Proper spacing of supply outlets off trunk

[ace064]

What is the proper spacing between supply lines off the main supply truck?

[hvacvegas]

What is the proper spacing between supply lines off the main supply truck?

As tight as I need to get the stuff to fit.

[ace064]

Is it better for two supply lines running to the same room to be close together or does that matter. Also, how does high or low air returns affect heating and cooling? It stands to reason that high returns work better in hot weather and low returns in cold weather. Which is best for both hot and cold weather?

[wraujr]

Before this discussion gets going (like all the previous high/low arguments) I have a question.
Has there ever been a study where a room was set up with high and low returns and supplies and smoke was used to observe air mixing, stratification, etc...... Sure would be nice if someone knows of a scientific study with real measured data to provide a real answer... Just a thought...

[skippedover]

The height of supplies and returns is always a topic of hot conjecture but no matter the opinions of the panelists, warm air rises. What this means is that no matter the season, the warmest air is always high in the room. Thus positioning the returns high in the room serves to move the warmest air out and back to the equipment for treatment.

In summer, it seems to be obvious that high returns are best but that seems to be lost on many when it comes to heating in winter. None-the-less, once the air has passed through a height of about 7-feet or that height in which people normally occupy, any further rise is wasted. Thus it is, whether the supplies are on the ceiling or the floor, warm air will exit the supply outlets and begin to rise. Since the very warmest air in the room will always be up at the ceiling, high returns are best.

However there is one caveat to the location of supplies and returns. When a room has cathedral ceilings, it is not always wise to try and condition all of the room volume. It may be not only desirable but also necessary to ignore the highest parts of the room and allow the warmest air to be stagnant high up, thus allowing a 'reduced volume' of treated air below. Under some circumstances for cooling, the cool air could enter the room at very high ceilings and before it ever reaches the occupants below, it is mixed with warmer air and begins to ascend. This can result in a room that is uncomfortable even though Manaul 'J' loads indicate the room is adequately cooled.

The one advantage to low floor returns is that much more dust and dirt will be removed from the rooms in which the return(s) are located.

[wraujr]

Well that addresses the effect of temp..
The other factor is the velocity and flow of air.
If I have a heat vent in floor that is tending to throw the air upward, couldn't that be considered bad as warm air already rises, so throwing it upward only makes the problem worse. So one could imagine that in heat season you might want to throw from the celing downward while in cooling season you want to throw upward.. Thats why I was curious about any testing using smoke, etc.

[beachtech]

Well that addresses the effect of temp..
The other factor is the velocity and flow of air.
If I have a heat vent in floor that is tending to throw the air upward, couldn't that be considered bad as warm air already rises, so throwing it upward only makes the problem worse. So one could imagine that in heat season you might want to throw from the celing downward while in cooling season you want to throw upward.. Thats why I was curious about any testing using smoke, etc.

please start a new thread

it's not polite to hijack another posters thread.

thanks :)

[wraujr]

No hijack intended, OP in Post #3 asked question about placement of returns and supllies, high and low. My comments were addressing air flow.

[genduct]

The height of supplies and returns is always a topic of hot conjecture but no matter the opinions of the panelists, warm air rises. What this means is that no matter the season, the warmest air is always high in the room. Thus positioning the returns high in the room serves to move the warmest air out and back to the equipment for treatment.

In summer, it seems to be obvious that high returns are best but that seems to be lost on many when it comes to heating in winter. None-the-less, once the air has passed through a height of about 7-feet or that height in which people normally occupy, any further rise is wasted. Thus it is, whether the supplies are on the ceiling or the floor, warm air will exit the supply outlets and begin to rise. Since the very warmest air in the room will always be up at the ceiling, high returns are best.

However there is one caveat to the location of supplies and returns. When a room has cathedral ceilings, it is not always wise to try and condition all of the room volume. It may be not only desirable but also necessary to ignore the highest parts of the room and allow the warmest air to be stagnant high up, thus allowing a 'reduced volume' of treated air below. Under some circumstances for cooling, the cool air could enter the room at very high ceilings and before it ever reaches the occupants below, it is mixed with warmer air and begins to ascend. This can result in a room that is uncomfortable even though Manaul 'J' loads indicate the room is adequately cooled.

The one advantage to low floor returns is that much more dust and dirt will be removed from the rooms in which the return(s) are located.

I AM IN YOUR CAMP!!!!!!!!!!

[timebuilder]

Ceiling fans are great at getting the warm air down to where the people are in winter.

[beenthere]

Low returns are fine for heating and cooling needs. The supplies have to installed properly, and chosen correctly. A floor register that only has a 5 foo throw, is not a good register for cooling needs when the ceiling is 8 foot above it.

Many times a contractor will just figure they are going to get 100 CFM from a 6 inch round duct. When in truth, they are often barely getting 70 CFM. There can be a difference in throw of 4 foot between a register that has 70 CFM going through it, compared to a register that has 100 CFM going through it.

High and low returns are ok to aid previous poor supply design, and register selection. But if your redoing the supplies, why not do them right.

[ace064]

wraujr- I do not feel highjacked at all. Keep sharing your ideas. We are all here to learn and share our experiences.
Along these lines about high and low returns, is it proper to vent thru the 16 inch wide space between studs and place both high and low covers that are closeable. That way you can use the high one in summer and the low one in winter. The exit would be out the bottom area of the wall space into the air return trunk. Is this crazy or does it make sense?

[wraujr]

I have no advice or opinion. I bet this is one of those things you ask five pros and you get five different answers. But if some HVAC industry lab set up a test room with vents and smoke gens and a stack of calibrated thermomer every 12" in altitude, you'd get a proof positive answer and this would never be debated again (like it has on this site before). Thats why I asked if anyone knew of a study...

I live in a house with floor vents and central return in hallway ceiling. I don't notice any layering/stratification, etc.
I also work in a office with supply vents AND return in 9' ceiling and once again I notice nothing. I'd like to see science provide an answer...(with data please)

[wraujr]

As to your spacing question. In my home and what I've seen in others they are minimally spaced about 16" apart which coincidently is the floor joist spacing :). And sometimes they are two takeoffs in the same joist bay (one going left, one going right). That said there is one feature of the trunk that somebody knowledgeable maybe designed, and that is that every so often (say 8 - 12') the trunk steps down a size. I beleive this is done to keep up pressure at the takeoffs closer to the furnace. So if you were creating a duct system from scratch a pro designer would be a good thing.

[beenthere]

wraujr- I do not feel highjacked at all. Keep sharing your ideas. We are all here to learn and share our experiences.
Along these lines about high and low returns, is it proper to vent thru the 16 inch wide space between studs and place both high and low covers that are closeable. That way you can use the high one in summer and the low one in winter. The exit would be out the bottom area of the wall space into the air return trunk. Is this crazy or does it make sense?

Wall cavities aren't always sealed. So you can pull in attic air.

[beenthere]

I have no advice or opinion. I bet this is one of those things you ask five pros and you get five different answers. But if some HVAC industry lab set up a test room with vents and smoke gens and a stack of calibrated thermomer every 12" in altitude, you'd get a proof positive answer and this would never be debated again (like it has on this site before). Thats why I asked if anyone knew of a study...

I live in a house with floor vents and central return in hallway ceiling. I don't notice any layering/stratification, etc.
I also work in a office with supply vents AND return in 9' ceiling and once again I notice nothing. I'd like to see science provide an answer...(with data please)

ACCA did do test on how much the position of a return makes. Minimal to no effect on comfort.

Remember, air is "pushed" into the return, not pulled.

[hvacvegas]

ACCA did do test on how much the position of a return makes. Minimal to no effect on comfort.

Remember, air is "pushed" into the return, not pulled.

Yep.

I seem to remember seeing that documentation on this site as a matter of fact.

[beenthere]

Yep.

I seem to remember seeing that documentation on this site as a matter of fact.
As I recall, it was Dash that posted it.

[jimj]

Ceiling fans are great at getting the warm air down to where the people are in winter.

Yep, I agree and just that is recommended in manual D.

[energy_rater_La]

about duct location on the plenum.

in my climate (hot humid) putting the ducts too close
together, where the outer liners are in contact with each other
will cause condensation.

also if duct take offs are too close to each other it will be
a pita to properly mastic seal the take off, the inner liner
and then the insulation and outer jacket.

if the insulation and outer jacket are not in contact with the
plenum then the exposed duct take off will condensate.
as will all of it if not sealed.

mastics last. duct tapes fail.

I usually install a 4' plenum instead of a 3' just
to have the space around the ducts.

just my own experiences but I don't tap off end cap
or too close (within 18") of ahu/coil.


best of luck.