# Calculating the coldest outside temperature my campervan can be in and still be warm inside

Summary:
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?

jrmichler
Mentor
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.

mess and Twigg
Twigg
Gold Member
first find the outside temperature where the heater runs 100% and barely keeps it at 21 C.

Just gotta turn the thermostat down on the weather 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.

mess
Baluncore
2021 Award
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

mess and Twigg
Twigg
Gold Member
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.

Baluncore
2021 Award
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.

Twigg
Twigg
Gold Member

P.S. I guess I'm the chump they marketeered those high altitude models at

mess
Bystander
Homework Helper
Gold Member
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.

Tom.G
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

Baluncore
2021 Award
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.

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.

Sorry I should have specified im 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.

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 dont 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!

russ_watters
Mentor
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...
 I get -50F as the temperature you'd be good to. It's actually worse than I expected.

Last edited:
russ_watters
Mentor
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.

Twigg
Gold Member
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.

russ_watters
Mentor
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.

Twigg
Twigg
Gold Member
I was suggesting the latter. I get what you're saying now. Thanks

russ_watters
Staff Emeritus
2021 Award
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.

Baluncore
2021 Award
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 ?

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.

russ_watters
russ_watters
Mentor
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.

Staff Emeritus
2021 Award
This was properly dealt with in his other thread.

A good argument for not starting a bunch of threads on the same topic.

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 im 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.