Sunday, April 7, 2013

Exhaust vents pulling air down your chimney?

So, I knew going in to the purchase of my home to be on the lookout for Pulte cutting corners when it came to energy efficiency. The builder doesn't have any incentive to actually build an energy efficient unit, only to say they're building an energy efficient home so buyers will buy. I never suspected that Pulte would install a B-vent or natural venting fireplace in a home built in 2012.

Some background. Fireplace inserts come in 3 flavors. Direct Vent, Natural Venting or B-vent, and Ventless.

Direct vent fireplaces are essentially sealed to room air. Combustion air is drawn in through the outside of a co-axial venting duct and the exhaust goes out through the center of the ducting. Here is a good photo.

Direct Vent
Room air circulated around insert, combustion air drawn in from outside.

Natural vent fireplaces use room air for combustion, and exhaust the room air up the chimney. These units, at least the one installed in my home, don't have dampers to close off the chimney. This has resulted in backdrafting in my home.
Natural or B-vent
Combustion air from room
Ventless fireplaces are just what they sound like. They don't have a chimney or vent. Ventless fireplaces are designed to have very complete combustion resulting in very little byproducts and very high effeciency.

So, what I'm going to explain next is a multi-faceted problem involving the pressurization difference between internal and external air pressure. If the home has a positive pressure, when you crack open a window air will flow out. If the house has negative pressure, air will flow in.

What affects whether a home has positive or negative pressure you might ask? Stack effect is a significant contributor. Hot air rises, so a tall building can act as a chimney, the warm air rises and leaks out the top of the building. Venting can affect the pressure in the building. Bathroom vent fans remove about 80 cubic feet per minute (cfm) when they're turned on, dryer vents exhaust air (100-200cfm), and kitchen vent hoods exhaust air. For homes with upgraded kitchens, the range vent hood exhausts 1,200 cfm at the high setting.

On the plus side, Pulte seems to have sealed our home fairly well. On the downside, they installed a natural venting fireplace. This means that with bathroom fans, the dryer venting or especially if the range vent hood on, our chimney back drafts into the house. We have to open a window, or the front door to get the fireplace to vent.  Once the chimney is hot, it seems to do ok, but it's really irritating to have to open the front door to turn on the fireplace.

The HVAC units in our homes are designed to bring in outside air via a 6" duct connected to the return side of the furnace. The furnace essentially sucks outside air into the home when it's turned on. I've seen homes with a timer to adjust how often the outside air damper opens, but in ours if the furnace fan is on, the outside air damper is open.

This back drafting is why modern high efficiency furnaces and water heaters draw combustion air in from outside and then exhaust to the outside. This prevents negative pressure in the home from affecting combustion or pulling exhaust back into the home.

Why Pulte chose an incredibly inefficient fireplace is beyond me (perhaps they chose direct vent and got screwed by their sub contractor). Looking at the HVAC diagrams, I noticed the fireplace ducting was supposed to be of the direct vent type, but that clearly isn't what they installed. I suspect the fireplace installers pulled a fast one on Pulte.

How Pulte got away with building a home that doesn't have make up air for a 1,200 cfm kitchen exhaust hood is beyond me. They're in violation of code with this one.

Air infiltration will either happen by accident or by design. Pulte didn't design enough make up air for our home, so it is coming through the path of least resistance.  In our case, that's the fireplace chimney.

What the building code says (Link):
2009 IRC
M1503.4 Makeup air required. Exhaust hood systems capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system."


So a couple things to note.

  1. Pulte's design doesn't seem to meet 2009 IRC code.
  2. Pulte's choice of fireplace model exacerbates the problem and is an inefficient choice
A couple issues solving the problem.
  1. Sticking a big hole in the kitchen wall to provide make up air is going to look horrible.
  2. Bringing in 1,200cfm of unconditioned air will cause problems (think 30deg or 90deg air).

A couple other folks have good information about the make up air issue growing with tighter modern construction the following link has good info and some interesting insights in the comments.
Make up air for range hoods

In my opinion, the only way to solve this problem is to have the furnace fan kick on with the range hood and draw make up air through the furnace return so it can be mixed, conditioned, and distributed throughout the house. I don't want icicles in my kitchen in the winter or condensing humid air in the summer.

The fireplace issue can only be solved by conversion to direct vent. Why would I want cold air coming down the chimney all winter?

Here's hoping Pulte figures out some solutions!

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