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Old 01-21-2022, 08:35 AM   #1
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2021 27' Flying Cloud
Salado , Texas
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Cold Weather Heat Balance

I'm sure there are some other engineers here that might find this interesting. We had cold weather last night, and conditions were perfect for a little field measurement. The purpose of all of this was to determine, within a some degree of accuracy, the heat required to hold temperature overnight in our Airstream.

So warning - this is a longer post than I intended and contains details that I did not fully describe.

Setup -

1. 2021 27' FC parked on concrete slab with mild shelter from wind.
2. Clear sky at Austin, TX Latitude, radiation is in full play.
3. Outside temp is COLD for here, dropping to 21 deg early this morning
4. AS is unoccupied, all systems are off except refrigerator is running on shore power. AS is plugged into shore power - 50A
5. A single 1500W "Smart" heater is plugged in mid-ship and running with a digital thermostat setpoint of 50 deg F in Automatic Mode.
6. Water tanks are drained, fixtures are drained, lines blown with air and traps filled with antifreeze.
7. Some but no all curtains are open - should have made sure all were closed!

Observations-
1. At daylight, OAT is 21 deg F.
2. Observed IAT is 44 deg F at plug-in heater.
3. Both heat pumps and furnace have been OFF the entire time.
4. Shower door, undersink cabinet doors (for both kitchen and lavatory) are open for airflow.


So, using the heat transfer formula of Q=UAdT, and knowing that UA is a constant, we can calculate the heat required to maintain different IAT with respect to different OAT. Sketchy hand chart attached. YMMV, especially depending on curtains, wind speed, cloudy sky and just your own body heat (assume that you are putting out 400B of Sensible and 400B of Latent body heat)

Disclaimer - entertainment purposes only. DO NOT use this to try and prevent freezing instead of winterizing, or doing something silly like freezing to death in the middle of nowhere...
Attached Files
File Type: pdf Airstream Heat Balance.pdf (2.03 MB, 28 views)
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Old 01-21-2022, 08:48 AM   #2
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Originally Posted by Gtx Eng View Post
I'm sure there are some other engineers here that might find this interesting.
Great work. I just finished installing dual diesel heaters in my 2022 AS and was thinking about doing something very similar.

Have you done the same analysis for propane heat?
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Old 01-21-2022, 08:51 AM   #3
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It's a generic chart of required MBh, so it would apply to any form of net heat input into the cabin. I can tell you from experience, we life-boated in that thing with LP heat last winter when the Texas SnowVID iced us in with single digit temps and no power for days. LP heat held that AS just fine...
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Old 01-21-2022, 10:00 AM   #4
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It's a generic chart of required MBh, so it would apply to any form of net heat input into the cabin. I can tell you from experience, we life-boated in that thing with LP heat last winter when the Texas SnowVID iced us in with single digit temps and no power for days. LP heat held that AS just fine...
I was thinking more about heat efficiency by source. A 1500W electric heater is roughly 7,500 BTU. Can't tell from your chart how many watts were actually used maintaining set point. Would be interesting to compute the watts used and pounds/gallons of propane required to maintain the same set points versus OAT by source.

For my electric conversion I considered using up to four 1,500 watt electric heaters since the F-150 could service that load. Based on your data three 1,500 watt electric heaters will maintain a set point of 70F down to an OAT of 0F. Better than I thought and you have a 27'. I moved to diesel heat to allow continuous heating when the F-150 is not hooked up.
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Old 01-21-2022, 10:51 AM   #5
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There's a few things that would have to be assumed, but you can certainly get there from that chart I posted.

1500W electric heater is assumed to be 100% thermally efficient - it will consume and dump 1500 W of heat into the AS. 1500W of heat is 5120 BTUh, not 7500.

The chart I made is just the MBh (1000 BTUh) required to maintain the Y axis IAT at the X axis OAT. Reading the chart, you are correct that 4500W of heat will maintain about 70 IAT with 0 OAT. I was surprised too, and caution that this is all projected off of one observation...

Combustion heaters are rated in BTU's (really BTUh). An indoor heater like some people use (Mr. Buddy) dumps all the heat into the AS and rejects none of it outside. Very efficient, but you create a pound of water vapor for every pound of fuel you burn, and you're going to breath up all those products of combustion. This is why I don't use them - but to each his own.

I prefer the AS furnace since it dumps all the products of combustion outside and keeps all the combustion water vapor from loading into the AS. The penalty for this is they typically can't operate at better than 80% thermal efficiency.

So - 1# of propane contains 21,550 BTUh (21.5 MBh). Only 80% is "usable" from the furnace, so a furnace can provide 17.2 MBh for every # burned. Looking at my chart, create new lines that are horizontal (steady IAT). You can see that you'd have to burn about a pound of propane an hour to hold 70 deg if it is -10 outside at night; about 2 gallons of propane to hold 70 all night (8 hrs) at -10. . You'd cut that burn rate about in half if you held 70 IAT with +30 OAT all night.

Useful info? I don't know. It's all very rule-of-thumby backed by experience in the field. Inefficiencies will draw these numbers out, such as underperforming equipment, bad thermostat placement, starting and stopping of the furnace, etc.
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Old 01-21-2022, 10:53 AM   #6
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Also - I like the idea of the diesel heater. You can get 50% more heat from diesel than you can from LP!
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Old 01-21-2022, 11:08 AM   #7
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Also - I like the idea of the diesel heater. You can get 50% more heat from diesel than you can from LP!
Thanks for the analysis. Diesel is very efficient and light weight. The little Espar heaters are also amazingly efficient (both diesel consumption and electricity).

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Old 01-21-2022, 11:39 AM   #8
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Quote:
Originally Posted by Gtx Eng View Post
There's a few things that would have to be assumed, but you can certainly get there from that chart I posted.

1500W electric heater is assumed to be 100% thermally efficient - it will consume and dump 1500 W of heat into the AS. 1500W of heat is 5120 BTUh, not 7500.

The chart I made is just the MBh (1000 BTUh) required to maintain the Y axis IAT at the X axis OAT. Reading the chart, you are correct that 4500W of heat will maintain about 70 IAT with 0 OAT. I was surprised too, and caution that this is all projected off of one observation...

Combustion heaters are rated in BTU's (really BTUh). An indoor heater like some people use (Mr. Buddy) dumps all the heat into the AS and rejects none of it outside. Very efficient, but you create a pound of water vapor for every pound of fuel you burn, and you're going to breath up all those products of combustion. This is why I don't use them - but to each his own.

I prefer the AS furnace since it dumps all the products of combustion outside and keeps all the combustion water vapor from loading into the AS. The penalty for this is they typically can't operate at better than 80% thermal efficiency.

So - 1# of propane contains 21,550 BTUh (21.5 MBh). Only 80% is "usable" from the furnace, so a furnace can provide 17.2 MBh for every # burned. Looking at my chart, create new lines that are horizontal (steady IAT). You can see that you'd have to burn about a pound of propane an hour to hold 70 deg if it is -10 outside at night; about 2 gallons of propane to hold 70 all night (8 hrs) at -10. . You'd cut that burn rate about in half if you held 70 IAT with +30 OAT all night.

Useful info? I don't know. It's all very rule-of-thumby backed by experience in the field. Inefficiencies will draw these numbers out, such as underperforming equipment, bad thermostat placement, starting and stopping of the furnace, etc.
Great work GTX, is the propane furnace efficiency of 80% an estimated number? I have the 34,000 BTU furnace in the Classic and guesstimated the efficiency around 50% based on burn rates.
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Old 01-21-2022, 01:01 PM   #9
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It's really hard to say. The government has mandated efficiency standardization for residential and commercial heating equipment. There's no such thing for RV equipment. I looked up Dometic's RV furnace website - the best you can get is a propane consumption rating per hour. What we don't know is the exact furnace output - they only give you a range, if that consumption rate includes a duty cycle or constant full fire, etc.

What I can tell you is that the Dometic furnace in the range you are talking about is their Large furnace, rated 35 - 40 MBH and burning 15 cfh. Since 1 cfh of propane has a Total Heating Value of 2488 BTUh, then the THV of 15 cfh is 37.3 MBh. That means they are rating their furnace at the BURN rate, not the NET rate. No furnace can exceed 80% thermal efficiency without condensing in the combustion process - and there is NO WAY these furnaces are built to be condensing high efficiency furnaces. The BEST efficiency you could expect is therefore 80% of the 34 MBh rating = 27.2 MBh (equivalent to 8000 W of electric space heater).

It is entirely possible that the furnace real world thermal efficiency could be as low as 50%. It is built for compact, thus has a minimal heat exchanger surface area (bad), less than optimal burner arrangement, etc. It is built for minimum space and weight and nothing else. Frankly, I'd be impressed if they can squeeze 70% efficiency out of it.

Jay
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Old 01-21-2022, 01:26 PM   #10
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It is built for minimum space and weight and nothing else. Frankly, I'd be impressed if they can squeeze 70% efficiency out of it.
One more issue with thermal inefficiency I discovered when I removed the propane furnace from my 25FB. There are 4 output ducts coming off the furnace. One goes aft to a vent under the couch. Two ducts go forward one coming out by the shower in the middle of the trailer and one going to the bedroom up front.

The 4th duct immediately out of the furnace goes strait down through the floor then runs forward below the floor toward the tanks underneath. So no matter what the outside temperature the propane furnace is configured in my trailer to dump 25% of the output outside of the trailer to an underfloor uninsulated cavity. Good thing when the temperature is below freezing but a waste of propane above 32F.
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Old 01-22-2022, 10:41 AM   #11
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Hi

Any time you look at a "burn" process for generating heat, part of the energy is in water vapor. The normal charts *assume* you condense this back to liquid water. If you look at a modern "high efficiency" furnace that's exactly what it does. Water comes out the bottom, hits a pan and is pumped off to a sewer line. Most RV setups don't do this. They vent it outside. Thus they do not get that last 10 to 20% out of the fuel.

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Old 01-22-2022, 10:52 AM   #12
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Hi

Any time you look at a "burn" process for generating heat, part of the energy is in water vapor. The normal charts *assume* you condense this back to liquid water. If you look at a modern "high efficiency" furnace that's exactly what it does. Water comes out the bottom, hits a pan and is pumped off to a sewer line. Most RV setups don't do this. They vent it outside. Thus they do not get that last 10 to 20% out of the fuel.
Interesting. I wonder how the net efficiency for heaters is impacted by those that only use outside air for combustion. A propane furnace burns using inside air then discharges the products of combustion to the outside. This obviously requires cold outside air to be drawn into the trailer to replace air used for combustion that must lead to a net decrease in heating efficiency.

The Espar diesel heaters source combustion air from outside then discharge the products of combustion back to the outside. So there is no air exchange inside the trailer as part of the combustion process.
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Old 01-22-2022, 11:03 AM   #13
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Did you have a use case in mind for this experiment?

I can think of one. Stocking trailer with food that could freeze day(s) prior to taking wintetized trailer south for snowbirding.

I do this but run the propane furnace instead of using a space heater. In order to not use up the onboard propane, I back feed regulated propane into the trailerís low pressure port. 20 lb tank from my home grill, grill regulator with hose from discarded grill, female propane quick connect on the regulator hose and made up double ended male quick connect adapter to connect the two females.


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Old 01-22-2022, 04:21 PM   #14
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Did you have a use case in mind for this experiment?



I can think of one. Stocking trailer with food that could freeze day(s) prior to taking wintetized trailer south for snowbirding.



I do this but run the propane furnace instead of using a space heater. In order to not use up the onboard propane, I back feed regulated propane into the trailerís low pressure port. 20 lb tank from my home grill, grill regulator with hose from discarded grill, female propane quick connect on the regulator hose and made up double ended male quick connect adapter to connect the two females.





Greg
I like that back feed idea.
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Old 01-23-2022, 08:51 AM   #15
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Originally Posted by goldenchase View Post
Interesting. I wonder how the net efficiency for heaters is impacted by those that only use outside air for combustion. A propane furnace burns using inside air then discharges the products of combustion to the outside. This obviously requires cold outside air to be drawn into the trailer to replace air used for combustion that must lead to a net decrease in heating efficiency.

The Espar diesel heaters source combustion air from outside then discharge the products of combustion back to the outside. So there is no air exchange inside the trailer as part of the combustion process.
Hi

Since the heaters push the combustion products ( and hot air ) out of the trailer, there must be an air source feeding air into the trailer to make up for that outflow. Put another way: it's using outside air either way.

The exception to this are the stand alone "catalytic" heaters. They don't vent anything outside. They get their super duper "high efficiency" numbers because the water vapor is assumed to re-condense inside the heated area. If you have any sort of well sealed area, you will notice condensation ....

Fun !!!

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Old 01-26-2022, 10:31 AM   #16
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Bob is correct, the exhaust on my Suburban furnace is fed out through an air intake tube. This accomplishes a few things, bringing in make-up air for combustion and keeping the hot exhaust pipe away from construction material, and keeping the combustion side separate from the hot air side in the heat exchanger.
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Old 01-26-2022, 12:06 PM   #17
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Great analysis and discussion. Thanks for posting.
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Old 01-26-2022, 02:03 PM   #18
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Hi

Since the heaters push the combustion products ( and hot air ) out of the trailer, there must be an air source feeding air into the trailer to make up for that outflow. Put another way: it's using outside air either way.
I think you missed the nuance. Diesel heaters source outside air through a separate combustion inlet tube from the outside. While combustion does take place inside the trailer within the combustion chamber of the diesel heater there is no exchange of air inside the trailer associated combustion with diesel heaters.

A separate fan in the diesel heater passes inside air over the hot surface of the diesel heater combustion chamber then back to the inside of the trailer.

I only brought up the issue around the discussion of efficiency comparisons. No cold air is pulled into the trailer for combustion, so in theory a diesel heater should be more efficient than a heater that uses inside air for combustion then forces the byproducts of combustion outside the trailer. The question is how much this may contribute to overall efficiency. No idea.
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Old 01-26-2022, 05:32 PM   #19
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Bob is correct, the exhaust on my Suburban furnace is fed out through an air intake tube. This accomplishes a few things, bringing in make-up air for combustion and keeping the hot exhaust pipe away from construction material, and keeping the combustion side separate from the hot air side in the heat exchanger.
Retraction, donít know what I was thinking but there are two pipes in the side of my van, one for fresh combustion air and one for exhaust. The exhaust does have a sleeve around it to keep the heat away from the construction material. The fresh air intake is just for the combustion chamber.
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Old 01-27-2022, 09:51 AM   #20
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I think you missed the nuance. Diesel heaters source outside air through a separate combustion inlet tube from the outside. While combustion does take place inside the trailer within the combustion chamber of the diesel heater there is no exchange of air inside the trailer associated combustion with diesel heaters.

A separate fan in the diesel heater passes inside air over the hot surface of the diesel heater combustion chamber then back to the inside of the trailer.

I only brought up the issue around the discussion of efficiency comparisons. No cold air is pulled into the trailer for combustion, so in theory a diesel heater should be more efficient than a heater that uses inside air for combustion then forces the byproducts of combustion outside the trailer. The question is how much this may contribute to overall efficiency. No idea.
Hi

Simple answer is no it's not.

Ok here's the math:

You burn some amount of diesel and get A joules of energy

You have B joules is in water vapor and either gets recovered in a fancy setup or goes out the pipe in an RV.

Your intake air is at some outside temperature, it gets heated to combustion temperature one way or the other. This takes C joules.

Some portion of the combustion gas (hopefully all) goes out the exhaust pipe. It has D joules left in it.

What you get is A - ( B + C + D ) into the cabin.

It does not matter what route this or that takes, the math is still the same.

Bob
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