Ductless Heads in a House With Ducts

Why this matters

The bonus room over the garage has been ten degrees off the rest of the house since the day it was finished. The ducts reach it at the end of a long run with one register, and no central system replacement is going to change that. A ductless head on the wall of that room fixes it in a day. Then the call comes back in February: the bonus room is perfect, and the kids' bedrooms down the hall are cold. Nothing is wrong with either system. They are fighting, and the person who installed the head never set them up not to.

A mixed system, a central ducted system for most of the house plus one or more ductless heads for the rooms the ducts never served well, is one of the most useful tools in a conversion. This card covers where it earns its place, what it should not be asked to do, how to keep the two systems from working against each other, and where the condensate goes. Duct fixes for the central system itself belong to Ductwork When a Heat Pump Replaces a Furnace.

Where a head earns its place

A ductless head (the indoor unit of a mini-split, mounted high on a wall or in the ceiling, with its own refrigerant lines to an outdoor unit) is the right tool for a room the duct system cannot reasonably reach:

• Rooms over garages or unconditioned space, with three or four exterior surfaces and a load far above their floor area's share.
• Additions and sunrooms tacked on after the original duct design, often fed by one branch off the nearest trunk.
• Finished basements, where the supply was a couple of registers cut into the trunk and no return.
• Rooms at the end of a long run, where friction and leakage leave little air by the time it arrives.

The test is whether the room's problem is reach, not capacity. If the central system cannot keep the whole house comfortable, a head in one room does not fix that.

What a head is not for

Most bad mixed systems come from asking the head to do one of these.

It is not a patch for an undersized central system. If the whole house runs short on the design day, putting a head in the living room makes the living room comfortable and leaves the rest of the house short. Fix the central capacity or the ducts first.

It is not a reason to close the room's duct register. Closing a supply register raises static pressure on the central system, by a little for one register and by a lot for several, and it cuts the room off from the central system's air mixing and filtration. The register the room already has stays open. If it was delivering almost nothing, it costs almost nothing to leave it.

It is not a hallway heater for a whole floor. A head mounted in a hallway or a large open area to "heat the upstairs" throws air into the space it is in and relies on doors and convection for the rest. Bedrooms with closed doors stay cold. One head per room or per open zone it can see.

It is not sized from the nominal number. A 12,000 Btu/h head in a room whose load is 4,000 at the shoulder season cannot turn down far enough, cycles constantly, and in cooling leaves the room clammy. Size from the room's own load in the room-by-room calculation, and check the head's minimum capacity against the room's mild-weather load, the same check as for the central system. See related: Sizing a Heat Pump for a Cold Climate.

Why the two systems fight

Each system has its own thermostat sensing its own air, and each acts on what it senses. A fight happens whenever one system's output reaches the other's sensor.

The thermostat near the head's room. The central thermostat sits in the upstairs hallway a few feet from the bonus room door. The head brings the bonus room up to its setpoint, warm air spills into the hall, and the central thermostat reads warm and stops calling. The rest of the house goes without heat.

Opposite modes in the shoulder season. Both systems left in automatic changeover, with their setpoints close together. On a sunny afternoon the head's room overheats, the head switches to cooling, while the central system is still heating the north side. Both run, against each other.

The head sensing at the head. Most wall heads sense temperature at the return air inlet on the unit itself, high on the wall where the air is warmest. The head reads the room warmer than it is at seated height, satisfies early, and the occupant turns the setpoint up, which makes the first problem worse.

Controls that keep them from fighting

Set modes by season, not auto. Put the head in heat mode through the heating season and cool mode through the cooling season. If the customer insists on automatic changeover, set a deadband of at least 4 F between heating and cooling setpoints on both systems.

Hold the setpoints together. The head's setpoint should sit at or no more than about 1 F above the central setpoint in heating, and at or no more than about 1 F below in cooling. A head set warmer than the central system in winter is the most common cause of the hallway fight.

Move the sensing. Many heads can sense at a wired wall controller or at the handheld remote instead of at the unit; use that, placed at seated height in the room. For the central system, if its thermostat sits where the head's air reaches it, relocate it or add a remote sensor in the rooms that are going cold, averaged with the thermostat.

Use an interface where the equipment supports it. Some mini-split systems accept an interface module that lets a conventional wall thermostat command the head. That gives the customer one familiar control per zone and makes schedules consistent across both systems.

Close the door, sometimes. A bonus room with a door that stays closed in heating season stops its air reaching the hallway thermostat at all. That is a habit, not a fix, but it is worth telling the customer why the problem gets worse with the door open.

Worked example: the bonus room over the garage

The house: a two-story with a central heat pump and the thermostat in the upstairs hall, about 5 feet from the bonus room door. The bonus room is 320 square feet over an unheated garage. The room-by-room load calculation gives it 7,200 Btu/h of heating at the 10 F design temperature and 6,800 Btu/h of cooling. Its single register measured 55 cfm; the duct design for a room with that load would call for about three times that.

Sizing the head. Take the room's zero-load point at 65 F, so its load falls by 7,200 over a 55 F span, about 131 Btu/h per degree. From the manufacturer's extended data (illustrative values; read the real table):

• A 6,000 Btu/h nominal head delivers 5,600 at 10 F, short of 7,200. Too small.
• A 9,000 Btu/h nominal head delivers 8,100 at 10 F, which covers 7,200 with about 12 percent to spare. Its minimum capacity is 2,800 at 47 F.

At 47 F the room's load is 131 times 18, about 2,360 Btu/h, below the 9,000's 2,800 minimum. The two lines cross where 131 times (65 minus T) equals 2,800, which is T of about 43.6 F. So above roughly 44 F the head cycles rather than modulates. That is acceptable for a handful of shoulder-season weeks; a 12,000 head, with a higher minimum, would cycle across far more of the year. The 9,000 is the call.

The February callback. The customer had set the head to 71 F and the central thermostat to 68 F, with the bonus room door open. The head held the room at 71, the hallway read 69, and the central system stopped calling. The two bedrooms at the far end of the hall measured 64 F and 65 F.

The fix was three changes and one deliberate non-change. The head went to heat mode only, at 69 F, one degree above the central setpoint. The head was switched to sense at its handheld remote, which was mounted at seated height on the wall opposite the unit. A wireless remote sensor went into one of the far bedrooms, averaged with the hall thermostat. And the room's 55 cfm register stayed open. The next week the far bedrooms held 67 to 68 F against the 68 F setpoint, and the bonus room held 69 F.

Condensate: where the water goes

In cooling, a head pulls water out of the room air, and that water has to leave by a drain line. In heating the indoor head is dry, but the outdoor unit makes water every time it defrosts, and in a cold climate that water freezes.

Gravity first. The simplest route is a drain line run with the refrigerant lines through the wall, sloping continuously downward to outside. The IRC's condensate provisions in Section M1411 (as adopted by your jurisdiction) require condensate to discharge to an approved place and set the minimum horizontal slope at 1/8 inch per foot, the same 1 percent figure the IMC states at 307.2.2. A line that sags or rises anywhere holds water and backs up into the head, where it drips down the wall.

Pumps where gravity cannot work. A head on an interior wall, or on an exterior wall over a walkway or a patio, may need a small condensate pump, mounted in the head, in the line-set cover or in the wall. Pumps fail, and a failed pump spills into the wall cavity. The IRC requires pumps in uninhabitable spaces such as attics and crawlspaces to shut down the equipment they serve on failure; for a head in a living space, wire the pump's overflow switch to stop the head anyway, because water inside a finished wall is the expensive outcome.

Where it discharges. Not onto a walkway, a patio, a deck or a driveway, where it stains in summer and freezes in winter. Not against a foundation that already has a water problem. Not into a gutter downspout that freezes shut. Your jurisdiction may allow discharge into a building drain only through an approved indirect connection; check before tying in.

Outdoor unit defrost water. Mount the outdoor unit on a stand or wall bracket that holds it above expected snow depth, and place it where the defrost meltwater runs off and freezes harmlessly, not on a step or a path. An ice sheet under a unit that sits on the ground can grow until it blocks the coil.

What changes the answer

Several rooms need help, not one. If three or four rooms are poorly served, a multi-zone system or a ducted mini-split air handler serving that wing may be cleaner than several wall heads, and some customers object to the look of heads on every wall.

The central system is being replaced anyway. If the furnace is coming out now, fixing the duct reach to that room during the conversion may be cheaper than adding a second system, especially if the trunk is accessible.

How to verify you got this right

After the install, record the head's setpoint, mode, sensing location and the central setpoint on the equipment record, with the reason, so the next tech does not undo it. Measure temperature at seated height in the head's room and in the farthest room on the central system, at the same time, with the door in its normal position. Run the head in cooling and confirm condensate flows freely at the discharge point, and if there is a pump, trip its overflow switch by hand and confirm the head stops.

References

• ACCA Manual J (room-by-room loads) and Manual S (equipment selection), in the editions your program requires.
• International Residential Code, Section M1411 (condensate disposal and condensate pumps), and International Mechanical Code 307.2.2, as adopted by your jurisdiction.
• Manufacturer installation manual for the mini-split system (extended data, sensing options, interface modules, condensate connections).
• See related: Ductwork When a Heat Pump Replaces a Furnace; Sizing a Heat Pump for a Cold Climate.