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Is your building positive, negative or balanced?
Tick what the building actually does. This works out which way it is running and what that usually means. It is the first thing we establish on a survey and it explains a great many complaints that get blamed on the cooling.
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What that pattern indicates
Work down the list with the building in mind. Every outcome is written out below, so nothing is hidden behind the tool.
More air is leaving than is being deliberately brought in, so the building makes up the difference by pulling outside air through every gap it has. In this climate that is cold wet unconditioned air arriving at the perimeter, and you pay to heat all of it.
- The usual cause is extract without matching make-up air — a kitchen hood, a washroom bank, a dust extract or a process fan that was added without anybody balancing what replaces it.
- It shows up first at the doors, because the door is the biggest deliberate hole in the envelope.
- It is why smells travel. Air moves from where it is made towards wherever the building is pulling from.
- It is measurable, and measuring it costs a great deal less than adding heating capacity to fight it.
- The serious version: a strongly negative building can interfere with a naturally drafted flue. That is a combustion safety matter, not a comfort one.
More air is being supplied than is leaving, so the building pushes air outwards through the envelope. Positive is usually the safer of the two — it keeps outside air and smells out — but it is not free of consequences.
- Warm moist indoor air driven into a cold wall assembly condenses inside it. That is the damp at the top of walls, and it is a building-fabric problem being caused by a mechanical setting.
- Doors that will not latch and drift open are the everyday symptom.
- Over-supply is often a control setting rather than a hardware fault — an outside-air damper at a minimum position nobody ever reviewed.
- Slightly positive is frequently the intended design. Strongly positive in a cold climate is not.
You have ticked symptoms that point both ways, which usually means the building is not one pressure regime but several — different zones, different floors, or a difference between trading hours and out of hours.
- Multi-tenant buildings do this constantly, because each tenant’s extract was added independently.
- Stack effect in a tall building produces genuinely opposite conditions at the top and the bottom in winter.
- Scheduled plant can make a building negative during occupied hours and neutral overnight.
- This is the case where measurement matters most, because guessing from symptoms will produce the wrong answer.
This reads a pattern of symptoms; it does not measure your building. Actual pressure is established with a manometer, with the plant running as it normally does, and it frequently differs between zones and between hours. Use this to know what to ask for — not to decide anything, and not to price anything.
Four readings and an afternoon
- Differential pressure across the envelope, with the plant in its normal running state, at more than one location
- Extract volumes at every hood, fan and grille that removes air from the building
- Supply and outside-air volumes at the plant, measured rather than read off a schedule
- Door force, which is the symptom everyone actually notices, checked against the readings
The output is a number and a direction, per zone. From there the options are usually obvious, which is the point of measuring.
Rarely more cooling
- Make-up air sized against the extract that is actually installed, not against what was installed when the building opened
- Interlocks so that make-up air runs when the extract it serves runs, rather than on a separate schedule
- Damper positions reviewed and recorded — frequently the whole fix, and frequently free
- Extract that is genuinely oversized turned down, which nobody ever suggests because it sells nothing
- Envelope work where the leakage is concentrated, which is somebody else’s trade and we will say so
What this page is not
It is not a survey, and it does not produce a number. Building pressure varies by zone, by floor, by hour and by wind direction, and the only honest way to establish it is to measure it while the plant runs as it normally does.
It is also not a sales route to new equipment. In a fair number of the buildings we measure, the answer is a damper position and an interlock — an afternoon of work and no capital spend at all. That answer is on this page because it is frequently the true one.