Long story short, I bought a new 5th wheel and paid to have a second AC installed. The ducts run from the bunk house to the front bedroom and I specified a 15,000 BTU connected to the existing duct work so that the increased capacity would be available to all parts of the trailer and not just the bedroom.The dealer made a mistake and installed a 13,500 BTU and not the 15,000. So far they appear to be willing to help resolve the issue since I discovered it after some minor wiring glitches had to be tracked down. The selling dealer is not local nor convenient.They are saying the Dometic rep is telling them that they don't recommend ducting a second 15000BTU AC as it will cause the evaporator to ice up. They are also saying that ducting a 13,500 "makes it 15,000" BTU.Now, I'm no expert here. The rating of BTU is the capacity for heat transfer which I see as a function of the compressor and evaporator sizing and not affected by ducting or not ducting.I'm trying to be a reasonable consumer and work with these folks and would like your expert opinion on the use of 2 15K units versus 1 15K and 1 13.5K. Remember this is deep South Texas where summer camping means high humidity and nearly 100 degree heat with regularity from June through September.Thanks in advance for your advice.

There may be something to that, as Monaco would only install 2 13.5 heat pump/AC on a 40' ducted system and those we know that have traded out, say that no dealer would install 2 15's on the duct system, only one 15 & one 13.5. I do know that the two 13.5 will just barley keep the 40' motorhome comfortable during the summer in south Texas(Pearsall). I am sure that one of each will keep your 5er comfortable. Just ask for proper compensation for the mistake and see how the summer goes.

You don't say how big your trailer is. Mine is 36' and has the same AC configure as you. Bedroom AC is not tied in to the ducting. We've had no problem with it keeping the trailer cool, including in Las Vegas and eastern Oklahoma in the summer time. We just leave the bedroom door open. The thermostat for the main AC is generally set at 68. If I remember correctly, the compressors even cycle off from time to time. I have redone the insulation under the trailer and I ordered it with double pane windows. I don't know how much that contributes to keeping it cool, but it can't hurt.

I have two ducted 15k low profile Coleman Mach AC's on my 5th wheel.I have had no issues with icing, or anything else, running them at the same time.Unless the Dometics are radically different, I doubt the info they've told you is accurate.

Thanks for the note. The trailer is 41 1/2 feet full profile. Maybe asking to speak directly to the Dometic rep is advisable.

The difference from adding the 15k BTU and the 13.5k BTU air conditioner is only 5% of the total capacity and then concentrated in the bedroom. It is not enough to worry about.

13,500 BTU vs 15,000 BTU second AC | Good Sam Community

dougrainer
Nomad
Jul 31, 2013
1. There is NO problem mounting 2 15k AC units on ANY RV---PERIOD.
2. Monaco did that because they were cutting cost NOT because there was an operational or ducting issue.
3. NEWMAR has installed 15k (2 and 3units) for 15 years on their ducted system with np problems
4. Tiffin installs 3 15k ducted AC units on their 40 to 43 foot motorhomes
5. The 13.5 and 15 k IF you have a wall tstat will have a freeze sensor that prevents ANY possible freeze-up.
6. There is NO WAY that Dometic told anyone that ducting a 13.5k will make it a 15k in operation. THAT IS COMPLETELY FALSE AND RIDICULOUS.
7. It seems the dealer is trying to NOT have to spend another $1000 to fix their mistake.
8. IF the extra AC has roof mount controls, you do have the possibility of freeze up, but you can stop that by always running the fan on HI/cool.
9. It is ALWAYS best to have the largest BTU AC units on a RV, as it is better to have MORE potential cooling capacity that nominal capacity. Doug
Chris_Bryant
Explorer II
Jul 31, 2013
It depends on the duct sizing and layout- some ductwork is barely adequate for a single 15k/btu unit, much less two.
j-d
Explorer II
Jul 31, 2013
I'd say Talk to Dometic is good advice.

Started to say it's possible that somebody doesn't think the ducting can handle the increased air flow that the 15K represents over the 13.5 but when I looked it up in Coleman it said their 15K uses the "biggest blower motor in the industry at 1/3-HP" but the rated air flow was 325-cfm and the 13.5 was rated 320-cfm with motor size not specified.

But it IS clear that if an A/C can't flow air, it will freeze over.

Excess capacity isn't good either. To me, two 15K on a fiver that big wouldn't be excess, but the 15K in our 31' Class C is overkill in our 95* high humidity FL conditions. Its cooling cycles are so short that it doesn't have time to condense enough humidity out of the coach. We just added a little dehumidifier to help with that, and it seems to have made a big difference.

I take it this is a 50-amp setup and you plan to use 50A sites all or nearly all the time? I mention this because our 15K causes problems with our 30A service.
I_am_still_wayn
Explorer
Jul 31, 2013
The difference from adding the 15k BTU and the 13.5k BTU air conditioner is only 5% of the total capacity and then concentrated in the bedroom. It is not enough to worry about.
TexasRedNeck
Explorer
Jul 31, 2013
Thanks for the note. The trailer is 41 1/2 feet full profile.

Maybe asking to speak directly to the Dometic rep is advisable.
drfife
Explorer
Jul 31, 2013
I have two ducted 15k low profile Coleman Mach AC's on my 5th wheel.

I have had no issues with icing, or anything else, running them at the same time.

Unless the Dometics are radically different, I doubt the info they've told you is accurate.
CentralCoastCam
Explorer
Jul 31, 2013
You don't say how big your trailer is. Mine is 36' and has the same AC configure as you. Bedroom AC is not tied in to the ducting. We've had no problem with it keeping the trailer cool, including in Las Vegas and eastern Oklahoma in the summer time. We just leave the bedroom door open. The thermostat for the main AC is generally set at 68. If I remember correctly, the compressors even cycle off from time to time.

I have redone the insulation under the trailer and I ordered it with double pane windows. I don't know how much that contributes to keeping it cool, but it can't hurt.
The_Texan
Explorer
Jul 31, 2013
There may be something to that, as Monaco would only install 2 13.5 heat pump/AC on a 40' ducted system and those we know that have traded out, say that no dealer would install 2 15's on the duct system, only one 15 & one 13.5. I do know that the two 13.5 will just barley keep the 40' motorhome comfortable during the summer in south Texas(Pearsall). I am sure that one of each will keep your 5er comfortable. Just ask for proper compensation for the mistake and see how the summer goes.

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13,500 BTU vs 15,000 BTU second AC

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