Residential AC System Installation

Bryan with HVAC School goes over residential AC system installation in his presentation from the BTrained HVAC training event in Birmingham, AL. He also talks about ventilation, dehumidification, ductless units, sensible heat ratio, the decay test, and triple evacuation.

We measure static pressure with manometers quite often, but they don’t actually measure airflow. In general, many people think that lower static pressure indicates better airflow and is more desirable than high static pressure, but many problematic conditions can also result in low static pressure (such as a conductor going to Y1 instead of Y/Y2). We can get a more reliable indication of airflow from using a TrueFlow grid than static pressure. However, in any case, the relative indirectness ad unreliability of static pressure should encourage us to look at the whole system for possible problems.

Installation requires a different skill set than troubleshooting and service; however, installation best practices can be useful for all sorts of technicians.

The installation can be divided into a few main phases: pre-planning, planning, demo, install, and post-install. However, many people focus on the demo and installation without putting a lot of care into the pre-planning, planning, and post-install steps.

The pre-planning involves Manual J, Manual S, etc., where we can use our tribal knowledge WITH those sorts of calculations and building standards (or special climate considerations, especially when you may need more dehumidification to meet those building standards in tightly built new constructions). If you enjoy being involved in the modeling and design process while using field principles, you might get a lot out of using Kwik Model 3D.

We can also use the pre-planning phase to consider the design, especially focusing on the envelope. We have to evaluate tongue and groove joints, insulation in the unconditioned space, can lights, radiant barriers (which keep heat out of the attic by reflecting the sun’s radiation), dryer vents, and mechanical ventilation in the design. If we have ventilating attics, the important thing to focus on is sealing the attic from the space. Pre-planning is also the time to make sure everyone working on the project is on the same page.

When designing an HVAC system and preparing for installation, you need to have a few discussions with the customer first. It’s prudent to have discussions about indoor air quality and when IAQ products may be appropriate for a building. (Some more controversial products include PCOs and UV lights, but there is a time and place for those, and it’s worth talking to the customer about why those things may or may not be ideal for the installation.) Done correctly, ventilating dehumidification is a good technology that can benefit many homes, especially if we also rely on ECMs to control the ventilation.

Ventilating dehumidifiers work best when they tie into the supply duct, NOT the return duct. (The relative humidity in the supply duct is high because the temperature is lower than the return; there is much less absolute moisture in the supply duct overall.) Dehumidified ventilation air going into the return derates the HVAC equipment’s ability to remove moisture. Dehumidifiers add a bit of heat, which increases the load and causes the system to have longer runtimes, which is better for dehumidification. The houses that need the most consideration are those with bad envelopes (leaking to the attic or outdoors), poorly installed ductwork, and inhabitants that run their thermostats too low.

Some duct upgrades are also worth discussing. You don’t HAVE to replace all the ductwork, but if you can oversize the trunks or ductwork near the equipment (and still fit it), it will typically be a good idea. Larger filters (and filter cabinets) slow down the air velocity over the filter media, which increases the filter’s ability to catch undesirable particulates; there is also a lower pressure drop across the filter. Upgrading the filtration can be beneficial to indoor air quality as well as system longevity, efficiency, and performance. You may also want to consider seeing if the equipment can be downsized and still allow for satisfactory cooling in the summer; downsizing equipment may lead to longer runtimes and may make undersized ductwork more appropriate.

When we’re planning ductwork, we also have to think of total effective length (with bends) and velocity in the throat. Turning vanes can help, but smooth transitions in the throat will be more beneficial. When fabricating metal ductwork, friction rate is typically less of a problem than leakiness.

As always, flow nitrogen while brazing during installations. There are no excuses NOT to do it (unless you’re using StayBrite 8 and can’t guild up oxides). In any case, you MUST purge with nitrogen.

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