Does closing most A/C vents create back-pressure?

[profdant139]

We have a very small trailer and a fairly big a/c unit -- over 13,000 BTUs, I think. (That's the way the trailer was equipped.) It gets cold in a hurry in there, which I personally like. But DW does not like it when the cold air blows on her, which I understand.

Aside from turning down the fan and turning up the thermostat, we have also tried shutting some of the various vents that are built in to the a/c unit.

My question is a simple one: if we close almost all of the vents, does that create back-pressure within the a/c unit? And if so, is that harmful to the unit?

Thanks in advance for your wisdom!

[01tundra]

If you close too many vents you restrict airflow across the evaporator coil.

If you restrict air flow across the evaporator coil the evaporator coil will start to freeze up.

If you freeze up your evaporator coil you run the risk of slugging the compressor with liquid refrigerant because it doesn't have a chance to flash to a gas in the evaporator.

If you slug the compressor you run the risk of it failing.

Now your too high of airflow problem is fixed because you no longer have an operating AC unit.

[profdant139]

Lots of helpful comments -- thanks! Although we do live in southern Calif, we live in the semi-humid part -- a few miles from the coast. (Can't afford a house on the beach!) But the a/c does reduce the humidity -- it puts out plenty of condensation that drips off the roof.

It is true that this a/c unit has no ducts -- it just has several small vents that can be opened or closed.

We could put up a baffle, but I am baffled about how to do that without destroying our already-limited headroom -- I am 6'3" and can't stand up under the a/c unit, although I can stand up in other parts of the trailer.

We tried the solution of closing most of the vents a couple of times, running the a/c for about 90 minutes at a stretch, during the hottest part of the afternoon. As far as I can tell, we did not experience any icing. We haven't tried running it during the whole night -- the temps where we live get down to about 70 at night, so we just leave the windows open.

Thanks again for all of your collective wisdom!

[way2roll]

pianotuna wrote:

Dick_B wrote:
Just slightly off topic but too large an A/C can cool off the air without reducing the humidity which leaves one cold and clammy.

Which can be solved by running an electric heater.

Are you suggesting running a heater and the AC at the same time?

[larry_cad]

wa8yxm wrote:

sch911 wrote:
Airflow CFM is directly proportional to current draw. Less airflow = less fan current.

For a free standing fan.. Likely true
For the blower in the A/C..... Complex

Closing most of the vents WILL cause back pressure but how much?
Back pressure means less air will flow (Hence the ice up comments)
But it also means the blower is working harder against the pressure
This INCREASES the current draw.

Come on Dave, you know better than that! Work is defined in physics as a force causing the movement (displacement) of an object. If you don't "move" the air, there is no work done. Less movement means less amps from the blower motor.

Did you ever put your hand over the end of a vacuum cleaner hose? The motor speeds up due to less work. Also draws less amps. Get an clamp on amp meter and check me out. You can totally block off the vents and the motor will speed up due to almost no load on the motor.

[valhalla360]

Did you try switching the fan setting to low (most units have a hi or low option).

In S. Cal, it's unlikely you will run into icing as as that's generally an issue with high humidity. The moisture condenses on the fins (like the outside of a glass of ice water) but the fins are below freezing, so it then freezes...gradually it builds up until air can't flow between the fins and you get no air flow. Higher airflow helps avoid that issue. If you close off the vents, it creates a similar issue. If it does happen, just switch to fan only (no air/con) for 10-15min and it will quickly melt the ice.

Another question, do you actually have a ducted air/con? That would be unusual for a really small trailer. Usually the cold air just blows out of the main unit because you don't have to worry about getting cold air to a separate bedroom.
- If you do have a ducted unit, closing off just the vent blowing on your wife, shouldn't cause much issue.
- If it's blowing out of the main unit, you are probably cutting off 50% of the airflow if you close one side. That would be a bit of an issue if you are somewhere humid.

[wa8yxm]

sch911 wrote:
Airflow CFM is directly proportional to current draw. Less airflow = less fan current.

For a free standing fan.. Likely true
For the blower in the A/C..... Complex

Closing most of the vents WILL cause back pressure but how much?
Back pressure means less air will flow (Hence the ice up comments)
But it also means the blower is working harder against the pressure
This INCREASES the current draw.

[pianotuna]

Dick_B wrote:
Just slightly off topic but too large an A/C can cool off the air without reducing the humidity which leaves one cold and clammy.

Which can be solved by running an electric heater.

[Old-Biscuit]

They do make adjustable roof ceiling vents.....

13.5K btu A/C Unit is standard size
15K is the large size

Close off the vent you want....
Unlike furnace---less duct/vents will not cause A/C harm
**Might cause Evap Coil to Ice up which is easy to correct

[time2roll]

I would be more inclined to put a baffle up to redirect the cold air away from her and toward you. Same as you do with the car vents.

[Dick_B]

Just slightly off topic but too large an A/C can cool off the air without reducing the humidity which leaves one cold and clammy.

[sch911]

The reduced airflow over the evaporator can cause ice to form, and lead to a possible freeze up which is not good for the compressor. So you'd have to try it out and see if that happens. As far as the blower fan is concerned that offers no issue. Airflow CFM is directly proportional to current draw. Less airflow = less fan current.

[stripit]

Someone may know more than I do, but we used to do that exact same thing in our travel trailer, never had any problems. I would think maybe with less air flow it might ice up in some conditions, but never had that issue.