Tonnage per Zone: Heads, Rooms and the Outdoor Unit

A zone’s tonnage comes from the rooms it serves, the head comes from a coarse catalogue, and the outdoor unit is sized to neither number exactly.

Tonnage per Zone: Heads, Rooms and the Outdoor Unit

A multi-zone heat pump is quoted as a list: an outdoor unit of so many
tons, and a head of so many tons in each zone. The numbers look like they
should add up, and they should not. Each zone’s tonnage is set by the rooms
it serves, the head that goes in it is picked from a catalogue with only a
few sizes, and the outdoor unit is sized to the way the zones behave
together rather than to their sum. Reading the three numbers as one
arithmetic problem is how New York houses end up with a large head in a
small bedroom that cycles all winter.

What a Ton Is, and Which Ton

A ton of capacity is twelve thousand BTU per hour, a unit left over from
the days when cooling was sold as blocks of ice. On a heat pump the nominal
tonnage on the nameplate is a cooling rating at a standard test condition.
Heating capacity at the outdoor temperature a New York house is designed
against is a different number, published separately in the manufacturer’s
extended data, and it is the one that decides whether a zone stays warm in
January. When this article says tonnage per zone it means the capacity a
zone needs, in heating and in cooling, compared with what the head serving
it can deliver at those conditions. The nameplate is a name, not a
measurement.

Where a Zone’s Number Comes From

A zone’s load is the sum of the loads of the rooms it serves, and each
room’s load comes from a room-by-room calculation: its exterior wall area
and what is in the walls, its windows and which way they face, its ceiling
or floor against unconditioned space, its air leakage and the people and
equipment inside it. That is the load calculation
that decides the whole electrification plan, and the reason a
Manual J is worth paying for is that it produces
this room-by-room table rather than one number for the house.

Two rooms with the same floor area can need very different capacity. A
corner bedroom with two exterior walls, a window on each and a cold attic
above will carry several times the load of an interior room of the same
size with one exterior wall. Square feet per ton, which is what a catalogue
design uses instead, cannot see the difference, and it is the reason the
head count on a quote so often has one head per
bedroom with no calculation behind it.

The Catalogue Is Coarse

Wall-mounted heads come in a short run of sizes, and the smallest in
most ranges is rated around half a ton to three-quarters of a ton nominal.
A small bedroom in a house that has been air
sealed and insulated may need a fraction of that at the design
condition and far less in the shoulder seasons. There is no head small
enough, so the smallest one is fitted, and the zone is oversized before
anyone has made a mistake.

What follows is cycling. An inverter-driven system
modulates, but every head has a minimum output below which it cannot go,
and a multi-zone outdoor unit has a higher minimum than a single-zone one.
When a room’s load is below the head’s floor, the head runs at its minimum
until the room overshoots, stops, and starts again, and the room feels
alternately warm and drafty. In cooling the same cycling means the coil
never runs long enough to wring moisture out of the air, and the room is
cool and clammy.

The Outdoor Unit Is Not the Sum

The outdoor unit’s tonnage is chosen against the total the zones will
actually draw at the same moment, which is less than the sum of the heads
because bedrooms and living spaces do not peak together. Manufacturers
publish a permitted range for the ratio of connected head capacity to
outdoor capacity, and a design that runs above the outdoor unit’s rating is
normal, within the range the maker allows for that model. What it means in
practice is that when every zone calls at once, on the coldest morning of
the year, each head receives a share rather than its full rating. The
outdoor unit’s heating capacity at the design temperature, not its
nominal tonnage, is the number that has to cover the house’s whole load at
that hour.

The same ratio sets what adding a zone later
costs: a spare port is only useful if the connected total stays inside the
allowed range once the new head is on it.

Heating Tonnage and Cooling Tonnage Are Different

In most of New York the heating load governs, so a zone sized to stay
warm at the design temperature carries more capacity than its summer load
needs. A head chosen that way will cool the room quickly and run short
cycles in July, and dehumidification suffers for the reason above. A ducted
zone can be tuned for this, because the blower speed can be set against the
airflow the coil wants in each season; a ductless head
cannot, so the choice of head size is the only lever, and it is worth
choosing with both seasons on the table rather than only the winter
number.

When the Room Is Smaller Than the Smallest Head

Three answers exist. The first is to serve the small rooms together from
one ducted air handler, so that the zone’s tonnage is the sum of three
bedrooms and lands inside the catalogue; that is the argument the
head-count article makes in full. The second is to
let a small room share a neighbouring zone through a
transfer grille and an open door, which works for
a room used lightly and fails for one with a closed door and a sleeper. The
third is to accept the smallest head and place it
where its throw crosses the room rather than hitting the bed, and to expect
some cycling in the shoulder seasons. None of the three is wrong; what is
wrong is not knowing which one the quote has chosen.

What to Ask For

The room-by-room table, with heating and cooling BTU per hour for every
room and the rooms grouped into the zones on the quote. Each head’s rated
heating capacity at the design temperature for your location, taken from
the manufacturer’s extended data rather than the nameplate. Each head’s
minimum output. And the connected-capacity ratio, with the maker’s
permitted range beside it. Four numbers per zone, and the design either
holds together or it does not.

Frequently Asked Questions

What does tonnage per zone mean?

The capacity a zone needs, in heating and in cooling, compared with what the head serving it can deliver at those conditions. A ton is twelve thousand BTU per hour. The nameplate tonnage of a head is a cooling rating at a test condition; its heating capacity at your design temperature is the number that matters in New York.

How is a zone’s tonnage worked out?

From a room-by-room load calculation. Each room’s load comes from its walls, windows, ceiling, air leakage and internal gains, and a zone’s load is the sum of the rooms it serves. A Manual J report is where that table comes from.

Why can I not just use square feet per ton?

Because two rooms of the same size can carry very different loads. A corner bedroom under a cold attic with two exterior walls needs far more capacity than an interior room of the same floor area, and square feet per ton cannot see that.

Does the outdoor unit’s tonnage equal the sum of the heads?

No. The zones do not peak at the same moment, so the connected head capacity is allowed to exceed the outdoor unit’s rating within a range the manufacturer publishes for that model. When every zone calls at once, each head gets a share. The outdoor unit’s heating capacity at the design temperature has to cover the whole house at that hour.

What happens when a head is too big for its room?

The head cannot modulate below its minimum output, so it runs at that floor, overshoots, stops and restarts. The room swings warm and drafty in winter, and in summer the coil never runs long enough to remove moisture, so the room is cool and clammy.

Is tonnage the same for heating and cooling?

No. In most of New York the heating load governs, so a zone sized to stay warm at the design temperature has more capacity than its summer load needs. A ductless head has no blower-speed lever to correct for that, so the head size is chosen with both seasons in view.

What about a room that needs less than the smallest head?

Serve it with neighbouring rooms from one ducted air handler, so the zone’s load adds up to a catalogue size; let it share a neighbouring zone through a transfer grille and an open door; or accept the smallest head, place it well, and expect shoulder-season cycling. The quote should say which.

What should I ask the installer to show me?

The room-by-room load table grouped into the quoted zones, each head’s heating capacity at your design temperature from the extended data, each head’s minimum output, and the connected-capacity ratio against the manufacturer’s permitted range.