A/C Coolness curiousity


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Old 08-09-12, 06:17 AM
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A/C Coolness curiousity

Just asking out of curiousity, not planning on DIY'ing anything unless it's a no-brainer.

Baseline: 1949 house, brick over block so a lot of thermal mass. We've been here since 1983 and added A/C the next year, this Ruud system is now OLD and we're always trying to "get one more season'' out of it. Heat is by hot water, so they stuck the A/C ducting in the attic (single level) via soft round tubes, maybe 6' diameter. Almost NO maintenence in all that time, once 10 years ago the outside fan was replaced w/a junkyard unit. Filter is fresh. All in all, hard to complain with the cost/benefit ratio (ignoring for the moment that it cannot be anywhere near as efficient as a new one)! We just paid off the house, thinking to replace the system next spring (we always say that).

OK, honestly, noticing that gee whiz, sometimes it takes a while to drop a couple of degrees (all that thermal mass?) Well, I got curious, it seems to blow just fine but I postulated that cooling ability is volume of air times the coldness of it. Hmm, how to check that? So, I went to the kitchen's gadget drawer and pulled out the brand new digital instant-read thermometer. The A/C had been blowing maybe 1/2-hour this cycle, so I presumed that the temp up in the ductwork had "stabilized". I stuffed the prob in there (end-of run vent furthest from the center), making sure the end wasn't touching anything.

The reading stabilized at 57 degrees; I cannot assure that the thermometer is accurate. No idea if that's good or not. What's the usual output of central A/C, or at least a range?
 
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Old 08-09-12, 07:52 AM
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You have to take the temp of the room at the return air grille, then we look at the temp-split according to the relative humidity in the conditioned area. You need to get a low cost humidity gauge at an Ace hardware store.

Want us to be enabled to check the ballpark performance of your system... so you can call an HVAC Contractor if it needs servicing?

All U need is a good thermometer (digital reading in tenths preferable) & and indoor Humidity Gauge

1) Helpful; Tonnage & SEER of Unit &/or outdoor condenser model number: __________________

2) TXV or, orifice metering device? _______. Only if U know…

3) Outdoor condenser’s discharge-air-temperature ______-F
Subtract Outdoor air temperature: _______
Outdoor Condenser Air-Temp-Split _______

4) Need the ‘Indoor’ percent of relative humidity - in the middle of the rooms or, at Return-Air inlet grilles ___
5) Indoor Return-Air Temperature ______
Subtract Indoor Supply-Air Temperature ______ -F
Indoor temperature-split _______-F

U don't have to do this...
If U can find a turn in the small liquid line insulate the temperature-probe; get its temperature for the subcooling analysis:
Small liquid line temperature ____F

U don't have to do this...
To measure return air the wet bulb temp, wrap a small/thin, wet (not dripping) piece of cloth around the probe and put it into the Return Air grille & then close to the closest Supply Air diffuser near the air handler (need air movement). Wait until the temperature stabilizes - may take a little while.

Return Air wet bulb ____F
Supply Air Wet Bulb ____F

Need the above information for troubleshooting & performance analysis.

Example below:
A Goodman 2-Ton 13-SEER condenser, @800-cfm indoor airflow; 80-F indoor dry bulb & 50% relative humidity; Indoor temp-split 18 to 19-F.
@ 85-F outdoors; 103.9-F - 85-F outdoors or around an 18.9-F temp-split;
@Indoor 75-F, 63-wet bulb around 50% RH - condenser temp-split is only 14.9F.
 
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Old 08-09-12, 11:33 AM
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Wow, that's a lot of input, and I'm not even sure if anything is wrong. I should look into that thermometer (no Ace nearby) especially if I don't replace the system by next spring.. and revisit all this.

Anyway, simple take-away: room temp 78F at thermostat, one presumes a wee bit higher at the return (in the ceiling, heat rises) if I felt like poking a hole in the new filter for the probe. Output temp 57, or 21 degrees colder than it went in, not too far outside your 18-19 range in your example. Maybe I should not worry... yet!

And I should print out all that other stuff and keep w/AC paperwork just in case.

And I think I'll try the wet-bulb approach to see if the results significantly change.
 
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Old 08-09-12, 11:38 AM
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Your temperature difference is 21 degrees and normal desired is approaching 20 so methinks this looks pretty good.
 
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Old 08-09-12, 12:52 PM
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If the room humidity is around 50% & the blower is delivering around 400-CFM per/ton of cooling, then a 21F temp/split is right on target.

I would always double check that with the temp-split off the outdoor condenser, as that tells you how much total heat, both sensible & latent, that it is being removed to the outdoors.

We can also compare that data to the mfg'ers published data on that particular SEER Rated & tonnage A/C.

When it comes to the indoor temp-split; the percent of humidity in the conditioned space is very important to know because, a low %RH will produce a high temp-split; whereas, a high %RH will result in a low indoor temp-split.
 
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Old 08-13-12, 12:54 PM
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The only humidity reading I get is from a decorative weather instrument set I got as a gift 30 years ago and probably not accurate then, nor now; but it has always given a RH reading of well over 50%, year round.

With 3-4 weeks left in the cooling season and what you've told me, I'll probably take no further action for now. That said, it's up to me if I want to play Checkbook Roulette and try to get more use out of it next year, or start shopping around for a replacement (I'd certainly consider another Ruud based on this performance).

Thanks for your input, I was probably just "starting at shadows", worried about the time it ran to drop the temp. Didn't occur to me to think out the drop in degrees; I guess if I keep the house at, say, 70 degrees, perhaps the output temp would be lower too -- but we're trying to acclimate ourselves to living w/the higher temps. I think with a few more ceiling fans, 78F is livable. Again, thanks.
 
 

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