I have a couple of HVAC questions

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Bronxguinea

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Just bought a house in Florida it's 2150 sq ft 1650 is the house with a/c and the garage is 525 no a/c,it has a 3.5 ton air handler that's dated 2012 . I'm figuring it is larger than the original one cause the air return is 20X24. Ok now for a few questions the return box is lined inside with fiberglass is this ok or can I cover it up inside with foam insulation board and alum tape. also should I make the return opening 32X24 at this time? last question can I run a duct to the garage and only open it when I work in there from time to time due it gets so hot in there Thanks for any input Dom
 

WorthFlorida

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  1. Leave it alone and don't mess with it.
  2. You move from the Bronx to FL so you better get used the heat. Hey man it's Florida, I'm in Florida for 27 years and the electric bill can really get away from you if you try to cool it as if you're back in NY.
  3. Do not increase the size of the return filter grate. It will do no good unless increase the duct work to the rooms. The correct way is to have a certified tech that knows how to read static pressure. All air handlers fan motors are auto speed if it is a high efficient unit or speed adjustable depending on the tonnage and air flow needed. The speed is set usually with strap options. If you switch the thermostat to heat and turn on the heat to check it out, you'll notice that the fan speed will be slower than in the A/C mode. Most air handlers (South Florida) will have an electric heater installed in it. In central Florida most units installed are heat pumps.
  4. Do not add a vent to the garage. You'll be pushing air into the garage and and there would be no return, therefore, you'll cause a slight vacuum in the house and then be pulling air from any leaks from windows and doors. Beside, you increase a chance of carbon monoxide entering the home. Also, the vent will always pass some air since they really do not close 100%
  5. Inside the return be can be anything you want. Fiber glass is not recommended because is will grab dirt and act as a filter. It can release fiber glass into the air.
  6. Many home owners install a mini-split system for the garage generally because it became more of a work shop than a garage for the car. If you really need to cool the garage it is the best way to go.
  7. Do as I do, install a ceiling fan or a wall mount fan.
 

Bronxguinea

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Ok Thanks but I was in Southern Cali for 6 years wanted to get back to the east coast lol, so I'll cover the fiberglass ,the return in the garage wouldn't have been a big deal being I could have put a return in the base pretty easy but I was also thinking maybe a attic fan instead in it and a roof vent the place gets like a oven , rebuilt trannys for 25 years and after seeing the way the well pressure tank and water softener was set up was kicking on every min and having to read up and fixed it , even on this unit the ez trap was glued closed and there was 2 " of water inside it which good thing one of the UV lights went out and I noticed it . so I have a new one ordered for a A-coil and not the universal ez trap View attachment 36819
 

Dana

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The 3.5 tons of AC would usually be ridiculously oversized for a 1650' house, but screwing around with the duct balance is inviting an efficiency disaster.

When it's time to updated the AC it pays to do real Manual-J load calculation (room by room) and go ductless (if you can.) The ratio of load/square foot is all over the place, but it's no where near a ton per 500'. Typical ratios for houses that size would be more like a ton per 1000', but with ducts running outside the pressure and thermal envelope of the house often adds a parasitic ton of load. A contractor in Georgia compiled calculated loads and the load per square foot ratio and plotted it against house size:

Bailes%20graph%20for%20Manual%20J%20blog.preview.png


For houses under 2000 square feet only one ~1500 footer came in at over 2 tons (600' per ton.) Yours COULD be that house, but it probably isn't.

If you want to air condition the garage 24/7 a right-sized mini-split can make sense, but if it's just for a few hours on weekends a 1.5 -2 ton PTAC or through-the-wall air conditioner probably has better economics (spending the difference on rooftop solar would more than make up the difference in power use.)
 

Bronxguinea

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http://asm-air.com/airconditioning/what-size-central-air-conditioner-for-my-house/ this is showing in Florida it's not that much over sized ,and I find it hard to trust what's here after looking at the set up for the ez trap and the fiberglass in the return plenum . There's 12 ducts in the house and never have to move it from 78 it keeps the very comfortable cool
2016-10-17 16.12.52.jpg

and through the summer the electric bill wasn't over $150 a month for the house . So when I saw that I figured why not do something in the garage too
 
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Dana

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Sizing air conditioning by square footage of conditioned space is barely better than a WAG. That ASM chart and their cheesy square feet x (pick your constant) divided by 12,000 is complete & utter crap! Cooling loads are a function of exterior surface area and solar gain, not the square footage of the interior space. That's why a Manual-J is called for.

what-size-central-air-conditioner-do-i-need-for-my-house-calculator.jpg

^^pure BS^^

Do the Manual-J! According to that silly chart my house should have a load of 4 tons, when in fact it's under 1 ton, and is not well served by the totally insane 5 ton central air conditioning previously installed. It's oversized to the point of being nearly useless for the latent loads, so I use a half-ton window-shaker in an upstairs window to cool the whole house, except at temperatures well above the 1% outside design temps, where I might have to run the central air for exactly ONE cycle per day during a heat wave to keep the place cool.)

The fact that an oversized AC system can keep the house cool should come as no surprise- it has the capacity, but it it's not particularly optimal, using more power than needed, making more noise than needed, and costing too much up front. It's pretty typical to see oversizing factors of 2-3x, (rather than the 5x at my house) but you'll be more comfortable if it's no more than 1.5x oversized for the 1% design load.

The very fact that your biggest cooling bills are $150/month is evidence that it actually IS oversized, possibly by quite a bit! If it was appropriately size it would be running a much higher duty cycle, and using much more power than that.

You can prove it by actually measuring the duty-cycle on the compressor with a data-logger the next time the outdoor temps are in the range of your 1% outside design temperature (about 90-93F for most of FL). I suspect you'll be pretty surprised at your oversizing factor. But if the thing is only 4 years old, you'll probably be using it for quite awhile longer, and there's no "payback" in swapping it out for a more appropriately sized system as long as it's keeping you reasonably comfortable.

Without knowing the time of day that picture was snapped it's hard to say for sure, but it looks like you have pretty good mid-day to afternoon shading factors and very little west facing window area, which would lower your peak cooling load compared to a typical house that size with less-favorable shading factors & window orientation.
 

Dana

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BTW: That inane chart on ASM's site is reminscent of this wonderful piece of disinformation, (same map & color scheme) and probably developed by the same source. According to their chart my house should need a furnace capable of 120,000 BTU/hr, whereas the design load at my 99% outside design temp of +5F is a bit over 35,000 BTU/hr, measured by both Manual-J, and fuel use against heating degree-day calculations. I'm radiation-constrained to only about 45,000 BTU/hr at the water temps I'm running in my heating system, but somehow don't get cold even when it's in negative double digits outside. This is in a sub-code 2x4 framed antique with single pane double-hungs and clear glass storm windows, not exactly a superinsulated wonder.

Whomever concocted that map and those methods of HVAC sizing wasn't doing anybody any favors.
 

WorthFlorida

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Bronxguinea, you're doing real well on the electric bill. Question, how did you get your tonnage reading? From the compressor or the air handler number? I have two air handlers that have a 3 ton reading but the compressors are 2 and 2.5 ton. All it means that the air handler can handle up to 3 tons. The true tonnage is from the numbers on the compressor.
 

Jadnashua

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For maximum dehumidification, the worst thing you can have is an oversized a/c unit. Most people are quite comfortable at higher temps if the humidity is lower, and the only way to achieve that is when it runs longer since it can't extract all that much moisture on a pass...lower capacity, lots more passes, less hysteresis, more comfort, cheaper to buy.
 

Bronxguinea

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Bronxguinea, you're doing real well on the electric bill. Question, how did you get your tonnage reading? From the compressor or the air handler number? I have two air handlers that have a 3 ton reading but the compressors are 2 and 2.5 ton. All it means that the air handler can handle up to 3 tons. The true tonnage is from the numbers on the compressor.


On both found the pdf owner manuals with model # break down on both FEM4P4200AT on the handler and N4H342AKE100 and both are 3.5 ton 42,000 btus. that's why I figured if it was oversized why not do the garage too. like I said doesn't seem like they people knew what they were doing anyway. Have vaulted ceilings in the living room ,kitchen,dinning room and foyer . It's a open room plan so figure that helps on the cooling
 
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Bronxguinea

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I think the one before this was smaller cause the grille is 20X24 and the top opening
2016-11-07 15.56.51.jpg
2016-11-07 16.05.53.jpg

of the return looks small for this air handler
 

WorthFlorida

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Yes with the high ceilings does add to the ac sizing. If your house is comfy and your electric bill is reasonable for Florida then you're sized ok. High efficient units want a lot of air movement at slow speeds but to increase the grill openings won't do anything unless your duct work is also up sized. 10" was the norm but new homes 12" in now preferred. It's a lot of expense for little changed in your kilowatt usage if you want to upgrade.
What is required now for most of Florida every room with a door must have a return. A grill in the bedroom ceilings duct to the open hallway to another open grill to the eventual return to the air handler. It allows unrestricted airflow for better efficiency and cooling. If you don't have the returns it's not all that bad to add them to your home.
 

Dana

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A fundamental fly in the ointment with this plan is that fire codes in most locations prohibit HVAC ducts that supply the house from having openings in a garage. Even plumbing & electrical penetrations between living space and garage are required to be sealed for fire blocking. If a fire breaks out in the garage that shares an HVAC system with the living space it would spread toxic smoke into the house if the air handler were running.

WorthFlorida has it completely right that despite a likely ridiculous oversizing factor, it will never be cost effective to scrap the existing system and start over. If it's keeping you sufficiently comfortable and the power bills are moderate there is no reason to swapping out a 4 year old system. (I haven't swapped out the 5x oversized system in my house- I don't really use it more than 20-25 hours/year, since I live in a ~500 CDD climate.)
 

Bronxguinea

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Well covered the fiberglass up in the return, replaced the one UV light that was out, and found the ez trap was run underground to the outside so capped the opening with a screen inside it being it was wide open pipe
 

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WorthFlorida

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Bronxguinea, what product is on the garage floor? It looks nice. When wet does it get slippery?
 

Bronxguinea

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View attachment 37098 The light gray is a oil base paint because that's what was on it which isn't good with rubber tires it leaves marks so I got Better Life Technology G-Floor Parking Pad Garage Floor Cover/Protector, Slate Grey from Walmart to lay on top they have ridges in them so dirt or liquid easy to sweep or hose out of garage

48f17861-5367-4391-b950-9b371ac5963a_1.4a4674baa38820fcdd557cd94f57779a.jpg
 
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