Hot Rodding the Mastervolt Inverter

Our Mastervolt MassCombi 24/4000-100 120V inverter works well and we generally like it but it has always seemed to go into thermal shutdown earlier than it should. The inverter is particularly vulnerable to thermal cut out when it’s more than 80F outside and the sun is shinning directly on the stern. The early thermal cutout on the Mastervolt system is more noticeable underway when we have higher 120V load.

Technically the Mastervolt specification is that the system can produce 3.6kw at 104F (30A@120V) and the Laz is rarely over 100F but, like many specs used to sell product, the spec appears to be optimistic or difficult to achieve in real world applications. This early cut out can be annoying both because it’s unpredictable when it will happen and because the system goes down for more than a minute when it does shutdown.

Mastervolt generally makes an excellent product but we find they can be a bit parental when “protecting the equipment.” As an example of an engineering approach we prefer, consider our two Victron Pheonix 3000/120V inverters that are wired in split phase to produce 6kw at 240V. This pair is rated at 6KW at 240V and yet it’ll produced well over 7KW for surprisingly long periods of time. It’ll start incredibly difficult loads like the SCUBA compressor where the inrush current can briefly exceed 40A at 240V. In fact, it will run any load on the boat. The Victron strategy appears to be to run until it simply can’t whereas the Mastervolt approach is to shutdown when the engineering team thinks it might put equipment longevity at risk.

I think the Mastervolt and Victron inverters are both well-made and well-designed. I just find the Mastervolt protection mechanisms a bit on the conservative side, and perhaps a bit too quick to drop the load. The Victrons run in the same environmental conditions and exceed their specs, while the Mastervolts don’t quite achieve them.

I thought the Mastervolt inverter early shutdown might be caused by dirty air intakes, dirty electronics, or a faulty fan. To a great extent, I was hoping to find a problem that could be fixed since I really would like to get a reliable 30A in a 100F operating environment (well inside the Mastervolt specification).

On Dirona, the inverter is both difficult to get to and a real chore to take down but I felt like I needed to do something. Unfortunately, I found all four internal cooling fans were operating correctly and the system was clean and flowing air freely. All was found to be as it should be so there really was nothing to “fix”.

I gave some thought to the design challenges facing an inverter designer in a marine environment. The design needs to reject water splashes while at the same time providing adequate cooling. Likely because of these design goal conflicts, the air flow on the Mastervolt inverter is pulled through small downward facing vents in the side and pushed out the bottom. Unfortunately the designers decided to fight nature and flow the hot air down. But the bulk of the problem is caused by the circuitous air path and fairly small openings to protect against water intrusion.

Since we operate in a dry environment where water intrusion at the inverter location is not even remotely likely, I decided to essentially hot rod the unit by opening up two 4″ holes in the inverter top to provide a much less flow-resistant air path. This alone makes a big difference and is now the normal operating mode. In addition, when the laz temperature is approaching 100F, we have 2 55CFM fans that blow down into the inverter case from the top. This dramatically increases the air flow when needed and it really seems to work. We tested the system for more than 5 minutes at a steady 30A to 31A and the system held the load without issue. Much nicer.

We feel like the inverter output problem is largely solved. The final step might be to automate the cooling fan control but it’s working pretty well at this point. A related problem where I don’t yet have a solution is the low voltage derating on the Mastvolt Chargmaster 24/100-3. As the voltage drops down to the 195V range, the chargers derate to about 75% of rated capacity. This charger derating is mostly not a problem and we have run these chargers all over the world on an incredible variety of voltages and currents without a failure.

Arguably it’s unlikely that a 240V system will ever drop down to 195V but some locations like South Africa are actually 220V nominal rather than 240V. The combination of South Africa using 220V as nominal and the high load on the grid means they frequently drop down below 200V, where Mastervolt begins to derate the charger making less output available. A related problem is many countries that use 120V end up supply 208V at the dock. This is common in nany parts of Canada, the US, and it is what we currently have in Barbados. 208V will often drop down below 195V as the electrical grid loads go up.

Overall, I like reliable equipment as much as the next person and there is no question that Mastervolt achieves respectable reliability. But I would really prefer that reliability be achieved with a wider environmental operating range prior to derating or shutting down.

P.S. An offer that is always open to all manufacturers of equipment installed on Dirona: If you want detailed data on environmental operating conditions and results at five-second resolution, we already store everything in a relational database and would be happy to share it with you. It costs us nothing, might improve your equipment, and it’ll certainly improve our experience with it.

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13 comments on “Hot Rodding the Mastervolt Inverter
  1. Rod Sumner says:


    Just a thought:

    What about fastening a ribbed heat sink on the outside of the inverter cabiinet (similar to heat sinks on CPU’s)

    Given that I don’t know the temperature of the cabinet (or its design) but if the delta T between ambient and the cabinet is large enough this may be worthwhile.

    If there is sufficient room inside the cabinets to mount heat sinks on the offending heat sources??????

    Hope you and Jennifer are better able to enjoy the sights of B’dos. Unless you have a very large restaurant buget beware of The Cliff – very good but $$$$$

    All the best


    • That’s a good suggestion Rod. There isn’t room in the cabinet to easily install more heat sinking but the heat rejection design is based upon a combination of active cooling with fans inside and conducting heat to the the case through direct physical attachment to internal heat sinks. Large heat sinks on the case would likely help but, at this point, more air flow appears to be solving the problem.

  2. Timothy Daleo says:

    Back in my PC building days I would get cabinet cooling products from and they worked well and have temp controllers. If those are standard 120mm fans you installed then they probably only use 10w each and are DC? I did not see how you wired it in but Amazon has inexpensive 12v temp controllers and even 12v remotes that would work nice and allow you to turn them off. I would even go lo-tech and epoxy a small mirror above the fans so I could see them working without having to go back in there. It looks tight.

    • I’m using 120V 4″ fans on this one. Right now it’s on a pilot house controlled switch but since my control softare knows the laz temperature and the inverter load, I might eventually automate it.

      Even when the fan isn’t running, the inverter can handle more load. I’ll probably leave it manual for at least a few months. Perhaps after I get digital ouputs driven by my Raspberry Pi. Right now I’m only using it for digital input.

      You are right about the space being very tight. Good idea on the mirror. The low tech solution is to (carefully) touch the hub of the fan to check for rotation :-).

      • Matthew Crocker says:


        Is your control system software/hardware documented on this site? I would be interested in seeing how you built it. I know you were playing with an rPi which is pretty cool.

        Have you open sourced your software by chance?

        Thanks for the inspiration you are living my dream. My wife and I hope to be doing the same thing in 10 years.

        • Brian Smith says:

          Yeah, what he said! (Matthew, about the source code to your control software.)

        • The control systems are not documented but enough people have asked that I will do that. They staerted more than 20 years ago on our previous boat when I got tired of NMEA0183 multiplexers failing and wrote my own in software. Then, since I’m seeing 100% of the NMEA0183 traffic, we might as well store it all in a relational database. Then since we had the data in a relational database, I worte a weather display that showed current and past weather conditions and another app that summarized conditions on recent trips.

          When we moved to the new boat, we went with NMEA2000. With help from a fellow Nordhavn owner, we found Kees Verujit’s CANBOAT. This is a very nicely engineered system that we use as low level access to the NMEA2000 bus. It’s a nice tight solution and isn’t hard to maintain and Kees has generously open sourced it in CANboat:

          With CANboat as the NMEA2000 interface, I rewrote the code that stores all the data into a relational database so we are back to having all telemetry on Dirona stored every 5 seconds going back many years. Jennifer ported my TripReporter software that reports summary data on single trips or groups of trips. I replaced my weather display system with Maretron N2kview. Maretron has done a super good job of N2kview and it supports both Windows and mobile (IOS or Android) and it is priced reasonably. It would take me forever to reproduce what Maretron has in n2kview and we are fairly dependent upon it. We show N2kview in the salon, the MSR, and the Pilot House and it’s installed on all of our phones and tablets.

          Jennifer wrote the software that displays the maps and takes some data from the nav database for display on the web site.

          I have recently started to use a Raspberry Pi for digital inputs and have 11 channels in use. I haven’t yet come up with a power efficient digital output solution but plan to implement digital output control as well likely using transisters for low power switching.

          I will document the system in one or more blogs in the future but the above gives a quick summary of what is there.

      • Ed Claunch says:

        You have probably solved your heat problem but there is one more low tech solution that helped me with the 3KW inverter on my airstream. Instead of mounting it directly to the floor (side wall in your case) I mounted it on 3/4″ by 1 1/2″ hardwood furring strips the length of the inverter. This creates a 3/4″ gap between the unit and the mounting surface allowing more air to circulate around it. It also saved my bacon in a freak plumbing leak since it was elevated off the floor.

        • That’s a good suggestion that I do follow with the chargers. They have 2″ under them and 4″ all the rest of the way around. On the inverter, I have a 1/2″ spacer on one side to help it fit better which gives it some air gap and the manufacturer designed in a bit of an air gap as well by spacing the back up from the connecting edges. So, I probably have a bit more than a 1/2″ on one side and a bit less on the other. There is no question that air flow really helps with these issues.

  3. Tim Morris says:

    Hi J&J

    I have commented before but no show or response, so not sure if I’m doing something wrong, or……

    Anyway, I have a half dozen photos of Dirona taken a few days ago whilst we were in Barbados. We had hoped to come and say Hi, but instead we passed you in a Pontoon boat from St Peters resort where Elaine’s cousin manages sales.

    If you would like them, please let me know where to send them.

    Best wishes, Tim Morris

    • Hi Tim. If I missed something on the web site from you, my appologies. I’ve not seen anything. Jennifer did get email from you and we did get back to you on that one. Perhaps it fell into your spam folder?

      Whatever the cause, feel free to drop by if you are still in the area. Yes, we would love the pictures. Please send them to or post them somewhere and I’ll download them. Whatever works best on your end. Thanks for chasing us down.

  4. Stewart Kelly says:

    Thanks for sharing that nice out of the box thinking about improving the inverter James. Hope Jennifer is making a speedy recovery. Glad to see she is still in good spirits!

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