Trailer (2017 T@G XL) Heating times and temps.

WilliamA

Hi all,
I've been doing some cold-weather data gathering so I thought I'd post some of my findings for everyone to mull over. I'd been wanting to do this since installing my heater but needed to wait for some cold weather to do it. In addition, I wanted to calibrate my wall thermostat and then mark it. The wall mounted line voltage thermostat I have doesn't have any numbers on it and I wanted to be able to quickly dial up the temp I wanted. A bit of research was in order.
Here's what I found:

My trailer is a 2017 T@G Outback XL. For my baseline tests, I took all of the bedding, pillows and mattresses out of the trailer. The trailer is in the shed (Pole barn. No heat but protected from the wind) and has been there for several days. Last night the temperature outside got down to 18 degrees so I thought that was adequate. When I started the test, the thermometer in my trailer read 32 degrees with a humidity of 69%.

Here's my test measurements over time with the trailer closed and heater on:

Ambient temp in shed: 32 deg No solar gain (fancy way of saying it's dark in the shed and there isn't any direct sunlight to skew the test)
Furnace set to low. (750 watts)

Start: 32 degrees
20 minutes: 40 degrees
30 minutes: 43 degrees
45 minutes: 52 degrees
60 minutes: 62 degrees
70 minutes: 70 degrees

Notice that the humidity has gone way down.

Now, this means a few things:
My furnace is forced air and I was running it on low. The wattage rating on that setting is 750W (plus the few watts it takes to drive the blower).
The humidity seems to have had a dramatic effect on heat time. For the first half of the test, temps didn't rise much but the humidity started to go down significantly right away. Keep in mind that I had removed my bedding and mattresses so I was heating a larger space with less insulation. Also, everything was cold. I wasn't in the trailer either.

It took 20 minutes to come up 8 degrees. The last 8 degrees took 10 minutes. A human body put out about 75 watts of heat, so assuming my son and I had been in there, the heat rise would have been faster. Also, there would have been less volume to heat, but more humidity. All of this pretty much mirrors my real-world use of the trailer. When I'm in there and it's cold, turning on the heater gets immediate results.

So for everyone out here who wants to know what it means, it means that it takes about 750 watts of heat at 110V to efficiently keep the trailer warm. These numbers (and my personal experience) show that even in a good wind, it will have no trouble keeping things comfy.
The electric furnace I had installed in my Little Guy 5 wide was a bit smaller (600 watt element) and did a great job of heating things. I had thought about putting the same equivalent heater in this trailer but decided because of the larger volume, I'd bump it up to 750. I think that it would have been adequate even at the 600 watt level but this is faster.

For heaters (a resistive load) it's the wattage that does the work. 750 watts at 110 Volts is about 6.3 amps. To get the same 750 equivalent wattage of heat at 12 volts would require 62.5 amps. That's why it's difficult to make heaters work on 12 volts.

I'm pretty happy with the results. In addition to finding out how efficient the whole thing is, I was able to find the "set" points for my thermostat. You can see them now, cleverly drawn in pencil on the thermostat cover. The low one is 60 degrees and the high mark is 70 degrees.
Hope some of this is useful....
WilliamA

TennTim

There are all sorts of ways I could “engineer geek out” about this. If you want to see some of the details look up “psychrometric chart.”

Contrary to popular belief, heating air doesn’t “dry it out.” Since warmer air can hold more water, heating it lowers the relative humidity while the absolute humidity stays constant.

Now your test didn’t perfectly follow the chart as you should have ended up around 20% RH. I would guess some moisture was liberated from the walls, etc. as it warmed up.

Heat capacity goes down as temperature goes up (i.e. slightly less energy to raise it a degree). But heat transfer slows down as the temperatures reach each other. Without putting pencil to paper I can’t guess which would win to support an initial slow heating for just those two variables. But I would surmise your slow initial heating on the walls thawing and the water evaporating (i.e. soaking up energy) as supported by the higher RH than expected based on the chart. If I wanted to continue geeking out I would see if the added Absolute Humidity supports the initial slow heating.

Anyway, I share these details as you strike me as someone that is interested in the details. You did a heat study on a trailer and I thought I only did things like that!

WilliamA

Tim,
Thanks so much for your insightful input! My mother always told people that I was "cursed by an interest in everything". I don't make any claim to understanding thermodynamics much beyond working out how much airflow it will take to heat the new addition (I'm a retired remodeling carpenter among other things) but I am nonetheless fascinated by how things work and delighted by how easy it is to conduct some rudimentary tests of ideas I may have.
I wanted to, as a minimum, work out how much heat (in watts) it actually takes to keep my trailer warm. I did my basic cubic foot measurements and worked from there. I was using one of those heater/fan space heaters and while it worked okay, it was either too hot or too noisy to be comfortable so I thought I could improve on it some.
For my test I wanted to have the highest volume inside so removed all bedding and mattresses. I also wanted to see how the humidity would react in a repeatable test and thought the bedding etc would provide an unpredictable amount of ambient humidity. Simple. Just remove all the "environmental sponges" so to speak. In addition, I simply wanted to see how long it takes in the worst-case. Once all the bedding (insulation) and me (supplementary heat) is in there, the time it takes to heat will inevitably go down. I was surprised by how long it took to heat up from ambient to a comfortable (60-70 deg) range. I wasn't surprised by how the longer the test ran, the faster the temperature rose. That, I expected. What surprised me was the drop in humidity as the temperature rose because it's essentially a closed space with no air transfer to the outside. Where was the humidity going? Reading the psychrometric chart on Wikipedia, I see now that the humidity (as you so rightly said) is still there but relative to the temperature, it's proportionately lower. More reading is in order there and I won't try to tackle the math, but it's fun to have a better understanding of some basics here.
All of the sausage grinding in things like this for me is based on a simple principle that we can all understand: "If you cut a hole, you'd better have an expectation that what you put back in it is going to work". That's the underlying principle behind my home-brew experiments. My interests aside, I also want to know that I've not hacked up my rig only to find out that I didn't do my homework first.
So thanks for chiming in and appreciating the effort if not the volumes I seem to generate in my ruminations. Now back to reading Wiki-psychrometrics. And where's my slide rule........
WilliamA

Michigan_Mike

I've been there and done it William... Well, kinda sorta! I pulled into Kentucky late one night on the way down to Florida in about 6" of fresh snow and below freezing temperatures. I set the trailer up, got power hooked up, flipped on the lights and fired up the electric heater, locked the trailer, hopped back into my SUV, drove down to the local restaurant, had dinner, came back a bit more than an hour later and the trailer was toasty warm!

Not as scientific as your write-up, but close enough for me and added to my learning curve too. The small space of the teardrops is deceiving in that many folks would surmise they heat up quickly. That isn't the case as it does take time to get things warmed up when surfaces are ice cold and mattresses are firmed up sitting in freezing temperatures.

I did the same thing about 6 years ago on a trip back to Michigan from California. I ran into white out conditions in Butte, Montana and a massive weather front was moving across the country and the wind was howling and the expressway was like a skating rink. I pointed the car and trailer into the storm, fired up the heater, drove down to the local restaurant, thawed out, ate dinner and when I returned the trailer was comfortable.

Thanks for sharing your findings as this is good information and does help if you've never set up in winter conditions, etc.

Trailpixie

For boondocking, What is stopping someone from putting four or five batteries in and running a 12v heater to warm the space?

Possibly something like this http://www.my12voltstore.com/12_Volt_235_Watt_Heater_Defroster_p/111-1224.htm

Could you use that for taking the edge off the cold?

TomD

There’s nothing stopping anybody from putting 4 or 5 batteries on board to run a 12v heater. Some of it’s logistics. Check out the search “batteries” and “heaters” to see what’s involved.

WilliamA

According to the spec sheet, the output is a couple hundred watts. The amperage requirements at 12 volts work out to about 15 amps. That's a lot for a little. 15 amps is about the maximum output of the converter. Keep in mind that the 15 amp draw assumes maximum battery charge. As battery voltage drops, the amperage draw will increase proportionately. The 12 volt output of the most popular generators is 8 or so amps. The short version of all that is unless you are on shore power, you'll need a truckload of submarine batteries to run that heater overnight. I have posted elsewhere that to do an efficient job of heating a teardrop at freezing temps, you need about 800 watts to get the btu equivalent. If you plan to heat comfortably at near freezing temps, you need to do that with 110 volts (shore power or generator) or an alternative fuel source like Propane or butane.

Sorry folks, but the physics are stubborn. If I were boondocking constantly and didn't want to run my gennie, I'd consider using one of the propane fired self-contained units posted elsewhere on the forum. They come with their own voltage requirements too, so that option isn't without its foibles. Bang for the buck in my math is still 110 volt electric heater and a gennie for boondocking. It is also probably the safest heat source.

"Everybody gets their own opinion
But nobody can have their own facts."
Al Franken

"It's all in the math."
Carl Sagan

WilliamA...providing a wet blanket to great ideas since 1956. (More often in the early years, but for reasons that have less to do with math...)

TomD

WilliamA..thanks for the physics lesson. I have zero degree sleeping bags that I use like blankets. Almost too hot and generally warm and comfy. However..I don’t generally do camping below the freezing line.

TomD

Thanks for the Franken/Sagen quotes too.

WilliamA

Tom,
I too have sleeping bags for blankets. I also have a comforter on top. The sleeping bags work well, are rugged and make putting the bed back in order a simple affair. If it gets too warm (hardly likely) I just throw it back. Mosquitos gotta eat too!

There's not a single mosquito in all of Wisconsin. They're all married with big families.

WilliamA

Michigan_Mike

I'm going to find out soon about trailer heating in these arctic like temps here in Michigan once I head southwest to Arizona. These first few days are going to be trying and hopefully the Alde heating unit in my trailer will crank out some heat in short order and make life bearable.