Field notes from Tay River Builders

You can’t suck your way out of a distribution problem

Jesse Smith — Wed, 03 May 2017 12:00:00 GMT

Here’s an excerpt of an email I wrote in support of adding large central returns in a finished basement, directly next to the unit, with transfer grilles at the top of the basement stairs. The overwhelming majority of HVAC technicians reject this as something that can never work.

One of the amazing things about trying this is that the effect can be tested before doing the work, by temporarily removing the blower cabinet door and taping the door switch shut. (Danger: this can cause serious safety problems by making combustion appliances spill, so it requires a continuous return path. Don’t try this if you’re not an HVAC professional!)

The point of why I went there was to test if the hypothesis was right. In this case, it appears to be right.

There are basically two parts to the problem:

Airflow is restricted, specifically on the return side. This reduces the total quantity of airflow. That statement is totally uncontroversial, and there are multiple ways to test it. You can check the total external static pressure of the duct system, estimate what a reasonable static pressure would be after the duct changes, and check both values against the blower chart for the unit. PSC fan motors in particular have a pretty sharp airflow curve, so reducing static pressure increases airflow significantly.
Delivery temperatures tend to improve, especially in summer, through better mixing. We’re (at least initially) moving the coldest air through the system.

Lots of technicians believe they can fix distribution problems by sucking air out of individual rooms. This rests on some false assumptions and leads to confusing outcomes. In general, you can’t suck your way out of distribution problems. In some cases you can increase total system airflow by adding return, much as I proposed on this job, but where the return is located doesn’t tend to matter much.

I think about it with Occam’s razor: the simplest answer tends to be the correct one. Why are rooms hot on the second floor in summer? Because not enough cold air is being supplied to them. At what point does a room being too hot stop meaning insufficient supply and start meaning insufficient local return? And how do so-called “hot air returns” know to pull the hot air out of a room and not the cold? When we pull air from a room, what temperature is the air that replaces it? Doesn’t the supply air displace the room air anyway, just by being supplied? (Yes, or the room would explode.)

I design the return side of a system to do two things: support the system airflow effectively, and ideally keep the pressure difference between rooms under 1 pascal. Local room returns are really good at reducing pressurization problems, which is what they should be used for.

If you still have skeptics, test it under hot conditions. Go to the house at 2pm on a really hot day and record the temperatures on all three floors. Then take the cabinet door off, leave the basement door open, and watch what happens. Ideally record airflow before and after. It might take a little while, but in most houses this will be highly effective.

Blower-door-guided air sealing isn’t real

Jesse Smith — Mon, 09 Jul 2012 12:00:00 GMT

I really like blower doors. There are a lot of building diagnostic tools most people have never heard of that I like: duct blasters and flow plates, to name a couple more.

Blower doors are mainly used to measure a home’s air leakage at a fixed pressure, nearly always 50 pascals. That lets us do two important things:

Compare houses. When people ask “How leaky is my house?” they usually mean “How leaky is my house compared with other houses?”
Track changes (hopefully reductions) over time.

We can do a lot of other things with a blower door, but we shouldn’t care nearly as much about zone pressure diagnostics as we do about that big-picture stuff.

I run a blower door several times a week, and I’ve worked on the shells of hundreds of homes. My crew has had tremendous success at making buildings tighter. One of our earliest discoveries was that a blower door is pretty much useless at directing air sealing. That runs counter to conventional wisdom in most energy-geek circles, so here is my case:

We can’t multitask very well (thanks, Daniel Kahneman). Either we’re running a blower door or we’re air sealing the house. We can’t do both at once.
A small area of sealing that doesn’t move the numbers doesn’t mean the rest won’t, and vice versa. Suppose we scamper down to check the blower door after finishing a quarter of the attic and the numbers haven’t changed much. Should we skip the other three quarters? Of course not. Get back in the attic and finish the job!
Walking back and forth between the work and the front door takes time away from the important work of sealing.
Blower door tests run sporadically through the day probably aren’t very accurate. Occupants and workers open and close windows and doors, which makes the numbers useless.

None of this is aimed at high-performance new construction, where homes are tight enough to find minor leaks with a fog machine.

The practical upshot makes air sealing existing homes fairly simple. Start with the accessible stuff up high (attics), then down low (basement and crawlspace rim joists). When other areas become accessible, say exterior walls during a re-siding project or a vaulted ceiling during a roof replacement, make those tighter too. Run a blower door test at the start and finish of each project.

Now get back in the attic!

Everything is sort of bad, but at least one thing is awful

Jesse Smith — Mon, 09 Jul 2012 12:00:00 GMT

I have two mental models for understanding and improving building performance. The first I call “Everything Is Sort of Bad.” It says a building is pretty bad (but not horrible) almost everywhere, in most of its systems: the thermal boundary, the pressure boundary, the ducts, the water-resistive barrier, and so on.

The second I call “Everything Is Sort of Bad, but at Least One Thing Is Awful.”

The first model is helpful. There are lots of places in a building where slight improvements yield large benefits. Attic air sealing is usually a good example. Cracks at the drywall ceiling tend to be fairly uniform, yet plugging all the holes in an attic yields a significant improvement, usually a 20 to 30 percent overall leakage reduction, or at least two thirds of the potential improvement on one of our projects. Here, geeky diagnostic tools are genuinely helpful: blower doors and duct blasters are great for verifying the effect of these measures.

But it’s often hard to tell whether the marginal improvement left hard-to-find problems behind. A 35 percent leakage reduction is great, but it doesn’t tell us whether the cantilevered second-floor joists, reachable only from a small roof over the garage, were sealed.

The second model includes all the lessons of the first, but tells us to be on the lookout for egregious problems. And when you’re looking for isolated but big problems, it’s unfortunate, but you may have to actually listen to the people who live in the building.

People who don’t pay your utility bills, #1

Jesse Smith — Sat, 20 Aug 2011 12:00:00 GMT

The world population is just shy of 7 billion people. Of those, the number who will pay your utility bills is probably even smaller than the number who have been in your kitchen. So we’re starting a series on people who don’t pay your utility bills, and the advice they give. The individuals will remain unnamed, but these stories are based on situations Tay River has actually encountered.

Neighbor who owns a truck says open an upstairs window, because hot air rises

Everyone knows hot air rises, right? Of course! So on the hottest days it makes perfect sense to take a two-pronged approach to beating the heat:

You’re definitely going to need air conditioning. Obviously. But what if that’s not enough?
On really hot days you need a bit more punch. If hot air rises, wouldn’t it make sense to open some upstairs windows, heck, maybe even put a box fan in them? Wouldn’t that force the heat right out the window? Obviously.

This is a really bad strategy for “saving” energy. Air pushed out of a building is always replaced by air pulled into it, so you end up drawing a lot of hot air inside. On top of that, stack effect is typically the biggest driver of air leakage. In summer, the pressure on an air-conditioned building pushes air downward through it, which feels a lot like heat rising, but isn’t.

So don’t listen to the neighbor who gives this advice, even if he owns a truck. Remember, he doesn’t pay your utility bills!