Month: May 2019

Have you ever seen a low voltage transformer like the one shown above? It has multiple input (primary) taps for a good reason.

It is common to find 3-phase and single phase equipment rated to operate on both 240v and 208v power. This is because three-phase power can either be 208v leg to leg when the building has a wye type transformer or it can have 240v leg to leg when a delta transformer is in use.

Single phase power (in the US) is almost always 240v leg to leg or 120v leg to neutral because one “phase” of the three-phase transmission line is being used and is wired in the transformer secondary to create two opposite sine wave (180 degrees out of phase) legs of 120v power.

The motors in a unit that is rated for 208v or 240v must all be designed to operate within that range of voltages. However, if the transformer is designed for 240v and only 208v is applied the secondary voltage will also drop below the rating. This can lead to issues with the controls such as chattering relays and contractors. In most cases the system will function normally, but in cases with long runs of control wire (high voltage drop) or sensitive electronic controls it can have a greater impact.

All you need to do when starting up or servicing a system with a multitap primary transformer is to ensure that the primary conductors are on the correct taps.  If you ever find a system where the control voltage is higher or lower than expected by about 10% then you will want to check for an improperly tapped transformer.

— Bryan

Erich Vinson is a tech from Colorado and one of the most entertaining people I interact with online. He wrote this quick tech tip on stripping back the outer jacket properly on control wires and it happens to also be something I preach. Thanks Erich.

In the first picture (above), you can see what happens when you try to use a pair of wire strippers to remove the jacket. It damaged the wire underneath. Instead use the pull string when you strip the jacket off of a low voltage (control) cable.

Use your strippers to remove about three inches of the jacket (or cut into the end like I show above – Bryan), and use the pull string to peel away the jacket, as is shown in the below.

Then, cut off the wires just below where you used your strippers.

The result will be low volt wires with no damaged insulation, and no hard to find low voltage short circuits.

— Erich

The primary role of setting an appropriate level of subcooling is to ensure that we deliver a full line of liquid refrigerant to the metering device.

We want to do this at –

  • A pressure differential required by the metering device
  • At a temperature and pressure no higher than required for maximum capacity and efficiency

But most important is that it is 100% liquid with no “flashing” or bubbles when it hits that metering device. Any amount of refrigerant that is already vapor when it hits the metering device is wasted energy and unwanted turbulence leading to noise and additional pressure drop.

We are generally safe to set the subcooling level listed on a the system data tag or the old 10° rule of thumb when you have nothing else to go  on.

We need to consider the adjusting the target subcooling in the following cases  –

  • Long lines or tall risers
  • Liquid lines run through high temperature environments

As soon as the pressure or temperature of the refrigerant in the liquid line hits the saturation point, bubbles will begin to form and the the dreaded “flashing”.

Let’s consider an example –

R410a system with a 110° liquid line saturation pressure (368 PSIG) with 5° of subcooling at the condenser so the liquid line is 105° but it’s a 100′ run of line with 20′ of rise and then through a hot attic that is 120°

First we can estimate the pressure drop of the rise based on the York / Johnson Controls rule of 1/2 psi of drop per ft of rise so this means we would see a 10 PSI pressure drop in the riser alone. Depending on the size of the liquid line there would be an additional pressure drop but it would not be significant so lets just estimate a 15 PSI total pressure drop and 2° of sensible heat gain into the liquid line due to the hot attic.

This would mean the liquid line would now be 107° and the liquid saturation temperature would also be at 107° due to the pressure drop from 368 to 353 PSI.

In other words the refrigerant could now begin flashing

In long long line applications Carrier instructs you to charge to 10° of subcooling or the listed subcooling whichever is greater becasue at 10° you have enough wiggle room to deal with most residential / light commercial situations.

In heavy commercial applications there are routinely longer line runs and the actual field pressure drops and temperature gains must be calculated to ensure flashing will not occur in the liquid line. Often this requires a higher subcooling.

— Bryan


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