Design & Installation
Complete Duct Systems Design and Installation
Shemtov Systems offeres complete duct system installations to service an existing or new addition. Manual J and duct pressure testing are available.
Shemtov Systems will design and install a complete duct system to service an existing or new addition utilizing manual J calculation, for complete new homes we will consult and coordinate with mechanical engineers. We will install the system according to local and international energy codes, we will get the job permitted and approved by your local government.
Duct design decides how much air reaches each room and how hard the blower has to work to send it there, which is why sizes come from measured numbers rather than from whatever was there before. Shemtov Systems designs and installs duct systems across Silver Spring and Montgomery County following ANSI/ACCA 1 Manual D, the recognized residential duct sizing standard, and to the 2021 energy code the county enforces. Call 240-355-3333 about a new system or addition.
Key takeaways
- Manual D sizes ducts from the pressure the blower has left over, not from the duct being replaced.
- Available static pressure is what remains of the blower's rating after the coil, filter, grilles, and dampers.
- Returns are undersized far more often than supplies, and a starved return raises pressure system-wide.
- Outside conditioned space the code sets duct insulation by diameter: R-8 at three inches and larger, R-6 below.
What does a Manual D duct design actually calculate?
It starts at the blower. Residential air handlers are commonly rated to deliver their airflow against about 0.50 inches of water column of total external static pressure, and everything that is not duct spends part of it: the wet coil, the filter, the grilles, the dampers. Subtract those published drops and what remains is available static pressure. Multiply by 100, divide by the total effective length of the longest supply and return path, and you have the friction rate in inches of water column per 100 feet. Manual D holds it between 0.06 and 0.18.
Why does static pressure matter as much as duct size?
A duct that looks generous still chokes a system when the fittings and accessories spend the budget first. Filters are the common surprise: the drop follows face area and media depth, not the MERV number by itself, so a one-inch filter in a small return grille can cost more pressure than a four-inch cabinet at the same rating. A constant-airflow ECM blower answers by drawing more watts and getting louder; a PSC blower just moves less air.
How is ductwork planned for an addition or a finished attic?
First, does the existing equipment have capacity left? A room-by-room load calculation answers that. If so, the design finds a tap point that will not starve branches already on the trunk, and adds a matched return path, because a room with a supply and no way back to the air handler pressurizes and stops accepting air. Where the trunk is loaded, a ductless mini-split system is often cleaner.
When should ductwork be replaced instead of repaired?
Replace when the trunk or returns are too small for the airflow the equipment needs, when flexible duct is crushed, kinked, or sagging across long spans, when interior liner has broken down, when metal in a damp crawl space has rusted through, or when the return is a framing cavity, which Section R403.3.7 of the 2021 energy code bars from use as a duct or plenum. Repair wins when the layout is sound and one branch is undersized.
How should ducts outside conditioned space be insulated?
The code sets the level by duct diameter, not by which unconditioned space the duct crosses: R-8 at three inches across and larger, R-6 below that, with the vapor jacket kept continuous so metal does not sweat in August. A vented attic has a second route. A duct insulated to R-8 and buried in ceiling insulation can count as inside conditioned space when the air handler is also inside the envelope and a duct leakage test shows 1.5 CFM25 or less.
Frequently asked questions
Does a heat pump need larger ducts than a furnace?
Often yes. Heat delivered is airflow multiplied by temperature rise, and a gas furnace rating plate lists a rise in a 30 degree band, commonly 45 to 75 or 50 to 80 degrees Fahrenheit. A heat pump works from a much smaller rise, so it moves more air for the same output. See our furnace versus a heat pump in a Maryland home comparison and our heat pump installation page.
Should the system use sheet metal or flexible duct?
Both have a place. Hard pipe carries trunks and long runs with far less friction. Flexible duct suits short, straight, fully stretched final connections, and is penalized when compressed or left to sag.
Will a properly sized duct system be quieter?
Usually. Most duct noise is velocity and turbulence, so larger trunks, larger grilles, and long radius elbows lower it at the same airflow.
Does a new duct system need to be balanced?
Yes. Dampers are set and airflow checked at the registers so each room receives close to its design value. Our duct system services page covers the rest.
Talk to Shemtov Systems
For duct design, a new duct system, an addition, or replacement of ductwork that can no longer keep up, call Shemtov Systems at 240-355-3333. We size, build, and balance ductwork for homes throughout Silver Spring and Montgomery County.
Homeowner guides
Straight answers to the questions we hear most from Montgomery County homeowners.