Coleman Mach a/c - twin compressors - stat replacement issue

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Dougie Brown

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Nov 16, 2007
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I've removed the original CM stat in the coach in readiness for replacement with a new Honeywell RTH221B.  I will also fit a SPDT switch to provide Hi/Lo fan which isn't on the stat.  I've identified the existing wiring without difficulty, but I'm left with two questions:-

1) The two gray wires provide freeze protection.  I'm reading that other conversions have simply abandoned them on the basis that the compressors will cut out anyway although with no error message on the stat.  Is this going to be a crucial problem?

2) I have two compressors.  They are fed by one yellow (Y1) and one orange (Y2) wires on the board.  The new stat has only a Y contact, and other conversions have simply connected both compressor wires together.  What are the consequences of this?  I understand that the Winnebago power management system will still power down one compressor in the event of more than 30A being demanded.

I don't want to compromise anything on the air side.

Dougie.
 
UPDATE: I've spoken with an RVP technician who advises that joining the yellow & orange wires (comp #1 & comp #2) will do no harm as the staggered start is controlled by the AC board.  He also advises that I can ignore the two gray wires (i.e. leave them unconnected to each other).

He did also advise that the stat needs the +12v red supplies to RC & RH, as well as the blue -12v wires.  I can't figure how to connect the blues; should I simply join them with a block?

The final thing he mentioned was that in his opinion, the new stat would be designed for AC (24v?) use, not DC.  He said this would eventually burn out either the stat or the A/C board.  I've read several accounts of people changing to a Hunter digital stat which is essentially the same as the Honeywell (which has AA batteries).  Any comments please?

UPDATE: I've spoken with an RVP technician who advises that joining the yellow & orange wires (comp #1 & comp #2) will do no harm as the staggered start is controlled by the AC board.  He also advises that I can ignore the two gray wires (i.e. leave them unconnected to each other).

He did also advise that the stat needs the +12v red supplies to RC & RH, as well as the blue -12v wires.  I can't figure how to connect the blues; should I simply join them with a block?

The final thing he mentioned was that in his opinion, the new stat would be designed for AC (24v?) use, not DC.  He said this would eventually burn out either the stat or the A/C board.  I've read several accounts of people changing to a Hunter digital stat which is essentially the same as the Honeywell (which has AA batteries).  Any comments please?

https://customer.honeywell.com/resources/techlit/TechLitDocuments/69-0000s/69-2448ES.pdf

Dougie.
 
OK. Final update.  It's all fitted and working.  The two compressors however both now start simultaneously which negates the last advice I got from the RVP tech.  It's the stat which controls the two-stage staggered start, not the board.  The risk is that I trip a circuit breaker when both compressors start together, but I'll cross that bridge if I need to.  Everything works fine, so for now, I'll concentrate on enjoying having a furnace timer which comes on without the need to shuffle out of a warm bed into the cold to turn the heat on. :)

Dougie.
 
Dougie,

Until you can figure out how to get the staggered start right, you could wire in a 12v relay with a delay timer to one of the compressors. They run less than $10.00 on amazon. It might just be the fix you are looking for, for the long run.

Steve
 
Ooooo - that sounds interesting.  Is this what you mean - http://tinyurl.com/bngstc4 ? Or http://tinyurl.com/bs4xbs9 ?

Dougie.
 
Thats close, there are some with settable delay

http://www.suntekstore.com/goods-14002896-h3y-2_dc_12v_delay_timer_time_relay_0--60_minute_with_h3y-2_base.html

I'm sure if you search you can find one that is perfect.

Steve
 
Thanks Steve.  The manufacturer's delay is 30 seconds, so anything more than that would be superfluous.  I'm liking the eBay H3Y-2 timer which provides that as a maximum.  Much obliged to you for the suggestion.

Dougie.
 
Thanks, It was just a wild idea that popped into my head. Might make an easy permanent fix, just have to see if it works like I think it should.

Steve
 
Well, each compressor has just one wire which becomes hot when the stat kicks power to them.  Both are currently joined together in one terminal instead of the two separate ones in the old stat, so delaying the power to #2 will work.  What I don't know is how the relay itself is powered.  I assume it may need an independent permanent +12v supply and perhaps a -12v one too.  I should figure it out if it comes with instructions. :)  I've ordered the eBay one for $8.99.  Thanks,

Dougie.
 
Our Coleman T-stat also kicks on the propane furnace in heat mode if the differential is more than 3 degrees, does yours address that?
 
Do you mean hysteresis?  Unsure what you mean by the differential.  The built-in hysteresis on the new stat is +/-1 deg which is much better than the previous +/-3 deg.

Dougie.
 
No, the normal range of temperature is +/-1 degree or so but if the heat pumps can't keep up with the temp selected (Or we suddenly select a temperature at least 4 degrees warmer than the current temp) our stat turns on the front propane furnace to add additional heat. Two of the wires coming out of our stat go to the propane furnace. It was finding a T-stat with these capabilities tat has me stuck.


We are not in AZ at the moment so I don't have the model number.
 
Ah, ok.  We only have a Surburban propane furnace so don't have that issue.

Dougie.
 
Not sure the brand of furnace has anything to do with that. Ours is a Suburban as well.

It is the original Coleman Mack thermostat that calls for backup heat  and kicks in the furnace when the set temperature is not realized within a certain delay. It also kicks in the furnace immediately if the new set temp is more than 4* higher than the current house temp despite the 'Electric' selection made on the thermostat. The heat pump will be locked out for 1hour 45 min. if back up was called in.

Heat pump operation, since it requires the two compressors  and fans on high (Auto) cannot normally run with only 30 amp main power unless a second source of power can be readily connected (as it can in my trailer...). The 30 sec delay between comp starts is still respected.

Unlike the thermostat Jeff describes, our model C-M thermostat does not permit the heat pump portion of the AC to run simultaneously with the furnace when it kicks in. Other C-M models do permit simultaneous running, at least according to my thermostat guide.

The call for backup heat etc is a complex function within the C-M thermostat and requires wiring to both  the AC unit and the furnace so I don't know if the other replacement thermostats would be able to duplicate it. ..... The simple solution is a second thermostat for the furnace and you to make the selection of heat source!!

And yes, your Tech was incorrect about the staggered start as you found out....
 
Stu, my air system doesn't have a electric heat pump. Or if it does, the old stat wasn't wired for it.

Dougie.
 
Jeff said:
Our Coleman T-stat also kicks on the propane furnace in heat mode if the differential is more than 3 degrees, does yours address that?
We came up with a slightly different solution to address this issue:
We have not changed out our original t-stat but didn't like the way it worked on some features.
We installed a single pole toggle switch so we could disable the gas furnace when we want to.
We also installed electric strip heat along with its own 24 volt system & separate thermostat.
It is wired in such a way so 3 elements come on when rig is connected to 220 volt service and only 1 element comes on when connected to 110 volt service so as not to use so much of available current when connected to 30 amp service, all automatically. It uses the ac blower & duct system. It has proven to be very versital for most any circumstance.
By setting the electric heat thermostat a few degrees below the ac thermostat, it works like a autochangeover thermostat.
 
My plan is to install a separate thermostat in the living room with a toggle switch as you did. Our biggest complaint is that the sensor for the Coleman is back in the bathroom hall and it does not keep the front of the m/h comfortable.
 
Dougie Brown said:
Stu, my air system doesn't have a electric heat pump. Or if it does, the old stat wasn't wired for it.

Dougie.

Ahhh!! Then you will have only the fan speeds to deal with in installing a Hunter to replace the C-M thermostat..... I have replaced 3 of these C-M thermostats in the first two years of my ownership, fortunately all were on warranty and the last one has now lasted 5 years..... The bad news is the 2nd compressor relay has given up and needs replacing so I am currently not able to use the heat pump!!! Has not been needed 'till this week fortunately..... so I have gotten by in the 30 amp site mode - single compressor for AC when required.
 
Correct.  I've installed a programmable Honeywell timer instead of a non-programmable Hunter.  It was only $11 shipped off fleabay which was a bargain.  We've already loving lying in bed listening to the 7:00 am furnace fire-up.  Just waiting on the delay timer to arrive so I can get the staggered start on the air.  Both compressors draw around 24 amps when running normally, but of course there's a huge spike on start-up.

The SPDT switch works a treat with the air fan.

Dougie.
 
Yay. The $8.99 delay relay arrived today and took around 30 mins to install. It works, and works beautifully. :D  I've set it to a 30-second delay to match the manufacturer's setting.

So a big phat thank-you to capsteve for the idea!

Dougie.
 
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