Multiple Ducts at Returns

[Anatol]

Fellow Inspectors,

Recently renovated town home in DC. The only return(s) are located in the second floor ceiling (attic). No return installed on the first floor or in basement. Any idea why they would install two sets of returns and seven sets of duct in this area?

Anatol

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[Terence McCann]

Where is the furnace located?

[Phillip]

With out seeing the attic view, my guess would be that they did not have room enough to run a large return pipe/duct so they had to run a bunch of little pipes to get the right amount of return air for the unit.

[Anatol]

Furnace is in rear basement. Low slope town home roof, not a lot of space to run large ducting.

Anatol

[Bain]

One system will seldom cool a two-level house or townhome adequately. The returns were installed on the second level to mitigate that reality. As we know, heat rises, so the idea is to suck in the warm air near the second-level ceiling to prevent stratification.

Sometimes it works. Usually it doesn't. Tell your peeps what's up, or they'll be calling you in August when the second level is 10 degrees warmer than the lower level.

[hausdok]

A quibble,

Heat doesn't rise. Heat radiates equally in all directions from its source - heated air rises.

I wouldn't make such a fine point of it, except that I frequently see folks misunderstanding radiant heating systems because they have this idea that radiant heat doesn't work unless it's placed directly underneath one's feet so that one will be in the path of what they believe to be "rising" heat.

Some folks quibble about water heater versus hot water heater; I quibble about heat rising versus heated air rising.

ONE TEAM - ONE FIGHT!!!

Mike

[Bain]

A quibble,

Heat doesn't rise. Heat radiates equally in all directions from its source - heated air rises.

I wouldn't make such a fine point of it, except that I frequently see folks misunderstanding radiant heating systems because they have this idea that radiant heat doesn't work unless it's placed directly underneath one's feet so that one will be in the path of what they believe to be "rising" heat.

Some folks quibble about water heater versus hot water heater; I quibble about heat rising versus heated air rising.

ONE TEAM - ONE FIGHT!!!

Mike

Dude . . . rock on.

[kurt]

It isn't about air temps and "rising heat". It's about the temperature of all the stuff you touch. So, I have no quibble with your quibble.

Radiant systems heat up the floor, the furniture, the walls, etc., etc.

If whatever you're touching is 78degF, you feel warm. Air temps should be around 64degF in a well designed radiant system. It's the physiology of comfort.

Right?

[Marc]

One system will seldom cool a two-level house or townhome adequately. The returns were installed on the second level to mitigate that reality. As we know, heat rises, so the idea is to suck in the warm air near the second-level ceiling to prevent stratification.

Sometimes it works. Usually it doesn't. Tell your peeps what's up, or they'll be calling you in August when the second level is 10 degrees warmer than the lower level.

Separate AC's for separate floors was born of a desire to save energy by shutting off the AC to the bedrooms during the daytime. A single system, if it possess adequate tonnage, can cool both floors just fine. If it's hotter upstairs during the cool season, the ductwork isn't properly sized. Between seasons it may be necessary to leave the blower on or run it intermittently, like my own thermostat provides for, to counter the rising hot air.

Marc

[Bain]

One system will seldom cool a two-level house or townhome adequately. The returns were installed on the second level to mitigate that reality. As we know, heat rises, so the idea is to suck in the warm air near the second-level ceiling to prevent stratification.

Sometimes it works. Usually it doesn't. Tell your peeps what's up, or they'll be calling you in August when the second level is 10 degrees warmer than the lower level.

Separate AC's for separate floors was born of a desire to save energy by shutting off the AC to the bedrooms during the daytime. A single system, if it possess adequate tonnage, can cool both floors just fine. If it's hotter upstairs during the cool season, the ductwork isn't properly sized. Between seasons it may be necessary to leave the blower on or run it intermittently, like my own thermostat provides for, to counter the rising hot air.

Marc

Perhaps in theory, but not in practice. I've been in too many two-story houses in which the second level was several degrees warmer than the lower level.

I've never heard the bedroom scenario before, but I do know that an assembly-line builder in my area, that's in the top ten nationally, began putting two systems in its houses because they fielded a gazillion complaints when they were only putting one system in a house.

[AHI in AR]

I tell my clients that they're fighting physics using one system to handle more than one floor: they can't win that fight. Returns on each floor help, but it's not a cure.

My experience says that the upper floor will ALWAYS be a little warmer -- heating or cooling season.

Running the fan continuously will help, but at what energy cost?

[kurt]

The real problems come from a lack of holistic view. Everyone focuses on the equipment, when they should be looking @ heat loads.

Glazing, orientation, insulation, cubic volume, floor plan, etc., etc., should dictate HVAC design. Of course, that never happens.

I mean, how many Manual J calculations has anyone seen in their entire career? I've only seen a couple in 27 years.

[AHI in AR]

I would make the argument that if we are looking at a single system, a Manual J load calc wouldn't truly be conclusive. I have to think that a system that handles more than one level -- and which doesn't run full-time -- can't be properly designed this way. It simply doesn't take into account all the parameters.

Sure, it can calculate total heating and cooling needs. But it is not going to tell you how to accomplish that with a single system since each level of the home has different heating and cooling needs.

Or am I missing something?

[Terence McCann]

You can cool the 1st and second floor correctly with one system however you have to find a contractor that knows what their doing and then you actually have to pay more for a correct temperature control system and balanced duct work.

When they build houses like their on an assembly line you get the contractor special. All I want to know is how cheap it is.

[Erby]

I"ve seen two Manual J calculations but they were for large shopping malls in a past life.

No one wants to make that effort for a house unless it's one of them multi-million things.

And it's likely gotta be a large house before anyone would suggests a variable air volume system.

Plug in two units and be done with it is the prevailing theory around here.

[Phillip]

The only time I've seen Manual J calculations is when I was learning how to do them.

Even if you have them done you still have to installed the duct properly.

A zone system is what you need to keep 2 or more levels at the same temp.