What Boiler Replacement Actually Costs in Denver
Search this question and you'll get a range. It will be wide, it will be confident, and it will be close to useless — because every one of those pages is written by someone who has never seen your basement.
I'm not going to give you a better range. I'm going to tell you what actually moves the number, so that when you're holding two or three quotes you can see why they disagree, and which one is telling you the truth.
The boiler is not the expensive part
This is the thing that surprises people most.
The appliance itself is a line item. On a lot of the jobs we do, it isn't even the largest one. What determines whether a boiler replacement is a straightforward day or a week-long rebuild is everything the boiler connects to — and none of that is visible in a phone quote.
So when two quotes on the same house are far apart, it's almost never that one contractor found a cheaper boiler. It's that they've scoped two different jobs.
What actually moves the number
The condition of the near-boiler piping. The few feet of pipe immediately around the boiler do most of the hydraulic work in the system. If the existing arrangement is sound, the new unit ties in. If it's a stack of fixes from four different decades — and in Denver's older housing stock it often is — that section gets rebuilt, and rebuilding it is real labor.
Whether the venting route exists. A standard-efficiency boiler goes up a chimney. A condensing boiler vents out a sidewall in PVC, at a specific pitch, a specific distance from windows and openings and property lines. On some houses that run is ten feet through a rim joist. On others it crosses a finished basement, or the only compliant exit is on the far wall.
Whether the water heater gets orphaned. If the old boiler shared a masonry chimney with the gas water heater, taking the boiler off that chimney can leave the water heater unable to draft — which is a carbon monoxide problem, not a comfort problem. Solving it means lining the chimney, relocating the water heater, or converting to an indirect tank off the new boiler. Any of those is a meaningful part of the job, and a quote that doesn't mention it hasn't accounted for it. We wrote about this one separately, because it comes up constantly.
The gas line. A modern high-input boiler may need more gas than the existing line can deliver, especially if a range, a dryer and a water heater are already on it. Upsizing a run from the meter is straightforward work but it is not free, and it's a common source of "we found something" mid-job when nobody checked first.
The distribution system you already own. Cast iron radiators on a converted gravity system behave completely differently from a 1990s baseboard loop. Getting a condensing boiler to actually condense on high-temperature emitters can require outdoor reset, primary/secondary piping, sometimes a buffer tank. That's design work and it's parts, and skipping it is how people end up with 95% equipment running in the mid-eighties.
Controls and zoning. How many zones, whether they're valves or pumps, whether the existing thermostats can talk to a modulating boiler at all.
Permit and inspection. Denver has its own building department; the surrounding jurisdictions have theirs. It's a normal cost of doing the work legally, and it should appear on the quote rather than being absorbed quietly or skipped entirely.
The one number that changes everything, and it's free to get right
Before any of the above matters, somebody has to answer: how much heat does this house actually need?
Most of the boilers we replace in the Denver metro are two to three times larger than the building requires. That happens because each replacement is sized off the last one, and the padding compounds down the chain.
Real numbers: an older Denver brick home with some weatherization runs around 40 BTU per hour per square foot. Newer, tighter construction is closer to 25. Not the 60 or 80 that a rule-of-thumb swap quietly assumes.
Getting this wrong costs money in both directions. Undersized, the house doesn't recover on the coldest week. Oversized — far more common — the boiler short-cycles, wears itself out early, and never gets its return water cold enough to condense, so you paid for efficiency you will never see.
A room-by-room heat loss on your actual house takes a couple of hours and it is the cheapest part of the entire project. It is also the part most likely to be skipped. More on why oversizing is the default failure here.
Add altitude to that. At 5,280 feet, combustion equipment loses capacity, and the manufacturer's derate table has to be applied — once. Not skipped, and not stacked on top of a load that was already padded.
How to read the quotes you're holding
Four questions. They'll separate the quotes faster than comparing totals.
1. "What heat loss number did you use, and how did you get it?" If the answer is a square-foot rule or the old boiler's nameplate, the rest of the quote is built on a guess.
2. "What happens to the water heater?" If the old boiler shares a chimney and the answer is nothing, that's not a cheaper quote. It's an incomplete one.
3. "Where does it vent, and what's the route?" Vague here usually means it hasn't been walked.
4. "What's the altitude derate on this model at this address?" A specialist has the table open. It's a fair question and it isn't a gotcha.
A quote that's meaningfully cheaper than the others is usually cheaper for a reason you can find by asking these. Sometimes the reason is legitimate — a genuinely simpler scope. Often it's that something on this list hasn't been counted yet, and it will turn up as a change order in week two.
Standard-efficiency is sometimes the right answer
A condensing boiler only earns its efficiency if the system can return water cold enough to condense. On some older Denver high-temperature radiator systems, it can't — not without work that costs more than the efficiency will ever return.
On those houses a standard-efficiency cast iron boiler is the honest recommendation, and it usually costs less up front. We'd rather say that than sell the more expensive unit and let you discover the problem in January.
Where our number comes from
We don't quote boiler replacements over the phone, and we don't publish a price list, because both would require pretending the six things above don't exist.
What we do: come look at the mechanical room, the distribution, the venting and the gas line. Run the heat loss on your house. Then give you a design and a number with the assumptions written down — the heat loss figure, the derate we applied, the venting route, what happens to the water heater — so you can hold it next to anyone else's quote and see exactly where they differ.
If that comparison sends you to another contractor, that's a fine outcome. It's better than either of us finding out in December.
📞 (303) 908-5658
— Ryan Van Inwegen, ByDesign Hydronics & Air, Littleton, CO
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