Log In | Register
Skip to main content
Topic: A/C won't work with the generator above 104F (Read 734 times) previous topic - next topic
0 Members and 1 Guest are viewing this topic.
A/C won't work with the generator above 104F
Note: this is about generators running air conditioners on RVs in general, nothing specific to LDs.

Problem: In the heat at Bishop, CA, last week, my 2005 Carrier AirV A/C (non-ducted 13,500 btu/h @12.8 amps - service manual) would stall my 4KW Onan generator when the outside air temperature exceeded 104F. Below that, both units appeared to operate normally, and I didn't have any problems cooling the RV (25' Class C). At 105F and the 4200' elevation of Bishop, the density altitude was 7,500', reducing the generator power by almost 20%.

Potential solution: Lower the power demand of the A/C by replacing it with a lower capacity or higher efficiency A/C; e.g., the Coleman Mach 3 Power Saver uses 10.2 amps (I think) and is still 13,500 btu/h. There may be other units with equal or lower drain; also, I think an 11,000 btu/h unit would cool adequately, given my Carrier 13,500 unit easily cools the motorhome in the sun at 105F, using the low fan speed.

Besides the lower operating current, I'd really like to have a unit that is noticeably quieter!

What I'd like to hear from the LD group...

  • Any ideas for improving A/C operation with the current Onan/Carrier units.
  • Suggestions for a new A/C unit with lower power operation and quieter running.
  • A link(s) to reviews and specifications for the various units (I've not had much luck getting real data beyond the "Woo hoo, is our unit good!" brochures.
2005 Jayco 24SS
Re: A/C won't work with the generator above 104F
Reply #1 –
Have you tried the altitude adjustment knob?

By my (possibly wrong) calculations, a 4000w generator should still have ~3000w of power under the conditions you describe.  That should be plenty to handle the A/C.

Is there something else (like the battery charger) that could be increasing the electrical load?

You've probably already seen this, but if not check pages 7 and 9:
http://www.joshwakefield.com/xplorer/onan-microquiet-4000.pdf

Rich
2003 MB
Re: A/C won't work with the generator above 104F
Reply #2 –
I would look to the generator which should be able to provide 24 amps at 4500ft. An AC clamp type meter should tell you how much current the generator is producing. If it is producing 24 amps then turn off the other appliance/s. If it is producing only enough amps to run the A/C then fix the generator. If it is producing 24 amps and only the A/C is running then replace the A/C.

A properly running generator is able to run a properly running A/C unit at 4500ft. Which one is failing is not yet clear.
Harry 2006RB
Re: A/C won't work with the generator above 104F
Reply #3 –
Quote from: Eric Greenwell  - 
Suggestions for a new A/C unit with lower power operation and quieter running.

Hello Eric ...

1.  Atwood RV Air Conditioner
2.  Atwood vs Dometic
3.  https://www.youtube.com/watch?v=LYPkqn6rfXw

Many of the Foretravel owners like their Atwood, which is discussed in link #1.   I'd very much like to find a new A/C with lower power requirements and quieter running.  Right now, Atwood is the leading candidate in my search.  I really hope you're successful in your search and post your results.
Re: A/C won't work with the generator above 104F
Reply #4 –

The traditional cure has been to install a larger starting capacitor, sometimes known as a hard start capacitor.
Like this.
Amazon.com: SUPCO SPP6 Relay/Capacitor Hard Start Kit with 500% Increase...
Dometic Duo Therm RV Air Conditioner Hard Start Capacitor

Larry

As an Amazon Associate Lazy Daze Owners' Group earns from qualifying purchases.
Larry
2003 23.5' Front Lounge, since new.  Previously 1983 22' Front Lounge.
Tow vehicles  2020 Jeep Wrangler Rubicon, 2001 Jeep Cherokee
Photo Collection: Lazy Daze
Re: A/C won't work with the generator above 104F
Reply #5 –
My thoughts.
With adensity altitude of 7500' the carburator mixture needs to be set at 7500'.  4500 ft. carb setting will make the generator run over rich and die out.  You may have to tweek the carb setting a little get proper operation. 

Operationally start the high current items first. Air Conditioner, then the converter.  The converter will likely have to be on  to charge the coach battery.  the coach battery is used to excite the field to generate 120 VAC power.
The microwave and hair dryer may have to stay off.

Generator surging causes havok with the old('88) tranfer switches, as they used an thermal overload device to effect the time delay. I start the genetator with the Air Conditioning Breaker OFF until the generator has warmend up to avoid brownout damage.

In the Foundry setting I work in we used 120VAC window AC units to cool 240 VDC bridge crane cab for a decade.  When they broke we got new ones.  The window units that came with R410 would not work inour high heat applications...The Compressor head pressures got too high.  We tried hard start capacitors but that did not help in the full day run. 
Rodney
1988 Mid Bath
Re: A/C won't work with the generator above 104F
Reply #6 –
Hello Eric,

If you're not a member of the foretravel Forum, you'll probably not be able to see the information in the above link.  So I did a cut & paste of a couple comments.

1.  "I replaced the old Coleman Mach in my 91 last spring with an Atwood 15k heatpump. Real quiet compared to the old one. Having the seperate fan motors for evaporator and condenser is the ticket. The inside fan can be set low and condenser fan outside can do its thing.When the bedroom unit starts giving out I'll put a,straight air unit back there when that time comes. I recommend Atwood"

2.  "We purchased the Atwood Air Command 13.5 without the heat pump last year. It is quieter than the Dometic we have in the bedroom, It does not however cycle off, the indoor fan runs continuously but it does change speeds as needed for cooling demand. The remote control feature is a nice plus for operation while driving.
Happy with it thus far."

3.  "Two years on our unit without incident. Unducted, w/ heat pump. I plan to replace our bathroom unit (first owners moved it forward) but we seem to use the Atwood most of the time.   Remote was a disappointment and I wish the filters were more robust. Overall, we are pleased w/ performance.

There's more, but these are representative of the comments about the Atwood A/C.
Re: A/C won't work with the generator above 104F
Reply #7 –
Have you tried the altitude adjustment knob?
 Yes, and it didn't appear to help any.

By my (possibly wrong) calculations, a 4000w generator should still have ~3000w of power under the conditions you describe.  That should be plenty to handle the A/C.
And it did run the A/C, but the generator stalled only when the A/C compressor cycled "on" in temperatures above 104F. Compressor cycling below 104F did not stall the generator, so I think the reduction in power due to high elevation and temperature), and the compressor being harder to start due to high temperature, were what caused the generator to stall. Maybe a workaround would be to turn the temperature setting as low as it would go to prevent the compressor from ever stopping, or raising the temperature to, say, 90F, to allow the pressure in the compressor to diminish more between "on" cycles.

Is there something else (like the battery charger) that could be increasing the electrical load?
I turned off the battery charger, but I now realize the refrigerator would have automatically switched to electrical operation (instead of propane), adding a 300 watt load to the generator. All other appliances except a laptop and an iPad were turned off.

You've probably already seen this, but if not check pages 7 and 9: http://www.joshwakefield.com/xplorer/onan-microquiet-4000.pdf
I had seen that. What I really want is an A/C with drawing lower power while starting the compressor, but the specifications I've seen don't show that.

Maybe I should contact Carrier for their opinion of the problem, and also get their recommendation for an A/C that is easier to start (and quieter). Asking Dometic, Coleman, Attwood, etc for their recommendations for a replacement might be the quickest way to find a better unit.
2005 Jayco 24SS
Re: A/C won't work with the generator above 104F
Reply #8 –
Quote from: hbn7hj  - 
A properly running generator is able to run a properly running A/C unit at 4500ft. Which one is failing is not yet clear.

Not really - because of the temperature, the density altitude for a carbureted engine was 7500', as announced on the Bishop Airport's automatic weather station. So, lower engine horsepower, and higher starting load from the compressor.
2005 Jayco 24SS
Re: A/C won't work with the generator above 104F
Reply #9 –
Quote from: Traxless  - 

Many of the Foretravel owners like their Atwood, which is discussed in link #1.  I'd very much like to find a new A/C with lower power requirements and quieter running.  Right now, Atwood is the leading candidate in my search.  I really hope you're successful in your search and post your results.

The articles were helpful - I'll now be asking about the minimum generator size as "figure of merit" for low starting power. And I will report what I find about the various units an mfg contacts.
2005 Jayco 24SS
Re: A/C won't work with the generator above 104F
Reply #10 –
Yep, the failure is that the generator cannot start the A/C when the compressor wants to kick in again. Maybe the suggestion of the start cap is the best one. I'm at 9300ft but temperature doesn't get above 80. Right now they are 50F. If it gets in the 70's in the next day or two I'll try it, not that it means anything. Will post what happens.

The internal differential pressure in a compressor that has been off for a while (2 minutes) is 0 isn't it, regardless of temperature? If that is the case the problem is the reduced power of a warm generator?. Clean air filter, make sure the choke is not partially on, adjust the mixture to it's leanest.

Harry 2006RB
Re: A/C won't work with the generator above 104F
Reply #11 –

With adensity altitude of 7500' the carburator mixture needs to be set at 7500'.  4500 ft. carb setting will make the generator run over rich and die out.  You may have to tweek the carb setting a little get proper operation. 
The altitude adjustment is limited to about 5500' on the Onan, so that is where I set it.

Generator surging causes havok with the old('88) tranfer switches, as they used an thermal overload device to effect the time delay. I start the genetator with the Air Conditioning Breaker OFF until the generator has warmend up to avoid brownout damage.
No surging - transfer relay doesn't have a delay built into it. I do start the generator with big appliances turned off.

2005 Jayco 24SS
Re: A/C won't work with the generator above 104F
Reply #12 –
Eric,

With regard to pressure equalization, yes, the pressures will equalize in the system regardless of temperature, but I wouldn't count on it in a couple of minutes, especially as the system high side pressure when running increases with the ambient temperature. It can take up to five minutes for the pressures to equalize in the system.

"Maybe a workaround would be to turn the temperature setting as low as it would go to prevent the compressor from ever stopping, or raising the temperature to, say, 90 F, to allow the pressure in the compressor to diminish more between "on" cycles".

I think either one of these are workable.  I'd go for the later.  Find the normal comfort setting on your thermostat you prefer in your coach so you can return to it easily, then raise to the highest setting for a few minutes.  Going low can have some complications if you forget to reset the thermostat as the coach\ambient temperatures go down. 

Bill




Bill
2003 -- 23' FL
Re: A/C won't work with the generator above 104F
Reply #14 –
Quote from: Craig P  - 
My plan
EasyStart 364 (3-ton) Single Phase Soft Starter for Air Conditioners |...
I like it, too: half the price of new A/C, and a lot less trouble to install. It wouldn't reduce the A/C operating noise, but I'm going to experiment with some internal duct smoothing that I hope will reduce the fan noise.

Will you be installing one soon? I won't be at high elevations or high temperatures for a while, maybe not until next summer, so I'd like to wait and hear how well it works for others. Though, I have to think it would work well, since it allows the Honda 2000 portable generator to run the same A/C units!
2005 Jayco 24SS
Re: A/C won't work with the generator above 104F
Reply #15 –
I have an Onan 2800 that can barely start the 10,000 BTU A/C. It always starts (except when it blew out the voltage regulator) but it makes one cringe. Maybe the Easy start would help. You can get $40 off the Easy Start by typing in Airstream in the discount space.

I still think a 4KW generator should be able to start/restart a 13,500 BTU A/C unit at a density altitude of 7500ft. I will try it at a density altitude Of 12,500ft in the next day or two just as a datapoint for everyone.
Harry 2006RB
Re: A/C won't work with the generator above 104F
Reply #16 –
Eric,
Under the heading of "been there, done that", I will say up front that, in our case, we replaced the A/C unit. It was on a much older Minnie Winnie and a long time ago. However, the fundamentals of the problem still apply. And, fundamentally, the issue is that the A/C is pulling too much current trying to start (at least, that's what I understand to be the problem), which is causing the generator to stall. Hence, Larry's,  and other's suggestion to install a "hard start kit". While it is true that, after a while, the pressure differential should be around zero, that doesn't mean the pressure inside the compressor is zero; that will be determined by the temperature and the type of freon ( say R22 vs R134a). As soon as the compressor starts spinning, it will be working against quite some pressure and the hotter, the higher. In you case, the system simply has reached the pressure (or temperature, if you please. PV=nRT, so basically the same thing)  where the generator can't supply enough electricity to spin the compressor. This is _most probably_  a compressor or starting capacitor problem. However, at 12 years old, I would do exactly what you are considering: replacing the entire unit, especially if you are planning to keep the motorhome for a while.

Ken
ex 2009 MB owner

PS It is also possible, but not as likely, that the size of the wiring to the A/C unit is too small. That would be nearly impossible to remedy. However, the connections to the unit could be loose or corroded enough to cause the problem, And, the problem could be at either end, ie, at the circuit breaker panel or the A/C unit. Just a thought.
Former 2009 MB owner
Re: A/C won't work with the generator above 104F
Reply #17 –
Quote from: hbn7hj  - 
I still think a 4KW generator should be able to start/restart a 13,500 BTU A/C unit at a density altitude of 7500ft. I will try it at a density altitude Of 12,500ft in the next day or two just as a datapoint for everyone.

It's not just the power loss due the altitude that I think is causing the generator to stall, but excessive compressor pressures due to the 105F+ temperatures. If I am right, you may have no trouble if the temperatures are lower, say, in the low 80s. Still, let us know how it works and air temperature at the time.
2005 Jayco 24SS
Re: A/C won't work with the generator above 104F
Reply #18 –
Quote from: Larry W  - 
The traditional cure has been to install a larger starting capacitor, sometimes known as a hard start capacitor.
Like this.
Amazon.com: SUPCO SPP6 Relay/Capacitor Hard Start Kit with 500% Increase...
Dometic Duo Therm RV Air Conditioner Hard Start Capacitor

My Carrier AirV wiring diagram shows a PTC and capacitor with 3 wires - does a kit like this install in place of, or in addition to the factory setup? Or perhaps the OEM capacitor has deteriorated over time and needs replacing?
As an Amazon Associate Lazy Daze Owners' Group earns from qualifying purchases.
2005 Jayco 24SS
Re: A/C won't work with the generator above 104F
Reply #19 –
 OK. Outside air temp 70F, inside air temp 79F. 4kw Onan starts a 13,500 BTU Dometic Penguin with little hesitation and restarts it after 4 minutes with little hesitation . Altitude 9300 ft. Density altitude is a guess at 12,000ft, mixture full lean.

QuickStart capacitor installs in addition to OEM cap, I think.
Harry 2006RB
Re: A/C won't work with the generator above 104F
Reply #20 –
Quote from: hbn7hj  - 
OK. Outside air temp 70F, inside air temp 79F. 4kw Onan starts a 13,500 BTU Dometic Penguin with little hesitation and restarts it after 4 minutes with little hesitation . Altitude 9300 ft. Density altitude is a guess at 12,000ft, mixture full lean.

QuickStart capacitor installs in addition to OEM cap, I think.
That's a 35 F difference in outside temperatures - Bishop and your location! Your temperature conditions were ideal for an easy start - low outside temperature, higher inside temperature, meaning the refrigerant was easily cooled (it's easy to move heat from a hot area to a cool area). At Bishop, it was 105 F outside, making it very hard to cool the refrigerant. Practically speaking, we would not even be running the A/C at those 70/79 F temperatures - just turn on a vent fan or open the door.

I have ordered a "hard start" cap - it doesn't do any harm, might help some, and it's only $12. I wish there was a way to test it effectiveness without finding 105 F temperatures at high altitudes, as it's unlikely we'll be in that situation again until next June!

If it's still not enough, the easiest, quickest workarounds appear to be running the indoor temperature at 85 or 90 instead of 80; turning the thermostat to full cold to keep the compressor from cycling; plugging in to a 30 amp circuit. More expensive fixes would be installing a $300 "EasyStart" or a new, low current start A/C at $600+.

Staying away from Bishop, CA, in July might be a good idea, too, and we are looking at going there in early June instead of early July, but it's not clear the weather is as good for flying gliders then.
2005 Jayco 24SS
Re: A/C won't work with the generator above 104F
Reply #21 –
I did spring for the easy start for my Roadtrek. The Onan 2.8 KW labors to start the A/C in the best of conditions and has already blown out one $300 voltage regulator. I will let you know if there is any improvement.

I tried to install a start cap in the Roadtrek but couldn't figure out how to do it or where to hook it in the Dometic Cool Cat 10,000 BTU unit. There was a space problem for it also. It is more straight forward in the rooftop models. The Easy Start is on it's way so I may have report on that soon.
Harry 2006RB
Re: A/C won't work with the generator above 104F
Reply #22 –
Quote from: Eric Greenwell  - 
My Carrier AirV wiring diagram shows a PTC and capacitor with 3 wires - does a kit like this install in place of, or in addition to the factory setup? Or perhaps the OEM capacitor has deteriorated over time and needs replacing?
Most Factory A/Cs have one run capacitor and are missing the optional start capacitor. Aftermarket A/Cs often come with them.
The hard start capacitor comes with instructions and is a two wire hookup. The hardest part is getting the A/C's cover off.

You will find these devices on many home A/Cs too, where the house or area has low voltage issues.
On hot days, many RV parks will have low voltage problems, the hard start capacitor will help in these situations too.

As for staying in Bishop in July, did that once and never again. I would find a place to stay up the hill and commute to the airport.
Is Bumper still flying?

Larry
Larry
2003 23.5' Front Lounge, since new.  Previously 1983 22' Front Lounge.
Tow vehicles  2020 Jeep Wrangler Rubicon, 2001 Jeep Cherokee
Photo Collection: Lazy Daze
Re: A/C won't work with the generator above 104F
Reply #23 –
As another datapoint. 101 outside temp, 107 inside temp, 1100ft altitude, 4400ft density altitude. The 2.8 Onan started the 10,000 BTU Dometic A/C with the same RPM drop it always has. The restart after 4 minutes seemed a little more labored but the generator handled it.

11.3 amps current draw, 1.6 amps of that is fan. A/C unit is essentially new with very little run time since 2015 install.

Easy Start arrives today!
Harry 2006RB