You’re at FL100, somewhere over the Cascades in January. Outside air temperature is -15°C. The heater is on. There’s no “more on”. The little plastic vents are pointed at the you, and you can hear the rush of air doing its best. You’re wearing a fleece, a jacket over the fleece, and gloves because the stick is too cold to hold without them. Cabin temperature is somewhere in single digits if you’re lucky. Your right knee, which is closest to the duct, is warm. Your shoulders are not.

This is what “cabin heat” means in a piston single. The heater isn’t broken. It’s working exactly as designed. The design is from 1948.

Now imagine the same flight in August, except it’s 32°C on the ramp and the cockpit is touching 50°C before you’ve even taxied. Cirrus owners have a button for this. Diamond owners have a button for this. Sling owners are about to get a button for this — Airflow Systems started shipping AC kits for the TSi in late 2024, and TAF has a factory option coming as part of the 916 iS firewall-forward redesign. The cooling problem is being solved. The heating problem, the one that’s been quietly making winter cross-country flying miserable since the Truman administration, has not.

That asymmetry is the whole story. GA is investing real money and real engineering effort in cabin cooling. It’s investing essentially nothing in cabin heating. And the more I dug into why, the less the answer had to do with weight, electrical budget, or any of the other things the industry hands you when you ask. It had to do with what customers will pay extra for, and a long-standing assumption that engine exhaust heat is “good enough” because it’s free.

It’s not good enough. And it doesn’t have to be free.

How GA heats a piston cabin

There are five ways general aviation moves heat into a cabin, and only one of them is common.

The dominant solution is the exhaust heat muff: a sheet metal shroud wrapped around a section of the exhaust, ram air entering one side, picking up heat by conduction through the exhaust skin, exiting into the cabin through a valve. There’s no consumer analog for this — cars don’t heat cabins this way because they don’t have to. It’s the architecture of every Cessna single from the 152 to the 206. Every Piper Cherokee, Archer, Arrow. Every Beechcraft Bonanza. Mooney. The entire Van’s RV line. Lancair, Glasair. And the Cirrus SR22 and SR22T, which retail for over a million dollars. A $1.2M turbo SR22T uses the same heat-source architecture as a $450K Cessna 172, which uses the same heat-source architecture as a 1948 Cessna 170. The muff isn’t a GA tradition. It’s GA’s default solution to cabin heat, full stop.

The second solution is a coolant heat exchanger — the same architecture as your car’s heater. A heater core fed by the engine’s coolant loop, an electric blower pushing air through it, ducted into the cabin. This only works on a liquid-cooled engine, which limits the population dramatically. Diamond’s diesel-powered DA40-NG, DA42-NG, and DA62 use this approach. Rotax-powered experimentals can use it because the 912/915/916 has liquid-cooled heads. Nothing in mainstream certified GA uses coolant heat because nothing in mainstream certified GA is liquid-cooled.

The third is a dedicated combustion heater — a small furnace burning avgas through a sealed combustion chamber, with a heat exchanger transferring the heat to cabin air. The consumer version of this is the Webasto or Eberspächer diesel heater that warms sailboat cabins, RVs, and overland Sprinter vans. In aviation, Janitrol, South Wind, and C&D make these as standard equipment in piston twins (Cessna 310/340/414/421, Piper Aerostar, Beech Baron and Duke) and a few high-end singles. They produce real heat, work on the ground without the engine running, and don’t depend on airspeed.

The fourth is bleed air, which means a turbine. King Air, TBM, PC-12, every business jet, every airliner. Hot compressed air tapped directly from the engine’s compressor section, regulated for temperature and ducted into a fully integrated environmental control system. It’s the central HVAC of an office building, applied to a flying machine.

The fifth is electric resistance heat — a glorified household space heater, powered from the aircraft electrical bus. Starting to appear in electric trainers like the Pipistrel Velis Electro but doesn’t pencil out on a piston aircraft with a limited alternator budget.

Look at where each solution clusters. Piston singles, regardless of price — Cessna 152 or Cirrus SR22T — almost universally use the exhaust muff. Twins step up to combustion heaters. Turbines get bleed air. The dividing line between “cabin gets warm in winter” and “cabin doesn’t” sits at about $1.5M. Below it, piston GA has accepted a heat source from 1948 as the default.

The two Sling systems

The Sling TSi has used two of the five solutions over its production history.

The original design is the coolant heat exchanger approach. A Siroco heater core behind the firewall, fed by the Rotax cooling loop. An electric blower pushes air through it. Five outlets distribute heat: two side vents at the panel, a Y-piece to the footwells, and a windshield demister. Three panel controls — a temperature knob, a 4-position fan speed switch, and a cabin airflow knob.

The newer design, which most current 916 iS airplanes have including mine, is the exhaust muff — the same architecture every Cessna single uses. A sheet metal shroud around the exhaust, ram air through it, into the cabin via a single butterfly valve. One panel knob — open or closed. No blower, no fan speed. Mechanically: a shroud, a duct, a butterfly, a cable.

The case for the simpler architecture is straightforward: fewer parts, no electrical blower, no coolant plumbing through the firewall, lower cost, lower weight, less to maintain. The case isn’t documented anywhere TAF has published — it’s a kit change that happened around the 916 iS transition and the rationale lives in conversation rather than in print. The harder question is what the change costs in heating effectiveness. The old coolant system had real problems — slow warmup, air-trapping in the heater core, ineffective heat per multiple builder reports. So the muff may be a net improvement at low altitudes. At FL100 in winter it almost certainly isn’t. Nobody has published the data either way.

Two systems, ten years of architecture between them. The newer one is simpler, lighter, and more reliable. It’s also the one that runs out of heat at altitude in winter.

Why these systems run out of heat at altitude

The muff was designed for a 1948 Cessna at 5,000 feet on a 50°F day. Take the same architecture to FL100 in January, in a turbocharged airplane that can climb to 23,000, and the design margins disappear in four ways that compound.

Inlet air is dramatically colder. ISA at FL100 is -5°C in standard conditions. In winter, you’re easily looking at -15 to -20°C. Where the muff was designed to take 20°C ambient air and warm it to 25°C cabin (a 5°C lift), you’re now asking it to take -15°C air and warm it to 20°C cabin (a 35°C lift). Seven times the temperature rise, from a heat exchanger that hasn’t gotten any bigger.

Air density drops to about 74% of sea level. Heat capacity scales with mass, not volume. Less mass per unit volume means less heat transferred per unit time. You’re heating a thinner stream.

Air density and mass flow both drop. At constant IAS, dynamic pressure at the muff inlet stays the same regardless of altitude — that’s what IAS measures. What changes is air density. At altitude you’re flying faster (higher TAS) through thinner air, and mass flow through the muff scales with density: at FL100, roughly 14% less air mass passes through the shroud per second than at sea level. Since heat capacity scales with mass, not volume, the muff exchanges less heat per unit time even though the inlet pressure is unchanged.

The cabin loses heat faster. Delta-T between cabin and outside air at FL100 in winter is 30–40°C versus 10–15°C on a typical ground day. You need substantially more heat delivered just to maintain cabin temp, and you’re getting less of it.

The exhaust itself doesn’t compensate. EGT is largely a function of power setting, not altitude. Same source heat, less effective transfer, more cabin loss.

Where the airframe makes it worse

Builder logs document specific issues that illustrate where the muff architecture runs out of reserve.

Heat ducts losing temperature in transit appears to be common. Kevin’s TSi build log notes that “many Sling builders have reported that the heated air from the cabin heater loses temperature during airflow because the outside cold air cools the heat duct while it runs through the fuselage.” Another builder, Lars at tsi.rabing.de, independently insulated the same duct “to keep the air warm when the heater is needed.” Two named builders, same retrofit, both saying it shouldn’t be necessary as-shipped.

Cabin vent sealing is a known failure mode for at least some airframes. A Sling Pilots forum thread documents one owner finding cold air leaking behind the panel where the silver ducting attaches to the vent. A second poster reports the standard plastic vent leaks even when fully closed. One forum thread — treat it as specific and verifiable rather than universal.

At least one well-documented case of a missing floor under the rear seats. Arthur and Gustavo at slingbuild.blogspot.com, who built at TAF’s Torrance Custom Builds facility, found “there is no floor under the back seat” with “a huge amount of cold air leaking into the cabin from underneath.” Their retrofit floor panels affected the static port indication enough that they recommend an external static port if you do this modification. One case, not a fleet-wide pattern — but it’s a Custom Builds aircraft, so it isn’t a homebuilder’s omission.

The point isn’t that every Sling has every issue. The muff system depends on the rest of the airplane doing its share — sealed cabin, insulated ducts, vents that close — and when the rest of the airplane doesn’t, the muff has no reserve to give. The heater works. The architecture is sound. It’s just sized for a different airplane, in a different envelope, in a different decade.

Meanwhile, on the cooling side

The same industry that accepts a 1948 architecture for cabin heating has been quietly investing real engineering effort in cabin cooling for the last twenty years.

Cirrus introduced factory air conditioning on the SR22 in 2006 — a vapor cycle system sold as a $19,990 option. In current SR22T configurations the AC system costs about 55 lb of useful load, 6 BHP, and 2 knots of cruise speed. The same airplane uses a 1948 exhaust muff to heat its cabin but has 2006 vapor-cycle AC with face/feet/defrost zone selection like a Toyota. The asymmetry of investment is visible inside a single aircraft.

Diamond Aircraft offers factory-installed air conditioning on the DA40 NG, DA42-VI, and DA62, engineered into the airframe with internal heat exchangers. Diamond is one of the few piston-single OEMs that takes both halves of cabin climate seriously as engineering problems.

In the experimental segment, Airflow Systems delivered the first composite-kit AC system in 1995 on a Lancair IV-P and has since developed kits for Lancair, Glasair, and Van’s RV series — proven on over 170 experimental aircraft. Flightline AC covers Comp Air, Epic, Glasair, Lancair, Van’s, Velocity, and Viperjet. Two independent suppliers, decades of installations. There is no equivalent industry for cabin heating in experimentals.

The Sling TSi has three viable paths to factory-quality AC. In July 2024, Airflow Systems partnered with Evan Brunye — the builder who produced TAF’s official Sling TSi Kit Build Guide video series — to sell an engine-driven compressor kit compatible with both the 915 iS and 916 iS, with initial deliveries from October 2024. Separately, TAF has announced a factory AC option as part of the 916 iS firewall-forward redesign.

There is an active engineering market for cabin cooling at every layer — OEM factory, OEM aftermarket option, third-party kit, DIY electric. There is no comparable market for cabin heating. The investment is real. The investment is also asymmetric.

Why cool but not heat?

Heat is free, AC isn’t. Engine exhaust is already there, already hot, already discarded. Building a muff costs sheet metal and a cable. AC requires a compressor, a condenser, an evaporator, refrigerant lines, controls, and engine power. AC is genuinely additional, which means it can be sold for $20K. Heat that already comes with the airplane can’t be sold separately, so there’s no revenue line item to fund improvements to it. The economics freeze “good enough” at whatever the muff produces.

AC is the more obvious upsell. A buyer climbing into a 50°C cockpit on a demo flight needs no convincing that AC is worth $20K. Cold-cabin discomfort is invisible to the buyer at the dealership — it only shows up on the trip the buyer hasn’t yet taken, at the altitude the buyer hasn’t yet flown. Cooling is sold during the buying decision. Heating is suffered after.

The customer base skews warm. A large share of GA piston flying concentrates in California, Florida, Texas, and the Southeast — places where summers are punishing and winters are mild. Pilots who fly cross-country in winter at altitude are a smaller market segment. Manufacturers build for the volume customer.

The system pays for novelty, not refinement. When Cirrus adds a vapor-cycle AC option, it’s a new STC, a new revenue stream, and a new differentiation point. When TAF could improve the muff system, there’s no certification milestone, no marketing benefit, no premium they can charge. Heat is old, so nobody refines it.

The failure mode is wearable. A bad muff at altitude in winter is uncomfortable. You put on a jacket. You fly anyway. Uncomfortable doesn’t drive industry change.

But here’s what the incentive story doesn’t justify: the same engineering effort applied to either problem would solve both. A heat pump — the architecture every EV is now using — heats and cools from a single unit, replacing both the heater core and the AC compressor. The industry hasn’t done this not because it’s hard, but because it has organized its investment around two separate product categories, and one of those categories doesn’t have a revenue line. GA invests in HVAC where the market pays for HVAC, and the market only pays for half of it.

One unit, two problems

The architecture that dissolves the heating-cooling trade-off already exists in production. It’s in roughly every electric vehicle sold in the last three years, and it’s called a heat pump.

A conventional vapor-cycle AC system moves heat in one direction: out of the cabin. A heat pump does the same thing, but reversibly. Run it one way and it cools the cabin; run it the other way and it heats the cabin. One compressor, one refrigerant loop, two functions. No exhaust muff. No heater core. A single climate system instead of two separate half-solutions.

This is exactly what Tesla’s Octovalve thermal management system does. When KITPLANES covered the Brunye Sling AC announcement in 2024, the editor noted that ultimately the way forward is automotive heat pumps — smaller, lighter, more efficient, capable of heating the cabin and eliminating the traditional heater core entirely. The heat pump doesn’t improve the muff or the heater core. It replaces both.

The honest engineering case is real but not trivial. Heat pumps become less efficient as ambient temperature drops. Tesla’s answer is to switch to resistive heat generation when the refrigeration cycle becomes inefficient — something a GA implementation would need to replicate from the electrical bus. The standard Sling TSi runs a 14V system, tight for a compressor-based climate system. The Rotax 916 iS B-model comes with 28V alternators, which approximately doubles the available electrical capacity and changes the heat pump math significantly — something I’ve explored in a separate post on this blog.

The technology is not speculative. It exists in production at scale in automotive applications from Tesla to Hyundai to Volkswagen. Nobody has built this for the Sling, or for the experimental piston single segment generally. That is a genuine product gap — one that the experimental category, with its builder latitude and absence of certification economics, is uniquely positioned to fill.

There is a nearer-term technology that bridges the gap between today’s muff and tomorrow’s heat pump — and that nobody in experimental GA appears to have publicly installed yet. The Eberspächer Airtronic D4 (gasoline variant) is a compact combustion heater rated to 5,500m / 18,000 ft, produces 2 kW of cabin heat, and burns 0.1–0.5 L/hr from the aircraft’s own fuel supply. Combustion is fully sealed and exhausted overboard, eliminating the CO ingress risk of the muff entirely. Unit cost roughly $1,500–$2,500. The Webasto Airtop 2000 STC is an alternative with similar specifications. Both are standard equipment in sailboat cabins and overland van builds, with hundreds of thousands of hours of cold-weather service.

No one in the experimental aircraft community has publicly documented doing this. The solution exists in adjacent domains. The experimental certificate is what allows someone to be first.

What you can do now

The experimental certificate on your Sling isn’t just a legal designation. It’s a repair order. The industry hasn’t fixed the cabin heating problem; the segment you’re in permits you to fix it yourself.

Insulate the ducts. Multiple builders have independently documented the same retrofit: wrap the cabin heat duct in thermal insulation before it traverses the cold fuselage. Cost is $50–$100 in materials and an afternoon of work. Do this first — it multiplies everything else.

Seal the cabin. Close the floor under the rear seats if yours is open. Address NACA scoop joint sealing. For serious winter flying, aerogel insulation panels at the firewall, footwells, and door cards are worth considering — thermal conductivity roughly one-third to one-half that of fiberglass at a fraction of the weight. Materials cost $500–$1,500 depending on scope. A weekend project, done once.

Install heated seats. Carbon-film heating elements from automotive interior suppliers: 50W per seat, PWM-controlled, under a pound installed. A pilot on a warm seat tolerates a noticeably cooler cabin than one relying on bulk air heat. Cost $300–$500 installed — one of the highest comfort-per-dollar upgrades on this list.

Do all three, DIY, and the total is roughly $850–$2,100. On a $300K–$400K airplane, that’s less than 1% of the aircraft’s cost to meaningfully shift the winter flying experience.

The cooling side has a path — kits that exist and are shipping. But given the weight penalty, the cost, the firewall work, and the engine power required, it’s worth being honest: buying a Sling TSi means accepting you probably won’t have AC, the same way you accept you won’t have FIKI. The platform has limits. The experimental certificate gives you the freedom to push past them — but most builders won’t, and that’s a reasonable outcome.

The heating side doesn’t even have a packaged path yet. That gap is the post’s actual argument. What the industry won’t invest in, the builder can build — but for most missions, most pilots, most of the time, the answer is a good jacket and a realistic preflight decision about the day’s conditions.