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Matthew
Matthew, HVAC Technician
Category: HVAC
Satisfied Customers: 1994
Experience:  14 years experience in the HVAC field of service, installation, Manual J & D calculations
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Hi,I have a Trane XR12 Unit (XE1000 I think) and a Variable

Resolved Question:

Hi,
I have a Trane XR12 Unit (XE1000 I think) and a Variable Speed AHU about 2002 vintage.

Been working fine till a few days ago. Now, compressor outside and AHU both running. But No Cool air. AHU also never seems to go faster than it's low speed mode.

Any ideas ?
thanks
Submitted: 4 years ago.
Category: HVAC
Expert:  Matthew replied 4 years ago.
Chat Conversation Started
Matthew :

Sounds like you are low on refrigerant and the indoor coil is freezing up causing the airflow to decrease to the house. When refrigerant gets low your delta T (the difference between the temperature of the supply air and return air) will dip below the acceptable range of 16-20 degrees. If your delta T is lower than this then it is a refrigerant issue.

Customer:

So, I need technicina to come out and rechrage system ?

Customer:

I have a 2nd system of similar age, would you reccommend I recharge that at same time ?

Matthew :

A system only needs recharging if there is a leak. Refrigerant doesn't dissipate if there isn't a leak it will never need more. Most often leaks form in the indoor coil first especially if it is located in an attic. Other common places for leaks on a system under 10 years of age is at the service valves of the outdoor unit. If the installer got the valve too hot when they soldered the lines in it will warp the ball valve and cause it to allow refrigerant out.

Customer:

Can you give rough guess what you think it shoudl cost for this service ?

Customer:

Thanks

Matthew :

You mentioned the blower running at low speed it is possible the blower is malfunctioning and this would cause it to freeze up if the variable speed board is bad and never ramping up to the high speed.

Matthew :

Service call $85, $85/hr charged by the 1/4 hr, $45/lb for refrigerant, Most systems hold 6-12 lbs of refrigerant most systems need 2-4 lbs any more than that and the leak is serious and you will not make it through the summer before needing refrigerant again.

Customer:

It was turned off all night. And when turned on this morning - same low air flow and no cooling. Does that help ? I have chagne the switches on that board before, but woudl not know how to detect a fault - unless visibly fried.

Matthew :

One other thing just to cross my T's and dot my I's.

Matthew :

Are you sure the internal compressor of the unit is running? Just because the fan on top is turning doesn't meant he compressor is pumping refrigerant inside. If it isn't this could simply be a bad capacitor in the outdoor unit.

Matthew :

The airflow issue is difficult to trouble shoot without being there I'm afraid. I could guess all day but with a variable speed set up it would take a lot of trail and error to see if it was the furnace board, the variable speed board, or the motor it self.

Customer:

Good point on the compressor. Fan is running, but when turning on - I did not get the normal sound (and house lights dipping) that I recall when the compressor starts. Does the start up cap go bad. I bet I could replace that. I am quite good electically.

Matthew :

yeah the cap is the most common service part I replace during the summers it is very simple to replace as well. Just make sure the MFD matches the one that is currently in there.

Matthew :

Full Size Image

Customer:

So (even though my wife is miserable) you think I have a btr shot at that, than the recharge service ?

Matthew :

If the compressor isn't running then it is either the cap or the compressor's internal windings are open(which you hope isn't the case) Until the compressor is running you can't charge the system if it even needs it so I would get the compressor running (if it is indeed not running) and then go from there.

Customer:

Great Thanks

Matthew :

If you pull the disconnect on the outside of the house and wait 4-5 mins then put it back in you should be able to hear for sure if the compressor is kicking on or not

Matthew :

It makes a very distinct hermetic sound when it is running, but it is very noticeable on start up.

Customer:

I assume these cap readily avialable at HVAC supply place? Just match specs nad replace ? Do they just wear out ? or get blasted by lightning ?

Matthew :

most of the time these caps bulge when they are bad the acid charge in them literally blows them up.

Matthew :

Full Size Image

Matthew :

Yeah they will have every size imaginably. They just get hot and overload they have a specific life span of starts and stops. If your unit is dirty, covered by bushes, covered by a deck, or any other obstruction that causes the unit to trap heat it will cut this life expectancy

Customer:

I will got inspect. If no visible damage, Any simple VOM test, other than a short ?

Customer:

sorry I am such a lame typist (I am used to a spelling checker)

Matthew :

If your multimeter has the option MFD on it then you can check to see if there is a charge on the capacitor. it will be a duel capacitor most likely this means you will have three terminals on it. C which is common, F which is fan, and Herm which is the compressor. Across C and F you will have normally 5 mfd, across Herm and C you will have whatever the capacitor rating is 35,40,60 whatever is on the capacitor listing it will read as follows 35/5 370 or 45/5 440 the 35 is the hermetic MFD the 5 is the fan MFD. Now you have to discharge the cap to test it this means unplug all the wires then take a screw driver and touch all the terminals together then take your readings. make sure you pull the disconnect for the unit when you are working on the capacitor it is high voltage being sent to it.

Customer:

Fixed. It was a burnt and broken wire on one of the capacator terminals

Customer:

Thanks !!!

Matthew, HVAC Technician
Category: HVAC
Satisfied Customers: 1994
Experience: 14 years experience in the HVAC field of service, installation, Manual J & D calculations
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