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Where you feel it tells you most of what it is
A cold spot at a window, cold air across the floor, cold air out of a register and a room that is simply cold are four different faults with four different answers. Only one of them is usually about the heating system.
Air that leaves has to be replaced, and cold is a sensation rather than a temperature
Every draught has two ends. If cold air is coming in somewhere, air is leaving somewhere else — through an extract fan, a flue, a chimney, a leaky attic hatch or a duct running through a crawl space. Sealing the end you can feel, without finding the end you cannot, usually just moves the draught to the next gap along.
And people do not sense air temperature. They sense heat leaving their skin, which air movement and cold surfaces both accelerate. A room at 21°C beside a large single-glazed window genuinely feels colder than a room at 21°C without one, and no amount of extra heat fully fixes that. Which is why the location of the feeling is diagnostic, and the thermostat reading mostly is not.
Where you feel it, and what kind of building it is
Nothing is sent anywhere and nothing is recorded. This is what we would ask you on the telephone.
A kitchen hood, a welding extract, a spray booth, a dust collector or a set of toilet fans can remove several thousand cubic feet of air a minute. That air is replaced whether or not anybody planned for it — through the front door, the roller door, the loading bay, a service duct or the lift shaft. It arrives at the outside temperature, moving fast, usually straight across whoever works nearest the opening.
Adding heating capacity to that is buying heat for the outdoors. You can usually feel the symptom before anyone measures it: a front door that is heavy to pull open or whistles, a dining room that is cold only at service, cooking smells drifting into the seating, or heating that runs flat out and never wins.
What to ask for: the total extract volume of every fan and hood, the make-up air actually being supplied, and the pressure difference across the building envelope measured while it is running as it normally runs. If the answer to the second one is “none”, that is the fault, and supplying make-up air deliberately is usually cheaper than the extra capacity that gets proposed instead.
A cold surface, not a draught — and it will not respond to more heat
Large glass is a cold surface, and your skin radiates heat to it. That reads as a cold spot even with the air at a perfectly normal temperature. There is often a genuine small draught as well, because a cold pane chills the air touching it and that air falls, which is why the sensation is strongest at the ankles a foot or two out from the glass.
What to do, in order: hold a lit incense stick or a strip of tissue at the frame, the sash and the sill on a windy day — if it moves, it is an air leak and it is a weatherstripping job rather than an HVAC one. If it does not move, the glass itself is the problem and the honest answers are heavy curtains, a secondary pane, or better glazing. The one thing we can do that helps is add a supply register that washes the glass, and we will tell you before we do it that it improves the feeling rather than the heat loss.
Air arriving from under the floor, and often through your own ductwork
In this housing stock the most common cause of cold air moving across a floor is not a gap in a wall. It is the crawl space: a supply duct that has come off its boot or split at a joint is pressurising the crawl space with warm air, and the crawl space is then pushing cold air up through every gap around pipes, at the skirting and under the floor itself. You are paying to heat the underside of your own house.
What to do: get the ducts inspected and pressure-tested rather than guessed at. This is a visible, photographable fault and a competent contractor will show it to you. Sealing and re-connecting is normally a day of work, costs a fraction of any equipment, and it is the repair we recommend most often in Vallejo. If the ducts are sound, the next look is at the floor penetrations and the crawl-space vents.
The one on this list that genuinely is the heating system
If air from a supply register feels cold while the system is calling for heat, something specific is wrong and it is worth a visit. The short list: the blower is running on the fan setting rather than on auto, so it runs between burns; the furnace is short-cycling on a limit switch because airflow is restricted; the burners are failing to stay lit; a duct is drawing cold air in from an unconditioned space between the furnace and that register; or the zone damper feeding it is not opening.
What to do: check the thermostat fan setting first, because it costs nothing and it is the answer surprisingly often. Then check the filter. After that it needs measurement — temperature rise across the furnace, static pressure, and the temperature at that register against one nearer the plenum. That comparison usually identifies it inside an hour.
A real air leak with a real partner somewhere else in the building
Doors, garage walls and stairwells are where buildings connect to spaces that are effectively outside. But a draught only flows if air is leaving elsewhere, and that partner is usually a bathroom or kitchen fan left running, an open flue, a leaky attic hatch, recessed lights, or duct leakage in the attic putting the house under negative pressure.
What to do: weatherstrip the door, by all means — it is cheap and it helps. But if the draught simply reappears at the next weakest point, stop sealing and start looking for the exit. A blower door test finds it properly. We will also say plainly when the answer belongs to a building contractor rather than to us, particularly on a garage wall, which should be sealed for air quality reasons whatever the comfort argument.
Not enough air is arriving, or none of it can get out again
A room that is uniformly cold with no felt draught is usually a delivery problem rather than a leak. Either too little warm air reaches it — a crushed or disconnected duct, a closed damper, a run that is far too long, or a register sized for a smaller room before an extension was built — or air reaches it and has nowhere to go, because the door is usually shut and there is no return path, so the room pressurises slightly and the supply stalls.
What to do: the measurement is airflow at that register compared with the others, plus static pressure on the system. The second cause is cheap to cure with a transfer grille, a jump duct or an undercut door, and it is missed constantly. Neither answer is a bigger furnace, and if a bigger furnace is the first thing you are offered, get another opinion.
About windows specifically
Ask whoever attends to distinguish, out loud, between an air leak and a radiant cold surface, and to say which one they measured. They are different faults with different costs, and one of them cannot be solved by the heating system at all.
About floor draughts specifically
Ask for photographs from the crawl space before and after. Duct faults are visible, and any contractor who has been down there can show you exactly what they found. If nobody went down, nobody knows.
About cold supply air specifically
Ask for the temperature rise across the furnace, measured, and compared with the range on the equipment label. It is a two-minute check that separates an airflow problem from a combustion problem, and it should be done before anything is quoted.
About door and garage draughts specifically
Ask where the air is leaving, not only where it is arriving. If the answer is a shrug, the sealing work will disappoint you. A garage wall should be sealed regardless, because what is in a garage should not be in a house.
About cold rooms specifically
Ask for airflow at that register in cubic feet per minute, against the others in the house. Without that number nobody is diagnosing anything, and the conversation will drift towards equipment because there is nothing else to discuss.
In an older timber house
Expect the crawl space and the attic to be the two places the answer is hiding, and expect the house to have been altered more than anybody remembers. Sequence matters here: seal and repair ducts, then air-seal the attic plane, then look at the equipment. Doing it in the other order means paying for capacity to serve leaks you are about to fix.
In a newer or recently renovated house
Tighter houses have fewer leaks and less forgiveness. Problems here are more often about distribution, balance and ventilation than about draughts — a room with no return path, a zone system commissioned badly, or a mechanical ventilation system switched off by somebody who found it noisy. A recent renovation is worth mentioning: new glazing and insulation change the load, and the equipment is often now oversized for the house it serves.
In a flat, condo or duplex
Shared walls and shared shafts mean your draught may have nothing to do with your own unit. Air moves between flats through service risers, plumbing chases and corridor doors, and a strong extract in a neighbouring unit can pull air through your walls. This is worth establishing early, because the fix may be a building matter rather than yours — and a contractor who sells you equipment without saying so has not helped you.
In a commercial building
The pressure question above comes first and it comes before everything. Until the extract and the make-up air are both measured, any heating or cooling proposal is a guess wearing a quotation, and it will be a large one.
What a proper visit produces
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Static pressure across the system
Measured against what the equipment label allows. High static pressure explains a startling proportion of comfort complaints and is almost never mentioned in a sales visit.
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Airflow at the register you are complaining about
In cubic feet per minute, compared with the other registers. A number, written down, not an opinion held in front of a vent.
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The duct run inspected and photographed
Somebody actually goes into the crawl space or the attic and comes back with pictures. If nobody went, nobody knows.
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The exit identified, not just the entry
Every draught has two ends. The report should name both, or say plainly that finding the second one needs a blower door.