# Strange HVAC condenser problem



## Shorty Circuit (Jun 26, 2010)

A mechanical engineering friend of mine reported that his maintenance people discovered that after a power outage, certain AEON condensers fed from panelboards wouldn't restart until they were disconnected by opening a breaker or switch and then reconnected. Others would. Checking with the manufacturer, some of their models have this problem, some models of some other manufacturers do also, others don't. Proposed solution is a contactor that will drop out in an outage.

Various theories advanced by the engineers and maintenance people involved included some sort of feeder cable capacitance with residual voltage across it, design flaw in the controls. 

Anyone ever encounter this? Any other ideas or suggestions?


----------



## Bob Badger (Apr 19, 2009)

............................



***Mod note: Bob, I know after that other thread tensions are high, but as long as things stay civil let's leave legitimate threads clean, OK?**


----------



## Bob Badger (Apr 19, 2009)

OK, but it was at least a little funny. :laughing:


----------



## Jlarson (Jun 28, 2009)

What type of controls? Plc, relay logic?


----------



## Shorty Circuit (Jun 26, 2010)

Unknown at this time. I'll have to call and ask.


----------



## RIVETER (Sep 26, 2009)

By condenser I assume that you mean the compressor. Did the condenser fan run? Did either run? For some of them to run with power back on and others of the same make not come on it seems as if the units that didn't were just defective. When an AC/HP cycles too quickly the head pressure will cause a back pressure strong enough to make the CEMF be too high and the breaker should trip or the internal overload may trip. If the internal overload trips and sticks in the open position your problem will happen. I have kicked them and they run forever after that. I would suggest a time delay. If you lose power, then the unit will not try to restart for 10 minutes. The pressures should have time to equalize between the hi and the low side.


----------



## Shorty Circuit (Jun 26, 2010)

RIVETER said:


> By condenser I assume that you mean the compressor. Did the condenser fan run? Did either run? For some of them to run with power back on and others of the same make not come on it seems as if the units that didn't were just defective. When an AC/HP cycles too quickly the head pressure will cause a back pressure strong enough to make the CEMF be too high and the breaker should trip or the internal overload may trip. If the internal overload trips and sticks in the open position your problem will happen. I have kicked them and they run forever after that. I would suggest a time delay. If you lose power, then the unit will not try to restart for 10 minutes. The pressures should have time to equalize between the hi and the low side.


I would have expected that if the compressor was switched off and then on again it would trip the overcurrent breaker on high head pressure. This didn't happen. The report I got is that the maintenance men went onto the roof, opened the safety switch and then when they reclosed it the units started again. Very strange, I never heard of anything like it and wondered if anyone else had. Not that it necessarily matters, I'm not all that familiar iwth AEON units but if they have a budget line this is it. Mechanical engineers in my firm occasionally spec out AEON units and they tell me that the ones they select cost more than Trane. These are owned by a county government and are specified because they are cheap.

I was thinking of a TD contactor but not a delay on, a delay off to prevent the units tripping out in a momentary transient hit. I'd assumed they have built in antirecycle timers to prevent restart on high head pressure, I'd figure 2 to 5 minutes. Large chillers usually give you one free start after an outage and then if it doesn't start on the first try you have to wait about 20 minutes to try again.


----------



## Jlarson (Jun 28, 2009)

Shorty Circuit said:


> I would have expected that if the compressor was switched off and then on again it would trip the overcurrent breaker on high head pressure. This didn't happen. The report I got is that the maintenance men went onto the roof, opened the safety switch and then when they reclosed it the units started again. Very strange, I never heard of anything like it and wondered if anyone else had. Not that it necessarily matters, I'm not all that familiar iwth AEON units but if they have a budget line this is it. Mechanical engineers in my firm occasionally spec out AEON units and they tell me that the ones they select cost more than Trane. These are owned by a county government and are specified because they are cheap.
> 
> I was thinking of a TD contactor but not a delay on, a delay off to prevent the units tripping out in a momentary transient hit. I'd assumed they have built in antirecycle timers to prevent restart on high head pressure, I'd figure 2 to 5 minutes. Large chillers usually give you one free start after an outage and then if it doesn't start on the first try you have to wait about 20 minutes to try again.


I would find out what time delays the unit has built in first an go from there you may be surprised it might be totally bare bones. I like the on delay idea, also I assume these are 3 phase so perhaps a phase monitor with a delay restart on it's relay contacts might be in order.


----------



## MDShunk (Jan 7, 2007)

My every encounter with an Aaon unit causes me to believe that they do strange things. I had one that you needed to get on the roof with the handheld programmer and reprogram it every time there was a power outage. Learning of one that needs the power cycled to restart it after a power outage is not surprising to me. I can't explain it, but it doesn't surprise me.


----------



## RIVETER (Sep 26, 2009)

MDShunk said:


> My every encounter with an Aaon unit causes me to believe that they do strange things. I had one that you needed to get on the roof with the handheld programmer and reprogram it every time there was a power outage. Learning of one that needs the power cycled to restart it after a power outage is not surprising to me. I can't explain it, but it doesn't surprise me.


Are they supposed to have an on-board EEPROM that should just re-load after an outage?


----------

