15k vs. 13.5k AC With Portable Gensets?

[GaryB]

My first RV (5er) is on order with 15k BTU ducted main AC and 13.5k BTU non-ducted bedroom AC.  Although I only plan to boondock infrequently, it would be nice to buy a generator that can run ONE of these AC's (if nothing else) and still be quiet/lightweight enough to lift easily (<80 lbs).  After much research here and RV.net, the Yamaha EF2400is seems like a winner.

My two questions are: 

1) Most say the EF2400is can run their 13.5k BTU AC, but it seems like most CANNOT run their 15k BTU AC (in "apples-to-apples" situations with similar amp draws/altitudes).  I'd like to understand this from "the numbers".  So in the link below, the compressor amp draw for the Dometic DuoTherm (the AC unit in my 5er) is only 0.8 amp more for the 15k AC than 13.5k AC (12.9 vs. 12.1 amps).  The "fan amps" are the same for each model.  So why do so many more people claim the Yamaha EF2400is cannot run their 15k unit than the 13.5k unit, since it seems like 0.8 amps is not very much difference?

http://www.dometicusa.com/pdf/os2132BriskAirRooftopLO.pdf

2) Assuming it will only reliably run a 13.5k non-duct bedroom AC, which of the following two options is the best way to keep the entire 5er cool during infrequent overnight stops en route to destination?

a) CLOSE pocket door between upper and main levels (w/ 13.5k non-ducted AC running in front bedroom), and rely on Fantastik Fan (in kitchen, near stairway) to cool the main level while opening the main-level windows?  In this case, cool air from the bedroom won't circulate to the main (living) area.

b) OPEN pocket door between upper and lower levels (w/ 13.5k non-ducted AC running in front bedroom), and try (somehow) to draw cool (air-conditioned) air from the bedroom into main living area?  In this case, I assume all windows should be closed to avoid cooling the "great outdoors"

Thank you!

 

[regularguy]

The rating for this generator is 2000 watts running and 2500 watts surge. The literature states that they loaded it to 3000 watts up to 10 seconds. Reading that would cause one to believe that the generator can handle the load of your AC unit, and in reality, it should if that is the only device you run at any given time. The concern with load comes mostly from the fact that induction motors (the compressor and fan drive) will draw up to 10 times run current for a moment on start-up and a 2500 watt (surge) compressor is a bit small. Your converter will be drawing at the same time to run charge the battery. One thing you could do if you use this compressor (I think it would be fine to use) is something that I do in campgrounds that have low voltage (105 - 110 volts). I used to shut the breaker to the converter and then start the air conditioner fan on high and quickly switch to cool hi. That way the fan is already up to speed and the start-up current for the fan motor is not a factor when the compressor starts. If you try it with an amp meter attached you will see a big difference in overall start current by the unit. Another helpful device to own is a "buck n boost". They are essentially a transformer that keeps a stable voltage on the line either by limiting or boosting. This is a good device whether you are on shore power of generator power (inverter type or not). They are not inexpensive though, ($300 - 500) but if you have ever experienced low voltage situations then they are a good investment for your rig. Actually, before I had one, when I ran my rig on the generator  I  would use an inverter to power the sensitive loads such as the microwave oven and TV. That way when the air conditioner compressor cut in and out (and the voltage dropped or would rise a little I was not fearful of the possibility of damage. Now that I have a buck n boost, I don't worry about surges or low voltage (to a certain degree). what will save you in all of this is the fact that it is an inverter type generator and won't be subject to the sure [and drop] in voltage that happens with non inverter type generators.  In closing: although the 2400 would be fine, I would get the 3000.

 

[Gary RV_Wizard]

I don't know why Dometic chose to recommend a 3500  watt generator vs the 2500 for the 0.8 amp increase in load - it may be they were just using round numbers that they thought represented the typical next step up in size from a 2500.

The additional amp load of the larger 15k btu unit will double during start up, so that 0.8 amp difference amounts to at about 2 x 0.8 x 120v = 192 watts. If we assume 2x during start up, then 2 x 12.9 x 120 = 3096 watts for a couple seconds and rapidly dropping to 1548 watts constant load. That suggests that a  generator with a 3kw surge rating would just barely handle it (most will take a 1-2 second excess surge with aplomb). 3500 gives some extra margin.

Dometic is estimating less than 2x start factor with the 13.5k btu model, since we see that 2 x 12.1 x 120V = 2880 watts and they are saying 2500 is sufficient. Working back from their 2500 watt generator requirement, we can deduce that they think the start-up load factor is 1.74. Applying the same load factor to the 15k unit, the start load would be 1.74 x 12.9 x 120V = 2694 watts, well within a 3000 watt genset capability but still beyond your Yamaha's 2400 watt surge.

Yamaha makes an EF2800i model that should handle the 15k a/c ok if Dometic's 1.74x start load factor is valid. The EF2800i is not as slick looking as the EF2400 but it weighs only  a little more and has the extra capacity you want.

 

[regularguy]

The figures that Dometic is using are an average of the curve. The initial start current is 10 times run and tapers off withing the start-up time. That would indicate that the generator would have to be capable of a 129 amp surge, but in reality all that happens is that by the time the generator starts to stall the initial surge is over and load is quickly falling to run current. During the initial start the voltage will drop which will increase load also. That 10 times initial is only momentary though. The best feature of an inverter type generator is that the voltage will stay closer to rating during the initial start because the battery takes over for the dc voltage loss as the generator slows from the load. Yamaha rates the 2400 at 2000 run 3000 surge. I would prefer a 3000 which has a 2500 run and 3500 surge, and would use my procedure as mentioned above to start the air conditioner. Note: I didn't mention that I also wired a switch so that I can opt to run my air conditioner fan constant regardless of the thermostat settings. That way my fan is always running and doesn't add to the start load as the thermostat cycles the unit on and off. Some therms already provide for that feature but mine does not.

 

[Karl]

Gary's numbers are quite accurate, and Dometic clearly states that their recommendations for generator size are based on user comments. Whether user data includes other loads running besides the a/c at the same time is unclear. Personally, I'd go with the larger 2800 too, if for no other reason than it provides you with a little more 'head-room'  for other electrical loads. 

 

[Karl]

The best feature of an inverter type generator is that the voltage will stay closer to rating during the initial start because the battery takes over for the dc voltage loss as the generator slows from the load.

Can you explain that? I'm not following your reasoning.

 

[regularguy]

I should have mentioned that the ratings by most air conditioner manufacturers are based on an ambient air temperature of 80 degrees F. Units of the RV type use a capillary tube not an expansion valve which means that during idle times, the system pressure is equalized and does not balance immediately during start-up. Of course as the ambient temperature rises, so does the system pressure and the subsequent load at start-up. It isn't relative that the compressor is creating a lower pressure on the suction side as it compresses the gas on the  discharge side because the capillary  tube is restricting flow. Capillary tube systems take longer to balance out. Add to that the fact that because of the nature of capillary tube systems, the compressor is not only compressing gas, it is trying to compress vapor. So beginning with a higher system pressure overall means that it begins with a higher load [and] for a longer time. Commercial systems that employ a "pump-down" solenoid do so to prevent the compressor from trying to start with liquid in the cylinder and to have less of a  load on start-up.

 

[regularguy]

Can you explain that? I'm not following your reasoning.

Inverter type generators that employ a battery (usually for electric start), usually have that battery in parallel with the inverter. When the generator slows from load, the battery will make-up the voltage loss temporarily. It would be like using an inverter powered by a coach battery while the battery is being charged but the charger is only getting [lets say] 100 volts ac. In that case the charger would not have the same output as it would if it were getting 120volts ac, but the battery in the circuit will make-up the difference for a certain amount of time.

 

[Ned]

That's not how inverter type generators work.  The generator puts out DC which feeds an inverter to create 120VAC, but there is no battery involved.  The design lets the motor run at lower speeds when the load is low, thus runs quieter.  As the load increases the motor speeds up to increase the output of the generator.  The larger Onan Diesel generators are inverter types and they have no batteries.  They are started from the house batteries in most installations.

 

[Karl]

RV a/c's DO use expansion valves. They are located in and are part of the discharge line. The only thing the capillary tube does is allow the suction and discharge lines to balance fairly quickly to reduce startup load. What's the difference between a gas and vapor?

 

[regularguy]

That's not how inverter type generators work.  The generator puts out DC which feeds an inverter to create 120VAC, but there is no battery involved.  The design lets the motor run at lower speeds when the load is low, thus runs quieter.  As the load increases the motor speeds up to increase the output of the generator.  The larger Onan Diesel generators are inverter types and they have no batteries.  They are started from the house batteries in most installations.

Robin Subaru R 1700 Super Quiet
How an Inverter Generator Works:

STEP 1
Produce 3 Phase
AC Power at up to
20,000 Hz.
STEP 2
Convert AC Power
to DC Power.
STEP 3
Convert DC Power to
Clean AC Power at
120 Volts/60 Hz.

Link: http://www.hayesequipment.com/robin_subaru_R1700i_generator.htm

Strengthening the core of the Yamaha inverter lineup are the Yamaha EF3000iSE and the EF3000iSEB, two of the most revolutionary inverter models available today. Ever need a little boost - extra power that gets your tool or air conditioner started quickly and efficiently? Well, now you can get that "boost" thanks to the EF3000iSEB generator from Yamaha. The ?B? on EF3000iSEB means it features Yamaha Boost Technology?, which provides additional power when you need it. Perfect for the RV enthusiast, the iSEB is an ideal source of power for either a fifth-wheel or travel trailer system. The Boost Control Unit in the generator automatically senses when additional boost is needed and it uses power from the internal 12-volt battery to produce additional amperage. In fact, the additional amps boost the alternator's 3000-watt output for up to 10 seconds, increasing the output so it's comparable to a conventional 3500-watt generator.

Link: http://www.hayesequipment.com/yamaha_generators/ef3000ise.htm

 

[regularguy]

RV a/c's DO use expansion valves. They are located in and are part of the discharge line. The only thing the capillary tube does is allow the suction and discharge lines to balance fairly quickly to reduce startup load. What's the difference between a gas and vapor?

If yours has an expansion valve, I stand corrected. The majority of RV AC's do not have one. The diff between gas and vapor is that vapor is comprised mostly of particles of liquid which cannot be compressed. That is why when checking a system, the temp of the evap discharge is calculated. Proper temp at that point  "insures" that there won't be any liquid getting into the compressor.

 

[regularguy]

BTW, if you look at the exploded view of most Duo Therms parts, you will not find an expansion valve. They are usually only on larger units.

 

[GaryB]

Thanks for the info!  I was erroneously assuming the "compressor amps" in the Dometic specs INCLUDED starting amps.  But it sounds like starting amps are in addition to those values.

Thanks again for the useful info!

Gary

 

[Karl]

Vapor: a.  The state of a substance that exists below its critical temperature and that may be liquefied by application of sufficient pressure.
b. The gaseous state of a substance that is liquid or solid under ordinary conditions.

Plagiarized from one of my chemistry manuals.
Note: Critical temperature is the temperature at which a gas can no longer be converted to a liquid regardless of the amount of pressure applied.

If you look at the diagrams (any of them for compressor'refrigerant a/c units), the expansion valve is part of the discharge tube and therefore is not listed as a separate part. They all have them.

 

[Ned]

That is an unusual generator design and not commonly found in RVs.  The most common inverter type generator is the Onan and it has no internal battery.

 

[Karl]

Ned,
I worked on one of those two years ago at QZ. The battery is a small gel cell used for starting. One couple had one and couldn't get the remote to work. It was an add-on by the dealer, and he wired it incorrectly. I must admit that throwing the battery power into the mix for startup loads is pretty imaginative, but their oscilloscope comparison to a conventional generator is somewhat misleading. A regular generator, in good working condition, will output as clean a sine wave as an inverter generator - without having to go through the additional steps of rectifying the output and then converting it back into ac.

 

[Ned]

The Onan inverter design is one of the reasons they're so quiet.  The motor is rarely running at more than half capacity.  I'm sure there are other engineering considerations, the Onan people are pretty smart.

 

[Karl]

I'm not questioning that at all.

 

[regularguy]

Plagiarized from one of my chemistry manuals.
Note: Critical temperature is the temperature at which a gas can no longer be converted to a liquid regardless of the amount of pressure applied.

If you look at the diagrams (any of them for compressor'refrigerant a/c units), the expansion valve is part of the discharge tube and therefore is not listed as a separate part. They all have them.

In refrigeration, we consider gas, vapor and liquid in the system. The low-side (evap side) of the system is where the liquid flashes out of the restricter (cap tube or expansion valve) and as the liquid absorbs heat it expands to gas. The compressor then squeezes that gas into the high side (condenser) and as it cools it goes to vapor then liquid again. I am sure you know the process, but i wanted to include the terms. If the compressor were to attempt compressing vapor it would not last very long. Either it would break mechanically or the motor would labor so much the windings would burn. We consider critical temperature as the temperature at which the vapor has absorbed enough heat that it is a gas. Considering that everything is always trying to change its state. Vapor can be gaseous (some gas in between the vapor droplets) but gas cannot be "vaporous" LOL.
As far as the expansion valve: I guess the guys in the factory were tired the day they built my Duo therm (2006 Rockwood) because they only put a cap tube and the one on my motorhome is the same. My buddy has a cigar boat with a Duo therm (15 ton) and it has an expansion valve.
I am not here to argue with you I just want to make my statements clear.