Chris brings up the value of economics of it. Is this an expensive service being offered in your parts of the country? I’m not seeing anyone selling it near me.

If I go ahead with the solar install I would certainly let them do it but would much sooner have it well done, and a measurement before and after with a blower test to know that it was of value.

This whole topic had me getting curious enough to try to get into my attic (unconditioned) and have a better look at my ducting. I’m not expecting too much being in a definite tract built area.

Thanks for any more info on this Aeroseal idea.

Martin: I think Aeroseal and Aerobarrier are great. The former is for sealing ducts and can be really helpful in existing homes where you can’t access all the ductwork, either because it’s trapped in building assemblies, covered by insulation you don’t want to disturb, or just too hard to get to. They use a water-based sealant that sticks in the holes when they pressurize the system. They monitor the pressure as the sealant does its job and stop when they get the target level of airtightness.

Hello! Another non HVAC expert homeowner here. I haven’t seen it explicitly mentioned but how do folks here feel about sealing methods like Aeroseal and Aerobarrier?

@JohnR, yes, manufacturers often rate air handlers with multiple condensers (or heat pumps) and it’s usually possible to do additional unofficial match-ups with condensers that aren’t officially paired. It depends on the specific AC and AHU model numbers, as well as the (duct system) external static.

If you post the AC and air handler model numbers, I can tell you what your options are, but you’d need to have someone check your air handler’s available speed settings and external static to see if your air handler and duct system can support higher airflow. If you’re willing to invest $50 in a Magnahelic gauge, I can show you how to do this test yourself (contact me privately for consultation).

you wrote: “will it make that much difference if we invest”…
Assuming it’s possible to up-size your condenser, of course it would make a difference! But before you do anything else, you should make sure there’s not something else that’s causing your system to under-perform (leaky ducts, dirty or undersized filter, unbalanced air distribution, improper airflow setting, inadequate duct system or restrictions, improper charge level, etc).

In any case, I’m not sure it makes sense to replace a relatively new AC because it couldn’t keep up with a once-in-a-century heat wave. You should at least eliminate other possible problems before considering a new AC.

Larry: Perhaps I am a bit dense and speaking “out of my expertise”, but you seem to want to be precise with some concepts (pressure) and loose on other ones (electricity). But I will keep trying.

Ohm’s Law is often expressed as V = IR. Ohm just said that the current through a material or object is proportional to the voltage across it. The constant of proportionality for this process is resistance (R) which was given the units of “ohm” in his honor. Some materials obey this relation, thus they follow Ohm’s “Law”, but many don’t, so this is really just an empirical relationship not a law of nature.

Another important equation for electrical analysis is P = IV. This is based on basic physics and is not an empirical equation. It has nothing to do with Ohm’s Law. You can combine Ohm’s Law with this electrical power equation and derive another equation for the electrical power dissipated as heat in a resistive material as P = IxIxR (I-squared R). This equation shows why appliance current requirements are important for wire sizing in order to limit the power dissipated as heat.

So now I will address your questions on pressure. Pressure is used many different ways depending on the issue being addressed. Let’s start with atmospheric or barometric pressure which is the actual pressure of the air around us, typically measured with a barometer. There have been many units developed for expressing it. At sea level under “standard conditions” it is 1 atm =14.7 psi = 29.92 in Hg = 101 kPa = . . . . This value is important for determining air density in an HVAC system when doing some energy calculations. It varies with weather conditions, local elevation, temperature, humidity, etc. So perhaps this is what you mean when you use the single term “pressure”?

The discussion on this thread has been mainly about pressure differences which are typically measured by a manometer with units of inches of water. These manometers (whether fluid-filled u-tubes or electronic) always measure a pressure difference between two points, typically between two points in an air handler or duct system, or between a duct and a space, or between the indoor and outdoor air (building pressurization). Since these pressure differences were typically measured with water-filled manometers, the typical unit of measurement is inches of water column (inwc). Note that normal atmospheric pressure as previously defined is equal to about 408 inwc. This is important since we typically talk about pressure differences in ducts or buildings on the order of 1 inwc or less which is insignificant compared to the actual pressure in a building for the purposes of determining air density. However, these pressure differences in terms of duct pressure drops and required air handler pressure rises are very significant in terms of their impact on blower airflow rate and power consumption. These pressure differences between the indoor and outdoor air can also be significant in terms of infiltration. These pressure differences between the ducts and adjacent spaces can also be significant in terms of duct leakage.

I won’t address the differences between static, total, and velocity pressures again. But I will repeat that many people in our industry (myself included) throw these terms around loosely because they are typically communicating with others who understand how these terms are commonly used.

Hopefully David addressed your confusion about the difference between thermal resistance and thermal mass. I can’t add much to what he said other than I prefer the term “thermal capacitance” instead of “thermal mass” since that is a better analogy to electrical processes.

I hope this answers your questions and addresses your misconceptions.

1. Static pressure is not the same as just ‘pressure’… as Roy and I have already said, total pressure = velocity pressure + static pressure. Although velocity pressure is relatively small in low velocity duct systems as Roy pointed out, we must measure static pressure if we want to use the manufacturer’s blower table to evaluate or diagnose airflow issues. Moreover, a static pressure probe is different than a pitot tube that measures velocity pressure. (Some pitot tubes can measure both.)

2. Thermal mass is not the same as R-value as your question suggests. In any case, duct insulation R-value has no bearing on Casey’s question regarding the impact of air sealing on external static pressure and whether air sealing can ‘do harm’ so I’m not sure how that became part of this discussion. Changing duct insulation R-value of course changes the heating or cooling load, and thus must be considered when doing load calculations.

3. Here’s why we’re concerned with blower motor watts: As HVAC equipment has become more efficient, the blower represents a larger and larger share of HVAC energy, which of course is the focus of this blog. Blower watts is a function of external static pressure, among other things.