Sizing a mini split for an RV

[Steve from Socal]

I want to get rid of the roof mount AC units on my 5th wheel.

The area is divided into a main area kitchen, living etc and bedroom/bath. The main area is roughly 25X12X8' with the slides extended. The bedroom/bath is 15X10X7' slide out.

There are two 15K BTU roof mount AC units both ducted into a central plenum one unit keeps the camper cool under "most" conditions.

The on-line specs for two zone mini's show 18K BTU able to cool up to 850 Sq Ft? Is an 18K two zone the right choice?

 

[PoorUB]

At a glace I would say the 18K should do it. but you are removing 30K and I wonder how well insulated a RV really is, so I question the sizing. Maybe 24K would be better? But I really have no idea.

How well did the two units cool the space in hot weather when they worked properly? Did they run all the time? Half of the time?

 

[dcg9381]

The on-line specs for two zone mini's show 18K BTU able to cool up to 850 Sq Ft? Is an 18K two zone the right choice?

Those specs probably don't take into account the "great" insulation common to most RVs. The roof top ducted units aren't that great because the ducts typically run along what's left of roof insulation.

And the splits are pretty directional. Might not be a big deal in Kansas, but a 5th wheel with a single AC would fry here in Texas front to rear.

Typically RVs only have 120V available, so how many BTU can you get on 20A?

 

[Steve from Socal]

The RV is well insulated for an RV R-25 roof and floor, R-15 walls. This is not wired as a standard RV, I have the original 50 amp RV plug and a 50 amp 240 plug and panel for electric stove, water heater and Mini?

The mini would be a two zone so one wall unit in the bed and one in the main. A 9K in the bedrom and 12K main? The roof tops are not known to be that efficient and the plenum is kind of a good/bad overall. I would not discount a 24K unit but, I have heard not to oversize these by too much?

 

[thunderalley3]

No real advice as I am not an AC guy but I would look at the new AXIOM 5th wheels as they are using mini splits in them from the factory. We were at an RV show and went through a couple of them that had factory mini splits.
This may give you some guidance on size and ideas for mounting the unit etc.

 

[PoorUB]

The RV is well insulated for an RV R-25 roof and floor, R-15 walls. This is not wired as a standard RV, I have the original 50 amp RV plug and a 50 amp 240 plug and panel for electric stove, water heater and Mini?

The mini would be a two zone so one wall unit in the bed and one in the main. A 9K in the bedrom and 12K main? The roof tops are not known to be that efficient and the plenum is kind of a good/bad overall. I would not discount a 24K unit but, I have heard not to oversize these by too much?

With that much insulation the 18K would probably do just fine. The 9k and 12K seems reasonable. You should look closely at specs and you can often over size the indoor heads by 125% You might be able to put an 18K head in the main part and a 6K in the bedroom. Granted on wide open you will only get what the outdoor unit will put out.

As for over sizing, pretty much all mini splits are inverter units and ramp down pretty far. Depending on the brand they might ramp down to 4K BTU, so it is pretty much impossible to over size one. A few years back for the brands I sold a 12K or an 18k would both ramp down to 4K.

 

[dcg9381]

The RV is well insulated for an RV R-25 roof and floor, R-15 walls. This is not wired as a standard RV, I have the original 50 amp RV plug and a 50 amp 240 plug and panel for electric stove, water heater and Mini?

That's amazing insulation for an RV. You've got 240V power in it... Go for it. 9k/12k should work just fine. You really can't get this too wrong with the way modern mini-spits work and they're certainly better than the roof top units in terms of efficiency. Climate matters though, I'd probably do 12k/12k in Texas minimum.

If you're going to heat with these, watch their low temp performance. Nothing eats propane faster than an RV in winter!

You may find it's easier (and more cost effective) to do two separate units over a dual-head deal.

 

[Steve from Socal]

That's amazing insulation for an RV. You've got 240V power in it... Go for it. 9k/12k should work just fine. You really can't get this too wrong with the way modern mini-spits work and they're certainly better than the roof top units in terms of efficiency. Climate matters though, I'd probably do 12k/12k in Texas minimum.

If you're going to heat with these, watch their low temp performance. Nothing eats propane faster than an RV in winter!

You may find it's easier (and more cost effective) to do two separate units over a dual-head deal.

This is an older premimum 5th wheel designed for ful time living in both cold and hot weather. It gets about as hot in central Kansas as Texas, mid 90's to 100 teens with moderate humidity. We do get much colder winters, as far as heat, a heat pump mini and a couple of electric space heaters would be more than enough. I have keeped the inside at 65+ in the low teens with space heaters alone. There is no propane in this, I removed all the gas equipment and went ellectric hence the 240 service.

 

[PCMusicGuy]

18k seems overkill to me. My folks just retired after living out of their RV (5th wheel) for 10+ years in the Louisiana and Texas area. The majority of the time was in their last camper which was a Forest River Cedar Creek Hathaway (38FBD in case you wanted to see the floor plan). It has 2 A/Cs but couldn't keep up in the Summer. They installed a 12k Pioneer mini split and mounted it at the very back of the camper and it was able to keep the entire camper cool, even without the roof unit running. At night, they would still run the A/C in the bedroom area and the mini split, but they like it cold to sleep.

 

[dcg9381]

It has 2 A/Cs but couldn't keep up in the Summer.

The roof top ACs installed in those are 13k or 15k. So thats 26k-30k of AC that "can't keep up". Those systems are kinda "lossy" if they are roof ducted, but I don't know if I'd go 18k.

 

[Mikes61]

Where are you going to mount the compressor and how are you going to run the lines? Also, I’m not sure how the flare fittings are going to like the movement of being mounted in a RV.

 

[Steve from Socal]

Where are you going to mount the compressor and how are you going to run the lines? Also, I’m not sure how the flare fittings are going to like the movement of being mounted in a RV.

I have been thinking about nose mounting it like a refer unit! The nose cap tapers from the base back as it goes up. That was an aero design for towing with a pick up. I tow with a Peterbilt 387 high roof truck the truck is a few inches taller than the trailer. The condenser unit will be mounted to the fifth wheel structure with a tube frame. As to the lines and fittings, I have not heard of any issues with them. The lines will run inside the fifth wheel frame to the closet in the bedroom. The lines to the living area from the closet in the ceiling duct 'old hvac plenum'

 

[bob smith1]

The RV is well insulated for an RV R-25 roof and floor, R-15 walls. This is not wired as a standard RV, I have the original 50 amp RV plug and a 50 amp 240 plug and panel for electric stove, water heater and Mini?

The mini would be a two zone so one wall unit in the bed and one in the main. A 9K in the bedrom and 12K main? The roof tops are not known to be that efficient and the plenum is kind of a good/bad overall. I would not discount a 24K unit but, I have heard not to oversize these by too much?

I do HVAC and very familiar with RVs. It would help to know what make you have. I don't need a model, more like length and amount of floor footage. Also what you plan to do with this. One factor people always forget here is it's a CAMPER! As is people going in and out constantly! this has to be factored. The other is hot pull down. You land at a site, you want it comfortable quickly. Lots of hot thermal mass there.

I'm going to reserve comment on insulation until I know what this is.

 

[Steve from Socal]

It is a Newmar Kountry Aire 37 KSCS it is just shy of 40' it has about 380? Sq ft As to what I plan to do with is LIVE in it. I have Teton that I live in at my shop, the Kountry Aire is a get out of Dodge or in this case Hutch mobile. The Teton would take lots of work just to get out of the shop.

 

[bob smith1]

Ok, I don't know what "get out of Dodge or in this case Hutch mobile" means. If being stationary or not.

Either way, those are pretty legit, but unless built with the 3.25" walls like DRV, you can't lean on that R value. The sales BS is rich in that space!!! Like the Aluminum framed walls and no one talks about the massive thermal bridging! I've run REAL math on my turd Forest River product, 44ft, 5 slide, and she is a comfortable R2!!!

All that said, I think your 18K is about right and if you want to do living only, which means normal door open periods, you might get away with 15k, but I wouldn't! The beauty of a modern mini is they throttle down nicely and you get insane EER values when they do that!

The roof unit performance is largely inflated because they are insanely inefficient! EER of like 7! What that means is your supposed 30k is probably close to 22-24k. I think an 18k will hold you fine.

On a 5th, you really have to chase for air seal!!! I mean it! They do NOT have any! ceiling lights, ducting, slide seals, etc, etc. They puke air man! But I will say Newmar is legit. They might have got some things right.

 

[Steve from Socal]

The Teton and Newmars of the vintage I have were the 'New Horizon' of that era. They were designed as full time living from the start, insulation is one of their main features.

 

[bob smith1]

The Teton and Newmars of the vintage I have were the 'New Horizon' of that era. They were designed as full time living from the start, insulation is one of their main features.

Let me tell you how the industry works. They sell you a 1.5" wall with an R-15.... Until you realize the physics breaks down and even Poly Iso can't get there. They I add thermal bridging and and solar gain, and we have a different beast.

Not knocking your rig man! Just saying. I used to own a DRV. Even that thing was not "impressive".

But either way, I'd be curious how it works out! Make sure to test on the hottest day. Some good data is once you're cycling at a set point, the duty cycle or time on vs total time helps paint the story how well it matches the load, and what true R values look like. Really U values because that's how we calculate them.

 

[dcg9381]

The Teton and Newmars of the vintage I have were the 'New Horizon' of that era. They were designed as full time living from the start, insulation is one of their main features.

You're fine.. You've got them from "real RV" days. Have a 2007 DRV used for the same reason, it's built like a brick ****-house.


I looked up Forest River Cedar Creek Hathaway (38FBD) - that was mentioned, Forest River claims crazy levels of insulation, like R-45 (I don't see how there is enough space for that to be possible) on the ceiling, but reality is the slides (walls are like R-7) - even that's optimistic...

I'd love to see this setup, but I don't think I'd do a dual head unit, it complicates the lines a lot.. I'd probably go with two units to avoid the copper HVAC lines.