Tuesday, January 25, 2011

Making Decisions Under Pressure

In our last major post we discussed how improperly sized and installed HVAC equipment can result in a quicker failure than normal.  We left you hanging with the question of how to equalize the pressure in your rooms so that they don't turn into a big wood and gypsum balloon.  If you recall:

  • More air goes into a master suite than out;
  • This is a result of no escape paths;
  • Air backs up into the furnace;
  • The furnace gives up and dies during a cold snap.
So how do you equalize this pressure?  The most common way is with a 1" door undercut.  Most homeowners find these unsightly, on top of which they are sized before carpet is installed.  Once carpet goes in place all of the 1" is used up and no other outlets are given.  Some might say to let the pressure leak to the outside.  To this we say "bad builder, no burgers for you".  If your pressure is leaking to the outside, where is the furnace's makeup air coming from?  Outside!  Let's try a different approach.

In order to equalize the air pressure, we need the same amount of air going out of a room as is coming in.  The idea is that warm enters a room and forces the cooler air out which goes to the furnace, gets warmed, and the cycle goes again.  If air is to leave a room, we need a space for it to happen.  There are four common ways to make this happen.

1. Door undercuts have been discussed.  These are generally not aesthetically pleasing.

2. High/low relief vents.  This entails cutting a hole high up in the wall of the room between studs.  Between the same studs a hole is cut near the floor in a hallway or other common area.  This allows air pressure to move freely.  Grills over both cuts give a more expected appearance.  This approach may not give complete privacy as sound can travel a little between both spaces.

3. Jumper ducts in the ceiling.  These are leftover pieces of flex duct used to create a connection between bedroom and hallway in the ceiling.  It is similar to the high/low vent but less obtrusive and slightly more private.

4. Dedicated return.  This is a connection in the ceiling of a room right into the main return for the furnace.  This allows for the most direct air balancing and the most privacy.  Depending on the distance to the furnace it may be noisy.  Further is better.

Bear in mind that these measures are only necessary in larger rooms that can be closed off from the main living area.  Smaller bedrooms and bathrooms don't gain enough positive pressure.  Great rooms and dining rooms are generally connected to the majority of the living area.  A media room or bonus room that is separated by a door and has more than one supply may benefit.

If you want to give it a down and dirty test, close all doors in the house and fire up the blower.  Then slowly open each door.  If it presents any resistance then some sort of relief is needed.  If you want to be even more savvy, get hold of a Duct Blaster and someone who can run one.  This will give you an accurate test of where your problems will be.  Then whip out your slide rule and estimate a size for pressure relief.

If your house is especially tight, you will definitely want to install a heat recovery ventilator.  This system brings fresh outside air into the furnace while tempering it with the heat from outgoing stale air.  In some cases this can be your blower with an in-line heat source providing comfort.

Istockhouseplans recommends doing away with ducted systems altogether.  For better comfort consider a ductless heat pump, radiant floor heat, or electric soft heat.  Or do away with heat sources completely and join the PassivHaus movement.

Tuesday, January 11, 2011

Lengthy Topic

Today's little bonus post is brought to you by the American Wood Council and their span calculator at

Don't get to excited, this is not a full structural calculator.  It is, however, a great alternative to checking span tables and they have an iPhone version, handy for field specification.  Mind the deflection.

Happy designing,


Tuesday, January 4, 2011

Does your equipment blow or suck?

Happy twenty-'leven from Istockhouseplans to all of our friends out there in cyberland.  We hope your holiday was interesting and thoughtful.  A quick note about our year-end report: despite the down economy, our books show that we made twice as much in 2010 as we did in 2009.  Thanks to everyone who made that possible, and here's a toast to all of you that your books did and will do the same for 2011.

Now, to the topic at hand.  A strange thing happened over the holidays.  The building that we work from is heated with a forced air gas system, commonly known as a gas furnace.  It was recently insulated and air-sealed.  Somebody kicked a heat register shut and the tighter house acted differently.  In this case something happened.  Before, nothing would have happened.  This lead us to a little investigation.

First off, a house is a closed system but not entirely.  It is impacted by the outside environment to some degree.  The more a house is insulated and air-sealed, the more you reduce the impact from and to the outside environment.  This means you get more reaction within the house when something changes.  What changed in this case was air pressure.  By shutting a heat register, suddenly more blown air is directed to other registers.  In a tight house, this air is supposed to balance by going into the return register, through the furnace where it is warmed, and back through the registers.

Now imagine that air that is blown into a bedroom can't return to the furnace.  That is, it is blown in but there is no way for it to get out.  This might be the case where there is carpet and the door is shut leaving a paper-thin space between the bottom of the door and the floor.  A little air will get out but now the system will be imbalanced.  Kind of like filling a balloon.  More air goes in, but none comes out.

But the air has to go somewhere.  With a balloon, it will stretch the environment.  Unfortunately drywall is not as pliable as rubber.  So the air goes where it can, that is through tiny cracks.  At some point the room reaches critical mass for pressure.  This is when things start to happen.

Much like a pipe that has been corked off, the air stops flowing through the duct.  It might back up and force more air through another register up or down the line.  If it is a dedicated duct run with no other outlets, it will back up into the furnace.  So what you have is two pressurized environments doing battle.  Try this: grab a straw and a loved one.  Each of you put one end of the straw in your mouths.  Now both blow as hard as you can.  Cheeks will turn red, eyes will bulge, and finally someone will get a mouthful of the other one's air.  This is called system failure.

In the case of your furnace you could simply blow a duct.  However with today's tighter duct runs, the weak spot becomes the furnace, most notably the blower fan.  So the fan has back pressure causing two forces to exert their will upon it.  At some point the fan gives in and stops blowing.  The furnace still warms but the forced-air part has been taken out of the equation.  This tends to happen on the coldest day of the year on a Saturday night.

Granted, one single room is probably not going to have major adverse effects on the HVAC system.  But imagine an 1800sf three bedroom house.  The furnace is likely to be oversized (60kBtu) and the master bedroom might have two heat registers due to it's size (~200sf) as well as a heat register in the master bath and maybe one in the master closet.  The door is closed and there is no appropriate 1" undercut.  In fact, you would need about a 4" undercut.  Air gets backed up in the room.  Your head hurts.  Then the furnace quits.  You thought it was just the end of the cycle but it doesn't turn back on and the temperature continues to drop.  At first you assume a power outage but the VCR clock is still blinking '12:00'.  The next day it's even colder in the house so you call the HVAC tech.  He says he'll be out there sometime between 9a and Friday.

The bugger is that your house was built last year.

Before buying a new house, ask if the system was properly sized.  There are a slew of manuals and related software out there for professionals to use.  As a builder, always ask your HVAC tech how they arrived at the necessity to install an 80kBtu furnace.  Also ask how they are going to mitigate zonal pressure relief.  If they stare at you or stutter or write it off, hire someone else.

In the meantime, Istockhouseplans recommends not using forced air systems.  We push for hydronic floor heat, so called 'soft' heat (electric baseboard) and mini-split heat pumps.  All of these systems take less space and use less or no air.  They can also be sized more appropriately than a furnace can.

Don't forget to check your filters and consider cleaning your ductwork.  Oh, and if your head hurts in your bedroom, simply try opening the door.  Stay tuned for next time when we'll look at some more zonal pressure relief options.