Terminology & Function

Post all other heating and cooling questions here.

Moderators: Greg, Mark

Post Reply
Mark440
Posts: 175
Joined: Tue Feb 26, 2008 8:33 am
Location: Anna, Tx

Wed Apr 12, 2017 5:56 am

...

I originally posted my quest to understand terminology and function about HVAC plenums. In the meantime, I've had 4 HVAC contractors come out and give me a bid to replace everything between the blower unit and the vents. We all know the ducting sucks in mobile homes - so why keep taping and patching, and hoping it will get any better?

The 4 bids were more than a bit revealing. First, all of them came within a couple hundred bucks of each other. And while I asked that any use of flex duct be minimal (if not none!), every bid used flex duct. No two bids of the four had the same configuration. And while I stuck right with them - not one pulled out a calculator or cardboard widget to size the ducts. Which leaves me with this:

HVAC ducting may well be a science - but there are few who know how to use the science and/or could care less. It's about opinion ("experience") and profit. So until I can 'validate' one of the configurations - nobody gets the work. Alternatively - I can do it myself, pocket the savings, and end up with just as big of a mess.
Opportunity has a shelf life.


David DeBord
Posts: 5
Joined: Fri Oct 20, 2017 5:01 pm

Sun Oct 22, 2017 7:41 am

Mark, ... Changing your Ductwork to "Flex Pipe" can be done efficiently, but, .... The "Design Static Pressure" will have to be maintained, other wise the "New Duct System" will cause you all kinds of problems, including the leading to the Failure of your Heating/ Cooling System.

"Ductboard", "If" allowed in your area by the "Building Department" (Because of possible Fiber-glass particles in the air), will be cheaper than metal duct, but it also will be Labor Intensive, & over time, if installed below the original location of the Factory Installed Duct, it will draw large amounts of moisture, which will lead to the loss of the Duct Board, as well as the possible spread of Mold.

The Ductwork System in your home can be "Replaced" by Superior Materials, but, .... The problems that caused the Re-occurring Failure of your present Duct System must be corrected, ... Otherwise you'll be throwing more $$$ on to that fire.

I'm also curious also, as to the condition of your existing Heating/ Cooling System, & if it is the "original System"?

Mark440
Posts: 175
Joined: Tue Feb 26, 2008 8:33 am
Location: Anna, Tx

Sun Oct 22, 2017 2:05 pm

I'll have to re-compose a reply offline, then cut, then paste - and dodge that idiotic facebook nag. Some sort of stupid timer dumped me....
Opportunity has a shelf life.

Mark440
Posts: 175
Joined: Tue Feb 26, 2008 8:33 am
Location: Anna, Tx

Mon Oct 23, 2017 7:50 am

David,
I purchased a new Goodman system in 2008 (exchange unit and compressor). The system has had only one fail (blower motor) in 9 years. However, since it's installation, the delivery of heated air to the two halves of this doublewide has been out of whack. One side is luke warm while the other is quite toasty.

This problem can be traced to the physical positioning of the heating coils in the exchange unit - coupled to a very small plenum box. Imagine a 12" circle - and the heating coils are positioned in one half of the circle - and directly over one of the outflow ports. The position of the heating coils + the position of the outflow ducts in the plenum box results in the heating imbalance.

The plenum box is one of those inverted triangle affairs. The each side of the triangle is tapped for airflow. The top side is input, with the two slanted bottom sides for outflow. Due to physical restrictions (I-beams, pier block stacks) the triangle will fit in only one orientation, which places one outflow side facing the inner length of the home towards the trunk lines, while the other faces towards the perimeter (short side) of the home away from the trunk lines. (Note: I don't have the 'cross over' duct. I have two runs of 12" flexduct from the plenum box.)

Due to the plenum box orientation, one piece of flexduct is 18 ft, while the other is 25 ft. The difference is primarily due to the 180° turn coming out of the plenum box. I will add that supporting long runs of flexduct so that it is not 'squeezed' (read: restricted air flow) is almost impossible without concocting some type of additional support structure beyond standard strapping.

The trunk lines: I know there are leaks in one or both trunk lines. Of course, I don't know if there are "holes" or if it is leaks at the registers. But! Inspecting the trunks is impossible without ripping the belly wide open. I've done my best to seal all connections leading up to the trunk lines. Bear in mind these are the original trunks - constructed of the foam board - and are nearing 20 years in place. At this juncture - I'm still working on how to deal with the trunk lines.

The plenum box and flexduct will be replaced with all metal units. I will be moving one stack of support blocks about 6" so that a rectangular box can be fitted without obstruction. The new plenum box will be roughly double in volume - but will allow both outflow ports to be on the same side - pointing towards the trunk lines (eliminates the 180° turn). The ducting from the plenum to the trunks will be 12" round standard fittings. So, other than the cubic volume of the plenum box - the "ducting" remains the same - albeit the metal ducting allowing for better flow - and the two runs are almost the same length. (Ducting will be insulated and wrapped).

FWIW - I have since had yet another contractor come out for a bid who actually stated that he would take all the measurements, calculate what was needed, then present a price. The catch? Their 'service' requires an upfront $300/year membership fee. Of course that includes one annual cleaning - and reduced rates for repairs.

I priced out materials - and the plenum box and ducting can be done for less than $300.
Opportunity has a shelf life.

Post Reply