Installation Mindset / Ducts, Venting, & Commissioning

Many problems that occur during installations stem from poor duct design. Ductwork requires a lot more thought and planning than just using ACCA Manual D. Duct sizing is important, and HVAC systems often lose efficiency to takeoffs and return drops that are too small; Michael and Neil like to make the ductwork as large as possible to improve the equipment operation and use large filters effectively. Some equipment coils also have more pressure drops than others, and the ductwork needs to be able to deal with that. Know the performance of your fittings and what their equivalent lengths are.

When you use filters, you want to make sure you maximize your surface area for the best filtration. The pleats of filters increase the surface area and effectiveness of each individual filter. Creating plenty of filter grilles and returns for your filters allows you to increase filtration without creating too large of a pressure drop. (Remember to use filters with high-quality media, and don’t be afraid to diversify your filter selection if the ductwork has unique needs!)

Oversized units may present issues with the ductwork. Whenever possible, look for opportunities to downsize the HVAC equipment during a changeout or retrofit.

Turning vanes can confuse some technicians. These are usually more beneficial in supplies than returns, and they shouldn’t present an issue if the ductwork is large enough in the first place. When installed correctly, these turning vanes can help even out turbulence and are great in cases where you have space constraints. When configuring elbows without turning vanes in the ductwork, make sure that the throat (inside) is large enough and that you have enough space for them.

When it comes to metal duct fabrication, you need to think about getting the ductwork off the shelf as quickly as possible. Retrofits can be tricky and may require some custom adjustments to the ductwork. Modifying fittings can take more time than creating your own transitions. You’ll also want to pay attention to the size of your air handler; larger air handlers can leave you without enough room to make a good supply transition.

You’ll also want to think about the ductwork’s relationship to the structure. Neil recommends decoupling the ductwork from the structure as much as possible, such as through canvases. Cross-breaking is not as important as one might think, but you can still do it if you think the sheet metal might not be rigid enough.

Flex duct provides a unique set of benefits and challenges. Flex ducts reduce noise, are well-insulated, and are leak-free. However, they can lead to poorer airflow if installed incorrectly. Keep flex ducts straight and tight, and you’ll have no problem.

When burying ductwork is a bad idea, strapping and routing are the processes that support the ductwork. Codes will give you a guide when it comes to the spacing between supports. Wider straps provide more support, especially if you can use a “saddle” material underneath.

Leak prevention is critical in sheet metal duct systems. The best approach to solving duct leakage issues is methodical, not reactionary. You’ll get the best results by approaching a job with the mindset of minimizing leaks from the beginning; use mastic on your elbows and do as much prep work ahead of time as possible.

You only need to worry about venting when you use gas appliances, not heat pumps. Venting can go wrong when people size the flue incorrectly or use the wrong pitch on condensing appliances. However, most issues can be avoided by simply reading the manufacturer’s literature. Pay attention to your terminations; avoid areas where terminations can be covered by snow. You should also check for signs of backdrafting, which is a dangerous condition.

Commissioning is the time to set airflow correctly. The more measuring and benchmarking you do, the more likely you will catch something that could cause problems later. Static mapping is a great commissioning practice. Of course, you’ll also want to check the refrigerant charges or combustion analysis before walking away from a newly installed system. Even though we highly recommend reading manuals and using all your resources during commissioning, your best tool will be good old common sense.

We also discuss:

Duct design modeling software
Balancing dampers and design strategies
Strategies for sealing fittings
Drilling register grilles into place
Addressing leak points at the boot
Using a Ductulator or duct slider
Using rules of thumb
Gas piping
Orphaned water heaters
Using manuals

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