ERV + AHU?

Everyone pretty much gets that continuous (or very frequent) ventilation is necessary in high-performance homes. And – at least in theory – most people get why balanced, heat recovery ventilation is better (than unbalanced and/or without heat recovery). But the devil’s in the details.

A couple years ago we started an R&D project with funding from DOE’s Building America program, and one of the first steps was interviewing several developers about ventilation (single- and multi-family residential, mostly on the East Coast). For none of these developers were HRVs or ERVs standard.[i] They all had some experience with ERVs, however, and when asked about these experiences the word “nightmare” came up shockingly often.

The ERVs on the market now can certainly work well in the right application, but we see problems more often than not. One of the biggest challenges is trying to add ERVs on to central heating/cooling systems in homes. Most ERVs aren’t really designed for this, and here’s what we see:

  • Ducts connected to the wrong places! Outlet and inlet ducts get reversed, or the supply air from the AHU getting exhausted (sad how often this happens).
  • ERVs are attached to supply and/or return trunks of the AHU. Unless the AHU fan is running constantly (or whenever the ERV is turned on), outdoor air comes into the AHU and is sucked right back out the ERV exhaust.
  • If the AHU fan is turned on, the relatively small fans in the ERV can’t successfully compete with the big AHU fan. People don’t get the ventilation flow rates they want and/or the flows are very unbalanced.
  • AHU fans can use A LOT of electricity. Hundreds of Watts is common – I’ve measured over 1 kW (though this is changing – more below).

Even if installers follow manufacturer instructions for attaching ERVs to AHUs, they could still end up with low flows, unbalanced flows, or high electricity consumption. Through this DOE R&D effort, we’re trying to do better.
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Does Your Exhaust Fan Suck? Part 2

If you recall from Part 1 of this article written back in September, we discussed why exhaust fans often don’t operate as they are intended. Now, let’s discuss how to rectify these issues. First, we need to understand that all fans are not created equal. To do this, SWA participated in a “blind” study that analyzed a number of today’s common exhaust fans. The study emphasizes the importance of fan selection. With this understanding, we will then discuss solutions and best practices for installing bathroom exhaust ventilation.

The “Blind” Study

To get a comprehensive performance dataset for a number of exhaust fans, the Riverside Energy Efficiency Laboratory (REEL) was engaged for a “blind” study. REEL is the HVI/ESTAR neutral, third-party testing facility. In total, 7 multi-speed fans, 7 single speed fans, and 6 low-profile fans from six manufacturers were sent to REEL without manufacturer markings. In general, ten-point airflow tests were conducted on each fan. Testing adhered to standards used in the industry, namely, ANSI/AMCA Standard 210 and HVI Publications 916 and 920, where applicable. While the dataset is extensive, this paper focuses on the 50, 80, and 110 cfm ventilation rates, as these are the most common specified fan speeds for bathrooms. These fan curves show the relationship of airflow that will be delivered at various static pressures of the duct system.

Figure 1 shows fan curves for single speed fans that were tested. The units are rated for 80 cfm unless noted otherwise in the legend (two are rated for 70 cfm and one for 90 cfm). While all of these fans performed in a similar manner, would it surprise you that two of the fan curves in Figure 1 are for exhaust fans that use DC motors? People often assume that all fans using DC motors are the same and result in constant airflow for a range of static pressures (let’s say up to 0.4” w.g.).

Figure 1

Figure 1. Performance Data for Single Speed Exhaust Fans

It is clear in this data (Figure 1) that flow rates decrease rapidly when static pressure rises over 0.3” w.g., as it often does in real world installations. Oh, are you still wondering which two fans have DC motors? It is actually SS-05 and SS-06. A bit surprising, isn’t it?

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Here’s What DC’s New Energy Efficiency Requirements Mean for Existing Buildings

Mayor signing legislationDistrict of Columbia Mayor, Muriel Bowser, signed a landmark piece of legislation known as the Clean Energy DC Omnibus Amendment Act this past Friday. With the mayor’s signing, Washington, DC becomes one of the first jurisdictions in the country with a binding, comprehensive law aimed at reducing greenhouse gas emissions. “It allows us to make significant improvements to the energy efficiency of existing buildings in the District,” Mayor Bowser said at the signing ceremony located at the American Geophysical Union (AGU) Building, which is slated to become the first net zero commercial retrofit in DC when it reopens later this year.

The new law has several sections which will impact the buildings in which DC residents and businesses live and work. In this post, we’re going to focus on Title III of the Clean Energy Omnibus Amendment Act, which is designed to make the city’s existing buildings more efficient.

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