A homeowner asked us this last week, standing in a Burnaby basement next to a boiler that had heated the house for a long time. We’d proposed a 90,000 BTU unit. He wanted the 120,000, on the reasoning that more is safer and it isn’t much more equipment.
It’s a fair instinct. It’s also usually the wrong call on a hydronic system, and the reason is specific enough that it’s worth walking through properly.
The strongest evidence we have is the boiler already in your basement
On a like-for-like boiler replacement, we’re not starting from a blank page. Your house has been running a heating experiment for years, and you have the results.
So the questions we ask on the walkthrough are the sizing method:
- Did it keep up? On the coldest few mornings of the year, did the house hold temperature, or did it fall behind?
- Did any rooms run cold? Which ones, and were they always cold, or only sometimes?
- Did it run constantly in January, or did it cycle on and off?
- Did you ever run out of hot water, if it’s a combi unit?
If the answers are “it kept up”, “no cold rooms”, and “it cycled”, then the house has already told us what it needs, and it needs less than you think. That’s not a guess and it’s not us cutting corners. It’s the only real-world data anybody on this job has.
Your old boiler is a reference point, not a target
Here’s the part that trips people up. The number on the old unit’s plate is not a measurement of your house. It’s a record of a decision somebody made, possibly decades ago, possibly with the same “go bigger to be safe” reasoning you’re using now.
Plenty of the boilers we pull out were oversized when they went in. If we match that number, we inherit the mistake. If we beat it, we compound it. So we read the old unit as evidence of how the house behaved, not as a specification to hit.
That’s also why “the new one is the same size, so it must be right” isn’t the reassurance it sounds like.
Oversizing a boiler doesn’t feel like anything, which is why nobody catches it
An oversized heat pump announces itself. It short cycles, the house feels clammy in the shoulder season, and you notice. We’ve written about that in why you shouldn’t oversize a heat pump.
An oversized boiler is quieter about it. Your radiators get hot, your floors get warm, the house hits temperature. Comfort is mostly fine. What you lose is the efficiency you paid for, and there’s no moment where you feel that happening.
Here’s the mechanism. A high-efficiency boiler is called a condensing boiler because it pulls extra heat out of its own exhaust, and doing that requires the water coming back from your radiators or floor loops to be cool. When the return water is cool enough, moisture condenses out of the flue gas and that heat goes into your house instead of out the vent. When the return water is hot, that doesn’t happen, and the boiler runs closer to the efficiency of an old non-condensing unit while still carrying the price of a new one.
An oversized boiler blasts the loop temperature up fast, gets its return water hot, and rarely spends time in the condensing range. The efficiency is on the label. It just isn’t delivered in your house.
Modulation and outdoor reset, in plain language
Two features do the actual work on a modern boiler, and both of them get worse when the unit is too big.
Modulation means the boiler can throttle its burner rather than being simply on or off. But the range is proportional. A bigger boiler’s lowest firing rate is higher than a smaller boiler’s lowest firing rate. In Metro Vancouver, where our winter design temperature is around -7°C in Vancouver and Richmond and gets to roughly -10°C toward the eastern and higher parts of the region (National Building Code of Canada 2020, Appendix C), your boiler spends almost the entire heating season nowhere near full output. A unit sized for a house colder than yours may not be able to turn down far enough to match a mild November afternoon, so it fires, overshoots, and shuts off. Repeatedly.
Outdoor reset means the boiler reads the outdoor temperature and lowers the water temperature it sends out when the weather is mild. That’s the feature that keeps return water cool and keeps the boiler condensing. It works beautifully on a correctly sized system running low and steady. It works much less well on a unit that has to fire hard or not at all.
The cold room is almost never a capacity problem
This is the objection worth taking seriously, because usually there is a real complaint behind the request for a bigger boiler. One room that never gets warm. A morning where the house feels behind.
On a hydronic system, that’s usually about how heat gets distributed, not how much heat the boiler can make:
- Zoning and controls. One thermostat governing a whole house with a north face and a south face will always leave somebody unhappy. Adding capacity doesn’t fix that. Splitting the zone, or correcting a reset curve that’s set too low, might.
- Emitters. A room with one small radiator on an exterior wall is limited by that radiator, not by the boiler.
- Air and circulation. Air trapped in a loop or a radiator, a seized zone valve, a pump that isn’t moving what it should. These present exactly like “not enough heat”.
- Slab response. A mid century radiant slab is slow by design. It doesn’t recover fast from a deep setback, and no amount of extra burner fixes that. What fixes it is not setting the thermostat back much in the first place.
Which leads to the point that ends most of these conversations.
Your radiators and floor loops set the ceiling, not the boiler
A hydronic system can only put into your rooms what the emitters can give off. Cast iron radiators in a Vancouver character home, baseboard convectors in a New West walk-up, in-floor loops in a 1990s or 2000s house: each has a maximum output at a given water temperature, and that’s a fixed property of the pipe and the metal in your walls and floors.
Put a bigger boiler behind the same radiators and the radiators don’t get bigger. You’ve raised the ceiling on a room whose ceiling was never the boiler.
That’s also why we look hard at what’s in the floor before quoting a hydronic replacement. In this region, a lot of in-floor systems from roughly the late 1970s to the mid-1990s were run in Poly-B, and that changes what a boiler install has to include. We cover that in what Poly-B is and what we do about it, and on those jobs the Poly-B Protection Package is a required part of the install, not an option.
When we do run a full heat load
We don’t run a full heat load calculation on every boiler replacement, and we’d rather say that plainly than imply otherwise. On a straightforward swap, the existing system is better evidence than a fresh set of assumptions about your insulation.
We do run one when the house has changed or is about to, specifically:
- Windows, insulation, or an envelope upgrade since the boiler went in
- An addition, or a finished basement that wasn’t heated before
- A suite conversion
- A system that never kept up, so the history is telling us the old unit was wrong
- A change of fuel or system type
If any of those apply to you, the old boiler stops being useful evidence and we go and measure. What a heat load report actually is covers what that involves.
So, the 120K instead of the 90K?
If your existing boiler kept the house warm and cycled on and off through January, going bigger buys you a higher minimum firing rate, less time in the condensing range, and no additional comfort. That’s a worse system for more money.
If it never kept up, tell us that, because it changes the whole conversation and it’s the case where a full heat load is worth doing.
If you’re getting quotes on a boiler replacement, ask each company what evidence they used to pick the size. “It’s what’s in there now” and “we always do this size” are both answers you can weigh. If you want ours, our starting prices are published, and you can reach us at (778) 800-8868 or book a free, no-obligation assessment.