in good shape - brunnhilde part 14

I conduct another round of tests in using the Honda EU2000i generator to power Brunnhilde's Kerstner roof top air conditioner. I want to determine once and for all if it is worth keeping and how reliably the Honda generator can power it, in Eco mode. While I am conducting the test at near sea level, I have to be aware of the reduction of the generator output at higher elevations.



Brunnhilde's air conditioner; it consists of common automobile AC compressor turned by a 12Vdc motor; the DC motor is chosen such that the AC may be power the the Sprinter alternator when the vehicle is on the road; for powering the air conditioner with 120Vac (or 240Vac in Europe) 2 big switch mode power supplies are ganged together to convert the alternate current to 12DC in excess of 100A! certainly qualifies as a ass-backwards design

all counted there are 8 DC motors in the entire Kerstner roof top air conditioner system - how ass-backwards can you get?

For the test, I wanted to observe the cycling of the compressor and how the entire AC system cope with the voltage sag bought on by the transient reactive loads upon the Honda inverter generator set to Eco throttle mode. To this I set the air conditioner temperature dial to fairly warm, to be able to observe it cycles fairly frequently. This was not too hard, and once I noted how frequency of the cycles I can go and observe it at the generator.

the Honda generator transient responses in Eco mode


In the above video here are the events in the timeline:
0 - 0:18 only blower is on
0:18 compressor switches on and the generator responded
0:24 - 1:03 compressor and the generator running in steady state
1:03 - 1:33 compressor switched off with only the blower on
1:33 compressor switches on and the generator responded
1:38 - 2:17 compressor and generator running in steady state

Judging from the RPM of the generator, it is running slightly over half of it maximum power.

I set up a DMM to see how low the voltage drooped. The DMM has low sampling rate.

even with a slow sampling DMM you can see the voltage sags very low


It should be note that the significant voltage droop and the stuttering of the generator is likely not the short coming on the generator's part. I suspect the transient current (of the very complex inductive as well as solid state switcher load) over a few seconds is very complex, and the generator have to try to spool up and down to try to keep pace. There is almost some hind of oscillation.

Observing the significant voltage sags from the generator, I have no confidence that the Kerstner can reliably power by the Honda set to Eco mode, especially in very hot days, or worst, in high elevations. Because of this, the inefficiency of the system, and the troublesome complex design I have decided to replace it with a common small US RV roof top AC. I narrow the candidates down to the Dometic Penguin II 11,000 BTU and the Coleman Mach 8 Cub 9200 BTU.

I did a lot of research on the web. In noise and performance, I think the Dometic is better. It is nearly 2 inches higher than the Coleman's 8 1/4" height. The benefit of the Coleman is lower 9200 BTU and hence lower operating, and hopefully lower transient surge current. Its lower height also make it less efficient and louder.

dometic penguin II

coleman mach 8 9200BTU

Evidently the early Coleman Mach 8 suffered very loud evaporator fan noise. Some one cancelled an order of a RV/trailer because of the high noise level of the AC.

The original 6 blade fan was later replaced by a quieter 3 blade design.

I tried to think through what it will take to remove the Kerstner and the effort to install the new AC. The greatest challenges of the replacement is working at the great height. As I think through this, the removal of the Kerstner can be made easier if I dismental as much of it as possible to reduce the weight. It is the most unwieldy because of the in-cabin evaporator and blower. Getting the new AC on the roof should not be that hard as they have flat bottoms.

It cannot be that hard, even with the enticipated modification and patching up the roof, and the reverse installation. I installed two mini-splits myself and one required an extended refrigerant line run. I don't own a vacuum pump so I used a compressor from a discarded refrigerator as vacuum pump - which is a tool you need to install a split air conditioner.

I struggled over the decision of choosing between the Dometic and the Coleman. In the end I decided to go with the Coleman as it stand better chance of working well with the Honda generator. Its 9200 BTU is a suitable fit for Brunnhilde's limited interior volume. I think the higher noise level (emited by the less efficient evaporator fan due to the low pancake aspect ratio) should not be too bad due to the Brunnhilde's great height. I suspect the upward facing evaporator fan noise is probably very comparable to that in the Kerstner.

I found a very good sale price of the Coleman without having to pay the common $120 freight shipping. I didn't want to miss the sale so I ordered it today. At this point I need to determine which ceiling unit (ADB) to get - to extract the needed thermostat and the rotary control switch. There are two. The two controls are grouped together in the more expensive one so that will be an easier transplant. RV ACs shares 85% the  parts as household window AC. You can pick up a 11,000 BTU units that are very nice (by that I mean with a nice rotary compressor inside, copper tubings, similar quality blower fans) for $250. Yet an RV AC of comparable BTU size sold for $800 and you have to buy the ceiling unit separately for circa $70.

I was in Home Depot yesterday returning all the unused plumbing fittings, and saw countless folks snapping up the portable ACs in preparation for the onslaught of heat wave here. I would wager that many have no idea the drawbacks with these units - need to deal with the condensate, and feed two hoses to through a window. They are inherantly loud and inefficient due to the back pressure of having to expel heat through a very restrict hose with a lot of friction. A lot of these impulse purchases are destined to the landfill after a few seasons.

Update 27.06.2015:
I didn't want to make the decision to buy the ceiling unit for the Coleman Mach 8 air conditioner. Since I will be installing it as a ducted configuration, all I want from the ceiling unit (air distribution box ADB) are the thermostat and the rotary control switch. I can either buy the standard issued one for circa $65, or the deluxe unit for $70 shipped. The advantage of the deluxe model is the two controls are grouped together and there is one metal junction box behind them, making it really easy to transplant to where the existing Kerstner control is.

the standard issued ADB

 the deluxe ADB with the controls grouped together

I let the sale day lapse (ended June 26) because I wanted to see if there is the possibility of reusing the existing from the Kerstner even though the circuit is 12Vdc instead of 120Vac. Why? I knew it is very likely the thermostat and may be the toggle on/off switch are rated for AC and DC.

i check this morning after the last of the sales day lapsed - but guess what? they play the perpetual sales to sucker you to buy now (I found much better price for the AC unit elsewhere); it now shows the sale prices are valid until the end of the month

I couldn't find a schematic of the Coleman ADB nor the Mach 8 air conditioner online. It is however very likely there is a circuit diagram inside the roof top AC. I have to wait until the unit arrives.

this morning i spent a few minutes to assess the feasibility of reusing as much parts of this Kerstner control panel as possible

note the cheesy prototype breadboard used for the fan speed control (that never work properly)

the thermostat is just a very common one used for air conditioner - i tried to take a photo with adequate resolution to see the markings on the body

in this view you can make out it says 230 250Vac - so i am very certain i can reuse this thermostat; this kind of thermostat is very common in European refrigeration appliances and there are mainly three versions - refrigerator, freezer, and air conditioner differ in their intended operating temperature ranges; i believe they operates based on the bi-metal characteristic it is a calpilary tube thermostat

I was wrong on my initial assertion the thermostat operates based on the bi-metal characteristic. The appearance of the snap action diaphragm led me to think it is a bi-metal action. The commonality is the snap action of both provides the needed hysteresis.

When the similar thermostat in one of my European Frigidaire refrigerators failed, I found the closest equivalent replacement in UK. I have to call the vendor to order it and the chap warned me that it is a 240Vac, therefore not for used in US's 120Vac appliances. I told him I just want to order it...

In addition to the thermostat I would also need the switch to control the main on/off, blower low speed, and blower high speed. In the Coleman ADB, these are all integrated into one rotary switch. I see however, no reason I cannot implement them with separate switches by reusing the On/Off rocker switch of the Kerstner, in conjunction with another switch that replaces the variable fan control of the Kerstner.

The decision for now is not to order the Coleman ADB.