Over air conditioned yuck

[JerryR]

Why do people think bigger is better when it comes to AC?

We recently bought a 1500 sq ft single story modular home that was built in 2000. This is a vacation home for us in central Florida zip code 34972.

Last year the previous owner installed a 3-1/2 ton straight cool single speed 16 seer Rheem unit with 10kw strip heaters. He bragged about how quickly the house cooled down.

What we are dealing with is noisy air ducts due to high CFM. Additionally being over air conditioned, the volume of cold air causes cold spots where registers are directed.

Using heat the other day heated fine but electricity usage for that day was 86.4 kWh. Rarely does it ever get below 40 deg F here. My primary 3200 sa ft home with a Daikin Inverter heat pump system has never used even 1/2 that kWh.

I would love to replace the unit with either an inverter system of at the minimum a 2 speed 2-1/2 ton system but my wife would kill me.

With the cooling short cycles my wife is complaining about being cold when AC cycles.

Just venting my frustration.

 

[Dana]

Even a 2 ton is likely to be oversized for your average load. Energy Vanguard is an outfit in GA that among other service runs Manual-J cooling & heating loads prior to specifying the equipment. They compiled this graphic plotting house size against the ratio of conditions space to tons of cooling over a few dozen houses, most of which were in the Gulf Coast states:

Two tons for for a 1500' house would be 750'/ton. Only the very worst performing houses under 2000 square feet had ratios that low. The median is about a 1000/ton, which means your house's cooling load is more likely to be in 1.5 ton range.

You could run the numbers yourself, or you could measure the duty cycle on your monster-0verkill unit on days where it's near your 1% outside design temperature to estimate the actual load.

A potential right-sized super-comfort replacement might be the 1.5 ton Fujitsu 18RLFCD ducted mini-split, which runs about SEER 20, and will modulate with load between ~3000-20,000 BTU/hr in cooling mode. With your ridiculously oversized ducts it should be able to use the existing duct system with few modifications despite the fact that it's blower isn't nearly as strong as a full-sized air handler. Like other modulating heat pumps the highest efficiency is achieved with a "set and forget" approach to the thermostat. (The Fujitsu thermostat is really a wired remote, and has to be programmed to use the temperature sensor in the remote, or it defaults to using the sensor in the air handler to sense room temperature.) During the highest-humidity times you'll have to run it in "DRY" mode to get sufficient latent cooling out of it, but it'll do it. The registers may have to be changed to smaller long-throw types to get adequate mixing during low load/low-cfm, but they're whisper quiet.

Daikin make a similar 1.5 ton mini-duct cassette, but IIRC it doesn't have as wide a modulation range, and is designed to work only with their commercial multi-zone VRF system.

 

[Sponsor]

 

[WorthFlorida]

Being a modular home, many are built quite good, the insulation may not up to current energy standards. The previous owner either got a good deal or because of the heat gain and the inefficiency of the old AC unit, going bigger took care of being too warm on humid summer days. If you have neighbors with the same home look at there unit and ask if they are comfortable with there size units. Also, window exposure plays into the picture. Dana has a lot of good information and probably with today's efficient AC/Heat Pump units, 2 ton is may all you need. Even with a heat pump you still want an electric heating element and 10KW is OK. It will heat the home faster and there is no waste. The thermostat controls the heat pump and the auxiliary heating elements. All new smart thermostat learn you heat load and depending on the outside air temp and set point on the thermostat, it determines when to run the electric elements. It works quite good and

My two story home (zip code 32828), 2400 square ft has a 3.5 ton replaced a few years ago, SEER ~15.9. All of the coils are aluminum (Carrier), as most brands are and the comfort level went way up from the previous SEER 14 Carrier unit. I do have a 10 KW electric element and with our current chilly weather the aux heating does turn on but once you get near the set point it is turned off and just the heat pump in on. The display on the thermostat indicates if the AUX heat is on. You can program the thermostat with "no" aux heat and it will never turn on the electric heat. The heating elements is a very low cost part of the entire system and it is always worth having the electric heat as a back up. Again it is quite comfortable.

After you do get it changed out the duct work will probably need cleaning. If it is the typical flex duct used all over Florida, a good chance there could be mold because of your over sized unit. more likely at the grills where the cold air hit the metal and condenses. The moister works itself into the fiber glass insulation.

 

[Dana]

Mind you, just the 10kw of heat strip on it's own could heat my sub-code 2400' + ~1500' insulated basement antique house in MA at 70F indoors down to about +10F outdoors. A single 1.5 ton Fujitsu could heat it all down to about 32F. (It has a control output for calling auxiliary heat, if necesary.)

I've never looked up the specs on the Rheems, but at 3.5 tons it's likely that you could run it in heat-pump only mode down to 0F (or whatever outdoor temperature it may turn itself off for self protection) and still have capacity to spare on any decent Y2K 1500' modular.