Home central air conditioner: Return path?

Factual Questions
1

I just turned on the central AC at my apartment for the first time since moving in, and it’s not very cold. Someone is coming to take a look soon, but in the meantime, I’m just trying to understand their design and how they work.

The model I have is a Gibson JS5BD-030KB. My understanding is that this sort of “split” AC system is a closed-loop system containing interior ductwork + an air intake, some sort of coolant, and an outdoor part that blows the heat away. The interior portion sucks in household air, transfers the heat content to the coolant, pumps the coolant outside where the heat is shed through heat sinks and a fan, and consequently pumps the cooler air around the house. Is that right so far…?

If so, what I don’t understand is how it manages to get air from other rooms back to the central unit (the “air handler”, I think, with the blower attached to it?) In some videos (like this one at 1:50), it shows what appears to be a return pathway for the ductwork, so hot air from a room get sucked in through a vent, through the duct, into the air handler, and then the cold air (after the heat exchanger) gets pumped back to the same room. That makes sense to me.

But my apartment, as far as I can tell, the blower motor intake is not attached to any ductwork, just a grille facing the kitchen. And all the vents in the rooms seem to blow, not suck. It’s a two-story apartment.

Does this mean that my particular system simply lacks a return path for the warm air, and there’s no way for hot air upstairs to get back downstairs to the air handler for exchanging? So it just keeps cooling the downstairs effectively (since it can suck all that air in through the grille), but for the upstairs rooms, the warm air never gets back downstairs? Sure, a little bit of cold air comes up through the vents, but there’s no real airflow back downstairs and hence very limited recycling/cooling of the mass of warm air upstairs?

Sorry for the newbie question… I never really thought about how central air conditioners work before, and this design seems really bizarre and inefficient to me. I’m not sure if I’m understanding it correctly. I hope there’s some method of making it work better short of just installing window ACs in all the upstairs rooms…?

2

In the upstairs cold air is blown into a bedroom. This new air will displace some of the existing air pushing it out into the hall (common area). The return air vent in the kitchen will pull air from the kitchen and common areas. You may also have a return air vent in the upstairs hall, I am going to assume you do.

By the way the return air vent in the kitchen is connected to the air handler either directly or with ductwork inside the wall.

In theory the hot air will return to the return air and you will have an evenly temperature through out.

Let describe house my house was built. 2 Story. Air register in each bedroom and bathroom upstairs. Down stairs one register each in living room, kitchen, and bathroom. Return air at the bottom of air handler in Livingroom and at the bottom of the upstairs hall.

The problems return airs are low, but hot air is at the top of the rooms not the bottom so cool get pulled back into the air handler. Upstairs return air is smaller than downstairs.

MY operation in the past during summer months throttle back the registers downstairs, except kitchen. Open used bedroom registers fully. On hot days put the fan from auto to continuously on. When the AC downstairs reaches temp and the compressor this will help pull some of the hot air from upstairs to downstairs. This help but on 85 plus days the temp difference between upstairs and downstairs would be 10 degrees or higher.

My modifications. During a upstairs remodel I modified the upstairs return air. In the winter the lower register open, in the summer I close the lower return air register and open the upper register. I added a return air fan to the upstairs return air, increasing the air flow from upstairs. And I also block off part of the 1st floor return air. (I know you can not make changes.

But some of the hot air from upstairs will get back to the air handler. The way you can improve you confort is to fully open the upstairs vents and throttle back down stairs vents. Set the fan to run continuously. Do not close off the downstairs vents you need to keep the air flow across the cooling coil.

Hope this helps.

3

in order for heat/ humidity to be removed from air, the air must pass through the indoor coil. while blowing cold air into a hot room will make the room less hot, it’s typically not going to create the ideal temperature/ humidity that most people have come to expect. where you live plays a big role in how big a problem this is.

in my experience, in my area, two stories and one system can be a real problem in the summer’s unless it’s a zoned system. in neighborhoods where a single system is the norm, you always see window units hanging out all of the bedrooms upstairs.

floorplans also can play a big role in how it all works out.

much better to have two systems. keeps the climate nice, and if one breaks down, you can always stay in the other part of the house till it’s repaired.

4

running the fan continuously on can help to make the house less comfortable during the compressors off time. it adds the heat from the fan into the system (if it’s a psc fan, this can be a fair amount of heat) and it also will add all of the water your coil still has trapped in it back into the indoor air.

I’m not saying some of what you’re doing isn’t helping, just that you may be doing something counterproductive without realizing it.

get some cheap hygrometers from Walmart, put one on each floor to base the results of your adjustments off of.

*make triple sure your return ducts don’t draw in any air from unconditioned space. the longer a duct is, the more that is likely. doesn’t take much 110-150 degree attic air to mix with your return air to completely offset a 15-20 degree drop across your indoor coil.

5

It has been a while since I have looked at the amp ratings of the fans. Usually they draw around 1 amp. That would be about 120 watts. The same as two medium sized lamps. That is not a great heat load.

Before making the changes to my system the delta between upstairs and down stairs on a hot day was over 10 degrees and higher really hot day. Now it is between -2 to 4 degrees. We can sleep upstairs now on hot days.

This is in San Jose CA. So for us a hot spell is maybe 5 days then we get god’s Air Conditioning. Fog from the coast.

6

Ideally you have return air ducts in each room. Some smaller systems like presumably yours they cheat and leave them out and rely on air moving from room to room until it reaches the master return air grill, and. It still kind of works but problem is if a door is closed the gap underneath isn’t enough to make it work right. I’ve seen it where people with this setup have put louvers in the doors or the walls between rooms, but this obviously increases noise transmission too.

Overall though if it’s blowing some cold air but not enough out the ducts, I’d suspect low freon. Since you have an R-22 system you will get sticker shock when you get a quote for charging it. You might consider the cost / benefit of replacing the entire system with one that uses a newer R-410A

I think my blower draws something like 600 watts. I still leave it on constantly because the benefits (white noise, balancing temperature, air movement, and pulling air through the filter) are worth it despite the cost.

7

It’s not uncommon for an apartment to have just a single return grille right at the furnace. As long as there’s a clear path for the air to flow down stairs, through hallways, and around doors (that’s the tough one), then it technically works. It would be better with returns on each floor and in each room, but that’s not usually practical. It’s also best for returns to be in opposite positions from the supplies. For instance, supplies are usually along exterior walls, so returns should be on interior walls, inducing flow across the room. Even more importantly, on systems that both heat and cool, if the supplies are at the floor then returns should be near the ceiling, and vice versa.* That reduces stratification.

To the situation at hand, any R-22 system is old enough by now that it likely needs replacing no matter what. Dirty filters or coils (inside or outside) can also cause lack of airflow and poor performance. Assuming the mechanics all check out, then it’s just a cheaply installed system that needs some manual re-balancing between seasons. Close off some of the downstairs supply registers by about 50% and make sure all the upstairs ones are fully open. Small downstairs rooms like bathrooms or halls could likely have their registers closed off completely. If the system starts to whistle or blow too noisily out the supply registers, then it’s choked down too much. Reverse the process as winter approaches otherwise you’ll roast your upstairs.

*For heating-only systems, the best setup is to have supplies and returns at the floor. The hot air is supplied at the floor where the room is already the coolest, helping to warm/lift that air. At the same time, the coolest air that’s still pooled on the floor is what’s sucked into the return to be heated, not the warmer air near the ceiling. It’s the opposite for cooling-only systems, where you want the supplies and returns near the ceiling. Retrofitting an old heating-only system to have air conditioning is where things go pear-shaped really fast, because the cooled air just short-circuits along the floor without mixing properly. A compromise in 2-story houses is to put all the first floor supplies/returns in the floor with ducts in the basement, and all the second floor supplies/returns in the ceiling with ducts in the attic.

8

If it’s low on freon, there’s a leak. They may not be able to find it, but systems in good shape are never just randomly low on freon. It’s a closed system. So if they just recharge it, you’ll have to get it recharged again at some point.

9

When I bought my house, it was 40 years old. It was a single level with the furnace under the floor. There were two returns in the common areas but none near the bedrooms. The bedrooms were cold in winter. When I was servicing the furnace, I noticed a return at the end of the hall near the bedrooms. Back in the house, I could not see the return because a closet was built into the end of the hall. I cut out the back of the closet and found the return that was blocked. I built a duct opening into the hall and the bedrooms were noticibly warmer after that.