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lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

lutful to robbin

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to robbin

Re: Room AC Unit Faulty?

said by robbin:

said by older dog:

...you AC is defective. The temperature of the air coming out of the unit should be around 50f 10c

You have no basis to state that. Without knowing the temp of air going into the unit you can't make an assumption of the temp coming out.

Temp difference = BTU rating / CFM rating ... is a good ball park estimate.

robbin
Mod
join:2000-09-21
Leander, TX

robbin

Mod

Point being is that the poster didn't state a temp difference but instead a set temp (unit should be around 50f 10c).

retired ac
@rr.com

retired ac

Anon

current draw should be between 10.6 & 13.7 amps with an outside ambient temp 102-108 f
from the rating plate the unit. If you can test the current draw and its in those limits unit ok. The best temp btween inside & out is 20 degress f with a room window ac.
54067323 (banned)
join:2012-09-25
Tuscaloosa, AL

54067323 (banned) to lutful

Member

to lutful
said by lutful:

Temp difference = BTU rating / CFM rating ... is a good ball park estimate.

And when working with a window unit where is the CFM listed??
older dog
Premium Member
join:2005-06-09

older dog to robbin

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to robbin
said by robbin:

Point being is that the poster didn't state a temp difference but instead a set temp (unit should be around 50f 10c).

You are correct that I should have qualified my statement with "in my case", but I did say he should make his own determination and I gave my results for comparison. Going by what the OP stated with a room size at 200sq the room should be cooling off at the very least overnight.

I have done this simple check out of curiosity to see if my AC has lost cooling ability over the years on different units I have owned.
lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

lutful to 54067323

Premium Member

to 54067323
said by 54067323:

said by lutful:

Temp difference = BTU rating / CFM rating ... is a good ball park estimate.

And when working with a window unit where is the CFM listed??

One should not "trust" the CFM value even if was listed on the manual. Same goes for BTU value. Both of them are easy to measure on window or portable designs.
lutful

1 edit

lutful to robbin

Premium Member

to robbin
said by robbin:

Point being is that the poster didn't state a temp difference but instead a set temp (unit should be around 50f 10c).

Yes. I was actually supporting your comment. When an A/C is trying to reach an unrealistic low setting, the outlet temperature will be approx [Room temp - (BTU/hr) / CFM ].

I posted link for a simple flap type air flow sensor in the electronics forum. The CFM is easy to estimate by assuming uniform flow over whole area of the outlet and measuring it near the middle of the outlet.
54067323 (banned)
join:2012-09-25
Tuscaloosa, AL

54067323 (banned)

Member

said by lutful:

I posted link for a simple flap type air flow sensor in the electronics forum. The CFM is easy to estimate by assuming uniform flow over whole area of the outlet and measuring it near the middle of the outlet.

And instead of rigging something up, why not just do it the way professional A/C techs do and use a thermometer and compare inlet temp to outlet temp and compare that to the service specs?
54067323

54067323 (banned) to lutful

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to lutful
said by lutful:

Yes. I was actually supporting your comment. When an A/C is trying to reach an unrealistic low setting, the outlet temperature will be approx [Room temp - (BTU/min) / CFM ].

Your formula doesn't seem to work.

Try it yourself, 80 degrees, 700 BTU/M 1750 cfm airflow = ?
lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

lutful

Premium Member

said by 54067323:

said by lutful:

Yes. I was actually supporting your comment. When an A/C is trying to reach an unrealistic low setting, the outlet temperature will be approx [Room temp - BTU/CFM].

Your formula doesn't seem to work.

Try it yourself, 80 degrees, 700 BTU/M 1750 cfm airflow = ?

I put the BTU/min just to see if anyone cared to do a sanity check. However, I wanted it to be someone other than you. FYI it is NOT my formula.

Even with BTU/hr, the formula will become inaccurate when outlet temperature approaches 0C because of latent heat during frost formation. Good A/C design will adjust CFM to prevent frost and also to lower noise when max flow is not needed to maintain setting.
54067323 (banned)
join:2012-09-25
Tuscaloosa, AL

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54067323 (banned)

Member

said by lutful:

Even with BTU/hr, the formula will become inaccurate when outlet temperature approaches 0C because of latent heat during frost formation. Good A/C design will adjust CFM to prevent frost and also to lower noise when max flow is not needed to maintain setting.

Good A/C design will not allow the evaporator temps to drop that low, so it's not a problem in the real world.

I put the BTU/min just to see if anyone cared to do a sanity check. However, I wanted it to be someone other than you.

Trying to bait a flame war with yet another member eh?
lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

lutful to 54067323

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to 54067323
said by 54067323:

said by lutful:

I posted link for a simple flap type air flow sensor in the electronics forum. The CFM is easy to estimate by assuming uniform flow over whole area of the outlet and measuring it near the middle of the outlet.

And instead of rigging something up, why not just do it the way professional A/C techs do and use a thermometer and compare inlet temp to outlet temp and compare that to the service specs?

a) air flow can't be measured using a thermometer
b) CFM can't be estimated without measuring air flow and outlet area
c) without CFM estimate, the real BTU can't be estimated
d) the real BTU is usually far less than the advertised BTU
54067323 (banned)
join:2012-09-25
Tuscaloosa, AL

54067323 (banned)

Member

said by lutful:

a) air flow can't be measured using a thermometer
b) CFM can't be estimated without measuring air flow and outlet area
c) without CFM estimate, the real BTU can't be estimated
d) the real BTU is usually far less than the advertised BTU

None of which is needed to do a performance test to determine if the unit is working as designed.

All that is required is thermometer to measure inlet temperature, outlet temperature and those readings are compared to a chart provided by the manufacturer to determine if the unit is within specs...

If one wants to get technical one can do a wet vs. dry bulb test to determine the humidity level, but for most purposes the two dry bulb tests are enough to get an "experienced" technician going in the correct direction.

garys_2k
Premium Member
join:2004-05-07
Farmington, MI

garys_2k

Premium Member

I think you need to know the wet bulb temp. of the inlet air, too.
54067323 (banned)
join:2012-09-25
Tuscaloosa, AL

54067323 (banned)

Member

said by garys_2k:

I think you need to know the wet bulb temp. of the inlet air, too.

Yes sir, that can be done for a bit more extensive test such as a performance test for a new install, but generally speaking the two dry bulb tests will get one in the ballpark when troubleshooting a system with a problem.

Actually many techs will use an IR based thermometer for the test as they provide an almost instant reading and in the A/C business it's how many in 8 one completes that counts.


garys_2k
Premium Member
join:2004-05-07
Farmington, MI

garys_2k

Premium Member

That's why I figured his ~14 F drop (assumed inside air temp) seemed alright (not gread) because it was likely wringing some water out of the air. That, and the likelihood that some ambient air was mixing with the outlet could have pulled that reading up a bit.
54067323 (banned)
join:2012-09-25
Tuscaloosa, AL

54067323 (banned)

Member

said by garys_2k:

That's why I figured his ~16 F drop (assumed inside air temp) seemed alright (not gread) because it was likely wringing some water out of the air.

While 16 is on the low side the unit in question is a low SEER window rattler so that reading is probably normal, on a central unit I would like to see at least 18 or preferably a 20 differential.

All measurements being F not C.
lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

lutful to 54067323

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to 54067323
said by 54067323:

All that is required is thermometer to measure inlet temperature, outlet temperature and those readings are compared to a chart provided by the manufacturer to determine if the unit is within specs...

Earlier you implied that even CFM numbers are not available for window units. Find the relevant chart.

P.S. OP already posted the label with brand/model.

StillLearn
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join:2002-03-21
Streamwood, IL

1 recommendation

StillLearn to 54067323

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to 54067323
said by 54067323:

While 16 is on the low side the unit in question is a low SEER window rattler so that reading is probably normal, on a central unit I would like to see at least 18 or preferably a 20 differential.

EER 10.8 is not so bad.

SEER numbers are different from EER, and SEER is not commonly used for window units. »en.wikipedia.org/wiki/Se ··· _and_COP says "Typical EER for residential central cooling units = 0.875 × SEER. SEER is a higher value than EER for the same equipment."
54067323 (banned)
join:2012-09-25
Tuscaloosa, AL

2 edits

54067323 (banned) to lutful

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to lutful
said by lutful:

are not available for window units. Find the relevant chart.

P.S. OP already posted the label with brand/model.

Then you should be able to find the chart, but it is not needed anyhow because that is not how to test such an A/C unit.

None the less this highly technical device is all, well other than experence, that is needed to test that type of A/C unit to determine the next path to follow.