What Makes Ducted Installations Hard

What we learned from a quarter of ducted swaps, & leak testing

The return plenum. That’s what makes it hard. Okay, and maybe several other things too.

Overview

We’ve done a lot of ducted installations. Dozens of them this year. This article talks about installations where you already have pre-existing ductwork¹.

Or, as we like to say: a ducted swap.

Ostensibly, these should be easy installations. Typically, there is a gas furnace in place, and maybe an AC coil on top. Your goal is to replace the furnace with a heat pump air handler. The ductwork is already there. You just need to swap out the furnace for the full-sized air handler.

In reality, we’ve found that 3 things can trip up these installations:

1. Custom sheet metal fabrication / return plenum procurement

2. Motorized damper controls with multiple zones

3. Leaky/unsuitable ductwork

Honorable mention goes to “refrigerant lines in tight/finished spaces (e.g. closets)”.

Preparing for these cases is key to ensuring that ducted swaps go smoothly. Let’s dive in.

Return Plenum Procurement

Oftentimes, a pre-existing ducted system will have a furnace that has a return duct entering the furnace from the side, like so:

Or like this:

So you can’t really do that with most heat pump air handlers, particularly the larger ones (2.5 ton+) due to the volume of air you need to move the heat up from the heat exchangers. Instead, you need a supply plenum box beneath the air handler, like this:

Whenever you do that, you need to connect the pre-existing duct return to the new box on the bottom. And while sometimes that’s easy, other times it’s quite hard.

Can you buy prefabricated versions of these boxes? Yes, you can. Like this one:

This one seems to assume that you’re familiar enough with sheet metal work to cut out the precise opening between the box & the bottom of the air handler. However, adjustable models exist. This one is our favorite (shoutout Supply New England in Needham for telling us about this):

Here’s the problem: if you don’t have a prefabricated plenum at the ready, you HAVE to custom fabricate the plenum, and probably do some custom sheet metal work in any case to attach the pre-existing return duct to the return box.

If the crew on the ground don’t have the right skills to do that, you’re stuck.

An example of a slightly less conventional return duct that we had to attach to a return box

Yes, we’ve run into cases where we’ve had an installation go long by 1 or 2 days because we didn’t get the plenum ahead of time. This gets harder if you’re constrained by space & need a custom low plenum, like a 12-inch one, because the prefabricated models are typically 15 inches+.

The Size Consideration

Air handlers are reasonably tall. They’re about 55 inches for a 30k or 36k BTU model:

LG’s KSSLA361A setup. Midea’s is similar.

And when you add on that ~15 inches for the prefabricated return plenum, and maybe an extra 10 inches of clearance from the supply trunk at the top (in a basement setting, of course), we’re talking about 55 + 15 + 10 = 80 inches of clearance you need from the bottom to the top.

If you didn’t measure that prior to the installation, you may end up in a scenario that we ran into: 3 days out from an installation and realizing that you don’t have enough space for this entire setup. And then you need to push back the installation, which messes up your schedule, etc.

Yeah, that happened here. We opted for a Mitsubishi instead of an LG, which I believe does have the ability to take are in from the side. That solved it.

So, unless you’re dealing with a small capacity install and you know your equipment is ok with a side intake, make sure you have around 80 inches and make sure you have your return plenum ordered ahead of time.

P.S. if your air handler is in a closet, will the return box fit in the closet?

Please make sure it fits in the closet.

Zoning & Motorized Damper Controls

So some people will have a single furnace or air handler, but two different zones in their homes (usually lower vs. upper floors) connected to 2 thermostats for individual temperature control.

To control for these zones, you’ll typically need a motor connected to the thermostats. And to make all of that happen, you have, ladies and gentlemen:

The humble zoning control panel.

If you don’t install these regularly, these get pretty intimidating because there’s a lot of wires, and you may need a dry contact board to help translate between the motors & the new control panel and its associated thermostats and the air handler control board:

For the techs out there, we did end up fixing the issue but I actually don't know if these photos are before or after, because LC co-founder Hannan is the engineer and actually knows how to wire this.

I’ve been here in person for one of these exactly one time and it kinda feels like defusing a bomb. But if you have any electrical engineering experience, or have done this before, apparently it’s a whole lot easier.

The important thing to remember is that pre-existing control boards that worked with a furnace is unlikely to be compatible right out of the box with heat pumps. For example, I don’t think this particular HZ311 panel works with heat pumps:

So unfortunately, you’ll have to buy a new one. And they are pretty expensive: I believe we bought the HZ432 for about $1,000 and the thermostats are a couple hundred each. This is why we are now adding a complexity adder for multiple zones in 2026.

Performance Tradeoffs

One trend in the past years is that heat pump manufacturers make proprietary thermostats for their heat pumps. One advantage of this is that their thermostats can better fine-tune the exact energy output of the heat pump, which is why these heat pumps are variable rate/inverter heat pumps, rather than conventional single-stage ACs like the ones you’d find 20 years ago.

However, if you use conventional thermostats, there’s a risk you’ll lose that fine-tuning functionality. And that MAY be the case with zoning control boards, but I don’t know enough to speak definitively about it.

Leaky Ductwork

Alright. Let’s talk about leaky ductwork.

For this research post, we had a HERS rater test for leaks at 18 homes we with pre-existing ductwork this fall.

1. About 6 homes passed right off the bat

2. Most were leaky

3. 3 failed the leak test

Remediation costs were our guess. We didn’t have a super strong take, so take these with a grain of salt. I personally think it’s totally worth it, so ask your contractor about it.

This largely lined up with a prior series of tests that we did back in the spring, with about a 10% failure rate.

Here’s the deal: half of the time, the leakage is around the return & air handler. And the reason was because some duct connection or the prior air handler just wasn’t sealed very well. Stuff like this:

Or more dramatically, like this:

Yeah, we replaced this elbow. You can’t just tape over that.

So when it comes to the fix, it’s not rocket science: just make sure you tape everything really well with mastic tape (not only aluminum!! Mastic > aluminum tape) when it comes to duct connections, like the return duct to return box I just wrote about in the first section.

Ok, to be fair, aluminum tape is fine on metal, but when you’re looking to secure ducts for 15 years, you probably want to use the best material possible.

So often we hear about people comparing SEER2 and HSPF2 ratings between different models (people = us lol) but the fact of the matter is that if your ductwork is leaking, none of that matters. For the average homeowner, the best thing you can do for your ductwork is to make sure it’s well-sealed.

I would recommend looking into getting a HERS rater come out to your house and test for leaks if:

1. You know your ductwork is really old

2. The existing air handler or ducts have clear places where you can put a hand next to the ductwork and feel air coming out, or simply doesn’t look good

3. You feel like your ducted system isn’t performing very well

At the end of the day, ductwork will leak some amount, which is why it tends to be less efficient than ductless systems. I won’t stop you from getting a heat pump if you have leaky ductwork. But I do want to ensure you’re not in the 10% that have ductwork so bad, it’s not even worth re-using it.

Takeaways

Ducted swaps have a reputation in the industry for being relatively quick (1 day, or 2 day jobs if everything goes right), but in reality there are some things that can make it more complicated.

I didn’t even talk about refrigerant lines yet. Sometimes, you’ll have homes that have existing refrigerant lines that run through finished space - this happened to us 3 times this fall, out of the ~18 ducted installations. In the future, we’re going to avoid these. We’ll want to make sure that air handlers have direct access to exterior walls, because otherwise you’re opening up finished spaces and that’s not really what we want to specialize in.

Fortunately for us, one of the installation teams we work with has a lot of sheet metal work experience, so they can fabricate something on the fly. That’s been essential for us. But in the future, we hope to procure as much in advance as possible so we’re not left in a situation where you need to do something crazy.

1. For new ductwork, read this: