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The remodel, and who decides where the air goes
Almost nobody is thinking about air when a house is being drawn. By the time anybody is, the ceilings are closed and the cheap moment has gone.
The same physical work costs wildly different amounts depending on when you ask for it. Moving a duct run in a house that is open to the framing is an hour. Moving the same run in a finished house is a ceiling, a plasterer, a painter, and somebody sleeping elsewhere for a fortnight.
There are two windows. The first is while there are still drawings, when a register can be moved on paper for nothing. The second is after demolition and before the ceilings close, which is usually a week or two and is the last time anything is easy. Almost every expensive HVAC problem we see in a remodelled house was decided in one of those two windows by somebody who did not know they were deciding it.
This page is not a complaint about builders or designers. A good general contractor is managing twenty trades and a designer is solving a different problem. The point is that air is nobody’s default responsibility unless somebody is asked, and the person best placed to ask is you.
Pick what the work involves and where you are. Everything below is written out in full.
Do not buy a bigger system, and do not add a mini-split to the problem room, until somebody has measured. Both of those are treatments for a diagnosis nobody has made, and both are expensive ways of arriving at the same complaint.
What a remodelled house needs first is three numbers: static pressure across the system, duct leakage, and the airflow actually delivered at the rooms that are wrong. Together they say whether the equipment is too small, the ducts cannot carry the air, the new work is stealing from the old, or the registers that were chosen for their looks are strangling the whole thing.
In our experience the answer in a remodelled house is the third or the fourth far more often than the first. Both are cheaper to fix than a system, and one of them is sometimes a morning’s work with different grilles.
One awkward thing worth knowing. If the work was recent, some of this may be the builder’s to put right, and a measurement is what turns that from a disagreement into a conversation. We will do the measurement and write it down whether or not the work is then ours.
The most common remodel here and the one with the least obvious effect on the air, which is why it catches people out.
Return air. Walls carry return paths, and a wall that comes out sometimes takes a return with it, or takes the route air was using to get back to one. A house that was fine becomes a house where doors slam, rooms hold pressure and the system cannot breathe. This is cheap to solve at framing and awkward afterwards.
One space, one temperature. Combining a kitchen and a living room makes a single large volume with very different loads in different parts of it — a hot appliance wall at one end, glass at the other. One thermostat now governs both, and it will be right in one place.
The kitchen extract. A serious hood in a modern, tighter house moves a great deal of air out, and it has to come from somewhere. Without make-up air the house goes negative, which pulls outside air in around every door and can, in a house with any gas appliance, affect how that appliance vents. That is the part of a kitchen remodel worth getting right rather than clever.
The assumption is almost always that the existing system will extend to cover it. Sometimes true, often not, and the question is answerable in advance.
Capacity. New floor area is new load, and the existing equipment was sized for the house without it. But the equipment is only half of it: extending duct from a plenum sized for the old house frequently means the new rooms get air at the expense of the old ones, and everybody is slightly worse off.
Garages especially. A converted garage has an uninsulated slab, a wall that was an exterior wall, and often a roof with almost nothing above it. It is a harder room to condition than its square footage suggests, and tying it to the existing system with one small run is how it ends up as the room nobody sits in.
The return, again. A new room with a supply and no return path will not work properly whatever you feed it. Door undercuts, transfer grilles or a proper return — one of the three, decided at framing.
This is the case where a separate ductless system is most often the better answer, and we will say so even though it is the smaller job, because a room with its own control and its own capacity solves the problem instead of spreading it.
The single most consequential thing on this list for the ductwork, and the one least likely to be discussed with anybody who understands air.
Vaulting removes the space the ducts were in. That attic is not decoration; it is where the runs and often the air handler live. Vaulting a ceiling means those runs have to go somewhere else — a soffit, a chase, a different route, sometimes a different position for the equipment entirely. It can absolutely be done. It cannot be done for nothing, and it cannot be done after the drywall.
Vaulting also changes the room. More volume, more glass usually, and warm air collecting at the top where nobody is. The register positions that worked for a flat ceiling frequently do not.
Lowering a ceiling is the gentler version and it is often an opportunity: a dropped section is somewhere to put a duct run that has nowhere else to go, and if somebody says so early it can solve a problem instead of creating one.
Small job, and the one where the same mistake is made most reliably in this county.
The extract fan has to terminate outside the building. Not into the attic, not into a soffit that vents straight back into the attic. We find this constantly, in expensive houses, in recent work. The fan runs, the mirror clears, and every shower puts moisture into the roof structure.
Size it for the room and duct it properly. A powerful fan on a long run of crushed flexible duct with three bends moves very little air, and the rating on the box is measured on a straight short duct. The duct is the fan.
Make-up air matters here too, in a small way. A strong fan in a tight house with the door closed will struggle simply because nothing can get in. An undercut on the door is usually enough and costs nothing if somebody mentions it before the door is hung.
The best case, because everything is decidable and nothing has to be worked around. It is also where the most money is quietly lost, because a full remodel has a hundred decisions and air rarely gets a seat at the table.
Do the load calculation on the house you are building, not the one you are standing in. New glazing, new insulation and new air-sealing frequently mean a smaller system than the one being removed — genuinely smaller, not a rounding difference — and a designer specifying the old size is carrying forward a building that will not exist.
Decide the duct routes with the framing, not after it. A run that has a straight path is quieter, more efficient and cheaper than the same run forced through three offsets because a beam went in first.
Name the special rooms early. A wine room, a home gym, a theatre, a server closet or a primary suite over a garage all behave differently from the rest of the house, and each one is easy at this stage and hard later.
And decide about ventilation on purpose. A house that has been properly air-sealed is a different building from the one you bought, and the fresh air it used to get through gaps has to come from somewhere deliberate.
You are in the cheap window, and the thing to spend is an hour rather than money.
Get a load calculation done on the drawings and have somebody look at where the ducts are going to run before the structure is fixed. At this stage a register moves for nothing and a beam that would have blocked a run can still be a different beam.
Two questions worth asking your architect: where does the equipment live, and where does the return air come from. If either answer is “we will work that out later”, later means a soffit you did not want.
Ask one question before work begins: who is responsible for the HVAC design, as opposed to the installation?
Those are different things and the answer is frequently nobody. An installer will install what they are asked to install; a general contractor will assume the installer has thought about it; and everybody assumes the drawings settled it. If the answer is unclear, that is the moment to sort it out, and it costs nothing today.
Also worth settling now: whether the duct layout is being drawn or improvised, who is measuring at the end, and whether the registers have been chosen by someone looking at free area or by someone looking at a photograph.
You are in the second window and it is short. While the framing is open, everything is still possible; once the drywall goes on, most of it is not.
Get somebody to look now, even for an hour. Duct runs that have been squashed to clear a joist, connections that will be inaccessible, a return path that has quietly disappeared, a run of flex with more bends than it should have — all visible today, all invisible next week.
And take photographs of everything before it is covered. Every wall, every ceiling cavity, with something in shot for scale. It costs nothing, it takes ten minutes, and in five years it is the only record anybody will have of where things are.
See the note above, which is the substance of it: measure before you buy anything.
The one thing to add is about expectations. A finished remodel can usually be improved a great deal without opening ceilings — grilles, dampers, balancing, sealing at accessible joints, and occasionally a properly sized return in one place where there is access. It can rarely be made perfect without going back into the structure.
We will tell you which of those you are in, with the numbers, and we will tell you honestly if the remaining gap is not worth what it would cost to close.
Ask where the return air goes once the wall is out, and ask what the kitchen hood needs for make-up air. Those two questions, asked at framing, prevent most of what we get called about afterwards.
Get the new space calculated as its own load, and ask whether it should be on the existing system at all. A separate ductless system is frequently better and cheaper than making the whole house slightly worse.
Find out what is in that ceiling before the drawings are finished. If ducts or equipment are up there, vaulting is a decision with a price attached, and everyone is better off knowing it now.
Insist the extract terminates outside the building, and look at the duct run rather than the fan’s rating. Ask to see where it comes out. It is a thirty-second check that prevents a roof problem.
Commission a load calculation on the new house, not the old one, and get the duct layout drawn before the framing is set. Then name the unusual rooms out loud while it is still a drawing.
We will not sign off a design we did not size. If a layout arrives already drawn and it will not work, we will say so in writing rather than install it and be blamed for the result.
We will not tell you your architect or builder was wrong on the strength of a web page. Air is nobody’s default job and the people we work with are mostly good at theirs.
And we will not talk you out of the grilles you want. Flush and linear registers usually cost airflow, and sometimes the answer is simply more of them or a larger run behind them. That is a cost, it is knowable in advance, and it is your decision to make once somebody has told you.
An hour, at the right moment
Bring us in while there are drawings, or while the walls are open. We look, we measure what exists, and we write down what we would change — whether or not the installation is ours.