Is there anything I can do to improve the heater effectiveness in my 1975 Sundowner? I just don’t get enough heat, especially rear seat heat.
Technical Editor:
A quick answer is that your 1973 Sundowner has only the firewall heat box that dumps into the front of the cabin on the aft side of the firewall, near the center (as you undoubtedly know). Effective in 1975, with airframe serial number M1600, a much expanded heat distribution system was added. This entailed a diverter mounted on the cabin side of the heat box; a Tee and a Wye on the RH side; a heat outlet on each side of the center pedestal, under the panel; and a heat outlet aft of the ankles of the RH and LH rear passengers. The latter two outlets do not have Wemacs on them; they are just open ports with trim. Most of the heat tends to warm the feet of the rear passengers, then blows forward under the two front seats; but they do help distribute heat to the cabin. And if the heat is off, they add fresh outside air to the same areas, unless the cabin air valve is closed.
No drawings are needed for this system. It is detailed quite well in Figure 411D, pages 2 and 3, of the Illustrated Parts Manual. Either you or your mechanic are supposed to have the IPC and the Shop Manual in order to legally work on the plane. The required parts are also called out in that figure. The system could probably be installed in a 1973 with a logbook entry and an informational 337 to the FAA; or even just logged as a minor change, as it affects no primary systems. It is essentially just a change to the interior trim. This assumes that the Beech parts are used, of course.
Please note that the IPC calls out 1.5″ underfloor ducts. Those will not work; they cannot be retrofitted after the plane has been ‘closed in’. There simply is not adequate room to route the ducts, without too tight a fit. You will need to use 1.25″ ducting. However, you will need to order the special ducts from Custom ducts, to get the 1.25″ ducts with 1.5″ belled, cuffed ends. That’s because the duct ends have to fit on the 1.5″ fittings. The exceptions are the ducts under the instrument panel, running to the LH and RH pedestal outlets; those are 1.25″ fittings and ducts. This is all described in detail in my recent post about replacing the underfloor and under-panel ducts. That is something you need to do at the same time, if they are still original. If they are original, you will be experiencing corrosion right now, wherever the old ducts are touching aluminum (airframe, brake lines, etc.).
Four final comments.
If the baffles are all burned out and missing in your muffler, you will not get enough heat in the cabin; regardless of the distribution system.
If you don’t use high enough power settings (75% or higher), and fly low enough to get that power (meaning under 7,000′), you won’t get enough heat in cold weather.
If all the cabin doors are leaking badly, due to bad seals, you can’t hold in enough heat to stay warm.
If the cabin exit air port is blocked or disconnected in the aft fuselage, you can’t draw enough hot air into the fuselage. Ditto if any of the heat control box parts are not working right.
The only practical way to accomplish this upgrade will be to find a ‘donor fuselage’ circa 1975 or later, and cannibalize the parts. The key parts are the firewall diverter, the 1.5″x1.25″x1.5″ RH Tee; the 1.25″x1.25″x1.25″ Wye; the two pedestal-mounted Wemacs, with their air duct adapters; and the two aft outlets that mount in the spar cover. The two long underfloor SCEET ducts can be ordered from Custom Ducts. I can furnish the needed short 1.25″ ducts for the short front pedestal runs. You also need to replace (if still original), the 1.25″ fresh air ducts that run underfloor, up the aft sidewalls, and into the overhead. Ditto for the short ducts that run around the corners of the instrument panel.