Furnace Repair in Sunnyvale, CA
Try four things before you call me. Check that the switch on the wall next to the furnace is on, because they look like light switches and they get flipped. Check the thermostat batteries. Pull the filter and hold it up to a light. And if your flue is white plastic pipe rather than metal, look for water sitting in the little trap on the side of the furnace.
There is a fifth one specific to this city. If your heat comes from hot water rather than a burner, and in Sunnyvale that is a real possibility, the fault may not be in the heating equipment at all. If the taps are cold too, you have a water heater or a boiler problem, and that is a plumbing call, not mine.
Past that, call me. I am John. 25 years in the trade, running this shop since 2017, and I do the work myself. 408-691-5940, days, nights and weekends.
Why furnace repair in Sunnyvale is its own job
Gas furnaces are the most common thing on my own Sunnyvale equipment records, so a good share of the no heat calls here are what you would expect. What is not expected is the third item on that list. Hydronic fan coils. That is my own book of work rather than a count of the city.
A hydronic fan coil is heat without a fire. Hot water arrives from a boiler, or in some setups from the water heater, runs through a coil, and a blower pushes house air across it. There is no burner, no gas valve, no flame sensor, no igniter, no inducer motor and no flue on the unit. The standard furnace diagnostic every repair company runs, the one that begins and ends with cleaning a flame sensor, does not apply to your machine at all. If somebody has been out and told you they cleaned your flame sensor on a hydronic fan coil, they were working on a furnace you do not own.
None of that is an accident of the housing here. Sunnyvale has real Eichler blocks with flat roofs, no attic and radiant heat in the slab, and a very large condo and apartment stock. Neither is a good home for a conventional gas furnace, because there is no attic to put it in and often nowhere sensible to run a flue.
The brands I see most on my Sunnyvale list are Carrier, AllStyle and Payne. AllStyle is worth naming because it is a fan coil and air handler brand rather than a household name, and its parts get ordered rather than picked up.
What actually fails on a Sunnyvale furnace
On the gas furnaces, two failures account for most of my Sunnyvale no heat calls.
A dirty flame sensor. The furnace lights, runs a few seconds, shuts the gas off and tries again, usually three times before it locks out. That sensor is a thin steel rod standing in the flame to prove the flame is really burning. A film of oxide on it drops the signal below what the control board accepts, and the board shuts the gas down. That is the safety working, not failing.
A blocked condensate trap on a high efficiency furnace. If your flue is white plastic pipe instead of metal, the furnace makes water every time it runs and that water has to get out. When the trap or the drain hose plugs, the pressure switch cannot close and you get no heat with nothing obvious to look at. In a closet or a garage where that drain travels a long way, it comes up often.
On the hydronic fan coils, set all of that aside. There is no flame to prove and no combustion water to drain. What fails is the blower motor or its capacitor, which leaves the unit humming or silent while the water side is fine. Or it is the water side itself: a circulator that has stopped, a zone valve that will not open, or a coil with air trapped in it.
The tell is simple. If the ducts blow cold but the taps run hot, the problem is at the coil and it is mine. If nothing in the house is hot, start with the water heater or the boiler.
Garage And Closet Furnaces
Most Sunnyvale tract homes have the furnace in the garage or a hall closet, often beside the water heater. Both locations get crowded. A garage furnace hemmed in by storage runs short on combustion air, and a closet furnace behind a solid door with a half blocked return grille starves for air. Either way it overheats, trips its high limit, and shuts down before the house warms. Furnace repair in Sunnyvale very often starts by clearing that space and checking the return path and filter size.
Eichlers With Radiant Heat
Sunnyvale’s Eichler neighborhoods were built with radiant heat in the concrete slab, fed by a boiler rather than a furnace. Many of those original systems have been replaced over the decades with forced air furnaces in the garage or attic, ductless heat pumps, or new boilers. When an Eichler has no heat, the first step is knowing which of those it has. A radiant system that still works gets checked for boiler operation, circulator pumps, and loop pressure. A converted system gets the same checks as any furnace.
Leaks in old slab tubing are a separate, bigger conversation. If a radiant system is losing pressure, the options range from repair to abandoning the slab loops in favor of ductless heat pumps, and each has tradeoffs worth walking through before anyone opens the floor.
Heat Exchangers And Safety
The heat exchanger is the part of an aging furnace that matters most for safety. Years of overheating from restricted airflow can crack it, letting combustion gases into the airstream or causing flame rollout. Every visit includes a heat exchanger inspection, a look at the venting all the way outside, gas pressure at the manifold, and a combustion test. A cracked heat exchanger means the furnace stays off until it is replaced.
Before You Call
Check the filter, the furnace switch, and the breaker, and clear anything stacked against the furnace or blocking the return. If the furnace lights and drops out after a few seconds, note the blinking code on the control board, since a dirty flame sensor is a common and quick fix. Every home with gas appliances should have working carbon monoxide alarms on each level and near the bedrooms. If one sounds, get everyone outside before calling. A yearly furnace check in the fall, with the flame sensor cleaned, the igniter measured, and the combustion numbers recorded, heads off most cold weather failures before they happen.
When an older Sunnyvale furnace reaches the end, it is worth looking at more than a like for like swap. A heat pump can replace both the furnace and the air conditioner, and in this climate it runs efficiently nearly every winter day. Dual fuel, a heat pump paired with a gas furnace for the coldest hours, is another option. The ducts and the return get measured first either way, because new equipment on a starved return runs just as hot and short lived as the old furnace did. You get the options side by side, with costs and the readings that support each one.
Send a photo of the furnace label before the visit so the right parts make the first trip.
When you book, mention whether the home is an Eichler and whether it still runs on radiant heat or has been converted to a furnace or heat pump. For a standard furnace, tell us whether it sits in the garage or a closet, and send a photo of the label. That way the right parts and test gear come along.
How I diagnose it
Temperature rise across the heat exchanger, against the range printed on the rating plate. Every plate gives a rise range, something like 35 to 65 degrees. I put probes in the return and the supply and compare. Run it above the upper end of that range and the heat exchanger is overheating, and overheating is how they crack. In a house with no attic and short duct runs, or a condo closet with one small return, running high is common and almost nobody checks it.
Flame sensor microamps. Not an opinion about whether the sensor looks dirty. A meter in series with the sensor lead, reading the current the flame is actually conducting against the minimum the control board needs. That tells me whether cleaning it truly fixed anything, or whether the board is the real fault.
Gas manifold pressure against the rating plate, read at the gas valve while the burner is lit. Underfired and you never get the heat you are paying for. Overfired and you are cooking the exchanger.
On a hydronic fan coil I read the water side instead. Supply and return water temperature right at the coil, and the air temperature rise across it. If hot water is arriving and the air is not getting warm, the coil is fouled or air bound. If hot water is not arriving, the fan coil is not your problem and I will say so rather than take it apart.
Repair or replace
Most furnace repairs are repairs. A flame sensor, an igniter, an inducer motor, a pressure switch and a blower capacitor are all parts, and on a machine otherwise sound they are worth doing.
The one thing that ends the conversation is a cracked heat exchanger. I do not say it to sell a system. If I find one I will show you what I found and how I found it, and I will shut the machine down, because a cracked exchanger puts combustion products into the air your family breathes.
The oldest thing on my Sunnyvale records went in in 1996 and the newest in 2021. Those are the two ends of what I have on file, not a pattern, so age by itself tells me nothing until I read the plate on your machine. If yours is recent it may still carry manufacturer parts coverage, worth checking before you pay for a part yourself. If it is from the nineties, parts still exist for most of them, and I will tell you when the repair stops making sense.
Hydronic fan coils are their own case. The fan coil is a simple machine, a blower and a water coil, and it usually outlives the boiler or water heater feeding it. If somebody is telling you to replace the whole heating system, ask which half actually failed.
Sunnyvale neighborhoods I cover
The Eichler blocks are where a Sunnyvale heating call gets interesting. Flat roof, no attic, heat in the slab. There is nowhere to put a conventional furnace and nowhere sensible to run a flue, so unless somebody converted the house at some point, the heat is hot water rather than fire and the repair is a different animal with different parts. If that is your house, say so on the phone. It changes what I put on the truck.
The Heritage District and the condo blocks around downtown hand you an access problem rather than a mechanical one. The unit is in a hall closet, a shared mechanical room or on a roof behind a locked hatch. Sort the fob, the key or the association contact before I come out.
Ponderosa Park, Birdland and Cherry Chase are the straightforward end. Low rise homes on flat lots with the furnace in a hall closet or the garage, where all I ask for is a clear path to it.
Straight Pricing, No Surprises
Residential Diagnostic
$99
Standard hours. After 6:00 PM, Saturdays, and holidays: $149. The standard diagnostic is credited toward your repair if you approve the work on the same visit.
Commercial Diagnostic
$199
Business hours. After-hours: $299. Light commercial, retail, and multi-family equipment.
Repairs and Installations
Quoted before we touch a wrench. Supplier invoice attached to your job sheet.
Furnace Repair Sunnyvale: common questions
My furnace lights and then shuts off after a few seconds. What is that?
That is almost always the flame sensor. It is a thin rod in the flame that proves to the control board that the burner really lit. A film of oxide on it drops the signal below what the board accepts, so the board shuts the gas off and retries, then locks out. That is the safety working, not failing. Small part, small job.
My heat comes from hot water, not a furnace. Do you work on that?
Yes. Hydronic fan coils are one of the equipment types that comes up most on my own Sunnyvale records, which is not true of most cities I work in. If the taps are hot but the ducts blow cold, the problem is at the fan coil and it is mine. If nothing in the house is hot, start with the water heater.
There is white plastic pipe coming out of my furnace instead of metal. Does that matter?
It matters for what fails. White plastic means a condensing furnace, which pulls so much heat out of the exhaust that the exhaust turns to water inside the machine. That water drains through a small trap. When the trap plugs, the pressure switch will not close and you get no heat with nothing visibly wrong. Look in that trap before you call.
Somebody told me my heat exchanger is cracked. Should I just believe them?
Ask to be shown. A cracked heat exchanger is a real and serious finding and it does end the life of a furnace, but it is also the most abused sales line in this trade. Whoever says it should be able to show you the crack on a camera, or the test that found it. If they cannot show you anything, get a second opinion.
How do I know if the problem is my furnace or my ducts?
Temperature rise tells you. Every furnace has a rise range printed on its rating plate. I measure return air and supply air and compare. If the rise is above the upper end of that range, not enough air is moving and the machine is overheating itself, so the duct or the return or the filter is the real fault. Replacing the furnace then gets you the identical problem.
Do I need a maintenance visit every single year?
Honestly, not always. If your furnace ran all last season without a hiccup, change the filter and you can reasonably skip a year. What I would not skip is a working carbon monoxide alarm on every floor with sleeping rooms, and keeping a closet or garage furnace clear of stored boxes.