I’ve been surprised at the number of people who think that their heat pump provides ventilation.

Thanks guys. I should have googled ‘portable ac’ before replying. I’ve seen two-hose units, especially those mobile outdoor units used for tents, construction, etc, but I wasn’t familiar with one-hose portables. Wow… every cubic foot of room air used by the condenser gets replaced by a cubic foot of outside air. That seems like remarkably like chasing one’s own tail 🙂

David,

The portable AC to which Alice refers typically operate not by being installed in a window but by being ducted to a window. The unit itself sits on the floor, typically on wheels. Nearly all (~90%) are single hose variants, which are horrible and should be outlawed, IMO. Air needed to cool the condenser / hot side is drawn from within the house, resulting in both a substantial imbalance of air pressure within the home and of course a huge amount of outdoor air sucked in through every available leak…I can’t imagine what one of those would do in a small tight house.

The better portable units have two hoses that serve to bring in outdoor air to cool the condenser and then discharge it outdoors. That is the only design anyone should purchase / deploy. They are horribly inefficient relative to conventional window units, but they do work, sort of. I have one in the loft of a ~300 SF shed whose windows are way too small for a conventional window shaker – it works, but draws roughly double the power of a similarly effective window unit.

Portable AC’s plug into an outlet and sit on the floor with a small duct that blows the hot air from the condenser out a nearby window. The cheapest ones with one duct get the inlet air for the condenser from the room. There are other models with two ducts connected to the window with one being a supply to the condenser and the other an exhaust from the condenser. The one duct system will depressurize the room which will cause outdoor air to be sucked in elsewhere, thus adding to the cooling load. The two duct system will be less efficient due to the condenser rejecting heat to the hotter outdoor air. So the net effect is one increases your cooling load at higher efficiency and the other has a lower cooling load at lower efficiency. Since you are noticing the depressurization, you probably should use a two-duct portable unit.

@Alice, ERV’s by design are intended to maintain pressure balance. Although it’s possible to adjust the intake and/or discharge fans to achieve an imbalance, doing so would significantly reduce the ERV’s recovery efficiency, thus defeating the purpose of having the ERV.

Not sure what you mean by ‘portable AC,’ but it definitely shouldn’t cause your house to *lose* air. You need to determine why that’s happening and fix that problem.

The exhaust fan over the stove and in the bathroom don’t have too strong of an effect, but the portable AC unit causes the home to lose air, so I’m trying to figure out what kind of fresh air intake system would help. Currently I have to keep a window open but that creates an extremely strong air current from the window and messes with the temperature in my 300 sqft tiny house. Anyone have advice?

If your objective is to ventilate the house with filtered air when outside conditions are favorable or moderate (‘poor man’s ERV’) then your best bet is to use a filtered window fan as the supply. The filter doesn’t need to cover the entire window opening, just the fan itself, since any openings around the fan’s frame will be at negative pressure. Forget about trying to neutralize pressure differentials when your HVAC and/or other fans are operating.

Extrapolating from this, even if I open all windows, the pressure will still not be uniform. So in essence it seems local and overall pressure differentials between various areas of the house and the corresponding outside areas are unavoidable. Back to: either provide forced make-up air if an exhaust fan is running, or open a (nearby) window and pray.

> Is there a mechanical ventilation system that will introduce fresh outside air based on actual pressure differential instead of calculated needs? Or is that = open a window if you know the house is negative?

I can’t imagine how something like that would/could work.

The problem with opening a window as a pressure equalizer is that it will increase (dramatically) the air exchange with the outside. Wind and stack effect guarantee that. The impact of wind is typically lateral (positive on one side, negative on the opposing side) and the impact of stack is vertical (top to bottom or bottom to top, depending on the season). That means the house can be positive and negative at the same time (and usually is, when all fans are off).

Theoretically, the only time a window (or any other leakage path) is neutral, or acts to benignly neutralize internal imbalances such as duct leakage, is if outside air is perfectly still and precisely the same temperature as house and the house is uniformly the same temperature (i.e., no stack effect).