# Not "no cool", but "less cool". What do head/suction numbers indicate?



## bob_cntrctr (Jan 30, 2008)

*Not "no cool", but "less cool". What do head/suction numbers indicate?*

Gentlemen,

I manage a small (~5,000 sq ft, 2 commercial & 4 residential units) mixed residential/commercial building that seems to have an hvac issue.

AC's not cooling as well as it used to. The air out of the vents is cool, but not cold like it used to be. 

Blower blows just fine. A/C unit's fan runs fine. Outdoor unit has had fins cleaned. The suction line feels cool to the touch, but not "cold", and the head line is quite warm - hot, but not too hot to touch.

So I called in an hvac tech who put his pressure guages on the head and suction. 250 psi and 60 psi. 3-ton unit - quite old - probably 15 years I'm sorry, I don't know the refrigerant, but I know it's been recharged several times already (and seals replaced).

So the tech says it's very obvious. At 60 and 250 there's no question - the head should be no more than three times the suction, therefore there's a blockage in the line which must in turn mean the compressor is dying and spitting carbon into the line, blocking the exchanger. It all has to come out and be replaced with an entire new system. This was stated with no more investigation than checking those two numbers.

So, my question: is it really that cut and dry? Or should I be getting another tech in who'll investigate a little deeper?

Thanks.


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## hvactech140 (Feb 23, 2009)

Sounds as if the coil is dirty. Mabee not on the surface, but deep in the coil. Check filter also, if not done so. I don't think you have a blockage of any kind. Sounds as if he is just trying for a quick change out. How hot is it in the apt? That will also run the head up, could be a little low on refrigerant.


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## bob_cntrctr (Jan 30, 2008)

hvactech140 said:


> Sounds as if the coil is dirty. Mabee not on the surface, but deep in the coil. ....


Thanks for the tip. You mean evaporator coil or condenser?


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## greengenius_A_C (May 27, 2011)

What was the outside ambient temperature that day? I'm assuming this is R-22 of course. A 90 degree day with a 10 SEER unit like they were 15 years ago would have a normal operating high side pressure of about 250 on a hot day like that. Looking at any standard pressure-temperature chart for refrigerants will tell you that (90 degrees plus 30 degrees for a 10 SEER unit= 120 degree high side saturation temp, which corresponds to 250 psig). 

The head line outside the unit should be about 10 degrees cooler than the sat. temp, or 110 degrees. This is probably what you meant when you said "hot, but not too hot to touch".

Obviously, I'd have to actually be there to take some measurements to be certain if this tech is accurate or not, but that's an issue in itself...you're saying he didn't take any more measurements. Superheat, subcooling, discharge line temp, outside ambient, indoor dry and wet bulb, evaporator temperature drop, motor amp draws, etc.

There is a certain minimum number of measurements a tech should take before coming to a conclusion as serious as that. Call someone else, or stick with this guy at your own peril.


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## flashheatingand (May 3, 2008)

sixty is a little low, especially if it's a hot day out.


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## mdiack77 (May 18, 2011)

flashheatingand said:


> sixty is a little low, especially if it's a hot day out.


i agree. and it looks like he is trying to upsell on a system.


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## GettingBy (Aug 19, 2010)

Now that you have specific readings you could get a pretty good take over the phone from HVAC places for what they think needs to be done. 

You might have to call several places just to get three good answers.


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## bob_cntrctr (Jan 30, 2008)

Thanks, all, for the helpful advice.

Seemed a little too easy - two numbers and we can confidently say you need a whole new system.

You can bet I'll be getting a second opinion.

Had a look at some evaporator coil cleaning videos and came across what they called "no rinse" cleaners - which seemed to suggest you could clean the coil right in the plenum without removing it. Anyone actually ever use such stuff? Otherwise, I'd be getting a tech to do a pull-out cleaning.


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## flashheatingand (May 3, 2008)

GettingBy said:


> Now that you have specific readings you could get a pretty good take over the phone from HVAC places for what they think needs to be done.
> 
> You might have to call several places just to get three good answers.


Pick just the first one that seems good to work with. No need to be playing a few of us.


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## beenthere (Mar 5, 2009)

> So the tech says it's very obvious. At 60 and 250 there's no question - the head should be no more than three times the suction, therefore there's a blockage in the line which must in turn mean the compressor is dying and spitting carbon into the line, blocking the exchanger. It all has to come out and be replaced with an entire new system. This was stated with no more investigation than checking those two numbers.



You had a bad tech. Those readings don't tell any tech that the compressor is going bad. In fact, they indicate the compressor is working fine. And you have other troubles.

Most good HVAC companies won't tell you anything over the phone. Other then, could be what ever you suggest. They will want to send someone out. Since its easier to diagnose when they can test things themselves.


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## bob_cntrctr (Jan 30, 2008)

Well, thank you all for the advice. Ya, threse guys' "troubleshooting" seemed a little superficial to me too - hence the question.

So, I did call someone else and told them up front I was looking for a competent technician who could accurately diagnose the problem.

And I'm happy to say that's what I got.

The guy checked head/suction pressure. Temperature drop over the evaporator. Checked the evap coil for cleanliness and cleaned it. Tested the system's reaction to a small addition of coolant (as it seemed a tad low to him), and when it reacted by upping the temp drop, THEN finally concluded that it was just coolant loss. Adding more back up to normal pressure would fix it.

Of course, competency is a double-edged sword.

'Cause then he went looking for the cause of the coolant loss with his freon sniffer. I crossed my fingers it would be a nipple or seal - but - alas - he finally found it - a ding on the condenser coil.

Darn rat son of a......'cause I know the story with a leaking coil.

So we ended up with the same prescription - all new system - but at least this time for the right reason.

Although, I'm gonna start a new thread on that coil just out of desperation.....

Thanks again.


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