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Thread: Measuring/confirming new AC/minisplit is up to duty

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    DIY Senior Member chefwong's Avatar
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    Default Measuring/confirming new AC/minisplit is up to duty

    Example, 90-93 degree day, typical Northeast humidity ...
    Relative indoor temp of 78 degrees. How long should it take for a new system to bring the temp down to 72 degrees ?

    Just had a minisplit upgrade. Yesterday, while comfortable (less humidity) in the air, it took 4 hrs to go from 78 degrees down to 75 degrees and this was only 2 people in the 500 sq ft space

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    Retired Defense Industry Engineer jadnashua's Avatar
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    There is a huge amount of stored energy in humidity (water vapor). It can take awhile. Once you get the moisture out, it will drop faster.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Senior Member Noth Jersey's Avatar
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    What size unit did you install? Is the area you're cooling tight and well-insulated? A two-ton unit is just about right for cooling our 650 square ft. main floor in the Northeast. You don't have to worry too much about oversizing the inverter units. You could always put your existing unit in the basement or garage if it's on the small side.

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    Plumber jimbo's Avatar
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    Around here, the " wing it number" used by a lot of guys is 600 sq ft per ton/

    That aside, what is the delta T, in other words what is the temp . of the cool air right at the outlet of the air handler?? Minis do have less powerful blowers than a full size air handler, so it is prudent to run for longer periods, and not expect to "blast cool" a place down. But if you don't have like 65 or less air temp at the blower, your unit may not be working well.

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    DIY Senior Member chefwong's Avatar
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    Our previous unit was a noisemaker 22K Friedrich unit.
    Our Hvac contractor recommended 18K BTU and while it's not a immediate ice box blowing out of the unit, I'm going to put it through it's paces even more...

    Seems like the pros advise, that it's best to turn it on early if not keep the windows constantly closed to keep the *conditioned* air instead of turning it on 2 hrs before guests arrive to give it a chance to wring the moisture out so then it can ~cool~ the air.

    Gotta love this inverter technology. Super quiet, I love how the unit ramps up or ramps down (no constant on/off) & instead of having it at 69 degrees at night to get comfortable, the sweet spot so far is 78 degrees at night !

    Plus the fact that energy wise, it's 3X more efficient SEER wise.

    We'll see how well the heat pumps do in the winter compared to my 70 year old boiler.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    There's something to be said for dry heat...you'll be comfortable in a warmer room if it is dry in the summer. This is one reason why it is not good to oversize the a/c unit. One just the right size on the hot design day will need to run constantly, which gives it the maximum chance to wring moisture out of the air. Now, if you lived in the dry desert, you may not want it dryer in the house. That's one reason why some people there use evaporative coolers. They just won't work where the humidity is high, though.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member Lightwave's Avatar
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    Quote Originally Posted by jadnashua View Post
    This is one reason why it is not good to oversize the a/c unit. One just the right size on the hot design day will need to run constantly, which gives it the maximum chance to wring moisture out of the air.
    The prohibition against oversizing ACs does not apply to minisplits, however, as they are variable capacity. Oversizing a minisplit--within reason--only means higher initial costs and much faster temperature pulldown.

    Unreasonable oversizing will cause a problem even with minisplits as the larger units have higher minimum cooling capacities. Installing a 3 ton cieling cassette unit in a space with 5000btu of cooling load would not be usful.

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    DIY Senior Member Noth Jersey's Avatar
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    My 2-ton (24,000 BTU) LG unit has a setting for rapid cool down.

    My anecdote:
    Here in New Jersey this size feels about right given the porosity of my house envelope and the occasional week in the 90s. I can cool down the whole upper floor significantly in about 20 minutes or so. The heat pump works beautifully; I wouldn't have gone any smaller, since the system works pretty hard when you get a record-setting -12 day like we did last winter. I was surprised the heat pump worked at all at those cold temperatures.

    I'm opening up the main floor by removing doors and partitions, so I won't need any through-the-wall fans to circulate the air, but I would definitely consider something along those lines if I were going to keep the existing floor plan. I'll be using a multi-zone system in the addition I'm building.

    On a different note, I've vented my tankless water heater in proximity to the condenser, which sits under a tall, unenclosed entry porch, so I imagine the heat pump will be a little more efficient than usual this winter.
    Last edited by Noth Jersey; 07-14-2010 at 05:16 AM.

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