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Old 07-23-2020, 03:34 PM
 
Location: plano
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Looking at variable speed HVAC systems where the AC and the blower fan operate at a variety of speeds as needed. Wondering if they also allow one to get a large capacity ac like a 4 ton to replace a 3.5 ton. The price of going up a half ton is near zero in equipment cost. Do these units adjust speed to keep humidity removal low enough or do they just optimize on lowest power need to reach set-point?

My current units during peek cooling season like today runs all day and night it seems. Might be useful to have a little extra cooling capacity if the downsides of an over sized unit are not critical in a variable speed design?
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Old 07-24-2020, 05:43 AM
 
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They are good, and work only as hard as they need to.

For example, if it’s 73 inside and your set point is 72, the compressor and inside blower might be at a very low setting and run longer.

But if you then set it to 68, the compressor and blower will go to a higher setting (or full) because there is a greater differential.

This is different than a single stage which is either on or off, and you get much fewer temperature and humidity swings.

Some models only come in full ton models so you would get say a 4 ton and not a 3.5. And you’re right, a variable speed is a bit more forgiving for being oversized from what I’ve read.

But - and it’s a fair but - you will pay considerably more for the unit, the increase of which likely will not pay for itself over a 10-12 year lifespan. There are more things that can go wrong, you need a communicating thermostat and parts may be harder to come by.

I just replaced my upstairs unit and went single stage, upsizing from a 3.5 to 4 ton. So far on the hottest day it ran around 14 hours mostly from late morning until late evening, with no problem keeping 72 when it was in the 90’s outside. That’s exactly what I want. On a slightly cooler and overcast day it ran around 10-1/2 hours. When I compare this to last July it averaged over 14 hours a day every day, so I know it ran longer and wasn’t keeping up in the afternoon (by 6pm the upstairs was 74-75 even when set to 72).

In my example a variable speed unit won’t give you any benefit because it would run on the hot days at 100% most of the time.
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Old 07-24-2020, 08:52 AM
 
Location: Virginia
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I have a variable speed unit, and there are times it just dehumidifies, rather than cools the house. I keep it at 76 degrees normally and the house stays very cool, which is probably due to the plaster walls in part. I do have to run a window unit in my upstairs room, but that's due to inadequate ducting to the room, not the unit itself (it's a bumped-out former one room space, not really designed for a master suite). I personally love this unit (Carrier Infinity 18 SEER) as it's extremely quiet and efficient, and it's decent backup heat in case my boiler goes out in the winter.
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Old 07-24-2020, 08:55 AM
 
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Quote:
Originally Posted by Bungalove View Post
I have a variable speed unit, and there are times it just dehumidifies, rather than cools the house. I keep it at 76 degrees normally and the house stays very cool, which is probably due to the plaster walls in part. I do have to run a window unit in my upstairs room, but that's due to inadequate ducting to the room, not the unit itself (it's a bumped-out former one room space, not really designed for a master suite). I personally love this unit (Carrier Infinity 18 SEER) as it's extremely quiet and efficient, and it's decent backup heat in case my boiler goes out in the winter.
a variable speed blower which is pretty popular is only for dehumidification .... it starts out at low speed giving the aor time to dry out before going to high .


a variable speed compressor is something else and that varies capacity . .
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Old 07-24-2020, 09:03 AM
 
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I'd avoid them too expensive to repair. I like things simple though and on a hot day will keep the AC high and in winter the house is about 60.


I'm the kind of person
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Old 07-24-2020, 12:01 PM
 
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I've experienced a few different types of variable speed systems. I was in a home that had a variable speed blower with a two stage compressor; that was pretty good, but my favorite version is my current home which has an inverter-driver infinitely variable compressor tied to the variable speed blower. I hate single speed systems- just too uneven, and I hate the way it's either full on or full off.

I echo another comment above too- I had a home with the Carrier Infinity system and it was very nice.

As the OP suggested- oversizing doesn't matter. This was important for me as this system also does my heating in the winter and I needed some extra capacity to cover the coldest days of the year, but I'm still able to keep the space low in humidity during the summer by setting the thermostat to the "dry" setting instead of the "cool" setting.

The only drawback is that many/most true variable speed systems need their own controllers and are not compatible with the standard wifi thermostats. Even the high-end wifi thermostats can only do 3 speeds- you really have to use the controller designed for your system to get all the features.
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Old 07-24-2020, 01:29 PM
 
Location: Virginia
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Quote:
Originally Posted by mathjak107 View Post
a variable speed blower which is pretty popular is only for dehumidification .... it starts out at low speed giving the aor time to dry out before going to high .


a variable speed compressor is something else and that varies capacity . .
Well, I have the whole system, so I would suppose it has both.
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Old 07-24-2020, 02:03 PM
 
5,086 posts, read 3,011,811 times
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Quote:
Originally Posted by Scrat335 View Post
I'd avoid them too expensive to repair. I like things simple though and on a hot day will keep the AC high and in winter the house is about 60.


I'm the kind of person
I agree, our last home had a variable speed blower where the inverter control that ran the motor was a proprietary gadget. A few months out of the 5-year warranty the inverter blew and the only fix was to spend $800 on a replacement. On a simple unit with an induction motor it would have been $10 for a capacitor or maybe $150 for a new motor and belts.
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Old 07-24-2020, 02:04 PM
 
2,684 posts, read 2,384,437 times
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Quote:
Originally Posted by Bungalove View Post
Well, I have the whole system, so I would suppose it has both.
Another perk of the variable speed blower is that it also runs in variable speed for heating too; when I had the Infinity system with a gas furnace, it ran nearly silent during the winter except for when there were big temperature swings, like coming home from vacation and needing a 20 degree rise.

Variable blower + variable compressor really is the best, both for heating and cooling.
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Old 07-24-2020, 02:26 PM
 
106,238 posts, read 108,237,907 times
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Quote:
Originally Posted by Bungalove View Post
Well, I have the whole system, so I would suppose it has both.
Not always ..many only have the variable speed fan for start up , not the variable compressor too
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