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

jrmichler
Mentor
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.

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 thats what i was thinking also. So maybe something for later. thanks!

russ_watters
Mentor
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.
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.

jrmichler
Baluncore
2021 Award
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.

russ_watters
Mentor
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.]