Sized by square feet
If nobody asked about your insulation, glazing or orientation, the size came from a rule of thumb. On a small, tight building that rule of thumb oversizes badly, and the error grows as the building gets smaller.
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Heating and cooling an ADU is its own discipline.
Not a smaller version of a house job. The arithmetic is different, the constraints are different, and the point at which decisions become expensive is much earlier than people expect.
The arithmetic
The load is smaller than the smallest thing most contractors stock.
A well-built accessory dwelling unit is small, reasonably insulated and reasonably sealed. Its calculated heating and cooling load is correspondingly small — often smaller than the minimum output of a conventional ducted system.
That is the whole problem in one sentence, and everything else on this page follows from it.
Sizing by rule of thumb makes it worse. Those rules of thumb — so many square feet per ton — were derived from older, leakier, larger houses. Applied to a modern small unit they oversize it substantially, and the smaller the building the larger the error.
An oversized system in a small building does not fail. It just behaves badly: it reaches the thermostat in minutes, shuts off, and starts again. A lot of starts, very little run time, uneven temperatures between the rooms, poor humidity control, and a machine that gets heard far more often than it gets felt.
So we calculate the load properly — from the drawings if the unit is not built yet, from the shell if it is — and we specify equipment that can run below that number as well as at it, because a small building spends nearly all of the year needing much less than its design load.
Which in practice means ductless
Not out of loyalty to a product category. Ductless equipment happens to be the thing that is available small enough, that modulates down properly, and that does not need duct routes a small building has no room for.
If a unit is large enough and laid out to suit a small ducted system, we will say so. It happens, and it is usually the better answer when it does.
The hard constraint
Where the outdoor unit can actually stand.
On a typical lot with a house at the front and a unit at the back, the number of legal, practical positions is often one or two. Finding that out at design is free. Finding it out on install day is not.
- Clearance
- The manufacturer's minimum, on the sides they specifyA unit pulls air in on some faces and discharges on another. Box it into a corner to keep it out of the way and you have starved it, which looks like nothing and costs capacity every day for the life of the machine.
- Setbacks
- Equipment placement is regulated, and the rules moveWhere mechanical equipment may sit relative to a property line is a local matter, and ADU rules in California have been revised repeatedly over the past several years. We do not quote a rule from memory. We check what applies to your address and your scope when the permit is pulled, and tell you what it means before you commit to a position.
- Noise
- Mostly a siting question, not a product questionModern equipment is quiet. What makes it a problem is standing it three feet from a bedroom window — and the window that matters most is often the neighbour's, on the other side of a fence, on a lot that used to be a garden.
- Condensate
- Water has to go somewhereBoth the outdoor unit in heating mode and every indoor head in cooling mode produce it. A route by gravity is best; a pump is fine where the levels do not allow one. What is not fine is discovering the question after the slab is poured.
- Access
- Somebody has to service it in five yearsIf the only way to reach the unit is past a new fence, through a planted bed and under a deck, it will not get serviced. We raise that at design because it is the cheapest possible moment to move something two feet.
The thing that stalls projects
Electrical capacity.
In our experience this is the single most common reason an ADU's mechanical work gets held up, and it is almost always discovered later than it needed to be.
A heat pump is an electrical load and it needs a dedicated circuit with real capacity behind it. Whether the unit has its own service or is fed from the main house, somebody has to have allowed for it — and on an older property, "the main house has spare capacity" is a claim worth checking rather than assuming.
We do not do panel or subpanel work. That is deliberate and it is not a gap we apologise for; it belongs to an electrician. What we do is give that electrician a written requirement — the circuit, the disconnect, the location — early enough that it can be planned rather than squeezed in.
The failure mode we see repeatedly: the mechanical scope is agreed, the build proceeds, and at rough-in it turns out the electrical design allowed nothing for the heat pump. Now you are choosing between a delay, a service upgrade nobody budgeted, or a smaller system than the building wants. All three are bad, and all three were avoidable with one conversation in the first fortnight.
What to ask your electrician, early
"Here is the mechanical requirement in writing. Does the design allow for it, and if the unit is fed from the main house, does the main house have the capacity?" If the answer takes longer than a day, that is useful information too.
Sequencing
Two visits, at two specific moments.
Mechanical work on a small build is not one long job. It is two short ones with the rest of the trades in between, and the first has to happen before the walls close.
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Design
Load calculation, equipment selection, outdoor unit position, line route and the electrical requirement — issued in writing before framing if possible.
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Rough-in
Line sets, condensate, control wiring and the wall penetrations, while the framing is open. Half a day to a day. This is the visit that cannot slip.
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Inspection
Mechanical rough inspection with the rest of the trades, on the programme's schedule rather than ours.
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Commissioning
Equipment set, system evacuated and charged, tested, and the owner or tenant shown the filters. Paperwork left on site.
Being called late
It happens, and we still take the work. What changes is the outcome: the line route goes on the outside of a new building instead of inside a wall, the outdoor unit goes wherever is left rather than wherever is best, and a condensate pump appears in a finished room.
None of that is a disaster. All of it was avoidable, and we would rather say so once, plainly, than pretend a late start costs nothing.
Warning signs
Four ways ADU mechanical work goes wrong.
Each one is visible in a quote or a programme if you know to look for it.
No outdoor unit position on the drawing
If the quote does not say where it goes, the decision has been deferred to install day — which means it will be made by whoever is holding the machine, under time pressure, from whatever positions remain.
The electrical requirement is "to be confirmed"
This is the one that costs weeks. A mechanical contractor should be able to state the circuit requirement from the equipment selection immediately. If they cannot, the equipment has not really been selected.
One visit in the programme
Mechanical work on a small build is rough-in and commissioning. A programme with a single "HVAC" bar somewhere near the end means the rough-in has not been planned, and the line route is going to end up outside.
And one about us
Our quotes are sometimes higher than the first one a customer receives, and this page is the reason. If you have a cheaper number, take these four questions to whoever wrote it. If they have good answers, use them — genuinely. If they do not, you now know what the difference was.
ADU questions
Asked once people have read this far.
Do I legally have to heat an ADU?
A dwelling unit in California has to be able to be heated — it is a habitability requirement, not an optional extra, and it will be looked at. Cooling is a separate question and is usually a choice rather than a requirement. The practical consequence is that a heat pump often makes sense simply because the heating has to be there anyway and the same machine cools. What we will not do is tell you what the code says today from memory; we check it against your scope when the permit is pulled.
Can the ADU run off the main house's system?
Occasionally, for an attached unit or a conversion sharing a wall, and it is worth examining before assuming otherwise. Two things usually stop it: the main system is not sized for the extra space, and a separate dwelling unit generally wants its own controls and its own metering of comfort — a tenant who cannot set their own temperature will call you about it. More often the right answer is a small dedicated system.
Is one indoor head enough for a one-bedroom unit?
It depends entirely on the doors. With an open plan and a bedroom that stays open, often yes. With a bedroom door that closes at night — which is the point of a bedroom — a single head in the living space will not keep that room even, and no amount of capacity fixes it. We will tell you which case you are in and what a second head costs, before the walls close.
How much noise will the neighbours hear?
Less than they fear, if it is sited properly, and we will not give you a decibel figure because the number that matters is at their window rather than at the machine. What we will do is walk the fence line with you, show you where it will stand and which windows face it, and suggest the position that keeps it furthest from anyone's bedroom. Raising it with the neighbour before it appears is worth more than any specification.
What does the tenant actually have to do?
Rinse two mesh filters in each indoor head, about monthly. That is genuinely it, and it makes more difference than anything else. We show whoever is there at commissioning and leave a note with the paperwork; if the unit is let later, pass the note on. Most of the ductless service calls we attend would not have happened if somebody had been shown the filters.
Call before the walls close.
Send a floor plan and an elevation if you have them. Most of the expensive decisions can be settled from those.