Month: May 2020

Some techs and contractors swear that flex ducts are an evil invention and should never be used in ANY circumstance. I agree with what duct design expert Jack Rise said on the podcast when I asked him about flex ducts he said:

“There’s a lot of problems with flex duct, there really is and it’s a good product but we abuse it…. It’s a good product, it’s just poorly handled”

While the proper sealing of ductwork in unconditioned spaces is nearly universally recognized as important, it is rare that a flex system get’s installed properly in these other important areas.

Fully Extend The Flex 

Some guidelines suggest pulling a 25′ piece of flex fully extended for 1 full minute before attempting to install it. This reduces the compression and the depth the of the corrugation (the accordion spiral inside the duct). The more compressed the duct is when it’s installed the greater the air resistance of the duct will be. The air duct council states that 30% of compression can result in 4 TIMES the air resistance. This means that fully extending the flex is a big deal and may be one of the most overlooked aspects of flex system installations. Cutting off that 2′ – 6′ of extra flex on the end instead of just “using the whole bag” can mean the difference between a good and a poor duct system in many cases.

Strap and Support the Flex 

Jack Rise spoke about how he tested a duct and measured a .2″ wc change in static when he altered a duct from sagging to properly strapped. In retrofit applications, many companies focus on “sealing” connections but they often don’t truly address sagging ducts with proper strapping. the allowable amount of sag is only 1/2″ per 4′ of length which isn’t much. Don’t ONLY rely on the code required strapping in your jurisdiction, just because a system passes inspection doesn’t mean it’s installed correctly.

Keep the Curves to a Minimum 

When designing a duct system you must calculate TEL (Total Effective Length) not just length. In a flex system each curve has a HUGE impact on the TEL and when a field install doesn’t match the design it can throw the whole system out of whack both from an air balance standpoint as well as a system performance by increasing the TESP (Total External Static Pressure). Every bend and angle matters so keep it extended, properly routed and well supported and all will be well so long as the design is correct.

Seal all the connections

As with all ducts the connections need to be well sealed. With flex, this will generally need to be done with mastic and the BEST way is to fully seal and allow the inner liner to dry before pulling the insulation over the connection. Also keep in mind that leaks, where the boot / can meet the sealing, are very cannon leak points and it’s a good idea to seal them from the inside and/or outside to the final floor or ceiling before installing the grilles

For more info go to the ADC (American Duct Council) website at flexibleduct.org or download their excellent guide HERE

— Bryan

I didn’t install this unit

First off, attic installations are among my least favorite applications from a serviceability, system longevity and a laundry list of other items. Here in Florida, it’s just a bad idea due to the high humidity and temperature in a vented attic and the condensation issues that can and usually does occur on the equipment.

Besides all of this, the IMC (International Mechanical Code) 2015 edition has some specific code requirements related to attic installation that you should be aware of. The IMC isn’t the “law of the land” and the final say on codes comes down the AHJ (authority having jurisdiction ) in your area, but in general the IMC is the basis for most local codes.

IMC 306.3 (2015) Appliances in Attics

This is a plain language paraphrase of the code

  • The opening needs to be large enough to remove the unit.
  • The opening/passageway must allow for unobstructed access.
  • The passage must not be smaller than 30″ high x 22″ wide and no longer than 20′ unless it’s at least 6′ tall and then no more than 50.’
  • Passageways must have a solid floor no less than 24″ wide.
  • The work area in front of the unit must have a solid, level floor of no less than 30″ x 30″ .
  • You don’t need a floor/passage when you can access/service/remove the unit from the opening itself.
  • There needs to be a light and an outlet by the unit with a switch located at the opening to the passageway.

Practical Considerations

  • Don’t ever cut trusses unless you have engineering that shows it is allowed.
  • If the unit is suspended make sure not to drill trusses in a location that will structurally compromise them. Follow manufacturer recommendations for proper mounting of the equipment.
  • In unvented attics take extra precaution to properly insulate drain lines/copper and seal unit penetrations to prevent moisture issues
  • Make sure that you CAREFULLY read manufacturer install instructions for horizontal installation and run test the system long enough in cooling to ensure there are no condensation issues. Coils and pans are OFTEN installed improperly in horizontal applications.
  • Install a secondary pan at least 3″ wider than the appliance (1.5″ in each direction) and condensate overflow protection to ensure that an overflow won’t result in MAJOR damage in accordance with IMC 307.2.3
  • When it comes to condensate in an attic CHECK EVERYTHING TWICE.
  • Be prepared with plenty of fluids and comply with OSHA confined space rules when applicable.

Mostly, avoid attic installs whenever possible and when required do them with care and prepare to do some extra prep work on the work area to comply with the code.

— Bryan

 

The term “short” has become a meaningless phrase in common culture to mean “anything that is wrong with an electrical device”.

A short circuit is a particular fault that can mean one of two things in technical lingo.

Any two circuits that are connecting in an undesigned manner. This would be the case if a control wire had two conductors connected together due to abrasion. Like a Y and G circuit “shorted” in a thermostat wire between the furnace and the thermostat. This would result in the condenser running whenever the blower is energized. This is disputed as to whether this is even a “real” short or not but is commonly referred to that way in the trade.

A short can also be described as a no-load path between two points of differing charges. This would be a traditional “short to ground” low voltage hot to common connection or a connection between legs of power without first going through a load of appropriate resistance.

Both of these conditions will result in something occurring that should not be occurring. Either something being energized when it shouldn’t be or fuses and breakers tripping or blowing or damaged components.

This is different than an Open circuit which is no path at all. So if a load has power applied and NOTHING is happening it is open. If power is applied and breakers or fuses trip or blow or something comes on at the wrong time or order, that is a short

Many techs advocate for using an ohmmeter to find a short circuit. We like that method but we often find that using the 24v as is and simply using a process of elimination to find the cause is easier for most techs as shown above.

 

— Bryan

If you work in refrigeration you may have heard the term “hot pull down”. This phrase is used to describe a condition where the load on the evaporator is above design due to the box temperature and/ or the temperature of the product in the box being higher than it would normally be.

My grandpa called me a few months back all upset “I just slaughtered a bunch of chickens and I’m going to lose all my meat because this freezer you got me isn’t working” he gasped into the phone. Now I had helped him pick out a commercial freezer a year or two back and he put it in his garage (a less than ideal location to begin with). What I had forgotten to mention to him was the importance of only loading with meat that was already down to temperature.

I showed up to look at it and sure enough, there sat a bunch of freshly slain birds PACKED into his freezer and the box temperature struggling to get below 15° instead of the 0° we really needed.

Most refrigeration equipment is designed to only maintain the temperature of the product, not to bring it down to temperature all at once, at least not in large quantities. This is due to two aspects of the design.

  1. Capacity – Most freezers and refrigerators just don’t move enough pounds of refrigerant to generate the necessary refrigeration effect to “pull down” warm product in a timely fashion. In other words, just like many A/C systems don’t keep up on a freak 98-degree day in Indiana, refrigeration equipment won’t pull down quickly if you add in more BTUs of heat than it is sized to remove.
  2. Coil Feeding Range – In the case of a cap tube or other fixed orifice metering device, the amount of refrigerant fed into the evaporator is directly proportional to the amount of refrigerant pressure differential between the liquid line and the evaporator. With a TXV the valve responds to superheat in order to open and close, opening as superheat rises and closing as the superheat falls. In a hot pull down the load on the evaporator is so high that the expansion valve goes wide open but still,  the coil “starves” or underfeeds refrigerant. This results in high superheat, high suction pressure and high head pressure but will also often result in low subcooling because so much of the refrigerant charge will move to the evaporator coil.

During a hot pull down the compressor will draw higher than usual amperage due to the increased density of the suction gas, this coupled with high superheat can result in compressor damage if it is allowed to run outside of specs for an extended period (Sporlan has a great piece on compressor overheating you can read HERE).

The conclusion is that most equipment should be allowed to get down to temperature before being loaded with product and the product should generally be at or near the design temperature. There are freezers and refrigerators that are designed specifically for “flash freezing” or pulling product down to temperature often called a “blast freezer”.

In the case of my grandpa’s freezer, we moved some of the meat around to other freezers and got it down in time to prevent salmonella… at least I hope so… I was feeling funny after Grandma’s chicken soup for Sunday dinner…..

— Bryan

 

Imagine a glass of ice sitting on a table.

Now imagine you place a lid on the glass so all the water and ice is contained in the glass.

If the ice and water are well mixed the water and ice will both be at 32°F because the ice is slowly changing state from ice to water which we call melting. Becasue this is happening at atmospheric pressure we can know what temperature this will occur at and the heat being transferred is going toward melting the ice rather than changing the water temperature which we call latent heat.

Let’s say the temperature in the room is 75°F. In this scenario, heat leaves the air molecules as they contact the exterior of the glass and heat moves through the glass into the water and ice. Becasue glass is a pretty good insulator this happens pretty slow but this heat still moves from hotter to colder.

This movement of heat from the air to the exterior of the glass transfers THROUGH the glass via conduction.

What happens if we blow air toward the glass? what changes? 

If we move more air over the side of the glass we deliver more air molecules to the glass via convection but it doesn’t change the fact that the heat makes it through the walls and into the glass via conduction.

By delivering more air to the glass we warm the outside of the glass more which causes the water melt inside the glass faster, in other words more air over the glass means more heat transfer even though we didn’t change the temperature of the water or the air.

This same basic thing happens inside an evaporator and condenser coil, when we increase the flow of air we also increase the transfer of heat through the walls of the copper tubing in the coils. In the condenser more airflow increases the heat rejection out of the refrigerant and in the evaporator more heat is gained.

Because the refrigerant circuit is dynamic (refrigerant moving) and under pressure more or less heat entering or leaving the system impacts the process and changes the pressures of the refrigerant inside.

If we move less air over the condenser the pressure on the high side increases, if we reduce the air over the evaporator coil less heat enters the circuit, and pressures drop.

This is a basic picture for you to consider next time you see high or low system pressures and how coil airflow impacts heat transfer.

A more advanced but similar thought experiment is what would happen if the evaporator coil had no fins.

— Bryan

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