Q: What's the difference between an aluminum and polypropylene core in an HRV, and how do I select the right size HRV?
A: HRV's save energy while ventilating your home. Understanding the differences in core materials used to transfer energy between airstreams is important.
The main difference between an aluminum core versus a polypropylene core is that the aluminum material conducts heat better than the polypropylene material. The product’s primary function is to transfer heat (sensible heat) from one air current to another without any cross-contamination between the exhaust and supply air streams. In order to achieve the same performance (relative to total efficiency of the units) from both types of materials, the main consideration is the core’s surface area. The polypropylene core would have to be bigger in physical dimension (requiring a bigger surface area) compared to the dimensions of an aluminum core to achieve the same heat transfer. The maintenance requirements for both types of cores are basically the same.
All HRVs are designed to run on a continuous low/med ventilation rate to optimize your ventilation and indoor air quality. HVI recommends you run the unit on a continuous basis rather than intermittently.
There are many brands of HRVs that offer various speed options. The most common setup in homes has the homeowner using the basic LOW/MED speed on a continuous basis and the HIGH speed intermittently. Some manufacturers offer HRVs with 100% variable speed control that allows the homeowner to select different speed ranges on demand to maximize comfort while reducing power consumption.
When choosing the size of your HRV remember that HRVs are intended to exchange equal amounts of fresh and stale air ("balanced") based on the size of your home. Bigger is not necessarily better. HVI recommends sizing your unit to provide the ASHRAE-recommended standard of .35 air changes per hour. Please consult with a reputable installer as there are other variables to consider which are too complex to discuss in this context.
And, of course, HVI recommends purchasing an HRV that has been tested by an independent laboratory and certified by HVI to perform according to the ratings stated in the product's marketing materials. You may also wish to consult the HVI Certified Products Directory.
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