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Make up air in reasidence

tomg66's picture

I have a 1983 contemporary home which backdrafts air down the fireplace chimmeys and allows air in any other crack or gap. I have upgrades to a closed combustion furnace and a power vent water heater with a fresh air supply in the furnace room. I would like to add outside air into the HVAC air return duct. I would like it to open when we are exhausting air with stove and bathroom exhaust fans and when ever pressure drops inside versus outside due to stack effect. I am considering a powered damper intake operating on a switch based on differential pressure inside the house versus outside. I currently run the HVAC fan 100% of the time to limit stratification from our many cathedral ceilings.Has anyone ever tried this? Any thoughts on pros and cons?

Not a bad idea, but before (post #207079, reply #1 of 10)

Not a bad idea, but before you make any changes, you ought to do a little simple diagnostic work to determine the real source of your problem.

I strongly suspect that the real cause of depressurization is  unbalanced airflow due to improper duct design . This is very common.

If certain rooms or spaces (especially in houses with basements) lack sufficient return air, those spaces become pressurized by the supply registers. In turn, other spaces become de-pressurized by the same amount. All this can be measured by using a differential manometer, but it's not really needed to determine where the problems are.

Before I go into detail that might not be applicable to your particular house, a little more information would help:  Does your house have a basement? Is the furnace in its own room?  How are the return air paths or ducts arranged?

Duct design (post #207079, reply #3 of 10)

The house has a finished basement with the furnace in a separate room. We have air return in every room of the house including the basement. I see the negative pressure in the basement and 1st floor fireplaces. The house has many recessed lights in catherdral ceilings and ceiling below the unconditioned attic and I suspent the leak a lot of air.

Keep in mind that the (post #207079, reply #5 of 10)

Keep in mind that the negative pressure only occurs when there is something expelling air from the house.  It could be simply a leaky upstairs ceiling, leaking into the attic due to convection forces, or it could be, eg, air ducts in the attic (or maybe an attached garage) that are leaking.

You probably should try to figure out what's going on there.  Otherwise, pulling makeup air into the return air duct will just increase heat losses.  The sort of makeup air setup you're contemplating is really only intended to produce a slight positive pressure in a well-sealed house.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

To test for depressurized (post #207079, reply #8 of 10)

To test for depressurized zones, or spaces, start by closing all doors (even interior ones) and windows.

Then set the thermostat high enough to cause the furnace to go to high-fire and the blower to high speed.

Next, if there is door that separates the basement from the main floor, position yourself on the basement side of that door (closed) and feel along the bottom to see if air is rushing thru--if so, it shows that the duct system is unbalanced, and is depressurizing the basement. This can easily cause the fireplace to backdraft along with "sucking" cold air into the basement from any air leaks, including excessive draw on your combustion air duct in the furnace room.

If a door separates the main floor from the 2nd floor, the same test at that door will show if there is depressurization between those spaces.

The most common cause of this sort of imbalance is under-sized return air pathways. Fortunately, it's fairly easy to correct. The fix may be simply to install transfer grille(s) to allow airflow from the main floor to the basement, or betweeen any rooms that are being pressurized or depressurized.

Your suspicion about leaks at the recessed lights is probably well-founded, and in a two-story house, the stack effect could cause substantial leakage there, and the leakage would be increased if the return air path from the upper floor is restricted compared to the lower floors because that would likely result in the upper floor being pressuized relative to lower floors. Adding that pressure to the stack effect will result in a lot of leakage.

As DanH said, the addition of a damper to allow outside air to be introduced will only increase you energy cost if you have other problems with imbalances.

Skuttle (post #207079, reply #2 of 10)

There is a perfect, simple product for what you need. It is called a Skuttle". This is an adjustable, barometric ventilation control damper. It works by opening as much as required based on differential pressure. Typically, when the house is more negative in relation to outside, it will open more. Connected between an outside air intake and the return side of your forced air system, it will remain shut when the fan is off (or with a variable speed furnace when it is in the low or ventilation speed). When the fan cycles, it will add a slight positive pressure.

In your case (if you have a multi-speed funace with the fan always "on") it will simply open more or less with the pressure variations in your house. The setup invloves a little trial and error to get it set the way you want it. I have one on my variable speed, 2-stage furnace. When the furnace fires on low, the house is nearly neutral. When it fires on high, I will get a slight positive pressure. I operate this system with constant fan on a very low speed that ramps up to higher speeds when firing.

You could use a power operated damper and associated controls, but in my opinion, that is unnecessarily complicated, troublesome and expensive.

furnace (post #207079, reply #4 of 10)

I have a variable speed 2 stage furnace and AC. The fan rins on low and ramps up based on tiem required to meet the set point. It sounds like it is configured like yours.

One thing you can do is turn (post #207079, reply #6 of 10)

One thing you can do is turn off the fan and see if you're still getting the downdraft.  If it stops when the fan stops then you probably have leakage out of the output side of the furnace, into "unconditioned" space, or else a problem with return air "balance".

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

If I'm hearing what you're (post #207079, reply #7 of 10)

If I'm hearing what you're saying, your complaint is simply that outside air is coming down the chimney and out an open fireplace?

There isn't anything wrong with allowing outside air into the house in those situations, although for it to work as intended I'll bet you will need a power assist to draw in enough air when exhaust vents are turned on - simple to wire, much harder to actually run the wires in existing walls, but not a nightmare if the person is experienced in doing such - most electricans are so-so and make way too many holes when they don't have to.  

I've only lived in fairly windy locations so any kind of simple flap setup or differential pressure switch would simply be going nuts depending on the speed and wind direction that day.   Nothing is more annoying than listening to a simple flap fluttering in the wind, or an automated system cycling on and off.



If I could edit my location it would say I'm now in Reno :-)

First things I would do (post #207079, reply #9 of 10)

are (1) put tight-fitting glass doors on the fireplaces, or just plug the chimneys with chimney balloons and stop using them, (b) do whatever you can to balance your ductwork so that it's not pressurizing or depressurizing the house, (c) seal and insulate the ductwork where possible, and (d) get a blower door test and do some air-sealing of the house. Adding a fresh air supply to your heating system is not necessary if you already have excess air leakage, which it sounds like you do.