A Little HVAC Advice For The Home Owners Here.......

[Robb]

I was sending an email to a buddy of mine earlier this evening about some advice in regards to residential HVAC...................and I realized that I got long winded, and instead of just letting it die, I would post it here just in case any homeowners here were interested. Most probably won't care, but it is a nickel's worth of free advice if you want it.

If you have "central A/C" in your home, take a read.............

In the commercial HVAC industry, outside air (or fresh air) is dictated by code in most localities. This is because most buildings are being built “tighter” than in the past, and because of this, internal air is being recycled over and over. In turn, germs are being spread all over the place, which leads to what is known as SBS (sick building syndrome). Modern homes are also being built tighter, which leads to the same result. Ever noticed that you and your family tend to spread “colds” amongst yourselves?

In residential construction, outside air has usually been introduced in various ways; opening windows, running exhaust fans, or just “loose” construction. All of which bring outside air into your home. Problem is, this air isn’t “conditioned.” It is just straight outside air, which if you live in my neck of the woods, could easily be 96*db air. This is air that your central ac has to work harder to compensate to overcome (ie, costs you money).

To also look at this from a duct technical aspect, your system is probably naturally creating a negative pressure inside your living space. A certain amount of duct leakage exists at every joint in the ductwork system. This includes the “slides and drives” of the hard ductwork, and at every “flex” duct connection. If you have a normal residential duct install, you probably have many supply outlets, and one or two return inlets per air handler. This means that you have many more supply connections than return connections, which means your supply ductwork is leaking more than your return ductwork. Now, since your ductwork usually isn’t in the same “space” as your living space, this means that you are returning more air than you are supplying into your living space…………………..which puts your living space into a negative. This means that your ac is pulling air from every opening or draft in your home directly into your living space, and is pulling a lot of unconditioned air into your space every time a door is opened.

This is what I have proposed in the past, and have gotten lots of good feedback from it. You want to introduce outside air into your living space, but just as in the commercial realm, you want to condition it first. This simply means running a small duct from your return duct to the outside of your home. A 4” (dryer or exhaust fan) flex duct will do this well (with a small volume damper for adjustment). By doing this, you accomplish two main goals: 1) you induce fresh air into the living space, reducing the recycling of stale air (and reducing the spreading of germs), and 2) you create a positive in your living space, which reduces inducing unconditioned outside air from entering your space. By doing this, if you open a door, a little of your conditioned air in the space leaves outwards from the door, but this is much more efficient than inducing unconditioned air into your space.

This small little trick leads to a more comfortable and efficient home. Expensive ERUs or ERVs (energy recovery units/ventilators) are becoming more popular in high-end homes, and these units “steal” heat energy from any exhaust air (bathroom, kitchen hood, etc), and transfer this energy to an outside air duct. These units are great, but $$$$$$$! My little trick is simple, and although not as efficient, starts you down that road.

[Cloak]

Interesting. I'm going to have to have a look at the duct work on the new house. It has proper AC instead of swamp cooling like the current house has. Now would be the time to make the change while it is still being constructed.

[Shaun (yota)]

What is the average duct leakage in a res. system?

[Robb]

Originally Posted by Shaun (yota)

What is the average duct leakage in a res. system?

That is a very open question due to the fact that there are great duct guys, and very lousy duct guys out there. A residential system (when properly sized and installed) runs at a very low pressure (external static pressure) of .5"WC or less. With such a low pressure, very small cracks and leaks are very forgiving. When I test commercial ductwork for leakage, I test anywhere from 4-12 times that pressure.

But, as with commercial work, there is plenty of "monday morning, friday afternoon" work out there! Especially if ductwork is suspended, there is a good chance of a bad seal. An average homeowner would probably be surprised to find the amount of leakage by simply walking their ductwork and just feeling for air around any joint. "Smoke bombs" are also readily available for a visible check.

But, if I had to throw a number out there, I would say that the average residential ductwork system easily leaks 5-10%.

[JeremyH]

Originally Posted by Robb

But, if I had to throw a number out there, I would say that the average residential ductwork system easily leaks 5-10%.

You are probably right. When I was younger, I used to do the ductwork at my Dad's AC business....and I saw some crap out there.

In my opinion, all ductwork needs duct tape, pookie, tin foil tape and zip ties (where applicable).