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Why 2 capacitors?

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wisepole

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Discussion starter · #1 ·
My friend has a maintenance contract with an HVAC company here in Northeast USA and was there when they came over. The guy checked the outside condenser unit and I noticed it has a 45/7.5 uF dual run capacitor AND A black cylindrical "Motorstart" capacitor (I may not have got the 2nd name correct as I asked and I think that's what he said). I did take a quick peek and I even think it's rating was variable like 160 - ??? MFD.
He said to my friend the fan's fine but the compressor was not turning on and he replaced the dual run. That's why I know its exact ratings - LOL.
I have my own HVAC guy doing stuff for me across my 2 homes and have thus seen at least 4 HVAC units but never seen this 2nd. black "Motorstart" capacitor. Just for my own knowledge wanted to know what that is for and why is it variable MFD. Asking since learning thru this forum I have understood these dual runs - can start the compressor (HERM) and the blower fan runs because of "FAN" side. So that is this 2nd. one for?
 
#2 ·
That start capacitor that you found is there to give the compressor a little boost on startup, and then a relay takes it out of the equation (which is different than a run capacitor which is in the circuit all of the time). They are usually optional and installed separately. The MF rating isn't variable, it's a range. Since these things are only in the circuit for a fraction of a second, the MF rating isn't that critical, so the tolerances can be a lot looser.
 
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Discussion starter · #4 ·
I always learnt from here - the dual run capacitor the HERM side is used to start the compressor. Then the FAN side is for the blower fan to 'run'. This extra capacitor as you mentioned is like the hard-start kits? Where it gives that extra "oomph" to the compressor when starting up. Got it!
The reason I asked is since the guy told me in motorstart ; I thought maybe it's a special capacitor for the BLOWER motor to START-UP. Like the HERM setting for compressor used only in startup - I thought perhaps because its a big blower fan motor also needs a fan startup capacitor over and above run capacitor (which is in the dual capacitor).
 
#5 ·
Your outdoor unit has a compressor and a condenser FAN motor.
A blower motor would be used with the furnace or air handler.
With a hard shutoff valve the pressures in the system will not equalize quickly on off cycle. This leaves the compressor trying to restart with a higher head pressure. To overcome this a start capacitor will give the compressor extra torque to start itself.
 
#6 ·
I think you get longer life out of the compressor with a start cap; compressors starts easier. I put a start cap and relay on my system since it didn't come with one.
 
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Discussion starter · #7 ·
VTP99 said:
Your outdoor unit has a compressor and a condenser FAN motor.
A blower motor would be used with the furnace or air handler.
With a hard shutoff valve the pressures in the system will not equalize quickly on off cycle. This leaves the compressor trying to restart with a higher head pressure. To overcome this a start capacitor will give the compressor extra torque to start itself.
Click to expand...
Sorry - I misquoted. I meant the condenser FAN motor.
 
Discussion starter · #8 ·
Thank you everyone. The condenser now is working properly, I just called my friend and he confirmed. i.e. after changing the dual-run capacitor. The HVAC person left the other one "motorstart" as-is. From all this I understood this ("motorstart" capacitor.) performs as a hard-start kit preinstalled by manufacturer,

It gives that extra "oomph" to the compressor during start-up and most probably as a consequence to have having hard shut off valve installed by manufacturer and why it is required as a result: head pressure being high and takes time to equate and how compressors require more power to start in the interim when pressure is still high.

This clarifies my misunderstanding as this capacitor has nothing to do with the condenser fan startup.
 
Discussion starter · #9 ·
As we are "talking" of hard-start kits MY preferred HVAC person states he does NOT recommend installing ANY hard-start kits unless and until the compressor is showing age/problems starting up. Even then he says if the recommendation is (example) 30/4 uF, putting instead a 35/4 uF works 99% of the time as all that old compressor needed was that extra 5 uF. He said only mandatory thing is to keep the FAN side same and changing it can have disastrous consequences to the balance and operation of the fan motor. In a nutshell, Home owner save money and its easier to swap out a capacitor than install a whole new component (hard-start kit) in the outside unit.

I recently had a situation where my outside compressor was not starting up due to a faulty capacitor. He came and replaced it and boom: the compressor started. When I mentioned about hard-start he said no.

I am an electrical/electronic engineer. So technically I understand what he mentioned. He said these hard-start kits like 5-2-1 provide that extra "oomph" for few milli-seconds. But if the compressor is fine, i.e. healthy buy doing this can actually shorten the life span of the compressor. He said wattage is a multiplier of volt, amps and power factor. During initial startup, the power factor is really very low. So all the marketing literature saying a 5-2-1 kit helps save on electricity, etc. by 5 to 10 fold is not correct as they don't factor in the power factor. That's issue #1 - savings aren't really that high. In fact he told me when he started his career way back these units (hard-start kits) were recommended only in locations where there are different utility rates due to time of day or commercial units which by default has varying utility rates.

Issue #2 he says compressors are tuned to certain voltage even at start-up. Quadrupling it or more via a hard-start and that too for a healthy compressor will only cause its health to degrade as you are doing it each and every time there's a call to the compressor to start. He said I don't design these compressors. The engineers who designed them know best. If they indicated a start-up voltage+amps we should respect and abide by it. After 10 to 12 years when the compressor is no more like it was when manufactured then perhaps these hard-start kits make sense, not before.
 
#11 ·
Your preferred HVAC guy is not correct in his judgements.

Starting is the hardest work your compressor ever performs. This effort creates heat in the motor windings. The longer the starting / getting to full speed process continues - the more heat is generated. Heat in motor windings does not create any benefit.

A 5-2-1 'hard start kit' shortens the time-to-speed period. Which is always a benefit for your compressor.

PHM
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wisepole said:
As we are "talking" of hard-start kits MY preferred HVAC person states he does NOT recommend installing ANY hard-start kits unless and until the compressor is showing age/problems starting up. Even then he says if the recommendation is (example) 30/4 uF, putting instead a 35/4 uF works 99% of the time as all that old compressor needed was that extra 5 uF. He said only mandatory thing is to keep the FAN side same and changing it can have disastrous consequences to the balance and operation of the fan motor. In a nutshell, Home owner save money and its easier to swap out a capacitor than install a whole new component (hard-start kit) in the outside unit.

I recently had a situation where my outside compressor was not starting up due to a faulty capacitor. He came and replaced it and boom: the compressor started. When I mentioned about hard-start he said no.

I am an electrical/electronic engineer. So technically I understand what he mentioned. He said these hard-start kits like 5-2-1 provide that extra "oomph" for few milli-seconds. But if the compressor is fine, i.e. healthy buy doing this can actually shorten the life span of the compressor. He said wattage is a multiplier of volt, amps and power factor. During initial startup, the power factor is really very low. So all the marketing literature saying a 5-2-1 kit helps save on electricity, etc. by 5 to 10 fold is not correct as they don't factor in the power factor. That's issue #1 - savings aren't really that high. In fact he told me when he started his career way back these units (hard-start kits) were recommended only in locations where there are different utility rates due to time of day or commercial units which by default has varying utility rates.

Issue #2 he says compressors are tuned to certain voltage even at start-up. Quadrupling it or more via a hard-start and that too for a healthy compressor will only cause its health to degrade as you are doing it each and every time there's a call to the compressor to start. He said I don't design these compressors. The engineers who designed them know best. If they indicated a start-up voltage+amps we should respect and abide by it. After 10 to 12 years when the compressor is no more like it was when manufactured then perhaps these hard-start kits make sense, not before.
Click to expand...
 
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#12 ·
wisepole said:
Even then he says if the recommendation is (example) 30/4 uF, putting instead a 35/4 uF works 99% of the time as all that old compressor needed was that extra 5 uF.
Click to expand...
I disagree with this, because with a "start assist" it drops out after start, where the extra 5uf he added to the compressor 30/4 run cap stays in.
 
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Discussion starter · #13 ·
Thank you guys. Its not like I am posing an argument and/or wish to argue. But rather for my own knowledge.

I feel although I am a home-owner and can quickly dial a number and get an HVAC professional who might be super competent I don't see why I should not or for that matter ANY HO should not try to get the basic knowledge and understanding. Most of friends question this attitude of mine saying why bother? Just call and get it done. Being an engineer let's just say I am implicitly curious. The problem arises when that thin boundary between DIY and seeking professional help is crossed, i.e. HO feels "empowered" to take on the costliest and perhaps the only mechanical item in their house. To be frank and honest - I do change my air filters myself, have checked the Amp draw using a clamp meter myself but if things aren't looking good or not turning on - then and only then make the call.

By virtue of this site and the knowledge you share I have a feeling whenever I called and spoke to a HVAC personnel he/she was more appreciate of this rather than being condescending and I felt the work done was better. So thank you.
 
#14 ·
wisepole said:
Thank you guys. Its not like I am posing an argument and/or wish to argue. But rather for my own knowledge.
Click to expand...
Personally speaking, my post was not meant to engage in an argument, it was just a meant to be an opinion from my experience. Trust me, I understand engineers, been followed around by many in my 36 years, do I like it? What it does to me personally is, it takes my focus off of my repair/troubleshooting because we're usually engaging in conversation. I'm an information sponge, but I sure don't want to be talking to my mechanic, or heart surgeon while they're trying to do their job, I can ask them any questions I might have after the fact.
 
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Discussion starter · #15 ·
Mr Bill said:
I'm an information sponge, but I sure don't want to be talking to my mechanic, or heart surgeon while they're trying to do their job, I can ask them any questions I might have after the fact.
Click to expand...
Apologies - by "argument" i meant from MY side, not yours. In other words wanted all to know I am asking these questions for my knowledge only.

That is a golden rule I have too. I let the person complete and THEN inquire. Alternatively, I ask then sometime I have seen HVAC guy put in a needle like thing here and there and then let system run for sometime. In other words while data is being analyzed/collected automatically by some device I sometime ask. But never which he's hands-deep doing something. 'Cause would hate it myself. Actually, that is why I end up asking it here instead most times. My guy came, did his stuff ; I asked some basic questions and if he did not respond satisfactorily I don't push him. Why? Over and above what you mentioned Mr. Bill - this guy has to go to his next address and that's how he's feeding his family. Me asking him questions is only holding him back.

The worst are some of my friends. Was in a guy's home and a repair person was repairing his dishwasher which required 3 parts overhaul. Every part my friend asked what's the part #, then looked it up on smart phone simultaneously asking the guy for the price of the part and then at the end discussing if your 3 parts in total costs $x and if your labor as per industry standard is $Y why are you charging me $Z and this means you are overpricing the part, etc, etc blah-blah.
Frankly that would be 100 time worse than asking technical questions while someone is doing work.
 
Discussion starter · #16 ·
Poodle Head Mikey said:
Your preferred HVAC guy is not correct in his judgements.

Starting is the hardest work your compressor ever performs. This effort creates heat in the motor windings. The longer the starting / getting to full speed process continues - the more heat is generated. Heat in motor windings does not create any benefit.

A 5-2-1 'hard start kit' shortens the time-to-speed period. Which is always a benefit for your compressor.

PHM
----------
Click to expand...
I called him. He said he NEVER puts and HATES it when HO asks him to put these: PTC hard-start kits. They are apparently sold everywhere and are dirt cheap. He said all his comments are geared to be against those. NO IDEA what all this means. He said he's okay to electronic, potential (his preference apparently) or mechanical.
He told me I happen to have a PTC one installed when I inherited my HVAC from previous owner and wants me to have him come in and replace it with a true 5-2-1 especially as my compressor is from 1983.
 
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wisepole
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