Minimizing Heating Needs in a Campervan at Low Temperatures

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In summary, the campervan can be in a cold environment with a heater turned on to max power and still maintain a CO2 concentration of 1600 ppm.
  • #1
mess
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I have a 5kw diesel heater that provides heat. I have a vent blowing in 15cfm of fresh air from the outside into the inside (in order to maintain healthy co2 levels for 1 person).
I am trying to determine the coldest temperature my campervan can be in, so that the heater is still able to keep the van at 21C, while still being able to blow 15CFM air into and out of the van.

Here is an earlier post figuring out the need for ~15cfm per person of fresh air to maintain less than 1000ppm co2.

https://www.physicsforums.com/threa...entration-in-a-campervan.996832/#post-6426271

I set this up today, let the CO2 ppm reach 1600 and then turned on the fan, it was slowly lowering with me in it, but the heater turned on to max power and it was using plenty of diesel. Outside it was only -3C. Does this mean that if i want to explore areas that are around -30C, I will need a much bigger heater?
 
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  • #2
The total heat loss from your campervan is the sum of:
1) The heat to heat the ventilation air,
and
2) Heat loss through the sides, top, and bottom.

The heat needed to heat the ventilation air is ##0.018 BTU/(hr-ft^3-deg F) X (70 - -22) deg F X 15 CFM X 60 min/hr = 1500 BTU/hr##. A 5 kW heater produces ##5 kW X 3412 BTUH/kW = 17,000 BTUH##. The heat loss from ventilation air is small, so your major heat loss is through your vehicle. To find where your heat is going, first find the outside temperature where the heater runs 100% and barely keeps the inside at 21 C. Then use that outside temperature to calculate the heat loss. Calculate the heat loss separately through the front, the rear, the sides, the top, the bottom, and the glass windows. When the calculated loss matches the heater output, you will know exactly where the heat is going. That will tell you where to add insulation, or if adding insulation is impractical. If adding insulation is impractical, you will need either a larger heater (with its larger fuel consumption), or a warmer sleeping bag.

To learn how to calculate heat loss through the vehicle, search heat loss through building envelope.
 
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  • #3
jrmichler said:
first find the outside temperature where the heater runs 100% and barely keeps it at 21 C.

Just got to turn the thermostat down on the weather :oldbiggrin: I'm just being sassy but I agree with the method. OP, just wait for a cold night and run the heater on full throttle for at least an hour beforehand. I assume you live somewhere with cold nights based on the question.

OP, you mentioned your heater consumes diesel. Is it linked to the engine cooling system or is it stand-alone? I imagine it runs off the engine's heat because I've never heard of an accessory diesel heater but if it happens to be standalone then watch your altitude. We don't want you freezing at 7000ft because your heater only puts out 5kW at sea level.
 
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  • #4
Twigg said:
I imagine it runs off the engine's heat because I've never heard of an accessory diesel heater but if it happens to be standalone then watch your altitude.
They work really well, for $150. Ebay search 'Diesel Air Heater' 12V 5KW
 
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  • #5
Huh. That's really neat stuff for not a lot of money. Thanks Baluncore.

In any case, it seems to me like "high altitude" models sell for more, so unless you bought a diesel heater specifically with a high altitude feature, I would assume that there's no altitude correction. I'm guessing on the special "high altitude" models they have a barometer and a servo loop on the forced air so that they always produce the correct fuel-to-air ratio. Or it could all be marketeering. Who knows.
 
  • #6
Twigg said:
I'm guessing on the special "high altitude" models they have a barometer and a servo loop on the forced air so that they always produce the correct fuel-to-air ratio.
Most diesels run excess air, or they blow black smoke. A stoichiometric mix is not required.
There are two paths for air through the heater. One air path goes to the internal combustor, then out through a dedicated exhaust. The other air path is heated as it passes outside fins on the combustor.
 
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  • #7
Gotcha, thanks for that explanation. Sorry, I learned about this stuff on propane systems.

OP, please disregard my concerns about elevation.

P.S. I guess I'm the chump they marketeered those high altitude models at :wideeyed:
 
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  • #8
You're planning on "what" wind speed? Duty cycle for my furnace is around 15 minutes; that keeps the house warm at a fixed thermostat setting for anywhere from all night to having the heat kick on every hour when the wind picks up.
 
  • #9
A rule-of-thumb:

1BTU per Hr. per Sq.Ft. per Degree F

This is valid for a single layer of metal or glass with natural convection. With a fan or wind, the loss doubles.

Conversions: 1BTU = 3.41Watts; °C = (°F-32)x5/9, °F = (°C x 9/5)+32
 
  • #10
mess said:
I set this up today, let the CO2 ppm reach 1600 and then turned on the fan, it was slowly lowering with me in it, but the heater turned on to max power and it was using plenty of diesel. Outside it was only -3C. Does this mean that if i want to explore areas that are around -30C, I will need a much bigger heater?
Ignoring the wind and the heating of exchange air, you can measure the power required by running the heater full on and measuring the outside to inside temperature difference = dT.

The heat lost through the walls is proportional to temp difference, dT. If you want 20°C inside when it is -30°C outside, you will need dT = 50°C. Scale the heater power up from 5 kW to get dT = 50°C. Or insulate the body.
 
  • #11
For having equal loss to 5kW heating at 21°C the math can be solid, but once you go camping you need sufficient reserve to heat up the interior within a convenient timeframe.
 
  • #12
Sorry I should have specified I am trying to figure out how to calculate just the heat loss from the fresh air exchange. I have already calculated the heat loss through my van insulation from my average R value.
 
  • #13
jrmichler said:
The heat needed to heat the ventilation air is 0.018BTU/(hr−ft3−degF)X(70−−22)degFX15CFMX60min/hr=1500BTU/hr. A 5 kW heater produces 5kWX3412BTUH/kW=17,000BTUH.

Thank you I think this is what I needed. So my fresh air exchange at 15cfm is only causing a 1500BTU/h loss? I am trying to understand how that was calculated, but i don't get parts of that formula

this part: 0.018BTU/(hr−ft3−degF)

Are hr, ft3, degF variable you plugged in or units?

what is this formula called so that i can look it up and play with it.

Thanks!
 
  • #14
mess said:
what is this formula called so that i can look it up and play with it.
I'm not sure if it has a name, but it's
specific heat x delta-T x flow rate

If 15kW is the heat output, that's a lot. You could bring in a wall of -30F air from outside and heat it to room temp. You could open the back doors to let the air out. Since you won't be doing that, there's basically no limit thermodynamically.

The temperature limit will be based on what the diesel can run in.

Might still be interesting to model the van as an uninsured glass box with a u-value of 1. You can Google how to use that...
[Edit] I get -50F as the temperature you'd be good to. It's actually worse than I expected.
 
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  • #15
mess said:
this part: 0.018BTU/(hr−ft3−degF)

Are hr, ft3, degF variable you plugged in or units?
Those are the units of specific heat for air. It's how much heat you need to add to raise the temperature of a cubic foot of air per minute by 1 degree.
 
  • #16
Russ_watters, correct me if I'm wrong, but the air that's blown in by the fan has to displace an equal flow of air leaking out, yeah? Cold air is blown into the van, and some of the heated indoor air has to be displaced. So isn't the total heat given by 2 x specific heat x delta-T x flow rate? I could be off my rocker here.
 
  • #17
Twigg said:
Russ_watters, correct me if I'm wrong, but the air that's blown in by the fan has to displace an equal flow of air leaking out, yeah? Cold air is blown into the van, and some of the heated indoor air has to be displaced. So isn't the total heat given by 2 x specific heat x delta-T x flow rate? I could be off my rocker here.
I guess I'm not following. Why would you need to heat the air leaving the van?

Or are you saying the warm air leaving is heat leaving the van? It is: the net heat flow into the van is Hin - Hout = 0.
 
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  • #18
I was suggesting the latter. I get what you're saying now. Thanks
 
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  • #19
mess said:
so that the heater is still able to keep the van at 21C, while still being able to blow 15CFM air into and out of the van.

Wait a second...

A cargo van is ~150 cubic feet, and I assume a camper van is similar. We're not talking a Winnebago, right? So at 15 CFM, the air is exchanged in 10 minutes - six times per hour. That is typical between one room in a house and another room in that house.

Are you sure you need that much ventilation? Because if you do, you are going to need a lot of heat. A lot of heat. Essentially you are heating outside air. Imagine a space heater in a room with the window open.
 
  • #20
Vanadium 50 said:
Are you sure you need that much ventilation? Because if you do, you are going to need a lot of heat. A lot of heat. Essentially you are heating outside air. Imagine a space heater in a room with the window open.
Has anyone considered a counterflow heat exchanger ?
 
  • #21
Vanadium 50 said:
Are you sure you need that much ventilation? Because if you do, you are going to need a lot of heat. A lot of heat. Essentially you are heating outside air. Imagine a space heater in a room with the window open
It's not that much heat.
15 cfm is about 0.5 kg air/min, so 10 KJ /K/min, or 17 W/K
Only 850 W with a 50 K temperature difference, and we have a 5kW heater.
 
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  • #22
Vanadium 50 said:
Wait a second...

A cargo van is ~150 cubic feet, and I assume a camper van is similar. We're not talking a Winnebago, right? So at 15 CFM, the air is exchanged in 10 minutes - six times per hour. That is typical between one room in a house and another room in that house.

Are you sure you need that much ventilation?
It's not a volume dilution problem, it's a continuous mixing problem. The volume and air exchange rate don't matter. This was properly dealt with in his other thread.

Not for nothing, but I thought it was a bit high myself, but in the ballpark. Prior to I think 2006, code required 20 cfm per person in most types of spaces (offices, retail, etc) and 15 cfm per person in schools. Today there is a more complex formula using both the area of the space and 5 cfm per person. So I expected it to come out between 5 and 10. Regardless...
Because if you do, you are going to need a lot of heat. A lot of heat. Essentially you are heating outside air. Imagine a space heater in a room with the window open.
Turning over all the air in a small space in a few minutes is still a small amount of air. Imagine the open window is on a dollhouse*, and then stick your space heater in front of it. Clearly not an issue.

*It's a 2 square inch opening with a fan in it blowing air in.
 
  • #24
russ_watters said:
This was properly dealt with in his other thread.

A good argument for not starting a bunch of threads on the same topic.
 
  • #25
Baluncore said:
Has anyone considered a counterflow heat exchanger ?
This is what i would like to calculate next, if it would be worth building. I could build it to allow the entering air to exchange with the exiting air. It would be interesting to determine how much of a help it would be to the heater. but my guess is that it would be negligable.

the biggest problem is that i placed the air out vent at the bottom of the van (in hopes that on average it will exhaust more co2, since its heavier) and the air in vent is on the roof. To run the in vent pipe down to the out vent pipe will take about 3 meters of pipe, and so I am worried about the efficiency of pushing 15-45 cfm through that to exchange, and if the losses will make up for the (i assume) small heat gains.
 
  • #26
If you want to build an air exchange heat exchanger, keep in mind:

1) When (not if), the inside air dew point is below the outside air temperature, you will get condensation inside the heat exchanger. This requires a design that drains to a low point, then to outside. Don't forget a P-trap to keep cold outside air from coming in through the drain line.

2) When the outside air temperature is below freezing, you will get condensation inside the heat exchanger. This requires a defrost cycle. The heat exchanger in my house runs its defrost cycle about every half hour in cold weather.

3) Given the various inefficiencies from cold air ducts and defrost cycle, expect to recover about 2/3 of the total heat, or about 1000 BTUH under the -30 C design condition. That's negligible compared to the output of a 5 kW heater.
 
  • #27
jrmichler said:
3) Given the various inefficiencies from cold air ducts and defrost cycle, expect to recover about 2/3 of the total heat, or about 1000 BTUH under the -30 C design condition. That's negligible compared to the output of a 5 kW heater

Yep that's what i was thinking also. So maybe something for later. thanks!
 
  • #28
jrmichler said:
1) When (not if), the inside air dew point is below the outside air temperature, you will get condensation inside the heat exchanger. This requires a design that drains to a low point, then to outside. Don't forget a P-trap to keep cold outside air from coming in through the drain line.
[emphasis added]
That should be "above". When it is humid in the van (high dew point) you will get condensation (on the windows too).

Due to the healthy ventilation rate it may not be significant though.
 
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  • #29
russ_watters said:
When it is humid in the van (high dew point) you will get condensation (on the windows too).
Due to the healthy ventilation rate it may not be significant though.
When the external air temperature is below –20°C it will be quite dry, so no moisture would be drawn into the space from outside.

Any 21°C air exhausted containing respired air will be rapidly cooled. It will condense water and freeze. A vent in the floor may be blocked by the pyramid of snow produced.

If a heat exchanger was used, the exhaust side would not drain, but would freeze solid.
 
  • #30
I'm not sure without calculating it:
People produce about 4oz per hour of moisture, or 1,750 grains.

Assume 0F dry bulb, -20F dewpoont outside air at 15CFM (1.8 gr/lb, 11.6 cf/lb, 76 gr/hr).

Exhaust is 27 gr/lb or a 32F dewpoint. So yep it could condense/frost on very cold surfaces.

[Sorrynotsorry if the English units make anyone's head explode.]
 

What is the most effective way to minimize heating needs in a campervan at low temperatures?

The most effective way to minimize heating needs in a campervan at low temperatures is to ensure proper insulation. This includes insulating the walls, floors, and ceiling of the campervan. Additionally, using thermal curtains or window coverings can help retain heat inside the van.

What type of heating system is best for a campervan in low temperatures?

The best type of heating system for a campervan in low temperatures is a diesel or propane heater. These types of heaters are efficient and can provide consistent heat for extended periods of time. They also do not require electricity, making them ideal for off-grid camping.

How can I conserve energy while heating my campervan in low temperatures?

To conserve energy while heating your campervan in low temperatures, you can use a programmable thermostat to regulate the temperature and only heat the van when necessary. You can also use a sleeping bag or extra blankets to stay warm while sleeping instead of relying solely on the heating system.

What are some common mistakes people make when trying to minimize heating needs in a campervan at low temperatures?

Some common mistakes people make when trying to minimize heating needs in a campervan at low temperatures include not properly insulating the van, relying on electric heaters that drain the battery quickly, and not sealing any drafts or gaps in the van. It is important to address all these factors to effectively minimize heating needs.

Are there any alternative methods for heating a campervan in low temperatures?

Yes, there are alternative methods for heating a campervan in low temperatures. These include using a wood-burning stove, which can provide both heat and cooking capabilities, or using a heated mattress pad or electric blanket for targeted warmth while sleeping. However, these methods may require a reliable source of electricity or a constant supply of firewood.

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