Quick answer
A boiler replacement takes one to three working days. A like-for-like cast-iron swap is usually one long day, with heat back that evening. Converting to a wall-hung condensing boiler takes two to three days because the near-boiler piping, the venting and the condensate drain are all rebuilt. Draining, flushing and purging air from every zone add hours to either job.
A boiler replacement takes one to three working days, and the spread comes from what is piped to the boiler more than from the boiler itself. Swapping one cast-iron boiler for another in the same spot is a long single day. Moving to a wall-hung condensing boiler means rebuilding the piping, the venting and the drain around it, and that is two to three days. Either way, water has to leave the system and come back without air in it, which is where hydronic jobs lose hours a furnace swap never does.
Like-for-like cast-iron swap: one long day
A cast-iron boiler replaced with a similar one is usually done in a day, with heat back that evening. The system is isolated and drained, the old boiler is cut out, and the new one is set on the same pad and tied to the existing supply and return headers, the flue connector, the gas line and the controls. The slow parts are physical: a cast-iron sectional weighs several hundred pounds and often has to be broken down to get it up the basement stairs. Then comes the refill, the purge and a combustion test before the crew leaves.
Conversion to a wall-hung condensing boiler: two to three days
A conversion takes longer because almost nothing from the old installation is reused. The work that adds the time:
- Near-boiler repiping: a condensing boiler needs a minimum flow through its heat exchanger, so it gets its own primary loop with the house zones on a secondary loop, joined by closely spaced tees or a low-loss header, plus new circulators.
- New venting: the old boiler used the chimney, and the new one vents through a side wall or the roof in plastic pipe certified to ULC S636, with a terminal that has to clear windows, the gas meter and the snow line.
- Condensate: the boiler produces acidic water that runs through a neutralizer to a floor drain, or to a condensate pump where no drain is close.
- Orphaned water heater: if a gas water heater is left alone on the old chimney, the flue is now too large for it and may need a liner.
- System protection: a magnetic dirt separator, a new expansion tank, a backflow preventer and fill valve, and an outdoor sensor wired to a shaded wall for the reset control.
The hydronic time sinks: draining, flushing and purging air
Water handling is what makes a boiler job slower than its parts list suggests. Pre-war brick houses that began as gravity systems have large-bore iron mains and cast-iron radiators holding a great deal of water, and the drain valve may not have been opened in decades.
- Draining: an hour or more on a big system, longer if the drain valve is seized or clogs with sediment.
- Flushing: the black sludge in old systems is iron oxide, and it will plug the narrow passages of a condensing heat exchanger. The loops are flushed until they run clear, sometimes with a cleaner circulated in the days before; see hydronic system flush.
- Refilling and purging: the system is filled to about 12 psi cold for a two-storey house, then water is forced through one zone at a time to push the air out, and every radiator is bled, top floor last.
- Surprises: a radiator valve that will not reseal, a leak at a joint disturbed by the work, or old pipe wrap that looks like asbestos. Suspect insulation on pipe that has to be cut stops the job until a qualified abatement contractor has dealt with it.
Replacing a boiler in heating season: temporary heat and the return visit
In winter the plan is built around the hours without heat. On a one-day swap the house coasts through the day and the boiler is firing again by evening. On a conversion, the new wall-hung boiler usually goes on a different wall, so it can be mounted, vented and piped while the old one keeps heating, and the cutover is held to a single day.
For that cutover day, a few electric space heaters on separate circuits keep the rooms with plumbing above freezing, and that plan belongs on the quote, not in a conversation on the morning of the job. Expect a return visit a week or two after start-up. Dissolved air comes out of fresh water as it heats, collects at the top-floor radiators and makes them gurgle or go cool at the top; the bleed, a pressure check and a second combustion reading close out the job. How to bleed a radiator covers what you can safely do yourself in between.
What decides whether your boiler job is one day or three
- Same boiler type in the same location, chimney in good shape: one day.
- Floor-standing chimney-vented boiler to wall-hung condensing boiler: two to three days.
- Adding an indirect water heater, or switching to a combi boiler that also makes domestic hot water: add half a day to a day.
- More than two or three zones, or zone valves and circulators replaced at the same time: more purge time.
- Permit and inspection: where the municipality requires a permit for the change, the inspection is booked after start-up, so it does not keep the heat off.
Boiler replacements are priced from a site visit because the piping, the venting and the number of zones differ in every basement; what a new boiler costs in Ontario explains what drives the number. If the question is still whether the old boiler can be saved, a diagnostic visit is $90–$150 and is folded into the repair if you go ahead. See boiler repair and replacement, find related questions under heating answers, or ask us to look at your system.