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Joe Cannarozzi
01-27-2008, 08:06 PM
I have been told that the reason the heat pumps are set not to function below 40 degrees is due to the fact that freon does not flow very good below that point.

If this is true then could someone explain why it is that every car with factory A/C is designed for the A/C to cycle in the winter with the defrost on with evidently no adverse effect from cycling the freon below 40 degrees??????????

Jerry Winchester
01-27-2008, 10:15 PM
Joe,

I don't think it is a freon problem, I think it is a thermodynamics problem in that there isn't enough heat to draw out of the air relative to the delta T needed to efficiently operate the refrigeration cycle. No liquid expansion to gas means no heat.

dalej
01-27-2008, 10:33 PM
Jerry, I'm impressed!

Joe Cannarozzi
01-28-2008, 12:32 AM
Thank you Jerry. That leads me to my next question.

We have OTR A/C and the switch for the control is a 3 position rocker switch. Off is in the middle, up is A/C, down is heat.

When we purchased our bus the fellow that ran through things with me suggested that in the winter when needing heat not to put the switch in heat but always in the A/C position and with the thermostat set warm it will in fact give heat and the windows will stay defrosted better in this position.

I never practiced this because I was concerned of doing harm to the A/C compressor but I guess from what I now know this is probably a better way to go.?????

Jim_Scoggins
01-28-2008, 05:51 AM
Joe:
Thanks for the info.

Quote
When we purchased our bus the fellow that ran through things with me suggested that in the winter when needing heat not to put the switch in heat but always in the A/C position and with the thermostat set warm it will in fact give heat and the windows will stay defrosted better in this position.
End Quote

I haven't really thought about it but that sounds like a good to know item that could be used selectively. I do believe that the OTR compressor running just to heat the coach would result in more, and in most cases, unecessarily increased fuel consumption. I will use it for those early launches then switch back to heat only.

Good tip.

A thinky. The other morning I was running a cruise air just puttering around trying to figure some things out about it. It was around 35 degrees out side. The cruise air was putting out warm air. Do some of these units have an additional heat element in them?

Just Plain Jeff
01-28-2008, 06:13 AM
A 'thinky?' From a Marine? Hmmm.

Anyway, here's 2 cents worth. In a camper, you have two types of heat from your A/C units: Heat sticks (very rare in a Prevert conversion) and heat pumps.

Heat pumps actually run the A/C units 'backwards,' and are designed to do just that. If you have a cruise air with SMX, you can go to the front of the POG site and download or look at the SMX control manual for heat pump operation. If you have roof airs, usually there is a setting on the thermostat which will invoke the heat exchanger.

Depending upon your altitute and ambient air temperature/humidity, a heat pump is good to go at between 28-35F, assuming a fully-charged and in good condition unit (clean those A/C filters troops).

Air conditioners with heat pumps like to blow hot air. Wait a minute....ah....anyway, you get it.

The alternative to using heat pumps is, well, y'know.

Jerry Winchester
01-28-2008, 11:14 AM
Not to dispute Sock Boy, but if you have a heat pump and are trying to get heat out of it at 28-35 degrees, you will freeze your cajones off. They are piss poor heaters at those temps.

Ray Davis
01-28-2008, 12:14 PM
I believe that Marathon also sets up the SMX controllers to not try to run the heat pumps (at least on roof airs) below 40 degrees. My Marathon manual specifically states that they will be disabled below 40 degrees.

A walk down the lane at ORA last week in the evenings would bring the sound of many webastos running in a row.

Ray

Coloradobus
01-28-2008, 03:06 PM
We have what Marathon calls, The "Canadian Heat Pkg" our our our last and present coach. Our cruise airs work down into the teens. One of our four isn't working correctly, and it used to blow hot, now its cool. Haven't yet addressed it since we are comfortable at present. Canadian Heat option pkg gives you also extra underbelly heaters both Webasto fired and electric. We also able to heat the coach's motor with the Webasto by a separate switch on the side dash.

Petervs
01-28-2008, 08:10 PM
Ok, thermodynamically there is no reason the cruisaire heat pump operates any different than a regular house heat pump. They routinely use the heat pump system on houses to provide heating down to outside temperatures of 25 degrees before the heat pump shuts off and the auxiliary electric resistance heaters take over. Remember, to suck heat out of outside air only requires the evaporator to be cooler than the ambient air. A freon evaporator gets much colder than 40 degrees. More like 0 or -10 degrees F. There may be some problem with coil freezup in humid conditions, but we have not experienced any.

The SMX controllers on Cruisare units on our busses are factory programmed to switch to straight electric heat at 40 degrees F, but only if you have electric resistance heaters attached to your cruisaire system. Mine does not have that.
You can easily program the SMX controller to swith off the heat pump at a lower temp that 40 F. This will allow them to run effectively when the nightime air is cooler.

While at ORA Aguanga last weekend out cruisaires ran all night just dandy, we never fired the webasto.

And by the way, that was a very fine gathering of POGers, and I would like to thank Ray again for instigating the shindig. Well done Ray!

Ray Davis
01-28-2008, 08:55 PM
Thanks Peter. It was a pleasure to spend time with you and Katherine last week!

So, this raises the question, as to why the SMX controllers are programmed to do "something" at 40 degrees. I have only roof airs, and my Marathon manual specifically states that the ac heat pumps are disabled below 40 degrees.

If it's simply a matter of changing the programming on the controller, why did they limit things at 40? Surely there must be some reason?

I spoke to someone (I think it was Kevin) who indicated his cruiseair heat did indeed turn-off in the middle of the night when things got cold enough.

Ray

Kevin Erion
01-28-2008, 09:26 PM
Yes Ray, that was our bus, it was 53 inside the bus at about 5AM, I had the heat side of the cruise airs set to 68 when we went to bed. The outside air temp was 41 on my SMX when I looked. The next night I used the base board electric heat but I think the Webasco is the way to go.
I am going to see if there is a reason to let it go down to about 35, that will take care of all our needs. I will let you know what I find out.

gmcbuffalo
01-28-2008, 09:42 PM
Check the manual for the roof air in it's heat pump section to see if the reasons for this 40 degree shut off is a A/C issue and not a arbitrary decision by Marathon.

Jerry Winchester
01-28-2008, 10:15 PM
Peter,

I agree that there should be no difference between a residental heat pump and the Cruisair unit. I do however disagree with the temp at which they provide much (if any) heat.

The chart shows the balance point at near 30 deg. but the reality is the discharge temperature of the air may only be at or near the temp you set the thermostat for, so the unit is A) drafty and B) runs all the time, so the efficiency is greatly reduced.

2203

The only heat pump that I have seen successfully combat this curve is a ground source unit that circulates water thru a loop in the ground and thus the effective temperature at the condensor is in the 50's so it continues to work regardless of the outside temp.

The Cruisair in our Marathon is also set to not function below 40 deg, so the resistance heat comes on or if you select the 12v Heat, the Webasto fires up. Mike and I had the Cruisair in his H running in El Paso when we went to bed and the temp dropped to 30 degrees during the night. The "heat pump" part was working when we turned the lights out, but it was a frosty morning when we got up. We did not know how to select the alternate heat function and learned first hand about the heat pump operating range.

gmcbuffalo
01-29-2008, 12:15 AM
I found this response to this question on one of the RV.net sites.
GregM


"Nick Badame Refrig.
Cape May Court House, N.J.
New Member
Joined: 08/23/2006
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Posted: 01/27/08 08:31am Link | Quote | Print | Notify Moderator

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Hi John,

Your basement HP does not have an electric resistance back-up like regular Home type heat pumps. So, below 35 deg. it will freeze up and have no means to defrost. That is why the controls cut it out around 40 deg. and usually will have a redundant system take over. I run 2 basement heat pumps with a 45,000btu diesel fired ProHeat as secondary.

Good Luck
Nick-


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MCI 102C3 40' 2006' Bus Conversion by Atlantic Custom Coach
Detroit 475hp 8V92t w/Allison Ht750 auto
Master Mason, Cannon Lodge #104 S. Seaville N.J.
FMCA# F-27317-S
""

wrongagain
01-29-2008, 06:12 PM
Since I am coming into this thread late,...
Heat pumps compressors "in residential applications" have an electric band around them to keep the freon heated and moving.
There has always been a concern that if it is too cold and all the freon is sitting in the compressor it will pop its internal non resetable overload switch, and it does happen.
Up here if we "have" to install a heat pump we always install electric back ups in the air handlers.
And most all manufacturers of residential a/c units say do not turn on below 50 degrees.
You can get around that by installing the electric band around the compressor.