Commercial HVAC durability question

[bjarni]

I'm wondering which factor plays more into commercial air conditioning systems wearing out -- total runtime of the unit or the number of times it cycles.

For instance, let's pretend we narrow the deadband on a RTU so it has approximately the same runtime but cycles 10% more frequently than before. Would this have more or less effect on the unit's total lifetime than if I increased the overall runtime by 10% but did not increase the number of times the unit cycles (perhaps by moving the unit's thermostat setpoint lower and widening the deadband slightly).

I recognize that this probably varies from unit to unit. Do you know of any tests that manufacturers run that could identify the impact of overall runtime and number of times a unit cycles on the unit's total lifetime? It seems to me that this is related to Highly Accelerated Life Testing, but I haven't seen a lot of HALT results posted for commercial HVAC units.

[Tool-Slinger]

I'm wondering which factor plays more into commercial air conditioning systems wearing out -- total runtime of the unit or the number of times it cycles.

For instance, let's pretend we narrow the deadband on a RTU so it has approximately the same runtime but cycles 10% more frequently than before. Would this have more or less effect on the unit's total lifetime than if I increased the overall runtime by 10% but did not increase the number of times the unit cycles (perhaps by moving the unit's thermostat setpoint lower and widening the deadband slightly).

I recognize that this probably varies from unit to unit. Do you know of any tests that manufacturers run that could identify the impact of overall runtime and number of times a unit cycles on the unit's total lifetime? It seems to me that this is related to Highly Accelerated Life Testing, but I haven't seen a lot of HALT results posted for commercial HVAC units.
Using your own example I would think the difference to be small enough to be negligible.

That said, I would expect a system would get more wear and tear from # of times it starts. Contacts arcing, momentary high amps on windings, mechanical shock of motors starting, and temperature changes being my reason to think so.

[Trevorjh101]

Im not 100% sure however, if a system is cycling off and on it puts a higher load on the system on Start-up. if its just runs without cycling its better for the system however less efficent on total power draw. York HVAC did a study on this and it showed there compressors Burned out faster on Cycler Controls then they did running all out. it must be the high amperages on Start-up over time wears them down.

[bjarni]

York HVAC did a study on this and it showed there compressors Burned out faster on Cycler Controls then they did running all out. it must be the high amperages on Start-up over time wears them down.

Do you have a reference for this study? I'm particularly interested in testing done by HVAC manufacturers that quantifies the wear and tear due to cycling.

[normally_closed]

In my opinion and experience, mechanical devices rate of failure is higher the more they cycle on and off. The torque of a motor starting exerts stress on the internal mechanical parts. Also more energy is consumed every time a motor starts. From what I've expierienced , most if not all customers are concerned with saving money or energy to you and me. Energy management systems play a large part in extending the life of mechanical devices. Best example is free cooling. Why run your compressors when you can energize a 24 vac motor and bring in out side air ? One newer compressor I've noticed seems to be designed to run all the time. The copeland digital scroll. Recently saw this on an air cooled chiller. First I was confused why the compressor was constantly changing speeds and not turning off , haven't had alot of expierience with these compressors yet. Then it was explained to me the compressor used less energy when changing speeds rather than shuting off and turning on to maintain the supply loop temp. What the rate of failure on the digital scrolls are I don't know , haven't seen enough in the commercial field yet. Seem energy friendly though , go green!

[DavyB]

In my opinion and experience, mechanical devices rate of failure is higher the more they cycle on and off. The torque of a motor starting exerts stress on the internal mechanical parts. Also more energy is consumed every time a motor starts. From what I've expierienced , most if not all customers are concerned with saving money or energy to you and me. Energy management systems play a large part in extending the life of mechanical devices. Best example is free cooling. Why run your compressors when you can energize a 24 vac motor and bring in out side air ? One newer compressor I've noticed seems to be designed to run all the time. The copeland digital scroll. Recently saw this on an air cooled chiller. First I was confused why the compressor was constantly changing speeds and not turning off , haven't had alot of expierience with these compressors yet. Then it was explained to me the compressor used less energy when changing speeds rather than shuting off and turning on to maintain the supply loop temp. What the rate of failure on the digital scrolls are I don't know , haven't seen enough in the commercial field yet. Seem energy friendly though , go green!

Is any cylinder head or scroll properly lubed at the moment of start-up? Is your car engine? Probably not. Every start-up creates a larger amount of friction partly seen with your ammeter.

[bjarni]

Is any cylinder head or scroll properly lubed at the moment of start-up? Is your car engine? Probably not. Every start-up creates a larger amount of friction partly seen with your ammeter.

Do you know of any studies done by HVAC manufacturers or others to quantify the durability effects of starting up the compressor?

[normally_closed]

Is any cylinder head or scroll properly lubed at the moment of start-up? Is your car engine? Probably not. Every start-up creates a larger amount of friction partly seen with your ammeter.

My point exactly..................:deadhorse:

[btuhack]

At 3 cph, 10 hrs a day, 5 days a week for 20 years- 15000 additional starts. You won't run the machine that much considering seasonal use, so chop the 15k down to 1/3, still 5000. That's a lot of opportunity for something to go wrong.

If lifecycle costs are your concern, don't forget to ad startup inefficiencies to the higher cph machine. I'd guess steady state rpms are easier on bearings with normal conditions on compressors and fans.

[DavyB]

Do you know of any studies done by HVAC manufacturers or others to quantify the durability effects of starting up the compressor?

Honestly, I have never looked into it. I always figured that a mechanical device of any kind is one and the same. starting and stopping is always harder on any device. Kind of like driving in stop'n'go traffic versus freeway driving.

[Trevorjh101]

It was in a book at work we get in the mail. I will Have to find out the name of the Artical.

[man from trane]

Distance from the ocean and wind direction! :angel:

Here, compressors start once per day, max 365 per year. T-stats are set at 50 Deg and units are controlled by circuit breakers when the store opens and closes. Compressors still only last 5-7 years average.

I think a compressor running non-stop under ideal conditions should last almost an unlimited amount of hours. Starting is not only hard because of the reasons mentioned, but also the slight liquid floodback that usually occurs immediately after startup.

[DavyB]

Distance from the ocean and wind direction! :angel:

Here, compressors start once per day, max 365 per year. T-stats are set at 50 Deg and units are controlled by circuit breakers when the store opens and closes. Compressors still only last 5-7 years average.

I think a compressor running non-stop under ideal conditions should last almost an unlimited amount of hours. Starting is not only hard because of the reasons mentioned, but also the slight liquid floodback that usually occurs immediately after startup.

Agreed, any oil is flushed away. Maybe you need to undersize the hell out of your equipment so that it runs longer. There has to be a way to keep those compressors lubed.