Are there power inverters without roar cooling?

[Artlav]

I have a battery backed 24V DC wiring at home, which powers most low power stuff directly or via DC-DC converters.
I also have a NAS capable of pulling 250W (always pulling 100W except when the disks are spun up), which is sensitive to power cuts and can't realistically be powered from DC directly.

So instead of getting a separate UPS for it, i decided to just connect it to the 24V bus using a small inverter. Simple, right?
Well, not really.
After going through several 24V->220V inverters in the 300-500W range from different origins (noname ebay, MeanWell from Mouser, one from a local store) i found all of them to have a critical flaw.

The flaw is that they all have a fan and this fan have only two modes - either quiet full off or roaring-loud full on.

Therefore the question is - where can i find a 300+ watt, 24V->220V (110V works too, the thing got universal input) power inverter with a gradual fan control (or enough efficiency to not need a fan at all)?

[NiHaoMike]

Just find a used APC that takes a 24V pack and wire it into your existing system.

If the NAS uses an ATX type PSU, you could use buck converters run from the 24V.

[David Hess]

Just find a used APC that takes a 24V pack and wire it into your existing system.

Common standby UPSes are not intended to operate continuously and may overheat.  An online UPS or standby UPS which accepts external battery packs should be suitable but they usually include a fan also.

Inverter design with a passive heat sink and no fan is tricky because of the high voltage isolation needed between the exposed heat sink and transistors.  Maybe look for an old used one?

[james_s]

You could get or build a temperature controlled fan speed controller with a thermistor attached to the heatsink so it has continuously variable speed and only runs full blast if it has to. Look around for a quieter fan, maybe see if you can replace it with a larger slower turning fan that moves as much air.

[Red Squirrel]

I noticed that too.  Seems they are setup to run the fan based on load instead of based on temperature.   I have a 350w inverter in my shed to power the lights and accessory outlets and it sounds like a data centre in there if the lights are on and I'm charging a drill battery at the same time. 

[Dr. Photon]

If the server/NAS is using something similar to ATX, you might consider one of the PICO-PSU type products sold by Mini-box, or other "carPC" type PSUs. They are generally designed for putting a PC into a car, but they do have models that can work from 24 volts as well as 12.