Need To Add a Return. I Have a Few ??'s

13 years ago

I need to add a return to my parents home. They had a 24' x 32' addition built on a few years ago, and it has never heated or cooled properly in that room. The furnace was replaced after the addition, and central air was added at that time too. The existing return is centrally located (in relation to the original floor plan and home size.) in the ceiling of the very small hallway. The original part of the home stays very comfortable year round but the addition just isn't comfortable on hot or cold days. Everyone I've talked to tells me I need to add a return to the addition. The room is one large, open room with no doors and a vaulted ceiling with 2 large ceiling fans. I have a perfect location for the return, and have found where to place a take off into the existing return. My main question is, what would be the best size insulated flex duct, and what size boot and grille should I use? I want to try to stay at or under a 25' run from the addition back to the main return. I realize after installing it, I may need to tweak the original return opening to create the proper draw from the addition. Any information, tips, and help is greatly appreciated. I 'm by no means an HVAC technician, and only know what I've been told, or what I've been able to read, but I do have the ability to do the install.

Thanks again in advance.

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Comments (5)

zippyhvac
13 years ago
I would be very worried about whether the HVAC unit actually has enough cooling/heating capacity for an addition of that size. Around my neck of the woods, 768 sq feet could require two tons of capacity by itself, and the vaulted ceilings make it even trickier.

That said, the room is already built, and the experiment will show its value in a short amount of time, and won't cost much. if it doesn't work, or if the additional load overcomes the hvac unit, you could probably just cap the return at your old unit and point the flex towards a separate, new unit for the addition, and re-use the new grille and flex...

My thoughts? Get the biggest boot and register you can find - 20x30" is common in my area. 18" flex is probably enough, but if you're running it 25 feet, personally , I'd bump it to 20" - the cost difference is probably
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zippyhvac
13 years ago
Random additional thoughts...

If you've got good access to the existing ductwork, spend a few hours with some "mastic tape" & a bucket of duct sealer to tighten up all of your existing ductwork, both supply and returns. Any steel plenums/boxes in the ceiling need additional insulation to prevent transferring btu's to the attic. Seal your return-air "can" to the ceiling with good quality caulk and mastic tape - a poorly sealed "can" will allow attic air to go into the return.

Ask for "r-8" flex duct - my supply house has both r-6 and r-8, and the cost difference is negligible - you need to assume your current hvac unit is marginal at best and you can't afford to give away any btu's. If feasible, you might consider replacing any existing flex duct with r-8 duct - some of the older stuff was only r-2.

I've read studies that showed that an "average" insulated ceiling "gave up" HALF as many btu's the ductwork - spend most of your time in the ductwork first.

Think about adding blown-in insulation to the whole house if you've got less than 10" over the entire house - you want to reduce the amount of work the current unit has to do. Weatherstripping the doors & windows will make a difference.

When running flex duct, be aware that kinks and bends cost you more flow than length - if you have an obstruction that requires you to kink the line, going around it will likely give you better flow.

Cut off any excess flex - leaving flex loose can cut flow 10-20% or more in flow capacity - pull it snug and cut off the excess -
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tigerdunes
13 years ago
bigbadstang

several questions.

what size is home including new addition?

what size furnace and AC do you currently have?

what size is original return?

returns are best located in central location preferably in an interior hallway away from heating/cooling source or supply vent.

I don't think I would waste my time on an additional return until I performed a load calculation for both heating and cooling. just adding a return may not be the solution you are looking for.

IMO
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zippyhvac
13 years ago
Respectfully, I call BS.

Adding a return isn't a "waste of time"

Advocating for a load calculation likely sends him back to the "pro" HACK who tapped into his existing ductwork without adding any return, thus GUARANTEEING a lack of comfort in the addition.

The "average" 10+ year old house has upwards of 15% duct leakage - fixing the leakage has the SAME EFFECT as adding 15% to his heating and cooling capacity. Weatherstripping & insulating any metal ductwork adds to that savings, even if he ends up adding a unit for the addition, or increasing the capacity of his existing unit.

His labor is FREE - pretending that only a contractor is capable of installing flex duct is silly. Anyone can, attics are hot and uncomfortable, and we all know that anything that makes a job hard = increased costs to the consumer.

The house might still be uncomfortable on the hottest or coldest days, due to lack of capacity, however, they've lived with the existing unit for a period of years. Advocating what could be a $10k fix FIRST is just as irresponsible as installing an addition without any air return was.

A couple hundred bucks in parts and a Saturday afternoon's labor could make a huge difference in performance.

AFTER the ductwork is tightened up, it might also be possible to tweak the fan settings a bit on his existing not-too-old setup to get even a little more comfort.

It might not be perfect, but very few new setups are either.
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tigerdunes
13 years ago
bigbadstang

perhaps "a waste of time" was a poor choice of words. but if you are undersized on cooling and/or heating, you will not get the degree of comfort that you are probably looking for. better airflow movement with an additional return, yes. comfort, I doubt it.

my point is to find out exactly what you are dealing with as far as load compared to the size of existing HVAC.

I am not opposed to reasonable DIY projects that a homeowner is capable of. And I am not advocating a 10K fix either. but it seams reasonable to tackle a problem with as much knowledge as possible rather than a hit or miss/trial and error approach. more knowledge equals a higher chance of success.

there are several homeowner friendly load calculation software that are available for an insignificant cost. I will attach a link to one.

IMO

Here is a link that might be useful: Residential Load Calc Software
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Need To Add a Return. I Have a Few ??'s

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