Month: August 2019


When mounting a TXV bulb or checking bulb placement there are a few important considerations (listed in order of importance)

  1. Mount the bulb on the suction line. Flapping in the breeze is no good.
  2. Mount TIGHTLY it with a proper metallic strap (usually copper, brass or stainless). Not zip ties, not tape.
  3. Position it on a flat, clean, smooth, portion of the horizontal suction line. Not on a coupling or an elbow.
  4. Mount it before the equalizer tube (closer to the evaporator than the EQ tube)
  5. When possible mount it at 8 or 4 o’clock on the suction line (or according to manufacturers specs) . This becomes more important the larger the suction line.
  6. When possible, insulate the bulb so that it is not influenced by ambient air temperature. It never hurts to insulate the bulb even inside the cabinet though not all manufacturers require it.
  7. If you do need to mount it vertically, make sure the tube points up not down

Poor bulb contact will (generally) result in a bulb that is warmer than desired, resulting in overfeeding and lower than desired superheat.

Finally… be gentle with the bulb and tube. They break easily.

You can read a more detailed description HERE

— Bryan

 

The Four P’s of IAQ

I’m not the first, second, or probably even the thirty-second person to write about improving indoor air quality problems by using the four P’s approach. It’s a well-known thought process in the building science community- not sure if that’s the case in HVAC circles.

The first P is pollutant. In this context, a pollutant is something that contaminates the air. It can be one or more of many things. Mold, moisture, animal feces, harsh chemicals, are some of the more common ones in an average home. If you remove the pollutant, there is no longer an IAQ problem.

The next P is pressure differential. If the pollutant can not be removed, creating a pressure differential will prevent it from entering or spreading throughout the occupied area… But you already know this, that’s why you turn on the fan when going to the bathroom, or the range hood while cooking.

Another P is pathway. If there is a pollutant, with an unfavorable pressure differential, there still needs to be a pathway to the occupants for that pollutant to become an issue. Eliminate the pathway and the pollutant can be contained.

The last P, but definitely not least is people. You’ve heard this before- If a tree falls in the middle of the woods does it make a sound? Well, If there is a pollutant with a pathway and a pressure differential but no people is it an really an IAQ problem?

In short, it’s always best to deal with the source of the IAQ problem, but if you remove any one of the four “P’s” there will be an air quality improvement.

— Neil Comparetto, Co-owner of Comparetto Comfort Solutions in Virginia

Most of the laws we refer to in air conditioning and refrigeration are pretty obvious and practical and Dalton’s law of partial pressures is no exception. John Dalton simply observed that the pressure of air was equal to the added pressures of each gas that make up air. This means that the pressure and density of air can vary based on the exact makeup of the gases contained in the air.

The law of partial pressures states that –

 In a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.

This simply means that if you take two gases and you place them together in a contained space, you simply add the pressures together to get the total pressure. The only case when this does not apply is when the gasses “react” with one another to create new molecular structures.

In practical terms, this is why nitrogen or air in a refrigerant circuit increases the pressures. The pressure of the nitrogen is added to the pressure of the refrigerant resulting in higher pressure.

It is also one reason that refrigerant manufacturers blend refrigerants to create ideal boiling and condensing temperatures based on the percentage of one refrigerant over another. A common example of this is R407C vs. R407A, they both are made up of  R-32, R134a and R-125 but the % of each in the mixture dictates the pressure/temperature properties.

Now clearly, this law applies only to gas (vapor), not matter (refrigerant) in the saturated state like refrigerant in a tank, but when the refrigerant is in the vapor state it obeys Dalton’s law.

— Bryan

Old timers used to always say that running plumbing and condensate drains wasn’t rocket science because “water flows downhill”, which may be true, but water also floats in the air, goes uphill and forces it’s way through concrete…

Here is a look at some of the ways that water moves that impact building comfort and integrity.

Flow in the Liquid State 

Water does run out or “flow” when it’s in a liquid state and this is the state we are most familiar with when we think about water moving. We know that flow in the liquid state often moves downhill with gravity, but it can often travel uphill due to the weight of a water column on the inlet side or the suction of a water column on the outlet side with a siphon. We see this every day with traps in sinks, toilets and condensate drains.

Capillary Action 

Water also travels against the forces of gravity in liquid form when it comes to porous materials or small gaps between materials. This force of capillary action is what allows nutrients to travel from the roots of trees up to the branches and leaves and cap be easily see when you dip paper towel into water and the water begins “wicking’ up the material against gravity. The more porous a material is and the smaller those pores are (so long as they are larger than water molecule) the greater the capillary effect.

In buildings we need to consider capillary action when designing wall systems to prevent water being held in or drawn up into walls. This can happen due to porous materials being immersed in water or damp soil or by having wall layers placed close together with a gap or “capillary break” when there is a possibility of moisture.

Convection (Air Movement) 

Air contains water vapor and the higher the dew-point of the air the more total moisture it contains. Let’s consider a situation where the outdoor air is at 75 degree dewpoint (as it often is where I live) and the indoor dew-point is 55 degrees, which is a good, comfortable dew-point that you will get at 75 degrees and 50% RH. When this outdoor air enters the building it brings it’s higher moisture content with it whenever a door opens, a widow isn’t shut fully or there are gaps around a can light into the attic. This water content carried in the air can be significant and is one of the reasons that air sealing is such a big part of good building design and construction.

Vapor Diffusion 

Water vapor can and does move through solid materials at the molecular level based on a difference in vapor pressure from one side to the other and the vapor permeability of the material. Often we call materials designed to slow down vapor diffusion “vapor barriers” as if they will stop all vapor diffusion and this simply isn’t the case.

These materials are really just vapor inhibitors to help slow down the movement of water vapor across the material. The “vapor pressure” is based on a difference in dew-point, not relative humidity, so if the outdoor temperature is 80 degrees at 42% RH and indoors it is 72 degrees at 55% relative humidity there will be no vapor diffusion because they are both at 55 degree dew-point which equals a partial pressure of 0.436″hg (inches of mercury column).

When there is a difference in dew-point (and therefore water vapor pressure) across a material such as drywall or insulation, there will be a transfer of water across that material via vapor diffusion from the higher dew-point to the lower dew-point.

So yes… water moves downhill and a whole heck of a lot of other ways as well.

— Bryan

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