Measuring System Airflow Using a Ductblaster

If you install systems in a state which requires duct leakage tests on all new installations, chances are you have a ductblaster which you use for that purpose. Your ductblaster is more accurate at measuring airflow than just about any tool you have. Afterall, the way it works is to pressurize the ductwork and measure the airflow required to maintain that pressure. You can use the ductblaster to measure system airflow using a procedure called pressure matching.

Operate the air conditioning system normally and use the ductblster manometer to measure the static pressure in the supply plenum or trunk. You should measure after the coil but before any takeoffs. Record this pressure. Now, turn the system off and connect the ductblaster to the blower on the return side. You will need to block the return air trunk off so air only goes through the unit and into the supply ducts.

Now turn on the ductblaster. Once again, measure the supply air static at the same location where you measured it with the unit operating.  Dial the ductblaster up until the measured supply air static equals the reading you took when operating the system. The amount of airflow the ductblaster is moving is the same as the airflow through the system when it was operating.

How does this work? You are matching the airflow required to create a supply static equal to the supply static created by the system blower. Note that this does not require any particular manufacturer’s data. This procedure allows you to make an accurate airflow measurement using a tool you may already have. It also allows you to get more from your investment in the ductblaster.

Refrigerant Don'ts

With summer now upon us and the price of R22 skyrocketing there are many questions regarding replacement refrigerants. This discussion could fill a book, so I am going to restrict this post to a list of don'ts. The intent is to help people avoid issues that can be caused by improper application of 400 series R22 replacements.

Do NOT use a flammable replacement refrigerant in ANY system originally designed for R22. There are some hydrocarbon (propane) based replacement refrigerants sold online. They are NOT EPA approved and represent an explosive hazard when charged into a system that was not designed for flammable refrigerant.

Do NOT add ANY replacement refrigerant on top of an existing R22 charge. This is an EPA violation. You are essentially creating a “new” refrigerant which has not been tested or approved. There are NO replacement refrigerants which are legal to add in on top of an existing R22 charge. You must first remove ALL of the R22 when doing a conversion.

Do NOT use ANY 400 series refrigerant in a flooded system. Even refrigerants which are advertised to work in systems with mineral oil will still separate in the flooded portions of the system because they are not truly miscible. There is a difference between miscibility and solubility, but that is the subject for another whole article.

Do NOT use ANY replacement refrigerants in ANY system using an electronic expansion valve. This would primarily be older R22 minisplits, multisplits, and VRF systems. Trane hyperion heat pumps can sometimes have an R22 charge. In that specific case, the indoor air handler is designed for both R22 or R410A, so switching to R410A and changing the refrigerant dip switch solves that problem for the indoor air handler. Unfortunately, you will still have to replace the outdoor unit with one designed for R410A.

Do NOT use ANY 400 series replacement refrigerant in systems which were originally designed for R22 and have Trane 3D Scroll compressors. The lubrication system that specific compressor design uses does not work well with HFC refrigerants, including ones advertised as being compatible with mineral oil.

This all come down to one main strategy for replacing R22 in most older systems: it is generally best to replace the whole system. Not only does this avoid application problems, it usually provides a significant efficiency upgrade as well.

Latent Cooling and Variable Capacity Systems

If you live anywhere other than the southwestern part of the US, you probably need latent cooling in the summer. The word latent means hidden. Cooling capacity is required to condenses water on the evaporator coil. This is referred to as latent cooling because there is no temperature change involved, you can’t sense, or measure the heat change using temperature, but cooling capacity is required. Two things help increase latent cooling: long run times and reduced airflow across the evaporator. These increase the percentage of system capacity used for latent cooling.

I have a brand new communicating, 20 SEER system with a variable speed scroll and an ECM indoor blower motor. One fun thing about the thermostat is that it reports the compressor speed and the furnace reports the blower CFM. I have been watching both.

The compressor is most often operating less than 50% capacity, but stays running most of the day once it starts. This ability to match system capacity to the load makes for long run times, which helps control humidity. It does not use more power, even though it is running a lot because it is using much less electricity while it is operating.

I have noticed that the fan almost never runs at the traditional 400 CFM per ton. For example, on one occasion I found the compressor running at 96% while the fan was moving 1230 CFM. It is a 4 ton system, so traditional CFM math would place the “normal” airflow at 1536  (4 x 0.96 x 400 = 1536). However, the system was operating at only 320 CFM per ton ( 1230 / (4 x 0.96)).

The thermostat also lets you set the indoor relative humidity and reports the indoor relative humidity. I set it at a fairly low 45% and the system has kept it between 45% and 50%. During the day when the system is running, it keeps it right at 45%. It accomplishes this by using long run times at reduced capacity with lower than normal airflow. Summer comfort in the southeast involves more than controlling temperature, it also involves controlling humidity. One bonus of the variable capacity systems is that they do a better job of controlling humidity than fixed capacity systems.