When is a duct booster fan OK?

[pstu]

Dash has mentioned Fantech as making duct booster fans which would really do something, so that brings to mind... can Dash or anyone say what might be an application where such a fan is really justified? Do you ever regard it as a legitimate part of a central forced-air system?

Not that I have any specific problem at this time. I know this question is vague, but if it is possible to give a generalized answer I sure would appreciate it.

Thanks -- Pstu

[OOC]

I was looking forward to seeing the replies on this. My brother has a long single story house and was condsidering a duct booster fan to reach the far end. I'd like to give him some things to consider when I also tell him to call in a qualified HVAC firm to see what's up.

[beenthere]

When there is no way to install the larger sized duct that is needed for that room.
EG: supply runs in the wall. Supply trunk is chaised in by drywall.

[beenthere]

My brother has a long single story house and was condsidering a duct booster fan to reach the far end. I'd like to give him some things to consider when I also tell him to call in a qualified HVAC firm to see what's up.

Single story. Is the duct work accessible from the air handler to this room/rooms.
If so, corrective duct work is better.

[OOC]

Thanks!

[perrybucsdad]

Single story. Is the duct work accessible from the air handler to this room/rooms.
If so, corrective duct work is better.

What if only some of it can be easily accessed? For example. In my basement, the main duct line can be easily gotten to, but when it goes up into the walls, I'd have to tear my walls apart. Would you recommend up sizing the ducts first in the basement and then see what happens, or will the small ducts in the walls prevent any benefits and just build up pressure?

[beenthere]

That has to be evaluated for that system, not a yes or no answer.
On some it will help, next one it won't.

[pstu]

Would you give the same answer or possibly different, if the duct were satisfactory for some applications and not for others? For example heating and cooling are two separate applications, and the best load calcs will not reveal the same airflow needs for each.

For areas with distinct summer and winter seasons, a once/yr changing of dampers might be recommended. But is that the only method acceptable? Is a duct booster fan a thing which has proper application given the right circumstances such as this? Or are there drawbacks that I don't see yet?

Thank you! -- Pstu

[coolwhip]

I install Fantec stuff. Correcting the duct work would be great, but sometimes you run into a lengthy branch that has minimal static pressure and virtually no throw.
So I install a 6 in in-line booster fan with a variable speed controller that's wired to come on with the indoor blower. These little fans can kick it out!, but you must realise that they are stealing air from someplace else.

[pstu]

In fact stealing air from somewhere else is exactly what seems desirable in some applications. For generalized heating and cooling, it would seem important to understand (or learn from experience?) just where it is being stolen from.

For one example when a 2-stage AC system runs in low, supply plenum static pressure could be 0.06 inch. Hard to believe that air just doesn't tend to dump out of whatever duct is closest.

[Shophound]

In fact stealing air from somewhere else is exactly what seems desirable in some applications. For generalized heating and cooling, it would seem important to understand (or learn from experience?) just where it is being stolen from.

For one example when a 2-stage AC system runs in low, supply plenum static pressure could be 0.06 inch. Hard to believe that air just doesn't tend to dump out of whatever duct is closest.

What is "ideal" is the correct amount of air at the correct temperature to meet the particular heat load of a room at a given time.

Since we don't yet have control technology that can accurately assess the heat gain or loss of any given room at any given condition, we design for an accepted maximum and minimum climate condition for a geographic region.

Realistically, short of tearing out construction to correct bad design and/or installation, dampers or duct booster fans might help right a temperature imbalance to an extent. The latter is less preferred, as it equates to adding more energy to correct a deficiency in design/installation; for had that been correct from the beginning, overall less energy would be consumed to attain comfort goals.

We're at a crossroads in the residential HVAC industry, IMO. Either maintain the status quo of slap-dash HVAC design and installation in the face of rising energy prices, or adopt a two prong approach of both consumer education and backing up such with good design and installation practices. Only the latter fork, IMO, is sustainable over the long term...and a wise choice for a contractor to pursue.

[beenthere]

For one example when a 2-stage AC system runs in low, supply plenum static pressure could be 0.06 inch. Hard to believe that air just doesn't tend to dump out of whatever duct is closest.

Low static pressure in the duct permits the air to travel easier in the duct. Since the pressure is lower, it doesn't force out more air then required to the closet supplies.

High static pressure causes the closet supplies to get more air then the rooms need.

[pstu]

Thanks Beenthere, that is the opposite of what I thought would be happening. I appreciate your expertise and your helpfulness.

Best wishes -- Pstu

[dash]

Low static pressure in the duct permits the air to travel easier in the duct. Since the pressure is lower, it doesn't force out more air then required to the closet supplies.

High static pressure causes the closet supplies to get more air then the rooms need.

Exactly!

Booster fan ,when ducts can't be corrected due to construction.


Improving ducts in an accessable basement ,could lower the static enough to mean that the ducts in the walls are "now" large enough.

Lowering static close to the indoor unit ,makes more available for the farthest runs.

[captube]

Good one Beenthere
Lightbulb came on when i read this, thanks!

Low static pressure in the duct permits the air to travel easier in the duct. Since the pressure is lower, it doesn't force out more air then required to the closet supplies.

High static pressure causes the closet supplies to get more air then the rooms need.

[beenthere]

Improving ducts in an accessable basement ,could lower the static enough to mean that the ducts in the walls are "now" large enough.

I didn't want to imply to anyone that may be a DIYer, that automatically the ducts in the wall will be large enough.

EG: A 7" oval running 8' vertical, and 14' horizontal to the master bedroom that should have 200 CFM.

[beenthere]

Good one Beenthere
Lightbulb came on when i read this, thanks!

Its one of the harder things to explain to a customer when you tell them the room that didn't cool well before, will now. When they say, "but you said it won't blow as much air in low".

[Joe Mascitti]

How much will the booster fan raise the temp of the air coming out of the supply?


Joe

[beenthere]

How many watts is the motor. 3.413xwatt=btu

[Bushum]

These little fans can kick it out!, but you must realise that they are stealing air from someplace else.

Wouldn't this also be true if the ductwork was sized correctly in the first place? A system only makes a max amount of airflow that is distributed right?