One box, one room, one open door
A ducted system distributes. A mini-split does not. Everything the indoor head produces goes into the air of the room it is screwed to, and it reaches anywhere else only through a doorway somebody has left open. That single property is what makes siting a mini-split a design exercise and not a fixing exercise.
The mistake it produces is predictable. A head goes on a hallway wall because the wall is central, the run to the outdoor unit is short and the customer likes the look of it. The hallway becomes the most comfortable space in the building and the three bedrooms off it become three separate complaints, all of which arrive in the same week in August.
Sizing therefore happens room by room, and rooms are small. A modest bedroom in a reasonably insulated house has a load that a great many manufacturers cannot supply a head small enough to match, which means the honest conversation on day one is about which rooms get heads and which get a door left open and a fan. That conversation happens with the customer standing in the rooms, not over a phone. Where the head goes, where the outdoor unit goes, and where the pipe travels between them are three decisions that are cheap while a tape measure is out and expensive once a bracket is bolted to a wall.
The load belongs to the room
A whole-house load figure is no use for selecting a head. What matters is the load of one room at design conditions: transmission through its own external walls and roof, solar gain through its own glazing on its own orientation, whatever it borrows from unconditioned space next door, and the people and equipment inside it. A west-facing bedroom over a garage and an identical north-facing bedroom over a heated room are not the same selection.
Latent load is the part that gets ignored and then complained about. A head oversized in cooling satisfies the room's temperature quickly and stops, having run for too short a time to remove much moisture. The occupant gets a room that is cold and clammy, turns the setpoint down to compensate, and the machine cycles harder. Inverter compressors soften this by modulating down, and there is a floor below which they cannot go.
Turndown ratio is the specification to read when a room's load is small. A head with a wide modulation range will loiter happily at a fraction of its rating; one with a narrow range will hunt on and off exactly as a fixed-speed unit does. Manufacturers publish minimum capacity alongside rated and maximum, and it is the minimum that decides whether a small bedroom will be comfortable.
Heating and cooling size different things in the same room. Glazing that dominates the cooling load contributes far less in heating, where infiltration and envelope area take over, and a head selected on the cooling number can be well short at the winter design condition. Check the manufacturer's extended performance data at the actual outdoor design temperature, since rated heating capacity is published at a condition considerably milder than most winters supply.
Heads are selected per room, and a room is small enough that orientation and glazing shift the answer more than floor area does — start from the room you are standing in.
The length of the room to be cooled.
The width of the room to be cooled.
Taller ceilings mean more air volume to cool.
Direct sun through windows adds significant heat load.
Recommended cooling capacity (BTU/hr)
2,990 BTU/hr
This is a rough planning estimate, not a Manual J load calculation — actual sizing should be verified by an HVAC professional, especially for whole-house systems.
- Room area
- 149.5 sq ft
- Baseline (20 BTU/sq ft)
- 2,990 BTU/hr
They open the calculator with your figures already in it
BTU Cooling / AC Sizing Calculator: 2,990 BTU/hr — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 2,990 BTU/hr — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- This is a floor-area rule of thumb, not a load calculation. It reads only room length, width, a three-step ceiling category and a three-step sun category — insulation level, window area and glazing type, air leakage, wall and roof construction, and your local design temperatures are all outside it. It will not satisfy a permit, utility programme or contract that asks for a Manual J (or equivalent) load calculation.
- No internal heat gains are counted. Occupants, cooking, laundry, and electronics or server gear add load this model never sees, so a kitchen, a laundry room or a room that regularly holds several people needs more capacity than the figure shown.
- Ceiling height enters as a fixed multiplier on floor area, not as an actual volume. The real height of a vaulted ceiling, warm-air stratification, and an uninsulated roof or attic directly above the room make no difference to the result.
- The output is sensible cooling capacity only; it does not size for moisture removal. In a humid climate the latent load decides whether a nominally correct unit feels comfortable, and a unit chosen on capacity alone can short-cycle and leave the room cold and clammy.
- It sizes one enclosed room and stops there. Open-plan connections to the rest of the house, multi-room or whole-house distribution, duct and vent sizing, condensate drainage, the electrical supply for the unit, and the capacities actually sold are all outside the calculation.
Where the head can actually go
A wall head throws its air along the ceiling and relies on that jet staying attached long enough to reach the far side of the room and fall.
Mount it too low and the throw dumps early into the middle of the space; mount it hard against the ceiling with no clearance above and the unit strangles its own return, since a wall head draws in across its top face. The manufacturer's clearance dimensions are the throw's working conditions.
Look at what is in the room before choosing the wall. A curtain track above the head, a wardrobe within the throw, a ceiling fan directly in front, a shelf across the return — each of these takes a properly selected unit and turns it into a badly performing one. So does mounting it above a bed, where the occupant experiences the jet directly and the answer will be to switch it off.
Wall construction decides the backplate. Masonry takes a proper anchor; timber frame needs the plate landing on studs or on noggins fitted for the purpose; a plasterboard-and-dab wall with nothing behind it will hold the plate exactly as long as the adhesive does. The head must also come off its plate for service and for cleaning the coil and the blower wheel, so leave the clearance that operation needs.
The drain pan inside the head slopes to one side, and which side is a fixed property of the unit. Choosing a wall that puts the drain connection on the far side from the penetration commits you to running the hose across the back of the unit, which is where the fall gets lost. Establish that detail from the installation manual with the box still shut.
The smallest head on a multi-split
A multi-split puts several heads on one outdoor unit, and it is not the same machine as several single-zone systems with the pipework tidied. All the heads share one compressor and one refrigerant circuit, which imposes constraints that do not exist on single-zone work and which surface as comfort complaints and never as faults.
The binding one is minimum capacity. When only the smallest head is calling, the outdoor unit still has to run above its own minimum output, and that output may exceed what the small room can absorb. The result is a bedroom that overshoots and a compressor that cycles, on a system whose selling point was that it would not.
Combination ratio is the second. Manufacturers permit the connected indoor capacity to exceed the outdoor rating within a stated band, on the assumption that not everything calls at once. Load the system to the top of that band and the diversity assumption becomes a promise the building has to keep; on a design day, it does not, and every room is short at the same moment.
Most multi-splits also cannot heat one room while cooling another, because one circuit runs in one mode at a time. A house with a hot west-facing office and a cold north bedroom will be given a choice it did not know it was buying. Where that pattern is likely, separate single-zone systems, or a heat-recovery VRF with a branch box, are the honest recommendation even though the pipe count goes up.
Ground, brackets and the clearance around the outdoor unit
The outdoor unit needs air to pass through its coil and leave without coming straight back in. Manufacturers publish a clearance for each face and they are not equal: the discharge face needs the most, the service face needs enough to get a spanner on the valves, and the coil faces need enough that a fence, a hedge or a neighbouring unit does not force the discharge back around. Recirculation costs capacity continuously and shows up as poor performance nobody can locate.
Ground mounting on a pad is the quiet option and the one that survives. The pad carries the unit plus its clearances, sits on ground that will not settle differentially, and lifts the unit clear of splash and standing water. Where snow is a factor, it also has to lift the coil above the depth that will drift against it, which usually means a stand and not a slab.
Wall brackets look neat and put a vibrating compressor in structural contact with the building. On a masonry wall with anti-vibration mounts and nothing sensitive behind it, that is acceptable. On a timber-frame wall shared with a bedroom, the customer will hear the compressor start every time it starts, and no amount of rubber added afterwards will fully undo it.
In heating mode the unit runs a defrost cycle and sheds meltwater from the base of the coil. That water has to go somewhere that is not a footpath, a doorway or a step, and in a cold climate it has to leave the base pan before it freezes there — which is what base pan heaters and a drain connection with a clear route are for. A unit sitting flat on a slab in a freezing climate builds an ice block under itself over a week.
The pad has to carry the unit plus the clearance the manual names on every face, which is why the slab a customer already poured is nearly always the wrong size.
The equipment unit's overall width.
The equipment unit's overall length.
The manufacturer's minimum clearance to be maintained around the equipment.
Required pad/curb footprint
20 ft²
Use the specific equipment manufacturer's minimum service clearance requirements (often different on the service-access side vs. other sides) rather than a uniform assumed clearance — this calculator applies the single clearance value you supply evenly on all sides as a simplified estimate.
They open the calculator with your figures already in it
HVAC Equipment Pad/Curb Footprint Calculator: 20 ft² — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- This is a plan area, not a pad. Thickness, reinforcement and what goes under it come from the unit's operating weight — the wet weight, with the unit charged and any water side full — spread over that footprint against the bearing capacity of the fill beneath. A pad poured thin on soft or uncompacted ground settles unevenly, and what shows it first is the refrigerant piping and duct connections working at the joints.
- A roof curb is dimensioned by the unit, not by clearance. Its plan size matches the equipment's supply and return duct openings and its base rails, so it comes off the manufacturer's curb drawing rather than any formula — and the roof structure under it usually needs framing added between joists to pick up point loads the deck was never designed for. Adding a service clearance to a curb dimension is the wrong shape of answer entirely.
- Service clearance does not have to be concrete, and the clearances that matter most are not service clearances. A technician can stand on gravel, so applying the allowance to all four sides can over-pour a pad by a wide margin. Meanwhile the non-negotiable ones sit outside this arithmetic: discharge clearance above and at the coil face of a condenser or heat pump, separation from a property line, an operable window, a gas meter or a dryer vent, and clear working space at the disconnect.
The length budget is spent by choosing two positions
Every model ships with a base line length included in its rated capacity, a maximum length that is a hard limit, and a maximum vertical separation quoted separately from both. Between the base and the maximum sits a derate: a published percentage of capacity given up per unit of extra horizontal run, and a steeper one per unit of rise. That is a budget, and it is spent entirely by where the two boxes end up.
The useful moment to spend it is while you are standing between the two candidate positions with a tape in your hand. Moving the outdoor unit two metres closer, or bringing the head to the wall the pipe already has to cross, can hand back a measurable fraction of capacity for the cost of a slightly less tidy elevation. Once the bracket is bolted up, the same conversation costs a day.
Route length is not tape length. Every direction change adds resistance expressed as extra equivalent length, and a run threaded around a soil stack, along a joist and down a cavity accumulates more than the straight metres suggest. Two long sweeps beat five tight bends over the same displacement, and the coil of surplus pipe left behind the outdoor unit because nobody wanted to cut it is a permanent addition to the run. Tell the customer the number. A derate that is explained while the positions are still moveable is a design decision they took part in; the same derate discovered at commissioning is a shortfall against the figure they were quoted. The second version is the one that ends in a dispute about a machine that is working exactly as its manual says it will.
Both positions are still moveable while you are standing there with a tape, so find out what the run between them costs in capacity while moving a bracket is still free.
The system's nameplate cooling/heating capacity at the manufacturer's base (included) line length.
The horizontal refrigerant line run beyond the manufacturer's base length allowance.
The percentage of rated capacity lost per foot of horizontal line beyond the base allowance.
The total vertical elevation change between indoor and outdoor units.
The percentage of rated capacity lost per foot of vertical rise.
Derated system capacity
19,300 BTU/hr
The horizontal and vertical derate rates MUST come from the specific manufacturer's installation/engineering manual for your exact model — they vary significantly between manufacturers and are not standardized. This calculator only applies the rates you supply; it does not know or assume any specific product's actual derate schedule.
- Total capacity derate
- 19.75 %
They open the calculator with your figures already in it
Mini-Split/VRF Line Set Length Derating Calculator: 19,260 BTU/hr — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 19,300 BTU/hr — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- A long line set needs more refrigerant, and that quantity is not computed here. Systems ship pre-charged to a stated line length, and past it charge is added at a published rate per unit of extra length — get that wrong and the capacity loss dwarfs the derate above, starving the evaporator when short and returning liquid to the compressor when long. The top-up is weighed in, not gauged in.
- A derate schedule has an end and this arithmetic does not. Manufacturers publish a maximum total line length and a maximum height difference, and beyond either the system is outside its listing no matter what percentage the rates would produce. Direction counts too: permitted lift is usually different with the outdoor unit above the indoor one than below it, and oil traps at intervals up a long riser belong to the same rule — the field above takes one rise with no sign on it.
- The nameplate being derated is a laboratory figure. Rated capacity is measured at standard test conditions, and on a design summer afternoon at a higher outdoor temperature — or in heating at a low one, where defrost cycles take their own share — the machine delivers less than its rating before line length is counted at all. Comparing a derated nameplate against a load computed at design conditions compares two numbers taken on different days.
The hole through the wall
One core drill decides several things at once, which is why it is worth resolving on paper first. Diameter has to pass the whole bundle — two insulated lines, the condensate hose and the interconnecting cable — without compressing any of it, because a bundle squeezed through an undersized hole crushes the insulation exactly where the wall is coldest.
Drill with a fall to the outside. The hole is going to carry a condensate hose, and any back-fall becomes a low point that holds water inside the wall. A level hole is a rain path in the other direction; a hole falling the wrong way is a slow leak into the structure that will be blamed on the roof.
Know what the wall is before the core goes in. A cavity wall needs a sleeve and a consideration of what happens to water running down the cavity face; a timber frame needs the penetration sealed to the air barrier and the weather barrier on their own terms and on their own faces; external insulation needs the sleeve carried right through and finished. Check for services in the wall on both sides — a core drill through a buried cable is a very short conversation with the customer.
Fire compartmentation is not optional because the penetration is small. Where the wall is a rated element, the penetration gets a rated treatment under the code in force locally, and a handful of expanding foam is not one. Seal both faces, and make the exterior seal a weathering detail, not a bead of silicone that will be gone in three summers.
What runs inside the cover
The bundle between the two units is four separate services travelling together, and each of them fails differently if the assembly is built carelessly. The two copper lines carry refrigerant. The hose carries condensate under gravity alone. The interconnecting cable carries power and communications, usually as a single supply routed through the outdoor unit so there is one isolation point.
Insulate both lines, for their whole length, on a heat pump. The habit of insulating only the large line comes from cooling-only work, where the small line runs near ambient; on a reversing machine each line spends half the year well away from ambient, and a bare line in an unconditioned space sweats in one season and loses output in the other.
Where the lines are taped together, insulate them individually first so they are not thermally short-circuited against each other.
Tape the wrap from the bottom of the run upward so every lap sheds water, and pull it firm without compressing what is underneath — squeezed insulation has lost the wall thickness it was chosen for and will condense at that point and nowhere else. Outdoors, the wrap or a proper cover takes the ultraviolet, and the failure of an unjacketed sleeve is gradual enough that nobody notices until it crumbles off in handfuls.
Keep the condensate hose at the bottom of the bundle. It is the only component in there that depends on gravity, it is the softest thing in the bundle, and a hose taped to the top of the pair and then pulled through a tight hole becomes the high point of a drain that was supposed to fall the whole way. The cable goes alongside, never spiralled around the pipe.
What runs inside a line-set cover
- Line-set cover — the exterior channel taking the ultraviolet and the ladder knocks, bought in lengths plus a fitting at every direction change
- Vapour-tight wrap — spiral-taped from the bottom of the run upward so every lap sheds, and never pulled tight enough to compress what sits under it
- Closed-cell insulation — carried over both lines for the full run on a reversing machine, since each line spends half the year away from ambient temperature Pipe Insulation Calculator
- Suction and liquid lines — the pair the manufacturer's table sizes against capacity and run length, flared at both ends and torqued to a published figure Refrigerant Line Set Nominal Size Reference Calculator
- Condensate hose and interconnecting cable — kept at the bottom of the bundle so the hose holds its fall and never becomes the high point of the drain HVAC Condensate Drain Pipe Slope Calculator
Sleeving comes in fixed lengths against a run measured around the building, and the shortfall always turns up at the penetration, where compressed insulation does the most harm.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
The total length of pipe you want to insulate.
Sleeve lost to mitring at elbows and trimming to fit between clips.
Pipe insulation sections needed
9 x 6 ft sections
- Pipe length (with waste)
- 53.9 linear ft
They open the calculator with your figures already in it
Pipe Insulation Calculator: 9 x 6 ft sections — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The count is driven by run length alone, so it does not tell you which sleeve to buy. Tubular insulation is sized both by the pipe's outside diameter and, separately, by wall thickness, and a sleeve one bore size out will either refuse to close around the pipe or sit loose with an air gap running the length of it.
- Nothing here checks whether the insulation is thick enough for the duty. The minimum wall thickness for hot water pipes is set by energy code against pipe size and fluid temperature, and the thickness needed to hold a cold or chilled line above its dew point depends on ambient temperature and relative humidity; both are jurisdictional, and neither is a function of how long the run is.
- The waste allowance covers mitring at bends and trimming to fit, not a fitting-by-fitting take-off. Elbows, tees, valves and flanges are normally covered with pre-formed fitting covers bought separately, and a compact run with many changes of direction can eat well past the 10% default in offcuts.
- The answer counts whole stock sleeves, so the imperial and metric figures are not restatements of one another. A 15 m run comes out as 10 six-foot sleeves, which is 18.3 m of insulation, or as 17 one-metre sleeves, which is 17 m; the piece counts differ because the products differ, and metric merchants also stock 2 m lengths that this count does not assume.
- Only the sleeves are counted. Self-seal tape or ties for the split seam and end caps are extra, and the figure assumes the run can be wrapped end to end: pipe clips, hangers and the points where a pipe passes through a wall interrupt the insulation, and those breaks are where heat loss and freezing concentrate.
Flares, and the torque that is not a guess
On this equipment the joints are flares, which means the entire refrigerant circuit rests on four mechanical seals made by hand on site with no filler metal to hide behind. A brazed joint is either sound or visibly wrong. A flare can look perfect, hold a vacuum, hold a nitrogen test, and leak slowly enough to empty the system over two summers.
The mechanics are unforgiving in small ways. Cut with a tube cutter and take the cut in stages so the tube is not ovalled. Deburr with the tube pointing downward so no swarf falls inside, and wipe the end. Put the flare nut on the tube before flaring, which every technician has forgotten at least once. Use a flare tool that produces the profile the manufacturer specifies, and inspect the finished face against a light for a crack, a ridge, an off-centre lip or a burr.
Torque is published per line size and it is a specific figure, not a firm feel. R32 and R410A systems run at pressures that make the values higher than habits carried over from older refrigerants, and both directions of error are real: undertighten and the seal never forms, overtighten and the flare splits or the nut cracks. Use a torque wrench with a backing spanner on the service valve so the valve body takes no strain.
A trace of refrigerant oil on the flare face, if the manufacturer permits it, helps the metal seat and lets the torque figure mean what it says. Sealant on a flare does not. If a joint weeps, the correct repair is to cut it off and make a new flare, and the wrong one is another quarter turn on a nut that has already been to its published figure.
The vacuum the joint work has to hold
Testing happens in two stages and they answer different questions. A dry nitrogen strength and tightness test at the pressure the manufacturer specifies proves the joints against pressure from the inside, which is the direction they will actually see. Hold it long enough to matter and read it against a thermometer, because ambient swing moves a gauge on a perfectly tight system and has convinced many people they had a leak.
Evacuation then removes air and moisture, and it is a measurement, never a duration. Pull through both service ports with short, large-bore hoses and a core removal tool, since a quarter-inch hose through an undisturbed Schrader core will let a pump run all afternoon and reach nothing worth having. Put the micron gauge on the system, at the far end from the pump, because a gauge at the pump inlet reports the pump's own performance and nothing about the system.
The decay test is what the whole exercise is for. Isolate the pump at the target vacuum, leave the gauge connected and watch. A reading that rises and then levels off is moisture still boiling out of the system and needs more pumping, ideally broken with dry nitrogen and pulled again. A reading that rises steadily without settling is a leak, and on this equipment that means a flare — go back to the joints, not deeper into the pump.
Only when the vacuum holds do the service valves get opened, fully, and the stem caps torqued to their own published figures. Those caps are a seal in their own right and they are the single most common place to find a slow leak on a system that was otherwise built well. Never use the system's own refrigerant to purge air out of the lines: it is a refrigerant release, it is an offence in most jurisdictions under the handling rules, and it leaves the air behind anyway.
Charge for the length you actually built
The outdoor unit arrives with a factory charge covering a stated base line length. Beyond that base, the manufacturer specifies an additional charge per unit length, and on some models a reduction where the installed run is shorter than the base. That figure is a weight, and it is added by weight, on a scale, with the number recorded.
Estimating it from gauge readings is the error this trade repeats most. On a fixed-orifice cooling-only machine, superheat and subcooling can be used to reach a correct charge given time and stable conditions. On an inverter mini-split with an electronic expansion valve, the machine is actively changing the very quantities you are reading, and a charge trimmed against those readings on a mild afternoon can be substantially wrong at design conditions.
Record what you did on the equipment itself. Installed line length, the additional charge added, the refrigerant type, the date and who did it, on a label inside the outdoor unit's service cover. The next technician's first question will be whether the charge matches the run, and answering it from a label takes a minute where recovering and reweighing takes a morning.
Refrigerant type is not a detail. Charge weights, line sizes, torque figures and safety requirements all follow from it, and A2L refrigerants add a limit on charge relative to the volume of the smallest occupied room the system serves — a constraint that can rule out a large head in a small bedroom entirely. That limit comes from the refrigeration safety standard and the appliance standard as adopted locally, and it is a design input rather than a commissioning surprise.
Additional charge is a weight tied to the metres genuinely installed, and it belongs written on a label inside the service cover as well as in the scale's display.
The liquid line as installed, measured along the route rather than point to point.
The line length already covered by the factory charge, from the model's own document.
The refrigerant mass the manufacturer adds for each unit of liquid line beyond the standard length.
The bore of the liquid line, used to report the volume of the extra run as a cross-check.
Additional charge to weigh in
1.537 lb
- Line length beyond the factory-charge standard
- 41 ft
- Internal volume of the run beyond standard
- 37.13 in³
- Charge rate applied
- 0.04 lb/ft
They open the calculator with your figures already in it
Refrigerant Line Length Charge Adjustment Calculator: 1.54 lb — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- This is a weigh-in figure, not a commissioning verdict. Subcooling and superheat measured at operating conditions are what confirm the charge is right, and they can disagree with the arithmetic when the metering device, the line insulation or the indoor airflow is not as assumed.
- Assumes the liquid line runs full of liquid. A line with a long lift, a sight glass showing flash gas, or a badly insulated run in a hot roof space will not.
Two drains, and only one is on the drawing
The indoor head produces condensate whenever it cools, and the hose taking it away has one requirement: continuous fall, every centimetre of the route, with no dips and no rise at the end. The drop available between the head's spigot and the outside face of the wall is usually a matter of tens of millimetres, which is why this gets designed at the point the backplate is offered up and not afterwards.
Whether the run needs a trap depends on where the fan sits relative to the coil. A wall head with its pan at close to room pressure generally does not, and the manufacturer will say so. A concealed or ducted head that draws air across the coil holds its pan below room pressure, and without a trap of the right depth the unit pulls air back up the drain instead of letting water down it. Follow the instruction for the specific unit. Where gravity genuinely is not available, a condensate pump is the answer and it comes with obligations: a discharge routed with a rise then a fall so the head is not left standing in the pipe, a reservoir that can be cleaned, and an alarm contact wired to shut the unit down when the float sticks. A pump without that interlock converts a blocked drain into a ceiling repair.
The second drain is the one nobody draws. In heating, the outdoor unit sheds defrost water from its base, repeatedly, all winter. It needs a route away from the unit that will not freeze under it, will not ice a path or a doorway, and will not discharge across a neighbour's boundary. Deciding that when the unit is being positioned is free; discovering it in February involves a chisel.
The drop between the head's spigot and the outside face of the wall is usually measured in tens of millimetres, so check the fall the run needs against the fall the building can give it.
The total horizontal run of the gravity condensate drain line.
The vertical drop per horizontal foot of drain line.
Minimum required drop
2.437 in
1/8 in per ft (10 mm per metre) is the commonly-referenced minimum slope for HVAC condensate drains — always verify against the applicable mechanical code edition and the specific equipment manufacturer's installation instructions, which occasionally specify a different minimum.
- Drain line length
- 19.5 ft
They open the calculator with your figures already in it
HVAC Condensate Drain Pipe Slope Calculator: 2.44 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 2.437 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The fall this returns is horizontal; the number that most often fails is vertical. A drain leaving the negative-pressure side of a draw-through unit needs a trap deep enough to beat the cabinet's static — the usual rule sets the inlet leg at the fan's negative static pressure plus a margin, with a shallower outlet leg — or the fan holds the water in the pan and it overflows past a perfectly sloped line.
- The drop has to physically exist. Stack up the trap depth, the pipe diameter and the total fall computed above, and the whole lot has to fit between the drain pan connection and whatever sits beneath it — which on a ceiling-hung fan coil or a horizontal attic unit frequently it does not. That is the moment a condensate pump enters the job, and it is far cheaper to find before the unit is hung than after.
- Slope alone does not keep a line clear. Drain size follows the equipment's capacity, with three-quarter inch the usual minimum and larger lines above roughly 20 tons (18 tonnes), and the code also wants cleanout access at the trap and at changes of direction — a correctly sloped half-inch line still blocks with biofilm. Where a blockage would cause damage, a secondary drain pan or an overflow switch is required as well, and neither follows from the drop above.
Start-up, where the gauges stop helping
Commissioning an inverter machine is not the exercise it is on fixed-speed equipment. Superheat and subcooling only mean anything at a known, steady operating point, and the compressor is changing that point continuously in response to room conditions. Most manufacturers provide a forced test run or a service mode that locks the machine at a defined output precisely so a technician can take a reading worth writing down.
Use the machine's own instrumentation while you are in there. Modern heads and outdoor units expose coil temperatures, expansion valve position, compressor frequency and fault history through a service display or an app, and that data set diagnoses far more than a pair of gauges hung on the service port — which, on a small system, also removes a measurable fraction of the charge each time they come off.
Take the measurements that do not depend on the machine's mode. Air-on and air-off temperature across the indoor coil, taken in the return path and in the discharge, tell you the head is doing work. Running current against the nameplate tells you the compressor is not struggling. Filling the drain pan with water and watching it leave the building tells you the drain works, which is the test most likely to be skipped and most likely to be needed.
Finish in heating mode if the season allows, and force a defrost cycle if the unit will let you. That confirms the reversing valve shifts, the meltwater goes where you decided it should go, and the indoor head does not blow cold air across the room while it happens. Then hand over the remote, show the customer the filters, and write the line length, charge and readings on the label before the cover goes back on.
The two positions, settled as one decision
Everything here is fixed the moment the bracket goes on the wall. Walk the route with a tape and agree both positions with the customer standing in the room.
- Room load per head, doors shut — Own walls, own glazing, own orientation; check minimum modulating capacity against the smallest room on the list.
- Line length between the two chosen positions — Route length with the bends counted, plus the drop down the cavity and whatever slack ends up coiled behind the outdoor unit.
- Vertical separation, measured, and which unit is higher — A separate published limit from maximum length; passing one does not clear the other.
- Additional charge for the run beyond the base allowance — Weighed in on a scale and written on a label inside the service cover with the installed length.
- Insulation for both lines, full length, uncompressed — Individually sleeved before taping together, jacketed against ultraviolet wherever the run is outdoors.
- Condensate route with continuous fall, or a pump with an interlock — Plus a separate answer for defrost meltwater at the outdoor unit that does not ice a path.
- Clearances at the outdoor unit on every face the manual names — Discharge, service and coil faces differ; recirculated discharge air costs capacity all year.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.