I’m about to replace a Mastervolt Combi with a Quattro II 24/5000 on a sailboat, currently berthed in Greece. The combi is mounted horizontally, below the saloon floor but at least 50cm about the bottom of the bilges. I understand that it is recommended to install the Quattro vertically with 10cm clearance on all sides, but if I did that the bottom would be uncomfortably low in the bilges. So I plan to install the Quattro horizontally, directly replacing the old unit. It will have unrestricted airflow, and if needed I could easily add a bilge blower or other fan.
Does that all seem reasonable… or might I regret it in the heat of the summer?
BTW… sustained load might be the inverter generating 2.5 to 3 kW for an hour or two.
I’ve been testing with better/ additional cooling of our MP-II 5000 GX that needs to provide high/maximum feed-in (inverter) power for prolonged periods of time to make best use of high price timeslots (as a trade only DESS system). From that I concluded that our priorities are as follows, high to low:
- Additional airflow to/from the location of the MultiPlus
- ‘Tunneling’ additional airflow to ‘force’ it to enter the MultiPlus from below/air inlet side
- Filtering the additional airflow against dust (and on a boat I’d add protect against water ingress)
- Removal and replacement of the original housing, specifically to better target the additional (cool)airflow towards the toroidal transformer (and MOSFETs if sound is of enough importance to want to disable the original fan)
- Mounting vertically to facilitate natural (convective) airflow when no additional airflow can be provided
So in your case I would not worry about orientation and focus on that additional airflow. And make sure to use large diameter DC cabling if you want to be able to draw 4kW plus from a 24V battery at times. 95mm^2 minimum I’d say.
PS, not adhering to my own advise I just removed the housing and during last months high temperatures I pointed a simple standing fan towards it (on a thermostat to limit sound / energy use when cooling wasn’t needed). That allowed 4kW continuous AC feed-in power for almost four hours straight before thermal throttling kicked in. 
Being a Victron forum, we try not to encourage people to remove housings etc, for warranty and safety.
There are various projects on the modifications category where members have 3D printed cowlings and mounting brackets for additional fans, to either force air into the chassis, or to extract it.
Personally, for multis, I prefer the extraction fans at the top.
The quattro has a different formfactor to the multi, so It is unlikely to have the same demands for additional cooling, providing you can prevent temperatures building up by extracting warm air and supplying cooler air.
Alternatively, upsize your unit so you have headroom should it derate, though at 24V you have limited choice.
Thanks Nick and Jan
Point taken about removing or modifying housings… I always try to avoid doing that, or recommending it. Airflow is complex and is such a critical design aspect of power electronics; messing with it could be counterproductive, to say the least 
Also good point about cable sizes, and that’s another benefit of my horizontal installation: it is very close to the VERY heavy duty busbar that connects batteries, chargers, alternator, etc.
I’m certainly not encouraging that either, just in case that was not clear. I guess I will put it back asap in combination with an external (filtered) fan I recovered from storage last weekend. Using filtered air is the only reason to have it blow in by the way, without filtered air I’d also put it on the exit.
These units are tough, especially the quattro. I am always amazed at the mobile installations we see posted on the forum - the compact spaces - and how well they work.
Here in hot climates, it is good design to always go up one size on the inverter. It buys you wiggle room when it gets really hot.
Design is always a compromise.
Good luck. Send some pics and post to the “show us your system” space when you have it all working.
Pics: will do.
I did think about going up one size, but that would be a (first-generation) Quattro 24/8000, rather than the Quattro II.
I was referring to the original quattro. The gen2 is closer to a multi in formfactor and architecture.
I apologize, I am unfamiliar with forum protocol, so please correct/educate me if I am not posing my question properly. I must install a MultiPlus 12/1200/50-16 120V VE.Bus and two (2) MultiPlus-II 12/3000/120-50 120V (UL458) all “sideways” (due to space limitations) in “warm” (95-105*F) compartments (boat in Caribbean). I wish to avoid thermal-throttling and extend lifespan, so would like to improve airflow. I have read the offered suggestions and deeply appreciate their value. Thank you. However, I also wish to consider one of the simplest solutions I have used on my computers and other electronics on the boat: upgrading the original interior fans, if possible. The MultiPlus 12/1200/50-16 120V VE.Bus uses a 60x60x25mm 2-wire fan, and the MultiPlus-II 12/3000/120-50 120V (UL458) uses 120x120x38mm (Jamicon Model: JF1238B2HR-R brushless 24v 0.32A 24614 A1). I cannot find a label for the 1200’s fan. Does anyone know the specs for the fan on the 1200? And does anyone know how much additional power the fan circuit is capable of supporting without modification? I was surprised to learn my 12v MP2s used 24v fans. I do not yet know the voltage of the 1200 fan. My thinking is if I can increase CFM airflow with no other modification, than a “fan-swap” that air will follow the engineers’ designed cooling path better than other modifications, and as I have read in several postings, body-mods are discouraged. A small increase in fan-noise is no problem. I would sincerely appreciate any information or advise. Thank you.