When the Old Furnace Finally Goes… A mechanical question in San Francisco homes.

4 Minute Read

If you’ve spent any time in the garages of old San Francisco homes, (or even have one yourself!) you probably know the scene already.

Somewhere in the garage there’s an old gravity furnace, and running out from it are the familiar sheet metal “tentacles” stretching through the house and feeding those floor grilles scattered around the rooms. In a larger remodel, that system is often one of the first things to go.

The ducting itself is frequently wrapped in old asbestos insulation too, so removal usually starts with abatement before anything new can even be considered.

Then the question inevitably comes up:Working Within the Existing Concrete Frame

Unlike single-family homes, high-rises come with boundaries that can’t be pushed. Plumbing, venting, and drainage are all tied into systems shared with neighboring units. You can’t just move a wall or reroute a drain without affecting an entire column of homes above and below.

That means you work within the framework that already exists. The structure is solid; thick concrete floors and ceilings that give stability, but it also sets the limits. Most walls are framed in steel, not wood, which makes every adjustment more like precision assembly than demolition and rebuild. It’s deliberate work where every fastener, every piece of framing, and every screw placement has to be right.

What replaces it?

Unlike newer homes in other parts of the country, San Francisco houses weren’t designed with modern mechanical systems in mind. Most of them predate air conditioning entirely. Over the decades, systems were added as needed. A furnace upgrade here. A duct run there. Sometimes a radiator system from an even earlier era still quietly doing its job.

By the time a house reaches a full remodel, you’re usually looking at layers of mechanical decisions stacked over generations.

Finding a new heating and cooling approach that works within that environment takes a bit of craft.

Most homes we walk into start from one of a few familiar places.

Older gravity furnaces or early forced-air systems are still pretty common, especially in garages or crawlspaces. Floor furnaces and wall heaters show up frequently in smaller homes. In some Victorians and early 1900s houses, you’ll still find hydronic or steam radiators.

And in many cases, ductwork was added later during smaller remodels. It tends to snake through framing wherever space allowed at the time.

The systems technically work, but they rarely resemble a well-planned mechanical strategy.

What Clients Are Asking For Now

Even though San Francisco still has a mild climate, comfort expectations have changed. We hear more questions about cooling, better airflow, and energy efficiency than we used to.

A few systems come up regularly:

Heat pumps
These have become a popular option because they provide both heating and cooling and support electrification goals.

Mini-split systems
Helpful when full duct runs are difficult and costly to route through an existing house.

Radiant heat
Often added in kitchens, bathrooms, or basement spaces where comfort underfoot matters.

Full duct redesigns
When the house is already opening up for a remodel, sometimes it makes sense to rethink the whole air distribution strategy rather than trying to work around older duct paths.

The technology itself usually isn’t the difficult part. The question is where everything can physically go.

Before getting too deep into mechanical options, we often step back and ask another question: Does the home have the insulation and windows to make the new system worthwhile?

Many older San Francisco houses have little to no wall insulation and still rely on single-pane windows. Heat escapes quickly in the winter and warm air can build up inside during the increasingly hot fall months.

In those situations, improving the building envelope can sometimes have just as much impact on comfort as upgrading the mechanical system itself.

Adding insulation where possible, tightening up air leaks, or planning for future window upgrades can dramatically improve how a home performs. When those improvements work together with the mechanical system, the result is usually a more comfortable and efficient house overall.

It’s another reason these conversations tend to work best when the house is looked at as a whole system rather than as a collection of separate upgrades.

Mechanical Systems Are a Bit Like the Veins of the House

Heating and cooling systems move through a home the same way veins move through a body. Air, refrigerant, water, and power all need pathways that run quietly behind the architecture.

Once a new system is introduced, those pathways often need to be updated or rerouted. And that’s where the work begins to expand.

Mechanical upgrades typically involve coordination with several other parts of the house:

Electrical systems for heat pumps and condensers
Plumbing for condensate drainage or radiant loops
Framing adjustments to create routes through tight structural bays
Roofing and exterior work where penetrations need proper flashing and waterproofing

What starts as a mechanical upgrade often turns into a small choreography of trades working together.

Most of the problem-solving happens when we start tracing the routes.

Older San Francisco homes tend to have tight framing, modest ceiling cavities, and architectural features everyone would like to keep intact. Crown mouldings, plaster ceilings, beams, original proportions.

That’s where we start running into moments like:

A duct run conflicting with framing or stair geometry
A mini-split line set landing in an awkward place on the façade
Ceiling drops that would change the feel of a room
Mechanical equipment competing with already tight floor plans

None of this is unusual. It just means the systems need to be thought about early enough that the architecture doesn’t end up paying the price.

Helping Homeowners Think Through the Options

Part of our role early on is helping homeowners understand what the different approaches actually mean for their home.

Some solutions are fairly light-touch. Others require opening up more of the structure to create proper pathways.

I often walk clients through both ends of that spectrum. Sometimes there’s a minimally invasive route that preserves most of the existing structure. Other times a more comprehensive upgrade makes sense if the goal is better long-term comfort and efficiency.

Having that conversation early helps people understand the trade-offs before design decisions and intended investments are fully locked in.

One thing we’ve learned over time is that mechanical systems behave best when their pathways are considered early.

Even a short walkthrough during schematic design can help identify routes that preserve ceiling heights, protect historic details, and avoid surprises later during construction.

If you’re exploring heating or cooling upgrades in an upcoming San Francisco project, we’re always happy to take a look at possible mechanical pathways early in the process.

Sometimes a quick conversation is all it takes to help the systems support the architecture instead of competing with it.

Zach with Heirloom Builders
San Francisco