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Joined: Oct 2005
Posts: 141
I need to figure out a way to automatically switch a load every 30 days. The loads are motor driven pumps for a chiller in a highrise. Only one pump is used at a time, they want the pumps to switch every 30 days.

The motors are controlled by different starters.

I can't find anything Intermatic puts out that will do 30 days.

Joined: Jul 2004
Posts: 787
Here is a link to a timer that will do up to 9999 hours. 30 days equals 720 hours.

The alternating relay just needs a trigger signal from the timer to change between one set of output contacts to the other. Some of the alternating relays will allow you to lock one pump out to overide the alternating process.

I would suggest hour meters to ensure that the run times are equal.

Larry C

Joined: Nov 2007
Posts: 763

Depending on the system, that could be a little more involved than just installing a simple timer in the circulator control circuit.
On many larger systems, the chiller PLC also controls the circulator pump operation based on inlet/outlet water temperature, among other things.
An interval timer’s input voltage would be disrupted every time the chiller’s PLC shuts down the motor, resetting the timer. You may need to tap a continuously energized circuit from ahead of the PLC output relay the to maintain input voltage on the timer. Maybe for your installation, you could then use a SSAC KSPU [up to 1000 hr.delay] interval timer to act as an input source for an SSAC ARP alternating relay to redirect the circulator motor control circuit.

Another issue that may arise is that many times the butterfly valves on the output side of the pump not being used are manually closed by maintenance personnel to prevent any pressure loss in the active loop and to keep backflow surge pressure from hammering the pressure gauges on the idle pump. These valves sets would need to be opened/closed when the pump change-over takes place.


Joined: Oct 2005
Posts: 141
Thanks for the replies. Larry, that looks exactly like what I need.

KJay, The chiller PLC does not control the circulator pumps in this application. Also the output valves have not been shut off in the past, what they are trying to do is not have the maintenance guy manually switch the pumps every month. I will bring this up to the customer, maybe they will want to change the manual valves to automatic actuated valves.

Joined: Aug 2007
Posts: 853
OHHHH, Out sourcing, MMMMM.(another kid with no toys')

Still KJAY raises a very good point on the valves. Unless these are centrifugel Pumps, they could have a big problem on changeover. Centrifugel pumps will allow the water to flow without building pressure (similar to the fire pumps, for testing).

Bring this to their attention and avoid a headache.
You must think ahead of them, or you WILL be the "Fall Guy".

Joined: Nov 2007
Posts: 763
Hi Lonegunman,
Just to be sure, I would still check with an actual commercial HVACR pro and not go by what the maintenance supervisor or anyone else says.

It sounds like you have a decent customer that wants to spend some money.
If you don’t happen to do a lot of this kind of work… her is just my 2-cents worth.

With HVACR today, it is all about system efficiency.
On some systems, those cooling tower pumps do not need to run when the chiller system temperature controls are satisfied and the chiller is not running. They are normally only for the purpose of heat transfer from the condenser and are not part of the chilled water supply loop, which feeds the fan coil units throughout the building. If the system is operating properly, the lowest leaving chilled water temperature should be around 40 degrees F, so evaporator icing should not be a concern. Many times, those pumps are at least 15HP. If they are not needed and are running 24/7 they are probably wasting electricity, reducing overall system efficiency and causing the need for more city metered makeup water for the tower as well as more on-site water treatment chemical injection, if present. You could come out smelling like a rose by both picking up the extra associated control wiring and helping them reduce their annual operating expenses if you team up with a qualified commercial HVACR pro.

As I said though, just my poopy 2-cents worth! wink

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