I'm working on a project to involving commercial custom undercounter refrigerators and having them listed by Underwriters Laboratories (UL). We are currently doing our pre-testing and are having troubles getting under the maximum low-side pressure as given in UL standard 471. Essentially it states the ultimate pressure of the low-side components must be at least 3-times the max as recorded during the pressure and temperature test, or 5-times for a Canadian UL listing. It further goes on to say the maximum low-side pressure cannot exceed 174 PSIG.

For the test we must condition the unit for at least 12-hours at 104F with all the doors propped open, and then begin recording the temperatures and pressures (low and high-side). The test runs until the unit achieves stabilization, somewhere about 10-hours or so, and the pressures and temperatures must stay within bounds. It is even allowable for the compressor to trip on its high limit for the first 8-hours.

Where our problem is coming in is in what appears to be our choice of refrigerant. We are using R404A with "off-the-shelf" Emerson compressor/condensers and Heatcraft evaporators. The maximum low-side pressure we are seeing is 248 PSIG. This is happening right at startup and drops immediately - but not before we fail for the pressure being to high.

I understand why the pressure is what it is - that is right from the temperature/pressure chart for R404A. My question is: If this is a legitimate problem, and I don't see how it could be, how could anyone hope to pass the UL test for Canadian approval using R404A?

Any thoughts or insights would be welcome. My test lab manager and I have been pulling our hair out for a few days on this.

Thanks, Wilade