residential duct design modifications

[a student]

I have a LR which, on hot days, remains 5-10 deg hotter than the rest of the house. The A/C is capable of cooling the LR, but that makes the rest of the house uncomfortably cool. If you close the registers in an effort to direct more air to the LR, it causes a lot of noise at the registers as airflow is very strong everywhere except the LR.

The LR is twice as big as the next largest room in the house, and has two registers. All other rooms have one. The LR registers are twice as far from the blower as the rest of the registers. The air flow is noticeably reduced at the LR registers when compared to other registers in the house and the temperature at the LR registers is also 2-5 degrees higher.

The LR registers collectively share exclusive access the same trunk. There are no significant leaks, but the duct path could be straigter and some excess length could be trimmed (currently just compressed). However, I do not believe straighting and fitting will sufficiently improve the situation.

There is a header between the LR and the return which prevent efficient return of hot air in the LR. There is a fan in the LR but it helps little.

The LR is also the hottest room in the house due to windows, exterior walls, and smaller attic space.

The A/C unit also appears to be oversized - typically on hot days the cycle is only 8 minutes - on for 8, off for 8.

My thought at this point is to increase the size of the duct (trunk and both branches) in order to increase airflow to the LR. Currently there are four 8" trunks coming from the blower, each with two 6" branches. I am proposing to increase the LR trunk to 12" and branches to 8".

Am I on the right track?

TIA, chris

[gofish]

yup, you're on the right track. all you need is another 5-7 years of hands on experience to arrive at a practical solution to the LR insufficient airflow problem.

btw, what did the manual D say you needed for the LR duct sizing?

what? no manual-D? errrr well, keep rolling the dice....

[Noon-Air]

manual J?? nawww..... the lowest bidder just uses a rule of thumb.

You can get it done, or you can get it done right.

[a student]

Yep. Or maybe a 5-7 minute conversation with someone who is willing (and able) to give practical advice.

[a student]

Yep. Or maybe I could have a 5-7 minute conversation with someone who is willing (and able) to give practical advice.

[udarrell]

If no dampers in ducts, install them to help you balance the airflow.

It is horrendously oversized! That is terrible! It takes 5 minutes to achieve near its efficient running capacity. What can be done with a little half-ton room A/C, over 900-sq.ft.

A real good "Return Air" in the LR & dampers should help! Here is some info on ductwork. - udarrell

[Tekkie®]

a student posted for all of us... I don't top post - see either inline or at bottom.

I think you got switched to the siding and ain't gonna get the highball for awhile...

[gofish]

Oh I'm willing to give you practical advice, the huge question is tho, are you willing to comprehend, understand & accept it?

In order to help size your ducts, you're going to have to provide us with the Blower Curve chart for your particular furnace/air handler, as well as the T.E.S.P. fo the air distribution system. For simplicity sakes, post Make, Model & Serial # of furnace or air handler. Also post your measured TESP, how and where you measured it.

From your past studies you will recollect that a blower will deliver a specific amount of air based on the TESP of the air distribution system.

What if your TESP is off the charts? What do you think that will do for airflow? It doesnt matter if you have a 20" duct, the fan still will not deliver the required airflow.

It's entirely feasible your entire duct system has to be replaced.

It doesnt make a bit of difference how big your LR is, where its located, how many walls or how many windows it has. Regardless of all of its physical characteristics, a person can determine how many btu's are required to keep the room at 75 when its 95 outside. This is called a load calculation. AKA Manual J.

How can you begin to size the duct if you're clueless on how many btu's are required for the LR??

OK so now you've did a room by room load calc, and have determined if your ac system has been properly sized, (or not). You did this by comparing the load calc data to the engineering performance data for your particular ac unit, AT DESIGN CONDITIONS.

OK ac unit is ok, blower is ok & within the fan curve chart, TESP is within manufacturers range, now do the manual-D, duct sizing!!!

Here's the kicker.....ya cant do an accurate manual-d until you know your TESP, and you can only measure your TESP once all the air distribution is installed !!! Of course it is entirely possible to calculate your TESP. if only you knew how, and had all the required literature & info.....

ok, wife says the moose steaks are off the barbie, so it's hasta-la bye bye for now.....

[a student]

Daryl, thanks for the advice. Your website is very informative!! Much more useful than most of the stuff I have come across on the web.

I can see now that the return is more of a problem than the supply in this case, and it does seem like dampers would help.

Obviously the ACCA manuals are a preferred reference source, and I see you mentioned ARI texts on your site...any other reading materials you recoomend?

Thanks again,

Chris

[a student]

Just so you know, I am trying to educate myself on the subject a) because I enjoy learning and b) because I have an interest in residential property management. I have a background in engineering so I think I am capable of understanding and measure things like TESP. I also have experience in all phases of residential construction, although the HVAC and electrical work was always performed by licensed contractors.

I think it would be best if I understand things well myself and then I will also be in a better position to gauge the competence of potential contractors when I need to hire help.

I understand the importance of properly sizing equipment, calculating loads and designing distribution systems and the part that the manuals S, D, and J play in that, but I am more interested in troubleshooting and pratical applications to existing systems. From what I can tell, most systems are not designed or implemeted all that well. Obviously it's not practical to replace every system that is improperly designed or implemented.

I am planning to get copies of the ACCA manuals and I am looking for additional resources if you have any recommendations. I am also interested in practical recommendations on cost-efficient and essential tools of the trade.

Thanks,

Chris

[udarrell]

Hi Chris, I am in agreement with your objectives, & applaud you for your initiative!

Low airflow is a number one problem of improper duct designs & blower applications!

The first priority is to reduce the heat-gain & heat-loss of the conditioned space.

After you learn what tonnage the ductwork & system will handle, you then decide which will be the most cost-effective way to go. Mid to Long-term it is usually finding ways to reduce the tonnage required in order to utilize the airflow being delivered, if that is within design Static Pressure specs.

There are ductless split-systems that can be added to meet extreme high heat-load cooling demands, & thereby further down size the central duct system equipment.

The blower curve would need to be checked to see the tonnage it could handle, at the equipment's required static pressure specs.

There is a possibility that could be done by using a proper sized TXV & changing out the condenser to a reconditioned 10-SEER unit, ha. (Where costs are the critical factor.) "When the sizing of equipment is right, SEER becomes less of a factor." Always look at the EER ratings, not the SEER ratings, the spread between them increases as the SEER goes higher!

Energy Conservation & saving the customer money with an effective payback period should be the paramount priorities.

Read my Web pages to learn more problems to look for, & remedy. - udarrell

[gofish]

My strongest recommendation is for you to attend a 1-3 day class taught by certified instructors on the topic at hand. Oh sure, you may pick up a few tidbits on some web page or discussion group, but to get the level of knowledge you are seeking is going to take a full on, hands-on class dedicated to that topic alone.

These people absolutely rock. And they have great specials on the required tools of the trade.