless you make the downstairs VERY warm. A 16x30" opening isn't that huge. Consider that an open stairwell and door won't fully condition the upstairs rooms. This is probably similar to the subject of using bath fans to move conditioned air between rooms that has been largely considered not effective.

1. | | #5

That's a good question. I've seen a fair amount of them in old houses, so I have to assume it works to a degree? I'll ask the tenants after they move in. At least in this scenario you'd taking advantage of convective currents and not trying to move heat laterally (or worse, down) with a bath fan.

2. Expert Member
| | #6

In order for heat to flow there has to be a temperature differential.

1. | | #7

I guess I see them working sort of in the scenario where someone stokes the fire downstairs just before bed so it is becomes unbearably warm downstairs in order to get some heat upstairs.

2. Expert Member
| | #4

Manual J won't help you in this situation. Manual J doesn't care about shoulder seasons, just extremes. When doing room-by-room calculations Manual J assumes that every other room in the house has its needs met and neither contributes nor removes heat from the room.

IRC requires that systems be sized according to Manual J, so when sizing your system you can't take those things into account even if you want to, assuming you live some place where codes are enforced.

Modeling things like room-to-room convection is notoriously difficult. Keep in mind that all heat flow is driven by temperature differences. So if there is to be meaningful heat flow between the rooms one has to be substantially warmer than the other. Whether that is acceptable depends on your tolerance.

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