Engineered Systems NEWSHVAC Engineering SectorsHigh-Performance Buildings & AutomationHVAC RetrofitsCommercial HVAC

Troubleshooting — Engineering 101, Part 2

This month we look at three scenarios designed to help solve water-side problems.

Last month I discussed the niche business of troubleshooting and the pertinent things to do in preparation of the data collection, data analysis, and eventually the potential solution(s) plan. This month, I’m going to continue with this topic, focusing on water-side problem solving.

I mentioned some of the skills necessary to problem-solve last month, but to be more specific to troubleshooting a water system, here are a couple of problem-solving basics you don’t want to skip over at the very start of the data collection phase:

Make sure the pump impeller diameter matches the impeller size on the printed pump curve. Over the years, I’d estimate that 98% of the time, when verifying the pump impeller, the impeller size on the manufacturer’s pump curve matched the actual impeller. Still, 2% of the time, while troubleshooting a problematic water system, I have found the actual pump impeller did not match the published pump curve. To verify the impeller size, perform the following steps:

“A coil or a heat exchanger output capacity can be compromised by approximately 20% if the two flows are in a parallel flow configuration versus the required counter-flow configuration.”

Close the discharge valve on the pump to dead-head the pump.
Using the same pressure gage, take a reading at the pump inlet between the inlet strainer and a second reading on the pump discharge between the pump outlet and the closed shutoff valve.
Convert the pump pressure differential by subtracting the inlet pressure psig from the discharge pressure psig and then convert it to feet of head.
Based on the actual feet of head, compare this value to the manufacturer’s pump curve feet of head value at zero flow (gpm) on the pump curve. If the two values don’t match up, then the actual pump impeller will most likely not be the same impeller size as the pump curve impeller.
If this is the case, then the pump impeller casing will need to be taken apart to confirm the actual impeller size.

The Results: Determining the actual impeller diameter will then allow the problem-solver to establish a new curve performance (gpm and pump head) based on the real impeller size, and the troubleshooting can proceed forward.

Verify the water coil is piped correctly. Over the years, I’d estimate that 85% of the time when verifying the coil or heat exchanger pipe configuration/counter flow arrangement, it matched the equipment manufacturer’s recommended piping instructions. This left 15% of the systems not achieving design performance because they were incorrectly piped. To verify the pipe arrangement, e.g., hot water supply inlet and the hot water return outlet for a heating coil, it’s important to look for the counter-flow arrangement configuration. To do that, one must perform the following steps:

For a water coil in an air system, the water supply should be at the downstream side of the air passing through the coil to create a counter-flow condition between hot water and cold air.
For a water-to-water heat exchange system, the same counter-flow piping arrangement should occur.

The Results: A coil or a heat exchanger output capacity can be compromised by approximately 20% if the two flows are in a parallel flow configuration versus the required counter-flow configuration.

Create a system flow diagram and do so as if you are drawing a riser diagram. By looking at the flow diagram in a vertical position you are also documenting the system high points where there needs to be automatic air vents to continuously vent air from the system throughout the year, especially when the system is off and air will be able to easily float to those high points and be vented, thus avoiding potential flow restriction from air blockage. To verify this air restriction concern, walk the facility, go to those high-point locations, and verify an air vent has been installed at each of these location.

The Results: At the time of verification, the troubleshooter should arrange to have the water system off and that a static pressure reading be taken at the highest point(s) in this system to verify there is adequate water pressure to assure that city water pressure has the capacity to pressurize the entire system without the assistance of a pump.

Based on the completion of the above, the troubleshooting can proceed.

Looking for a reprint of this article?
From high-res PDFs to custom plaques, order your copy today!

Howard McKew is author of Integrated Project Delivery for Building Infrastructure Opportunities for HVAC consultants and mechanical contractors and can be reached at hmckew@bss-consultant.com or at www.buildingsmartsoftware.com.