Supplementing central AC w/ portable AC unit

[Gonad the Barbarian]

I'm apprehensive about relying solely on my 13 y/o heat pump to get me through another summer. Those portable AC units that are popping up have caught my eye. I figure a dual hose one would be necessary so it's not sucking more hot air in for my central air to handle. Good idea? Bad idea? Anyone tried this? Any noticeable effect on the power bill either way?

 

[3chordcharlie]

The efficiency of portable units is poor at best, so I would guess your hydro bill will go up if the portable unit is running much.

 

[Greenman]

Used a twin pipe AC for several years, it worked very well. Gave it to a friend and he's still using it.

 

[Gonad the Barbarian]

^ Thanks for that info. Several years would be about my life expectancy for one based on what I've seen with dehumidifiers. How was the noise level? I assume that would be about the same as well.

The efficiency of portable units is poor at best, so I would guess your hydro bill will go up if the portable unit is running much.

Well, I've got an older system with attic ducting so I'm thinking any parasitic loss I can avoid by going around that would help make for any inefficiency from the smaller unit. That's really what I should address, but I could put one of these portable AC's in almost every room of my home for a quarter of the cost of a new HVAC system.

 

[bamx2]

Is there any way you can use Window units ?

I am in a similar situation (40 year old 2-story house w/ 2 HVAC systems , 2nd floor HVAC 2.5 ton system and ductwork in attic) . Due to some substantial tree removal aound my house, I lost a lost of shade which has now caused my HVAC system to be under capacity. The attic tempertaures can now reach 125 deg F on a hot summer day (in Atlanta,GA). It was even hotter before I made some inprovements to the attic ventilation

I should replace the system with larger capacity (at least 3.0 ton) and more efficient unit and install better insulated ductwork but this is the wrong time of year to get a decent job/decent price. BTW, about 20% of the supply ductwork in locations that will probably require ceiling drywall removal to replace them. I would love to get the both unit and ductwork out of the attic or get mini-splits but I don't think that either of those options is worth it (effort /$) since it world be odd compared to the rest the houses in the neighborhood and I don't plan on living in house more than another 5 ys.

In the meantime, I am contemplating buy some window units for the few rooms (3) that are used the most ("zone cooling) and set of the cental unit t-stat up pretty hogh.

"Well, I've got an older system with attic ducting so I'm thinking any parasitic loss I can avoid by going around that would help make for any inefficiency from the smaller unit. That's really what I should address, but I could put one of these portable AC's in almost every room of my home for a quarter of the cost of a new HVAC system.[/QUOTE]"

 

[Gonad the Barbarian]

I'd probably run into some HOA issues with a window unit. One place I could probably get away with it, it's an inset window that's really only visible from my back yard.

Sun exposure makes a massive difference from my observations as well. One thing I should have already done (but really hadn't thought about it until now, and today is not the day to address it) is move all the ducting runs as far from the roof line as possible. Some of them run out from the handler, go overhead for clearance, then back down along the roof to the register. One of these is of course my computer room that's already on the hottest corner of the house.

Did you add an attic exhaust? Right now mine is just vented with soffit vents. I'm not sure if I should change this or not. I don't want positive pressure blowing insulation and outside air down into the house or negative sucking up and out the conditioned air.

 

[bamx2]

Yes, I added a powered exhaust vent fan. A few additional soffit vents might help but no easy DIY (the areas are essentially 3 stories high ). Depending on your roof design I would say add a power vent - hip roof , ridge vent- gable roof.

Did you add an attic exhaust? Right now mine is just vented with soffit vents. I'm not sure if I should change this or not. I don't want positive pressure blowing insulation and outside air down into the house or negative sucking up and out the conditioned air.[/QUOTE]

No official HOA issues with using window unit for me but I suspect some of the newer neighbors may raise their eyebrows but I don't care. The only concern that I would have is whether my wife (a light sleeeper) will put up with the additiional noise.

 

[Greenman]

^ Thanks for that info. Several years would be about my life expectancy for one based on what I've seen with dehumidifiers. How was the noise level? I assume that would be about the same as well.



Well, I've got an older system with attic ducting so I'm thinking any parasitic loss I can avoid by going around that would help make for any inefficiency from the smaller unit. That's really what I should address, but I could put one of these portable AC's in almost every room of my home for a quarter of the cost of a new HVAC system.

They are loud on the high setting, tolerable at lower settings.
Be careful comparing portable units to central AC. The initial cost is lower, but they have higher operating cost and a shorter life expectancy, depending on use. If you're actually cooling an entire house using 3 or 4 units cost will be more than central ac. But if they only cool the 1 or 2 rooms you actually live in they can save you some money.

 

[Gonad the Barbarian]

I haven't moved on this yet and after looking more at them am reconsidering getting a small window unit for that inset one instead of a portable because of the price difference. I have vinyl windows and it looks like there may be mounting issues with the lip on the bottom sill and having to screw into the sash. Anyone know if this would this be easy to work around? I suppose I could shim the bottom to sit flush with the lip and just forgo the screws in the sash.

 

[Red Squirrel]

The twin pipe will be more efficient, so it will help, get the biggest one you can, mine is 1 ton (12,000btu).

I converted mine to dual pipe:



Though I'm getting central A/C soon so I can stop using it. Downside with portables is they're loud. I'd compare it to having a blow dryer running behind you, except at a slightly lower frequency so not quite as bad. You can't watch TV or watch videos or anything like that if it's in the same room, and even if you're a heavy sleeper or need a fan to sleep, this is louder than you'd want to try to fall asleep with.

It will cool a place decently though. Sometimes I leave it on when I leave the house and when I come back I can actually notice the house being slightly cool. I have it point towards the hall and that's where one of the return vents is, so I'll just run the furnace blower to circulate the air.

 

[Gonad the Barbarian]

If anyone's interested, I picked up a small 5000 BTU window unit earlier this week
and got it mounted this morning. I had to shim the sill on the outside with some lattice scrap to raise it enough to clear the inside lip at the bottom. It's probably good enough as it is for the rest of the season, and probably no more leaky than my crappy window fully shut. I'll pull it out and keep it stored in my basement in the fall/winter and do a better job of mounting it next spring. It's definitely colder than anything my HVAC spits out, and I can already tell that's running quite a bit less given the 90+ outside temp today. Running the window unit 24/7 should translate to about $30 a month, I'll try to keep up with the usage and do some math on my next power bill.

 

[Aikouka]

I'm rather tempted to do something like this just for my garage. The biggest reason is that no matter what, the garage sits at about 82-85F all the time. The other day, it dropped down to 70-75 (depending on time of day), and the garage still never dropped. If I'm just accessing my car or whatever, it's not a huge deal, but if I'm spending an hour or more in there working on projects, then I'd rather not be super hot.

Now, the garage actually resides under some of my rooms, so technically, there are HVAC lines running in the ceiling, but tapping into them just seems like a bad idea, because the garage lacks a return vent. It's a garage, which means I'm not as bothered by noise in it (I'm pretty sure the super manly power tools make more noise), but I would need to ensure that the garage is fully insulated before I worry about trying to cool it. I'm also wondering if keeping the garage cooler will help keep the rooms above it cooler.

I am in a similar situation (40 year old 2-story house w/ 2 HVAC systems , 2nd floor HVAC 2.5 ton system and ductwork in attic) . Due to some substantial tree removal aound my house, I lost a lost of shade which has now caused my HVAC system to be under capacity. The attic tempertaures can now reach 125 deg F on a hot summer day (in Atlanta,GA). It was even hotter before I made some inprovements to the attic ventilation

What sort of improvements did you make? I've heard the reflective paneling that you can put up does wonders for reducing attic heat.

 

[Gonad the Barbarian]

If there's a window in your garage get a window unit. If there's not and you can manage it, run an exhaust duct outside for a portable AC. You can do it with a dryer exhaust through the wall, and you'll just need that and not a dual hose with intake because it's a garage. Don't cut into you're existing house ducting. 85F is really not that bad for a garage, maybe one of those giant heavy duty garage fans might be enough, but just glancing at the pricing it's probably better to go with an AC if possible.


I've been looking at that radiant heat paneling too. I'm due for a new roof in a couple of years, I'm definitely going with lighter colored shingles than the pitch black ones that are up there now and see how that goes first I think. All this was done 15 years ago so that's a consideration, but man the more I look into it the more I see just how sub-optimal my hvac system is. Leaky ceiling, leaky ducts, return mounted in ceiling, no attic exhaust, and I'm sure everything is under-insulated for the conditions up there. Really should have moved on addressing these things sooner.