Most of the difference between two furnace quotes is not the appliance, it is what the appliance needs where it is going. A modern gas furnace does not use the old chimney, an oil furnace brings a tank and a filter with it, and an electric one is an electrical job as much as a heating one. Reading a quote by fuel is the quickest way to see which parts of the work somebody has left out.
Gas: the flue is the line most quotes hide
A condensing furnace vents through plastic pipe out of a wall rather than up the existing flue, and it produces condensate that has to be drained and sometimes neutralised. Where the old furnace shared a chimney with a water heater, taking the furnace off it can leave that appliance venting badly, which is a safety matter rather than a detail. Ask where the new flue runs, where the condensate goes, and what happens to whatever else was on that chimney.
Oil furnace installation adds the tank, the line and the filter
Oil brings its own infrastructure: the tank and its condition, the supply line, a filter and a nozzle sized to the burner, and a service regime the owner has to keep up. On a replacement, the question worth asking is whether the existing tank and line are being reused and what warranty covers them, because a new burner fed from an old tank picks up whatever is in the bottom of it.
Heating furnace installation on electricity is an electrical job
Electric heat is the cheapest appliance to buy and the most expensive to run, and its installation cost sits mostly in the panel: a large dedicated circuit, sometimes a panel upgrade, and staged elements sized to the load. Where the house has the capacity, it is a straightforward swap; where it does not, the electrical work can cost more than the furnace.
Sizing, and why how much to replace a furnace varies so much
A load calculation, not the plate on the old unit, decides the size. Oversizing is the common fault: the furnace short cycles, the house is unevenly warm and the equipment wears, and it happens because matching the old output is quicker than measuring the house. Two honest quotes for the same house can differ by a lot when one has priced the duct changes the correct size needs and the other has not.
The permit, the commissioning and the paperwork
Replacing a furnace normally needs a permit and an inspection, and a firm that treats that as optional is telling you something. Commissioning is the other half: gas pressure and combustion figures measured and recorded, airflow checked, and the results handed over. Efficiency ratings are what the label promises; the commissioning record is what the installation actually delivered.
Questions people ask about gas furnace installation
Does a gas furnace installation need a new flue?
A condensing one does. It vents through plastic pipe out of a wall rather than up the chimney, and it needs a condensate drain, so the old flue is abandoned or reused by whatever else was on it.
Is oil furnace installation more expensive than gas?
The appliance is comparable and the surroundings are not. Oil brings a tank, a supply line and a filter into the scope, and on a replacement whether those are being renewed is the line to look for.
How much does it cost to replace a furnace?
It follows the work around the appliance rather than the appliance: the flue and condensate on gas, the tank on oil, the panel on electric, plus any duct changes the correct size needs. That is why two quotes on one house can differ so widely.
Do I need a permit?
Almost always, and it is worth insisting on. The inspection is the only independent check that the venting, the gas work and the electrical are right, and it costs a fraction of what putting them right later does.
How much is a furnace replacement once the flue work is counted?
On a condensing gas furnace the flue, the condensate and any duct change are a real part of the total rather than extras, which is why a quote that names only the appliance is not a price.
What does the cost of new furnace and ac together include?
One mobilisation, one permit, one set of duct changes and a matched pair, which is why doing both at once is normally cheaper than a year apart. Ask for the phased price beside it and compare.