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Understanding BTUs


TherapyBound
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Total newbie question, but we're in the process of comparing two Class A gas motorhomes of similar size (more on that in a moment), and while most specs are clear to me, I'm unsure about the difference in their air conditioner and heater BTUs, which are significant (I think).

Motorhome A's length is 36.83 ft (8.46 wide, 6.67 interior height) and the automatic air conditioner BTUs total 17,000, while automatic heater is 40,000 BTUs.

Motorhome B's length is 36.25 ft (8.5 wide, 7ft interior height) and the automatic air conditioner BTUs total 30,000, while automatic heater is 34,000 BTUs.

At this point I don't know any more about the brands/etc, but why such a big difference in similar sized rigs, and which would you say is most effective and efficient (if it's possible to tell using only those stats)?

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First impressions say the 2nd MH has better total cooling and nearly as good heating; however, insulation ratings and double--pane windows play a huge role in cooling or heating any MH. It sounds like the 1st MH has one air conditioner and the 2nd MH has two air conditioners; which begs the question, what time of year do you anticipate doing  most camping?

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Most "full-size" roof-mounted RV A/C's are rated at 15,000 btu, though there are quite a few rated 13,500 .  It sounds like the first rig you are looking at has a single unit (though the 17,000 btu is a bit of a puzzle) while the second has two, 15,000 btu units. Our similar sized (35') fifth-wheel has a single 15,000 btu A/C and has worked fine for us, but we carefully avoid hot summer climates. If we were to stay in the south during summer we would definitely add the second A/C, which we are pre-wired for. 

Note that running two A/C units simultaneously generally requires a 50 amp hook-up, which is available in many parks, but certainly not all. 

Another thing to consider is if either R/V features Heat Pump/AC units. We tend to stay in cool/cold areas and use the Heat Pump side of our unit extensively. It is a very nice feature, especially if you are staying at sites where electricity is provided. The heat pump heats efficiently with electricity while the furnace burns propane. 

As far as the furnace, we have a dual speed 23,000/34,000 btu unit that seldom runs on the high (34,000 btu) setting. It is only needed when in cases of "first setting up" where the trailer is very cold and needs to be warmed up.  The 23,000 btu setting keeps up very easily once the trailer is at "normal" temperatures. So I wouldn't think that the difference between 34,000 and 40,000 btu would be significant unless you are planning to stay in very cold areas. I agree with Ray that insulation and windows would be much more important in that case. 

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The BTU (british thermal unit) is a measure of energy in the form of heat that can be added to or removed from the air around the air conditioner or heater. Both are usually rated in BTUs.

I would bet that Mark is right about the number of air conditioners. My suspicion is that the first one only has 1 unit and I suspect the number of BTUs was misquoted and is 1 unit sized at 15000 BTU. While motorhomes do occasionally have 2 air conditioners and only a 30A power cord, those that are that way usually also are set up to only use one at a time if on shore power but both if on the built-in generator set. I would definitely ask if it has a 50a power cord. 

I have found that once an RV is more than 30' in length, it will be a struggle to keep it comfortable in weather where the temperatures exceed 100° F and if in direct sun or if the insulation value of the RV roof and walls is not good, it may start to drift upward once themperatures reach 90°. I would not buy an RV for fulltime living that was more than about 32' long without 2 air conditioners and 50A power. It is very rare today to be in an RV park that does not have 50A unless far off the beaten path and an older park. We have volunteered for a site in more than 30 different locations and all but 2 have had 50a power available. 

RV furnaces available range from 15,000BTUs to as much as 40,000BTUs. Either of the furnaces would do the job but the smaller one would probbly run more often and longer in severe cold. In addition to the quality of insulation in the RV impacting the ability to keep it cool, the same is true of heating it and while both are also effected by the amount of outside air penitration into the RV, heater is probably impacted even more than cooling for that part. Higher quality construction makes a very major impact on the ability to keep the temperature inside comfortable in extreme heat or cold. 

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Hmmm...interesting. That's great information, and it led me to do a bit more digging. I rummaged around on various Winnebago sites regarding the rig (Adventurer 36Z) whose specs list the A/C as 17,000BTU, and while one set of specs have it at that number, another set list the A/C as 2 units each at 13,500. I was pretty sure I'd checked for two units in all of the rigs we liked, but that number just had me thrown.

Thanks for your help! We'll keep it on our short list and start looking at the finer points. 

PS: Our goal is to avoid any real extremes of weather, hot or cold, but we know both heat and A/C will be important no matter where we are. Weather is a funny thing.

Edited by Bigthinkers
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3 hours ago, Bigthinkers said:

PS: Our goal is to avoid any real extremes of weather, hot or cold, but we know both heat and A/C will be important no matter where we are. Weather is a funny thing.

You will be the most fortunate RV fulltimer that I know if you manage that one. We followed the weather too, but we still experienced nights below freezing in places where that rarely happens and we saw high temperatures which were near records. Spend enough time in your RV and you will experience weather that requires both heat and cooling. See this picture from another thread.

hqdefault.jpg

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We live in Florida, and also lived in Michigan, which is why I used the term "goal" rather than "expectation" or "requirement". ;) We know there is no way to avoid big fluctuations, which is why we know how important the A/C and heater will be. 

We do get a chuckle every time we see that photo though, Kirk W. That was taken in northern Florida'. We're in Orlando, precisely for that reason!

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18 hours ago, Bigthinkers said:

We live in Florida, and also lived in Michigan, which is why I used the term "goal" rather than "expectation" or "requirement". ;) We know there is no way to avoid big fluctuations, which is why we know how important the A/C and heater will be. 

We do get a chuckle every time we see that photo though, Kirk W. That was taken in northern Florida'. We're in Orlando, precisely for that reason!

I still remember walking on ice at the Orlando Thousand Trails one morning, a couple days after it was 80 degrees.  Also remember the space shuttle that blew up because the temperatures were in the 20s during launch from the Cape.   A few years ago it went down to 20 degrees a couple nights in a row in both Indio (Palm Springs) CA and Yuma AZ, so it is difficult to get away from freezing weather.

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It's been up and down like a fiddler's arm these past few weeks. Into the 40s, then into the 70s. We had a frost warning a week or two ago. Yes, we can have ice now and then, but very rarely, and very thin ice that doesn't last the day. The last time Orlando had "snow" was 1977, though, so that's a pretty good track record. 

Just to be clear, part of the purpose of my original post was to understand the difference in the two rigs' AC/heater stats (which we seem to have gotten to the bottom of!) BECAUSE we know we'll encounter variations and we want to make the best decision for dealing with those variations. That doesn't mean we won't "follow the weather" and hope for the best. What we won't encounter are serious extremes. For example, we won't be in Florida in summer and we won't be in Alaska (or Michigan, or New York, or Wisconsin, or Minnesota...etc) in winter.  :)

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