Month: June 2020

When I first started in the trade we used to run into shielded control wires on the Carrier Comfort Zone 1 zoning systems and also on a Carrier VVT system I used to maintain at a bank. I knew it has something to do with electrical “noise” and that communicating systems often called for it but I never looked any further into it.

Over the last decade there has been a lot of different residential communicating systems that have come out. Some require shielded cable, some recommend it and others don’t mention it all.

The fact is that whenever controls work on a low voltage “signal” rather than a simple “on/off” control they are more susceptible to induced charges from other nearby conductors, electronics and even transients from electrical storms.

A shielded cable has a  metallic jacket that surrounds the individual conductors and routes the induced charges to ground, keeping it away from the conductors inside.

As an example of this, I installed a Carrier Infinity system at my own house WITHOUT using shielded cable and almost every time there are lightning strikes nearby the unit will throw a communications fault, since I’m in florida that happens quite often.

If you do have the wisdom to run shielded cable you need to remember to bond (ground) one side of the shield securely to a good equipment ground on one end and ONE END ONLY. If you ground both ends you risk the sheild becoming a path in the case of a ground fault which could cause some bigger issues. If you ground both ends you can also create a “ground loop” that can cause the very noise you set out to eliminate.

In some cases, you can perform a similar function by grounding leftover/unused conductors on one end if you failed to run a shielded cable. There is no guarantee it will solve the issue depending on the severity because the other conductors don’t fully surround the conductors being utilized.

The lesson being, when working with communicating “signal” controls run shielded cable whenever possible. I was looking around and found this spec sheet from Southwire on their shielded 8 wire.

— Bryan

Most controls and thermostats will have some sort of cycle rate per hour setting that kicks in to prevent over cycling once setpoint is reached.

These cycle settings don’t kick in until the system starts achieving setpoint, so don’t worry that it will shut off if it’s set to 70° in heat mode and it’s 60° in the house. Once it gets to the setpoint the cycle per hour programming will prevent the system from running MORE cycles per hour than the setting.

When the cycle rate per hour is lower the run times will be longer and the off times will be longer.

When the cycle rate is higher the on times and off times will be shorter. 

As a general rule, a lower cycle rate is better for efficiency and a higher cycle rate is better for comfort. This is because most equipment is inefficient for a portion of its start-up time so more starting and stopping impacts efficiency due to this “ramp up” time.

Older homes that were poorly sealed and insulated often required more cycles because they would get cold fast when the equipment went off. Nowadays, it is great if we can use 3 cycles per hour with furnaces and 1 cycle per hour with heat pump systems to maximize efficiency.

If you get comfort complaints due to wider temperature swings between cycles then you would need to increase the number of cycles but this is unlikely to be an issue with modern construction.

Also, keep in mind that colder climates may require higher cycle rates due to more extreme differentials between indoor and outdoor temperatures.

You will notice that the thermostat manufacturers show that electric systems can have higher cycle rates, this is because there are no ramp up losses in electric systems so there is no real downside to shorter cycles.

— Bryan

 

When a system has abnormally high head pressure (high condensing temperature over ambient) and compression ratio, one of the easiest things to look for is a dirty condenser coil and more often than not, that will be the cause.

However…

There is another category of issues that can cause high condensing temperature (high head pressure) that result from improper practices rather than dirt and grime.

When a tech comes across a failed condensing fan motor or a damaged blade they will often go to their van and see what they have as a “universal” replacement part. I don’t have an issue with using aftermarket repair parts in some cases but you need to make sure that the part you are using will operate properly without sacrificing capacity, efficiency and longevity.

Often when using aftermarket parts a tech may be sacrificing one or more of these things and that can lead to issues.

When replacing a fan blade you need to ensure –

  1. The pitch is a match
  2. The number of blades is a match
  3. The Diameter is a match

If you change the pitch you will also need to change the # of blade and vice versa to end up with the same CFM airflow output which can be very tough to determine in the field.

The diameter really cannot change or you won’t have the proper gap between the blade edge and the shroud (Usually 1/2″ – 1″) which can greatly impact air movement.

When replacing a motor you need to ensure –

  1. The RPM (# of poles)  matches
  2. Voltage and phrasing matches
  3. The HP is the same or greater
  4. The physical size will allow proper installation

In some cases, the technical specs may work but the motor body may be deeper. When this happens you need to make sure that the fan blade can still sit high enough in the fan shroud for proper movement of air. In many cases the blade/shroud are designed so that the middle/center of the blade matches up with the bottom of the fan shroud (cowling) and if it isn’t it can decrease airflow.

This issue comes into play often in cases where a factory motor fails on smaller tonnage residential units with a less than 1/4 HP motor. In these cases when you replace the factory motor with a universal motor the larger physical depth of the motor and sometimes the width can result in less than designed airflow. Make sure when replacing the motor that you are still able to place the motor blade in the same position in relation to the blade to ensure proper air flow/condensing temperature.

Sometimes you will come across systems that are running higher head pressure than they should be. In these cases you will want to check and make sure the motor HP and RPM are correct and that the blade is properly sized and positioned in the shroud.

As always, being attentive is key to finding issues, even issues caused by others

— Bryan

 

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