another HVAC question: interfacing new air handler w/ existing ductwork

[RustyShackleford]

I'm installing a new 3-ton heat pump and am trying to plan the installation of the indoor unit (air handler with heat strips) and its connection to the existing ductwork.

This scale drawing shows the location of the existing ductwork and my proposed location for the new air handler. (The air handler is shown without any elbows or square-to-round transitions that may be added to it to interface with the ductwork).

The old air handler was mounted so that the airflow was upwards (in the drawing), with the outlet flowing directly into a big rectangular elbow that leads up into the bottom of the supply trunk (roughly at its mid-point).

The inlet ductwork was far complex that shown here (legacy of an even earlier system) and I'd measured an unacceptably-high inlet-side static pressure of 0.5-0.6". I ripped all that out so that the stack from the return-air grill (mounted in a hallway of the house) comes straight down where I'll install an 18" starting collar and an elbow pointing towards the air handler. A short run of 18" flex-duct will lead to a round-to-square transition at the inlet to the air handler. This will be a significant improvement, with the return air path being both shorter and larger diameter.

But it occurs to me that I could do better, either by mounting the air handler with the air flow from right-to-left (in the drawing), or diagonally as shown here. I seek opinions on these options. Either option will make the return air path even shorter, at the expense of adding an elbow (with angle between 45 and 90 degrees) to the supply side path. It seems to me that it's the total airflow resistance (and thus static pressure) that's important, and if I can reduce the total by reducing the return side at the expense of the supply side, it's a good thing.

Of course this would also require fabricating that big elbow (that goes from the air handler outlet to the existing elbow that leads up into the supply trunk), but I assume that's easy enough for the moonlighting cryogenic tech friend who's helping. I'm run this by him too, of course, but though he's an ace with refrigeration I'm not sure he's that good on ducting.

• hvac
• ducts

 

[gruesome]

Our installer told us that for noise absorption purposes you do not want direct line of sight from the air handler intake through the duct work to the return filter & grill, since that would allow sound to propagate straight from the air handler into the hallway. Of course you don't want pressure drop, so you need to go with a duct cross section matched to the air handler intake.
To me the argument makes sense, and we therefore have an extra U-turn of large diameter insulated flexible duct hanging in our attic, adding a few feet and a 180° turn to what would otherwise be a very short intake run, with direct view of the air handler inlet from the return filter.

If you don't have it already, I would also recommend retrofitting a large (20" by 30" or thereabouts) return filter box that can take 3" thick higher MERV filters. The larger thicker filters last longer, filter better, and restrict the air flow less.

 

[RustyShackleford]

My setup has the filter in a box in the hallway, not next to the air handler. I hacked it to handke a 4" filter (20x25") awhile back, so I feel good about that.

I'll have an elbow and a short piece of 18" flex between the filter and the air-handler proper, so hopefully that'd kill the noise.

 

[gruesome]

That's what I meant (filter right behind & accessible through the grill).

We had decided to get rid of the old asbestos-containing ductwork when replacing the air handler; it was very accessible in the attic. Is your existing ductwork already insulated?

Added: we also decided to replace the existing gas burner; I think it only added $1000 to the total cost. Gas is not an option for alternative heat in your area, instead of the strip heaters? Here in California it unfortunately almost never makes sense to run the heat pump in heat mode, since gas is more than a factor five cheaper than electricity.

 

[RustyShackleford]

Is your existing ductwork already insulated?

Yeah, it's got fiberglass duct liner that's in pretty decent condition. Thought about replacing the whole ductwork, but decided against it. Did replace (and improve) a lot of the runs of flex duct though.