Perhaps it’s a symptom of the times, but I have been attending to a rash of comfort complaints that have come through the new home warranty program in houses less than three years old. The complaints are usually expressed as rooms that are too hot, too cold, stuffy or stale. These are in homes that have gone through the building permit process and have had a proper heat loss, heat gain and duct design done. The resolution is often a pretty simple adjustment of fan speeds and a balance of flows.
It is interesting to note that in five of the last six houses I attended, the homeowners, as they recounted their concerns, very quickly and emphatically stated, “It’s not because I haven’t changed the filter.” It seems they were told repeatedly by every contractor they called to resolve the issue that the problem was most likely due to a dirty or restrictive filter. Of course, this is a simple, first order item to check. In fact, these homeowners had made some really good decisions, or at least their builder and HVAC contractor had made good choices.
Let’s review what are considered good choices when it comes to filtration in modern forced air ducted heating and cooling systems.
The notion that the furnace filter is just to keep the equipment itself clean is long gone. There are ample choices available now that can legitimately offer more for homeowners without compromising the airflow performance of the system. To make the right choice, a review of the most common industry metric for filtration effectiveness is warranted. That metric is described in the ASHRAE Standard 52.2 Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. The metric is distilled down to a single number, known as the Minimum Efficiency Reporting Value or MERV rating for a filter. Arriving at this number is a measure of a filter’s initial efficiency across 12 dust particle sizes averaged down into three particle size ranges to determine the single MERV rating. The chart attached shows the particle size ranges and corresponding effectiveness levels required to achieve a specific MERV rating. For example, a MERV 8 filter would be expected to remove more than 70 per cent of particles in a size range of 3.0 to 10.0 microns and at least 20 percent of particles in the 1.0- to 3.0-micron range but no expectation for removal of particles in the 0.3- to 1.0-micron range.
Compare these effectiveness ranges to air quality industry objectives to reduce exposure to particles of 2.5 microns, or less, in size. This size range, often referred to as PM2.5, is of concern because particles of this size are respirable and typically make their way through the nose, the throat and deep into the lungs. The Government of Canada recommends “Indoor levels of PM2.5 should be kept as low as possible” and Health Canada recommends “that indoor PM2.5, at a minimum, be lower than PM2.5 outside the home.”
A MERV 13 filter with an expectation of removing 85 per cent of particles in the 1.0- to 3.0-micron range may be of more interest to clients concerned about the quality of air in their home, office or school. Indeed, of the six houses I attended, three of them had filters clearly labelled as being rated at MERV 13. Not all filter manufacturers choose to label their filters to this metric. Professional contractors should have the resources on hand to accurately communicate the performance effectiveness against particle sizes for filters they choose to sell.
The next consideration is the impact the filter may have on airflow of the heating and cooling system. To reinforce the homeowner’s decision, I immediately measure the pressure drop across the filter, both the one in place at the time of the visit and even a new or dirty filter that happens to be nearby. Many HVAC designers are now allowing for a filter pressure drop of 0.1 in. to 0.15 in. (15 Pa to 37.5 Pa) of external static pressure in their designs. I fully appreciate that allowance may not have been made in older duct systems. However, given how easy and quick it is to measure the pressure drop across a filter, it should be a routine part of every site visit.
All three of those MERV 13 filters were nicely between 0.1 in. and 0.15 in. static. They were each four-inch thick pleated media filters. Two of the other houses had MERV 8 filters, one labelled, I had to look up the rating for the other. Both of those MERV 8 filters were one-inch thick media filters purchased at a big box store by the homeowners. One of those filters was just over 0.1 in. static and the other was just over 0.25 in., even though it was a clean filter. I showed that homeowner the results and explained how to shop for a better choice in the future, even though I was able to confirm the filter was not adversely affecting the total airflow of the system by more than a few cubic feet per minute (CFM). This is due in large part to the benefit of having the electronically commutated style fan motors or equivalent that are effective at overcoming higher levels of static.
Finding a balance
There are at least three ways to help your customer address the airflow resistance of filters and find a nice balance between filtration effectiveness and air flow delivery. Start by assessing whether the proper filter size has been used. I couldn’t find pressure drop labels on any filters I looked at in three retail centres, but I was able to find data online from a leading manufacturer. It lists pressure drops at two different face velocities, 300 feet per minute (FPM) and 500 FPM. This is a good hint to selecting the proper filter size. For example, a 16 in. x 25 in. filter has a face area of 2.78 sq. ft. at a face velocity of 300 FPM. The air flow would be 834 CFM and at 500 FPM face velocity the air flow would be 1,390 FPM. Thus, a 16 in. x 25 in. filter is ideally suited for a two- to three-ton system (assuming 400 CFM per ton). It was no surprise then when I measured a pressure drop of over 0.3 in. across a 16 in. x 25 in. filter on a four-ton system. My recommendation was to modify the return duct to allow for at least a 20 in. x 25 in. filter. It worked out nicely, since the only way to do it was to create a flair out in the return duct drop and install the filter in a horizontal drop. This is a much better location for a filter, as it gets the filter away from that sharp restrictive bend right at the mouth of the fan and provides two pressure drop relief measures in one: a larger face area and a less restrictive location. The homeowner also noted it made the filter easier to change as well.
Another measure is for you as the professional to use a pressure gauge to test three of four filter options on behalf of your client. I did this just last evening for a client whose daughter recently had a severe asthma attack and they are scrambling to ensure the air in their home is as healthy as possible. I took over five filters, from three manufacturers, ranging from MERV 8 to 13. We chose a MERV 11 that showed a pressure drop at cooling speed of 0.2 in. That was manageable for their system in the short term while they make other changes to the house and the HVAC system over the next year or so.
The filter experiment
Knowing that higher MERV rated filters will capture more particles and become dirtier quicker, adding surface area by using deeper four- or five-inch thick pleated filters should increase service life. I was disappointed to find that manufacturers of commonly sold filters don’t publish surface areas of their filters, so I purchased three filters and tore them apart to measure the expanded surface area. I was pleasantly surprised to find the expanded surface area of the MERV 11 filters was five to six times that of the Nominal face area of a 16 in. x 25 in. filter. It lends credibility to guidance that these filters only need to be changed once or twice a year. The one-inch MERV 11 filter was only $4 more expensive than the MERV 8 option and can be considered to be a cost-effective choice.
Back in business
I recently came across an HVAC contractor who works with a builder of about 40 high performance homes per year. The contractor’s bid included the cost of four four-inch pleated MERV 11 furnace filters, as well as filter replacements for the ERV. They would provide two filter changes per year over the two-year new home warranty commitment. A filter-change reminder on the customer’s smart thermostat links back to the contractor, who brings two filters to the annual service. The contractor is back in the filter business, which with the growing interest in air quality, is a smart move.