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Looking to start air sealing house - Concerns with combustion air / backdrafting

Looking to start air sealing house - Concerns with combustion air / backdrafting


  #1  
Old 11-04-13, 11:30 AM
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Looking to start air sealing house - Concerns with combustion air / backdrafting

Hey guys,

We bought our first house about 6 months ago. It is a ranch house on a full basement. It was built in 1968, and based on it's age, it isn't the most sealed and energy efficient house in the world. I am getting ready in the next few weeks to start air sealing the house. I'm hoping to get the obvious areas first, but eventually want to try to get it fairly tight.

We have a gas furnace, gas water heater, and a gas dryer. From what I have read, I have some concerns that if I air seal too much, I might be introducing a safety risk with those devices. There isn't any combustion air venting right now.

What do I need to do to make sure I have a safe situation with my furnace, water heater, and possibly even dryer? The room that contains the furnace & air conditioner is an unfinished space, and we plan to keep it that way. We want to finish the rest of the basement. The dryer will end up being in a finished area.

I've read about combustion air ducts that come in from outside, and just dump air into the open space in the room containing the furnace & water heater. I also know that some instances will have combustion air ducting that feeds into the cold air return for the furnace. Are there other methods, what method would work best for my application?

When we do get around to finishing the basement, we wan't to be able to have a warm and comfortable living area down there.
 
  #2  
Old 11-04-13, 12:29 PM
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Hi Mark,
It is almost impossible to determine how tight the house is and whether it is affecting the supply of combustion air without some test equipment. As an energy auditor we use a manometer to conduct a worst case depressurization test. Illinois is loaded with energy auditors who can do that for you.

The reality is most existing homes are always going to be a bit leaky as many of the leaks would have had to have been sealed during construction.

The ducts for the forced air system will need to be sealed as well. Foil tape or duct mastic on all seams. Imbalances in air flow can be a cause of backdrafting as well.

Let's see what the hvac pros say.
Bud
 
  #3  
Old 11-04-13, 01:01 PM
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We had an energy audit done a few months ago. The auditor specifically mentioned air sealing and rim joist sealing/Insulation as two ways to make big improvements in our house. I'll have to go back and see if he did any air flow measurements.
 
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Old 11-04-13, 04:20 PM
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If he set up a blower door, big fan, and depressurized the house, get the CFM-50 number that test generated. That tells us/you how leaky the house is to start.

He should have also done a CAZ test (combustion air zone), although before the sealing is done it tells you little.

If no testing, then it wasn't an energy audit, a lot of that being done.

Bud
 
  #5  
Old 11-05-13, 06:12 AM
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air infiltration

Hi, in my situation I added air for combustion to my boiler area using a 4" duct from outside hanging down into in a 5 gal bucket near my boiler. Pretty low tech, but it did wonders to eliminate problems with fumes and smells. My house is 20 years old, and I didn't think it was especially "tight", but I guess with exhaust fans and clothes dryer running, there wasn't enough make up air. I got this idea from NJ Trooper on the boiler part of this site- works great for me- the bucket acts as a trap to prevent lots of drafts.

Steve
 
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Old 11-05-13, 08:03 AM
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I've been reading up on the same setup. Combustion air ducting that just runs into the room with the furnace & water heater. The concern I have is that the room with the furnace and water heater is also where the plumbing and gas lines enter the house. I don't know how cold it would get in there, and if pipes freezing would be a concern in the winter time.

Are there other options to directly vent fresh air into the units? It doesn't appear that my furnace is a sealed combustion unit. I don't have the details at the moment, but there are a few grill slats on the top of it. However, there aren't a ton. There is also a 4" round hole on the side that is currently just closed off with some yellow insulation. I've always wondered if that was an air intake area. I'll need to see if I can find some more information about the furnace. It's only about 2 years old. The previous owners replaced their old furnace in 2011 with this new one. I believe the new one is a York.

The room that contains the furnace, water heater, and dryer (for right now) is roughly 18'x24'. It is about 1/4 of the footprint of our house.

As my house stands right now, everything seems to be operating fine. I'm going to start air sealing in the attic and upper levels first. My concern is once I start air sealing the rim joists and around the basement, that I will start blocking the fresh air for the gas appliances.

I looked at the paperwork from our energy assessment. There were some CFM50 numbers on there, but my wife says they didn't seal up the door and hook a big blower up, so I'm wondering if those were just sample numbers.
 
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Old 11-05-13, 01:27 PM
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Ignore what I said about my furnace. I looked at some product manuals online today. It doesn't look like those 4" openings are for intake. They appear to be alternate vent options.
 
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Old 11-05-13, 02:01 PM
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I have a similar "bucket" arrangement in my 1980 townhouse (actually a quad occupying on corner of a 2 story building). The bucket is in my small utility room (furnace, water heater, refrigerator, washer and old electric dryer), but is in the interior portion. The siphon system works amazingly well with no detectible drafts coming in even at -20F. I run my fan 24/7 during the heating season for comfort and uniformity. The only problem is in the early season when the house is closed up and the furnace does not run for heating much and the air is not real fresh. The utility room is not sealed in any way with a large (1 gap at the bottom).

It (the bucket) works to create a gap and break the thermal siphon effects. When the heater or furnace run, the exhaust goes up and out thermally and fresh air is drawn in. My gas bills have never been over $80.00/month. This is a very common system here for homes that do not have ducted combustion air that suffer from being stuffy in the winter. I considered going to a 90+ heating system, but venting/exhaust would be a big problem for little cost savings, so I am happy with my 80% units and variable speed fan.

I do wonder if my cold air is being drawn up somewhat when the house is on AC for weeks or several months in the summer. Not too likely because of the temperature difference between the interior and exterior on a hot summer season.

I did what air sealing was possible by foaming around each rim joist cavity and immediately putting in a slightly undersized piece of 2" XPS and let the foam do the sealing and adhering. I do have some old slider windows (3x5+) that seem fairly tight, but probably do leak and they are next on my list. I did have 2 oversize 6' sliding doors installed (facing north) by Renewal by Andersen that are like trying to open a freezer door. The next is to have the new old, heavy windows replaced by them (all on upper level).

The siphon does work for me in MN.

Dick
 
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Old 11-05-13, 02:21 PM
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I'm sure you have much colder temperatures in MN then I do in Illinois. Does the room with your siphon "bucket" stay comfortably warm? The room is not insulated at all from the rest of the basement or even the room above (our master bedroom). When we finish the basement, I would probably insulate that room to keep the cold out of the rest of the house. I'm just not sure how cold it will get.

I've been reading other threads about people having freezing temperatures in utility rooms because of combustion air vents.
 
  #10  
Old 11-05-13, 02:27 PM
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I have the same setup as bud and the bucket does a nice job of isolating the cold air; my utility room feels pretty much like the rest of the basement.
 
 

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