A furnace installation starts with the BTU input the house actually needs, not the number on the old data plate. The old unit and its flue come out, the new cabinet is set and connected to the existing return and supply plenums, the gas line is reconnected and leak-tested, and the venting is rebuilt to suit the new appliance. Gas manifold pressure is then regulated to the manufacturer's specification and the result verified by measuring the temperature rise across the furnace before the job is handed over.
How a furnace is sized before installation
Furnace capacity is measured in BTU of input, and the units we install run from 60,000 BTU in a small house up to 120,000 BTU in a large one, with 70,000, 80,000, 90,000, 100,000 and 110,000 BTU covering most of the work in between. Picking the number off the old data plate is the shortcut that causes trouble, because the old unit may well have been wrong.
- An oversized furnace short cycles. It satisfies the thermostat before the house has evened out, so the rooms furthest from the equipment never catch up, and the burner and the heat exchanger take the punishment of constant starting and stopping.
- An undersized furnace runs continuously on the coldest nights and still loses ground.
- The duct system is part of the sizing. A furnace can only deliver what the duct can carry. Where the distribution is the limit, that gets addressed too — see ductwork repair, sealing and venting.
- The cabinet has to fit. Furnace cabinet widths vary, and 21-inch cabinets are a size we fit regularly where the existing alcove or closet dictates it. We have also framed and drywalled a closet with bifold access doors around an exposed furnace as part of a remodel.
80% or high-efficiency: choosing a furnace
The efficiency rating decides how the appliance vents, and the venting decides a good deal of the installation.
| Type | How it vents | What the installation involves |
|---|---|---|
| 80% efficiency, single stage | Metal flue pipe into an existing chimney or vent stack, often shared with a gas water heater | The most common replacement we make. Flue pipe is renewed, and 4-inch and 3-inch pipe with a water heater tie-in is typical |
| 90% to 92% efficiency | Sealed combustion through PVC, vented out through a wall | New 2-inch PVC venting has to be run and terminated correctly, and a condensate drain added |
| 96% to 97% efficiency | PVC, condensing, with a condensate drain | Same venting work, with an EZ trap and drain line for the condensate the appliance produces |
| Variable speed, two stage | By efficiency rating as above | Quieter, steadier heat and better distribution. We have installed a 90% variable speed two-stage unit at 120,000 BTU |
| Oil fired | Metal flue | Different fuel handling entirely; the oil lines are bled after the unit is set |
Brand matters less than the installation, but for the record the equipment we standardize on is Bryant and Carrier, and we regularly remove Trane, Rheem, Comfortmaker and Ruud units that have come to the end of their lives.
What a furnace installation includes, step by step
- Shut off and disconnect the gas, then remove the old furnace and the old flue pipe and haul them away.
- Deal with access. On one job the water heater had to come out before the equipment could be moved in and out of the mechanical room at all.
- Set the new cabinet and reconnect the return and the supply plenum. Where the new cabinet is a different size, a sheet metal transition is fabricated so the duct meets it squarely.
- Reconnect the gas line and leak-test it. Drip leg, shutoff valve and flexible connector included.
- Rebuild the venting. New flue pipe on an 80% unit, or new PVC venting on a condensing unit. Where the chimney connection has to be redone to suit the new appliance, that is part of the job.
- Reinstate the electrical and the thermostat wiring, and set up the thermostat.
- Regulate the gas pressure to the manufacturer's specification, run the appliance and verify the temperature rise.
Where the cooling side is being replaced at the same time, the coil sits on top of the furnace and the two are done as one job — the refrigerant side of that work is described on air conditioning installation.
Venting and flue work on a furnace installation
Venting is where corner-cutting hides, because nobody looks at it after the panel goes back on. It is also the part of a heating installation with the most direct bearing on whether combustion products stay out of the house.
- The flue is renewed, not reused blindly. New flue pipe goes on with the new appliance. On a replacement where the existing flue and chimney connection did not suit the new furnace, the connection was redone as part of the scope.
- Shared venting is checked. Many houses here vent a gas water heater into the same stack as the furnace. When a high-efficiency furnace moves to PVC and leaves that stack, the water heater is left venting alone into a chimney that may now be oversized for it. That has to be addressed, not ignored.
- Condensing furnaces get proper PVC. Two-inch PVC venting, correctly pitched and terminated.
- Roof and exterior penetrations get sealed. On a roof leak call, the leak path proved to be failing vent boots and deteriorated caulking around the furnace vent pipe, which was stripped and resealed with heat-resistant exterior caulking.
Gas line and gas pressure on a new furnace
A new furnace is only as good as the fuel supply feeding it, and this is a recurring problem with work done by others.
- The supply line has to be sized for the appliance. On one full system replacement the original gas line was judged too small for an 80,000 BTU furnace before the equipment went in, and was addressed as part of the scope.
- A regulator is not a substitute for adequate pipe. We were called to install a gas line regulator and flagged that it might not be sufficient to operate the appliance, because the company that installed the homeowner's new furnace appeared not to have calculated the correct BTU gas requirement against the pre-existing line.
- Manifold pressure is regulated at commissioning. Every furnace we install has its gas level set per manufacturer requirements, and the gas pressure checked as part of start-up.
- The connection is rebuilt properly. Drip leg, tee, nipple, shutoff valve and braided flexible connector. Where we have repaired gas leaks at a furnace, that is exactly the list of parts involved, along with replacing the run of pipe back to the nearest tee.
- Gas work is permitted work. Where new gas piping is run to a furnace and other appliances, that is covered on gas line installation and repair.
Do you need a permit for furnace installation in Virginia?
Yes. A repair does not require a permit; replacing a furnace does. We pull and process the permit ourselves, the work is performed according to the Virginia building code, and a final inspection closes the job out. On permitted work the last payment draw is tied to passing that inspection — which is how it should be, and how we have always done it.
Permit practice also drives items a homeowner would never think to compare between quotes: correct vent sizing and pitch, proper combustion air, a sediment trap and shutoff at the gas connection, condensate handling on a condensing appliance, and electrical connections made to code. If the previous installer skipped them, they get put right rather than reproduced.
How a new furnace is tested before we leave
A furnace that lights is not a furnace that is commissioned. Two measurements settle whether the installation is right:
- Gas pressure. Manifold pressure is regulated to the manufacturer's published figure for that model, and checked rather than assumed.
- Temperature rise across the furnace. We measure the supply air against the return air and confirm the delta-T falls inside the range on the data plate. On one installation that meant verifying roughly 120 degrees supply against 70 degrees return; on another, a 120 over 20 check. A rise that is too high means airflow is restricted; too low usually means the unit is undersized for the duct or the gas is under-set.
Where cooling shares the system, the cooling charge is set by subcooling and the heating verified by delta-T, so both sides of the equipment are proved before the technician leaves. Thermostat setup and calibration finish the visit, and where a homeowner wants a Wi-Fi thermostat fitted with the new equipment, see thermostat installation.
How long does a furnace installation take?
A straight furnace changeout — old unit out, new unit in, gas, flue, plenums and start-up — is usually a single day. What extends it is everything around the appliance:
- A full system where the evaporator coil and outdoor unit are replaced alongside the furnace, which normally runs into a second day.
- New venting on a high-efficiency unit, where PVC has to be routed and terminated through a wall.
- Gas piping that has to be resized, rerouted or replaced back to a tee, and the permit work that goes with it.
- Access problems — equipment behind a water heater, a tight mechanical room, a furnace being enclosed in a new closet, or a crawl space with no working clearance.
- Added equipment: a whole-house humidifier, a media filter or air purification fitted at the same time. See humidifiers and indoor air quality.
What furnace installation costs, and how we price it
We do not quote heating equipment by phone and we do not publish prices. The same model of furnace is a different job in two different houses: BTU input, cabinet width, whether the venting stays metal or moves to PVC, whether the gas line is adequate, what the plenum transition needs, and whether cooling is being done at the same time.
An estimator visits, inspects what is actually there, and gives you a written price for a defined scope before anything is ordered. Permit and processing fees are written into the contract, and the final draw is tied to passing inspection.
