Because 6,000 BTU per hour will heat and cool that bedroom, and 12,000 will fight it.
“12K” is trade shorthand for 12,000 BTU per hour of rated capacity, which is also roughly what people mean when they say one ton. It’s the size of unit you’d hang in a large open living area. Put it in a bedroom and you haven’t bought yourself a margin. You’ve bought a unit that will spend most of its life trying to make the smallest amount of heat it’s capable of, and still making too much.
That feels backwards, so here’s the mechanism.
What the 12K actually does in that room
A heat pump head doesn’t decide how much heat the room needs. The room decides. The room’s demand comes from its size, its windows, its insulation, how much wall is exposed to outside, and how cold it gets outside on the coldest design day.
That last number is where this gets local. Under the National Building Code of Canada 2020, the winter design temperature for Vancouver and Richmond is -7°C, and across the rest of our service area it runs to about -10°C as you move east and uphill. That is the coldest condition the system has to cover. It is not a hard winter. A bedroom in a Metro Vancouver house, with one or two windows and a normal amount of exposed wall, is a small heating load at -7°C. In summer, with our short cooling season, it’s a small cooling load too.
So the 12K unit satisfies that room quickly. Then it has nothing to do, so it backs off or shuts down. Then the room drifts, and it fires again. That pattern is called short cycling, and it’s the actual answer to your question.
If you want the longer version of how the demand number gets calculated, we wrote it up separately: what a heat load report is and what’s in it.
Short cycling, and what you’d notice at 3 a.m.
Short cycling means lots of starts and stops instead of long steady runs. You notice it three ways.
Temperature swing. The unit blasts, overshoots, quits, and the room cools past where you set it before it starts again. You wake up because the room got warm, then you wake up again because the fan kicked on. A correctly sized head instead runs long and quiet at low output and holds the room close to the setpoint. The comfort difference is not subtle, and it’s the thing homeowners actually call us about after the fact.
Humidity in the summer. Cooling removes moisture only while the indoor coil is cold and air is moving across it. That takes runtime. A unit that satisfies the room in a few minutes and stops hasn’t had time to pull much water out of the air. You end up at the right temperature and still feeling clammy, which is a particular problem on our muggy July and August nights near the water. Then people drop the setpoint to chase the feeling, which makes the cycling worse.
Equipment life. Starts and stops are the hard part of a compressor’s job, not steady running. The City of Vancouver’s own multiplex electrical guidance, prepared by Dunsky Energy + Climate Advisors, makes the oversizing point plainly: oversized heat pumps short cycle, and that costs both efficiency and equipment durability. Worth noting that this is a document commissioned by the City, not by anyone with equipment to sell.
“But it’s variable capacity. Doesn’t it just turn down?”
This is the right question, and it’s why the 12K quote isn’t crazy, just wrong for the room.
Modern inverter-driven heat pumps modulate, meaning the compressor can run at a range of speeds instead of just on and off. A good unit can throttle a long way down. That genuinely narrows the gap, and it’s why oversizing a modulating heat pump is less punishing than oversizing an old single-stage system.
But every unit has a minimum. It can only turn down so far, and below that it’s back to cycling on and off. The bigger the unit, the higher that floor sits. A 12K head’s minimum output can be more than that bedroom needs for most of the year: the long mild shoulder season where the outdoor temperature sits well above design conditions and the room barely needs anything at all. The unit isn’t modulating then. It’s cycling.
The same Vancouver guidance is blunt about how real this floor is. Dunsky’s modelling found that the smallest multi-head mini split available was already oversized for a typical suite’s load. If the smallest multi-head unit on the market is already too big for that load, sizing up on purpose isn’t a margin.
If your quote is a ducted system, the ducts have a vote
Sometimes the oversized quote isn’t a head on a bedroom wall, it’s a larger central heat pump on the ductwork you already have.
Bigger equipment moves more air. Your existing ducts were sized for the airflow of whatever was in the basement before. Push more air through the same trunk and branches and static pressure goes up, which is just the resistance the fan has to overcome. What you hear is a whistle at the registers, a rush in the hallway, and a noticeable roar when the system starts. What you feel is uneven distribution: the rooms nearest the air handler get hammered while the far bedroom, the one you were trying to fix, still doesn’t get enough.
This is common in the older Vancouver and Burnaby housing stock with ducts that were added or extended over the decades. The fix is not more capacity. It’s the right capacity with ductwork that can carry it, and sometimes it’s a ductless head in the problem room instead. The mechanism here is the same one that applies to gas equipment, which we covered in why you shouldn’t oversize a furnace.
What going bigger costs you before it ever runs
Two things, and neither shows up in the comfort conversation.
The larger unit costs more up front. You’re paying for capacity that the room will never ask for. Our starting prices are published, so you can see where a system like this starts before anyone comes out.
The other one catches people out. Larger equipment draws more current, which means a larger circuit and breaker, and in some houses a panel that can’t accommodate it without work. Plenty of homes across the North Shore, East Vancouver and Ladner are running older electrical service that’s already close to full. Adding a heat pump that’s twice the size it needs to be can turn a straightforward install into a panel discussion. If you’re coming off electric baseboards, the panel question is worth settling early anyway, since those homes may qualify for the BC Hydro Home Renovation Rebate. Program amounts and eligibility change, so check BC Hydro’s own pages rather than any figure a contractor quotes you.
What to do with the two quotes in front of you
For a ductless install like this one, ask each company what heat load they used for that bedroom, and what the selected head’s minimum output is. Those are two specific numbers, and any company that sized the job properly can give you both without hesitating. A quote that came from a rule of thumb about room size can’t. That question fits a ductless job, where each head is sized to a room. A like-for-like furnace or boiler replacement is sized differently, from the system already in the house, so it isn’t the test to apply there.
If the answers don’t line up, or nobody will show you the calculation, that’s the information you were looking for.
On a ductless system like this one, we size each room from its own heat load before we recommend equipment. If you’d like a second set of numbers to compare against, book a free, no-obligation assessment or call us at (778) 800-8868. Bring the other quote. We’d rather talk about why the numbers differ than ask you to take ours on faith.