UNICO Chiller Problem

[stbjersey]

Hi,

I had a UNICO 5 ton chiller installed about 2 1/2 years ago by a contractor/friend. Inside it is zoned with one unit in the attic, and one on the first floor. It worked great after the initial installation. Last year it was set up so the outside unit would only come on when either of the inside units started up. Since then, it has been running poorly (air temp for A/C was not very cool), and about a week ago it stopped working. The chiller will start up, the fans on the unit run and then it shuts down after a couple of minutes, and needs to be reset. One thing that concerns me is that when the changes were made, erie pop top actuators were installed on each of the chilled water modules. These seem to be operating ok, but the three way valves are 1/2" instead of 3/4". Could this be the problem? If not any ideas would be appreciated.

Thanks,

Steve

[Shophound]

Let's see if I have this straight...on the original installation, there were no actuators at either chilled water module (air handler) so when the chiller ran, it circulated water through both coils, all the time. Then it was modified so the chiller would run only when one, the other, or both modules called for cooling. The Erie Pop Tops were put in to isolate any idle module. They're on 1/2" three way valves but the chilled water lines feeding them are 3/4". Sound about right?

If so, I'd say that without actually being there to see it for myself first hand, your chiller may be trying to protect itself from freezing up due to low chilled water flow.

[stbjersey]

Yes, you have it right. Do you think the 1/2" valves may be the problem?

[Shophound]

Yes, you have it right. Do you think the 1/2" valves may be the problem?

I think the problem is that the chiller was designed to flow both coils simultaneously. The valve size may be the culprit as there's too much pressure drop even when one of the two valves is in bypass mode.

If the system worked beautifully without the valves, the valves are likely the problem.

[DavidNJ]

Does it have a storage tank?

Smart man, Mr. shophound. The chiller is rated 12gal/min min flow, 14gal/min max flow. 45 feet of head to at the minimum flow. The 1/2 Erie Pop Tops I think have a Cv of 1.5 or 3.

Does it make any difference if they are open?

[Shophound]

Does it have a storage tank?

Smart man, Mr. shophound. The chiller is rated 12gal/min min flow, 14gal/min max flow. 45 feet of head to at the minimum flow. The 1/2 Erie Pop Tops I think have a Cv of 1.5 or 3.

Does it make any difference if they are open?

This link might shed some light on it:

http://www.engineeringtoolbox.com/water-valve-cv-diagram-d_326.html

Going by the chart in the link, if we're flowing a median 13 GPM and the CV of the pop tops are 3 each, there's about a 20 psi pressure drop through each valve (provided each valve is seeing 13 GPM).

Either bump up the pop top valves to 3/4", or put in a buffer tank and extra circulating pump so all the chiller tries to do is keep the buffer tank cold (buffer tank would offer thermal flywheel cushioning...call for cooling from a given zone would not necessarily immediately energize chiller...only when deadband exceeded on buffer tank would chiller start). The other pump would then circulate on call for cooling...could even have zone pumps like on hydronic heat vs. three way valves.

[flange]

I personally would consider two way valves with either a differential pressure bypass, or a hydraulic decoupler. also consider that on low load days, the chiller may not be at temp if cold starting, so some warm air currents may occur.