Ducts under the floor
A ducted heat pump in a crawl space house depends on the ducts under the floor. Many Memphis houses run them through a crawlspace a few feet high, which holds humid air for much of the year.
Bluffline Memphis Heat Pumps looks under the house before quoting any system that relies on those ducts, because they decide how well a new heat pump works.
What a crawlspace system looks like
- A trunk duct, rectangular sheet metal or duct board, running down the middle of the house.
- Branch runs, round metal or flex, to a floor register in each room.
- A return, often one large grille in a hallway, with its own duct back to the air handler.
- Sometimes the air handler itself, on a pad near the access door.
Why crawlspaces are hard on ducts
Memphis summers bring air with dew points in the 70s. Under the house, that air meets a supply duct carrying air in the 50s.
Sweating
If the duct’s insulation is thin, torn or missing, the metal surface drops below the dew point and water condenses on it. The water drips onto the vapor barrier, soaks the remaining insulation and makes the problem worse each summer.
Leaks
Supply seams that leak push cooled air into the crawlspace instead of the rooms. Return seams that leak do the opposite: they pull damp crawlspace air into the system, adding moisture the coil then has to remove.
Sagging and disconnection
Flex runs sag between hangers and can pull loose from the trunk or the floor boot. A disconnected branch leaves a room with almost no air, and the conditioned air it should carry pours into the crawlspace instead.
What a heat pump needs from the ducts
A heat pump moves about 400 cubic feet of air per minute for each ton. In winter it delivers air in the 90s rather than the 120s or more that strips or a furnace deliver, so leaks and drafts are easier to feel.
Enough return
The most common restriction is the return. One 14 or 16 inch return for a 3-ton system starves the blower. A second return or a larger one is often the single biggest improvement.
Low resistance
The blower is built for about 0.5 inches of water column of total external static pressure. Readings near 0.8 or higher mean the ducts are fighting it. We take that reading on the first visit.
What we fix under the house
Each problem under the floor has a matching fix:
| Problem | Fix |
|---|---|
| Seams sealed with cloth tape that has let go | Mastic or rated foil tape on every seam and joint |
| Torn, wet or missing insulation | Rewrap to at least R-8 |
| Sagging flex | New hangers or straps at the right spacing |
| Disconnected branches | Reconnect and seal at the trunk and the boot |
| Undersized return | Add or enlarge a return, boxed and sealed |
| Crushed or rotted runs | Replace the run |
Most of this fits on the install day, and the duct work is priced as its own line. Heat pump ductwork describes the work in more detail.
Air handlers in the crawlspace
Some houses keep the air handler under the floor. That works when it is set up for the space.
What a crawlspace air handler needs
- A pad above the vapor barrier, so it never sits in water.
- Sealed return connections, so it doesn’t pull crawl air.
- A condensate drain that leaves the crawlspace, not one that empties onto the dirt.
- Room for filter changes, or a filter grille moved up into the house.
If the air handler is failing, air handler replacement covers the indoor half, and the replacement is a chance to fix all four.
The vapor barrier and the crawlspace itself
Duct work and crawlspace moisture are linked. A torn or missing vapor barrier lets ground moisture into the crawl air, which makes every duct problem worse.
What we look at
- The vapor barrier’s coverage and whether it has been torn by previous work.
- Standing water after rain, which points to drainage problems.
- Vents, open or closed, and whether they are blocked.
We put back any barrier we move, overlapping the seams so ground moisture stays below it. Full encapsulation, drainage and foundation work are separate trades. Where the crawl is wet, we say so on the estimate.
Measuring the result
Duct work you can’t measure is duct work you have to take on faith. We take the static pressure reading before and after and write both down.
What good looks like
- Static pressure at or near the blower’s design, about 0.5.
- Supply air temperatures close from room to room.
- No sweating on the trunk on a humid afternoon.
- Indoor humidity settling in the 45 to 50 percent range within a week or two of summer weather.
The humidity guide explains why that last number depends on the ducts as much as the equipment.
Where this comes up most
Crawlspace ducts are common in three kinds of houses here:
- Older Memphis ranches from the 1950s to 1970s.
- 1990s ranches in Bartlett on their original ducts.
- West Memphis houses on flat Delta ground with damp crawls; see our West Memphis page.
Questions to ask any contractor
Whoever quotes your heat pump, a few questions show whether they have looked at the ducts:
- What was the static pressure reading? If nobody measured it, nobody knows whether the ducts can carry the new system’s air.
- Did anyone go under the house? A quote written from the driveway can’t account for loose branches or sweating trunks.
- What will you do about the return? A new system on an undersized return is a common source of noise and weak airflow.
- What sealant goes on the seams? Cloth duct tape fails; mastic and rated foil tape last.
- Will I get the readings afterward? Before-and-after static pressure is the proof the duct work did something.
Comparing two quotes
If one quote includes duct work and another doesn’t, the cheaper one may simply leave the ducts as they are. That can be the right call when the static reading is fine. When it isn’t, the duct line is the part of the job that decides how the new system performs. The buying guide covers comparing quotes line by line.
Costs and rebates
Bluffline’s typical range for duct repair and sealing done with an install is $800 to $3,500. Replacing a whole trunk or many runs is priced per house. Your written estimate after the visit sets the price.
For MLGW customers, TVA EnergyRight pays $300 for qualifying duct sealing and insulation, done by a participating contractor. The cost page covers every range, and the sizing guide explains how ducts and equipment are sized together.
The order to do it
The sequence matters as much as the fixes:
- Measure static pressure and look at the ducts before choosing equipment.
- Size the heat pump from a load calculation, not the old nameplate.
- Fix the return, seal the seams and rewrap the insulation on install day.
- Measure again and keep both readings.
The Memphis buying guide covers the rest of the decision, and the commissioning checklist lists the start-up readings.