Product Update August 2019 Issue

Ultraflex Debuts Variable Speed AC

“Soft start” variable speed compressor reduces peak loads to permit limited cooling at anchor.

In the June 2018 article “Air Conditioning for Sailboats,” we compared several options for 12-volt air conditioning on boats, and more recently we looked at the power requirements for running our air conditioner without being tethered to shorepower, “see Air Conditioning at Anchor,” PS June 2019). Since that article was published, we were told about the new i-Line VSD Series of compact air conditioners from Velair—an Italian company that is part of the Ultraflex Group.

The VSD stands for variable speed drive, which, as the name suggests, varies the speed of the compressor to manage current loads. In the case of the i-Line series, the compressor is driven by a synchronous brushless DC inverter. According to Velair this drive, combined with a low-speed reverse fan blade, a permanent magnet fan motor, and an electronic expansion valve, makes it six percent more efficiency than competitors. Most interesting to cruisers, though, is what happens at startup.


The Velair i-Line VSD series from Ultraflex has a synchronous DC inverter driven compressor.

As we say in the 2019 article, reducing peak current loads every time the compressor starts up is essential to running an air conditioner on a small generator, or on battery power alone. Typically, the these peak loads are reduced with a “soft start” capacitor, an optional accessory designed for this purpose. The Dometic DTU 10 unit that we tested for the 2018 report was equipped with a Dometic soft-start device. There are some aftermarket soft starts such as the EasyStart, although we’d not recommend this as a DIY project.

In our 2018 report, the Dometic DTU unit was the most efficient in that size (10,000 BTU) that we could find at the time. The start amperage did create a slight bump in current, but as long as you had a generator, or shorepower to back up the inverter it was connected to, this bump was nearly invisible.

According to a technical report and specifications from the manufacturer, the Velair VSD series (which we have not yet tested), can start and operate using battery power alone (via a 12-volt DC to 120V AC inverter), without the need for an onboard AC generator. Ultraflex said it was able to start and run its 10,000 BTU unit at partial load for 6 hours using two Group 29 batteries (about 180 Ah capacity, or 60 Ah usable). At full load the unit will use 450 Ah in 6 hours, but as the boat cools, you should not need to operate at full load for the full 6 hours.

We were able to achieve similar results with the Dometic system on our test boat, starting and running the Dometic DTU 10 for 4 hours on three Group 27 batteries (about 180 AH total), so we believe it is possible. A portable 120V AC unit (also Dometic) that we also used for testing, could also operate on battery power alone (via the 12-volt DC to 120V AC inverter), but it could not start using this power source setup.

The variable speed motor has many advantages. Inverters and generators can be sized for the running load rather than the starting peak. Vulnerable shore power cords aren’t stressed by high loads. And it should prolong battery life, as repeated peak loads can shorten the battery’s life.

Another advantage of variable speed AC is longer run cycles, which helps keep humidity down. By reducing the compressor speed without turning the unit off, you have, in effect, a variable-size AC unit, cooling fast when needed, and then dehumidifying once the boat is cooled down.

Another interesting feature is the possibility of interfacing your air-conditioner with your smart phone via the Velair App. This would allow you to cool down the boat before you get to the dock—assuming you left the cooling intake seacock open (something we are reluctant to do when leaving our boat).

We haven’t had a chance to live with a Velair variable speed AC unit, to see how it runs and how it holds up. But we think variable speed drives and soft start technologies have a definite place in marine air conditioning.

Comments (8)

Boston Barry has nailed it. Any way you slice it, running an AC unit on a boat or RV using battery alone is, in a practical sense, an impossible task. The engineering numbers done properly simply do not add up. You would not even get one short night out of the average battery bank! Gary Wiseman, P. Eng, M. Eng. Marine Engineering, M. Eng. Naval Architecture.

Posted by: Garywiseman | August 18, 2019 7:10 PM    Report this comment

Watts = volts x amps. 5 amps at 110 VAC = 550 watts. Ignoring inverter inefficiencies 550 watts at 12 vdc = 46 amps. Boston Barry's conclusion that running on batteries is not practical is still valid. Just not sure where the "12" multiplier comes from.

Posted by: Del | August 18, 2019 9:40 AM    Report this comment

Thanks you all for your comments, The Velair units we have in stock here in the US are 115V volt. We do offer 230V units at our European division. We can offer them here in the US on a special order basis. We are in the process of setting up a distribution network, but we can sell factory direct for a limited time. please feel free to contact me directly at (941) 351-2628

Posted by: Velair Marine A/C | August 14, 2019 8:32 AM    Report this comment

It is impossible to run an air conditioner off the battery bank of a recreational vessel.

Multiply the AC amp draw by 12 to get the DC amp draw. The smallest air conditioners draw about 5 amps at 110 volts AC. That's about 60 amps at 12 volts off the battery. See how many amp-hours capacity your battery bank has. The largest batteries typically used on boats is size 8D. An 8D battery has less than a 250 amp-hour capacity, but only half of that is usable if you want decent battery life. Also the amp-hour capacity is generally specified at a 20 amp draw. It is less at 60 amps. So a small AC unit will empty an 8D battery in between 2 and 4 hours. The calculation also doesn't include inefficiencies of the inverter or loss of capacity as the battery ages.

Posted by: Boston Barry | August 12, 2019 10:14 AM    Report this comment

I am also confused about the input voltage. The Velair website says that the Compact i10 VSD model runs on 230 volts AC at 3.1 amps. That would equate to at least 59 amps at 12 volts with the proper equipment to invert the 12v DC to 230 volt AC. Please clarify.

Posted by: vulcan213 | August 10, 2019 2:53 PM    Report this comment

This is not a 12V powered system. The spec on the product page specify 115V AC.

Posted by: Veritas | August 10, 2019 2:14 PM    Report this comment

This is not a 12V powered system. The spec on the product page specify 115V AC.

Posted by: Veritas | August 10, 2019 2:14 PM    Report this comment

Looking for a distributor in FL

Posted by: tomdaisho | August 7, 2019 12:25 PM    Report this comment

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