Phoenix has had a Dometic FreshWell 3000 under-floor air conditioner since the conversion — installed in the rear section, distributing air through Ø60 mm ducts. But the duct layout is designed primarily for the rear of the vehicle, with only a single duct running forward — and that simply isn't enough to cool the front sleeping area eight metres away. At night, when you need to sleep up front, this had long been the most noticeable compromise of the entire conversion.
The solution wasn't stronger fans or a better air distribution system, but a second, separate air conditioner for the front of the vehicle.
Where to put the second unit
I was looking for a place to install the second air conditioner so that it would cool directly overhead in the sleeping area, while not interfering with the already-finished wiring and interior. Above the front bed there was originally only a small roof hatch — without much benefit for light or ventilation, and at the same time exactly in the spot where the cooling was missing the most.
I replaced the hatch with the roof unit, so the second air conditioner didn't take up any new space in the interior — it just took over the position the hatch had already occupied.
Why an inverter, and why the SINCLAIR ASV-35BIS
I deliberately didn't go for the same type of unit as the rear Dometic. Classic ON/OFF air conditioners run either at full power or not at all — in the small space of the cabin, that means cold/warm cycles and a noisy compressor start over and over again. The inverter unit SINCLAIR ASV-35BIS, on the other hand, smoothly regulates output according to current demand, so at night it runs at low speed and noticeably quieter, instead of constantly kicking on and off.
For the front part of the vehicle, where you sleep, that's exactly the difference that matters — a quiet, steady operation at night is more important than maximum cooling power.
How the two air conditioners work together
The rear Dometic FreshWell and the front SINCLAIR ASV-35BIS now work as two independent zones — each one only cools the part of the vehicle where it actually is. Instead of one unit trying to blow cool air through the entire interior, I have two smaller units that cover exactly their own space and don't need to expend as much energy doing it.
Energy-wise, it even handles operation away from a standard mains connection. Thanks to the LiFePO₄ battery bank and the Victron MultiPlus-II, it's possible to run both air conditioners at the same time along with other appliances without worrying about a fuse blowing or running out of power — the system handles it without problems in normal operation.
📦 Components used
- SINCLAIR ASV-35BIS (roof inverter air conditioner, front of the vehicle) — specifications
- Dometic FreshWell 3000 (under-floor air conditioner, rear — 2.7 kW / 9,200 BTU, ~620 W)
- Sinclair air conditioner for caravans (short video about the same generation of the indoor unit) — video on YouTube
Frequently Asked Questions
Is one roof air conditioner enough for an 8-metre (27 ft) motorhome?
In a vehicle over 7 metres (23 ft) long, a single rear unit usually can't cool the front sleeping area adequately — the cold air doesn't travel that distance in sufficient quantity. Phoenix is over 8 metres (26 ft) and with just the rear Dometic FreshWell, cooling was barely noticeable at the front bunk. A second unit solved the problem.
Why choose an inverter air conditioner over an on/off unit?
An inverter AC modulates its output to match the current demand — it doesn't start at full power but maintains temperature smoothly. This means lower current draw, quieter operation, and fewer temperature swings. For powering from a 12V battery and solar system, the inverter is essential.
Where to place a second air conditioner when the interior is already finished?
The ideal spot is to replace an unused roof vent above the sleeping area. Phoenix had a small front skylight with no real benefit — that's exactly where the SINCLAIR ASV-35BIS now sits. No need to touch the finished interior or wiring.
How many kWh do two air conditioners use overnight off-grid?
At around 35 °C outside with cooling set to 22 °C, both AC units in Phoenix used approximately 4–5 kWh overnight. With 400 Ah LiFePO₄ and 780 Wp of solar, the system handles it without grid connection.