Three weeks a year, Puget Sound gets genuinely hot, and every summer the same question lands in our inbox: we already have a furnace and ducts — can't you just add air conditioning to that?
Usually, yes. The furnace already owns the two expensive pieces of a cooling system: the blower that moves the air and the duct network that delivers it. Adding AC means bolting on the cooling half. Here's exactly what that involves, what it costs, and the honest cases where it's the wrong move.
What actually gets added
A "furnace add-on AC" — the trade calls it a split-system add-on — has four parts:
- Outdoor condensing unit. The box in the side yard with the fan and compressor.
- Indoor evaporator coil (A-coil). Sits in a sheet metal plenum directly on top of your furnace, in the supply air stream.
- Refrigerant line set. Two insulated copper lines running from the outdoor unit to the coil, plus a condensate drain line and, in a lot of Washington basements and crawl spaces, a condensate pump.
- Electrical and controls. A dedicated 240V circuit, an outdoor disconnect, and a thermostat with a cooling stage wired to the furnace control board.
Your furnace's blower does the air moving in summer. That's why the furnace's condition and capability matter so much to whether this works well. Full scope and scheduling live on our AC services page.
What it costs in 2026
| Job | Standard market | Affluent market |
|---|---|---|
| JobAdd central AC to existing furnace | Standard market$5,000 – $12,000 | Affluent market$6,000 – $15,000 |
| JobFurnace and AC replaced together | Standard market$10,000 – $16,000 | Affluent market$11,500 – $19,200 |
| JobDucted heat pump (heating + cooling) | Standard market$12,000 – $18,000 | Affluent market$13,800 – $24,300 |
| JobDuct modification / replacement | Standard market$3,000 – $8,000 | Affluent market$4,000 – $10,000 |
Where a specific home lands in the AC range depends on tonnage, efficiency tier, how far the line set has to travel, whether a new circuit is needed, and whether the plenum has to be rebuilt to accept a coil. Prices are installed — labor, materials, permit, and startup.
Want a number without waiting for a visit? Run your details through the instant estimate tool.
The five things that decide whether your furnace can take AC
1. Blower capability
Cooling needs roughly 350 to 400 CFM of airflow per ton. Heating needs less. A furnace with a variable-speed ECM blower handles the switch easily. An older single-speed PSC blower sometimes can't deliver cooling airflow at all, or can only do it at a noise level you'll hate. We check the furnace's rated airflow against the cooling load before we quote a tonnage.
2. Room for the coil
The evaporator coil needs vertical clearance above the furnace cabinet. Tight utility closets, low-ceiling basements, and horizontal furnaces wedged into crawl spaces sometimes require custom transition ductwork. That's fabrication work, and it moves the price.
3. Ductwork sized for cooling
This is the one people skip and regret. Undersized returns are common in 1960s–1980s Puget Sound homes because those systems were designed for heating only. Add cooling to strangled return ducts and you get long run times, a whistling register, and in the worst case an evaporator coil that ices over. If your ducts are the real bottleneck, we'll say so rather than sell you a condenser that can't perform.
4. Electrical
Most outdoor units need a dedicated 240V circuit at 20 to 40 amps plus a service disconnect. Homes with a 100-amp panel or larger usually have room. Full panels and old fuse boxes are where extra cost and an electrician enter the conversation.
5. The furnace's remaining life
The deciding factor. A new AC coil, line set, and condenser are matched to that furnace's blower. If the furnace dies in three years, part of that labor gets paid twice.
Why a heat pump is often the better buy for the same money
A ducted heat pump is, mechanically, an air conditioner that can also run backwards and heat your house. Same outdoor unit style, same indoor coil, same ducts. The difference is a reversing valve and controls.
So compare honestly:
- Add AC to an aging furnace: $5,000–$12,000, and you still own an aging furnace.
- Ducted heat pump: $12,000–$18,000 standard, and heating and cooling are both new, with a 10-year parts and labor warranty on the whole system.
Rebates widen the gap. PSE's incentive programs are written for qualifying heat pump installations — up to $4,400 toward a qualifying heat pump when it replaces an existing electric or fossil-fuel system, with the exact amount depending on what you're replacing and what you install. Standalone AC generally isn't in that lane. Programs change, so we confirm your current eligibility and handle the paperwork. Two things worth knowing: the federal 25C tax credit ended December 31, 2025, and starting April 2, 2026 PSE rebates require a PSE-certified installer. Varsity is PSE Certified. The details are in our PSE heat pump rebates breakdown.
Western Washington also happens to be excellent heat pump country. Winter design temperatures across our service area run 17°F to 29°F — the range where modern equipment delivers full comfort efficiently. We laid out the running-cost side in heat pump vs. gas furnace.
When standalone AC really is the right call
We say this to customers regularly, and it costs us the bigger ticket:
- The furnace is newer, efficient, and healthy. Ripping out a five-year-old 96% AFUE furnace to electrify makes poor financial sense.
- You want cooling this summer on a tighter budget, and the ducts are in good shape.
- You have cheap gas heat you're happy with and no interest in changing how the house heats.
- The home is a rental or a near-term sale where the payback window is short.
There's also a middle option: pair a heat pump with your existing furnace as backup — a hybrid. Reusing a healthy furnace as the backup stage typically lands in the same $12,000–$18,000 range as a standalone ducted heat pump, and the system picks whichever fuel is cheaper for the conditions.
Permits, and what a real estimate looks like
Adding AC is a mechanical permit job in every city we serve, and adding a new 240V circuit usually pulls an electrical permit too. We pull them. Unpermitted work can complicate an insurance claim and shows up in Washington's seller disclosure form.
On an estimate visit we measure the home and run a load calculation, check the furnace's blower rating and cabinet clearance, inspect return duct sizing, look at the panel, and walk the outdoor unit location — away from bedroom windows, off the property line, on a solid pad. You get written Good / Better / Best options with real numbers. It's a sales appointment, but it's not a pressure appointment; if the answer is "your ducts first" or "keep the furnace and add AC," that's what the proposal will say.
Most add-on AC installs are done in one day. Book at get an estimate or call (253) 785-4622, Monday through Friday, 8 to 6.