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Jguerriere 01-23-2011 08:11 AM

Negative pressure - what are my options
My house was built in 1971. HVAC unit is about 5 years old and I just replaced all the duct work after sealing the crawl space. Duct work all runs under the house in the duct work.

The house is very loose. When the hvac unit turns on, doors close. If the front door is cracked it will open (it opens in).

Cold air gets pulled in from gaps around sun room and other openings.

Local hvac guy did a test with a device and said the house was negative -2. I have no idea what the -2 means, but what it confirms to me is more air is leaving than is coming back in.

I was told the way to correct was to seal up the house but this seems to defy logic. Doesn't the problem have to be something like a leaking duct on the warm side of the unit?

If more air is leaving the house than is returning my uninformed minds tells me there must be a leak. I was assured this is not the case by my hvac guy. We have tested thigs with all bedroom doors open and vents open. Same. Cold air rushes in from any crack.

Before going nuts with spray foam or a make up air system I want to confirm I have eliminated the root cause.

Any ideas? Where should I start.

Thanks in advance.


How do I trouble shoot?

yuri 01-23-2011 08:16 AM

Where do you live and what size and style of house is it. 1 or 2 story. Where you live tells us something about the type of construction used in your area as it varies greatly across the continent.

Jguerriere 01-23-2011 08:24 AM

I live in Charleston SC. House is 2700 sf 2 story with sep units for upstairs and downstairs.

Orig single pane windows. Very little insulation in general in walls. Walls and floors get cold in winter and warm in summer.

Entire house sits on a 3 foot crawl space.

thanks again for the help.

Jguerriere 01-23-2011 08:27 AM

Also, when unit turns on you can hear a rushing sound of air near the return. Paper or plastic bags will get suck onto the return grill which at on the wall at knee level. Can post a video if it helps.

yuri 01-23-2011 09:02 AM

Hang in here for a day and you should get some useful responses.:yes:

tcleve4911 01-23-2011 09:06 AM

The furnace needs air and you have depleted it's supply by sealing up the house.

One idea would be to introduce an outside air supply to the furnace.

This could be as simple as a dryer vent or insulated flex duct with one end vented to the exterior and the other end located near near the furnace air intake.

I'm no HVAC guy. Just heard of this before.:)

Marty S. 01-23-2011 09:06 AM

Get up in the attic and seal every ceiling penetration for wires and plumbing. Warm air is rising through those holes and the cracks in the house are where it's coming back in. Google "stack effect" for more information. Could be more to the problem then that but it's a very good place to start.

Jguerriere 01-23-2011 09:32 AM

I have been holding off on sealing up the house. I have a foam guy on standby to seal up the cracks and the underside of the floor, but before I do I want to make sure I don't have an issue with the return, ducts, or something else. I just can't get over the fact that doors are closing or opening. My understanding is the HVAC system should be like a pump in a pool where the pump is taking water out and returning it to the pool.

The water level should never change UNLESS there is a leak on the return side of the pump resulting in water getting on the ground and not into the pool.

My return takes air out of the house. It heats and, then then returns the very same air back into the house via the vents.

The house should not go negative unless some of the air is not making it back in. That is my understanding of the physics.

I read a great deal on the stack effect, which is why I had the crawl and attic sealed up (foamed just the attic).

Even if the house was 100% sealed, should it go negative? Air quality may suffer, but my understanding is the air going into the Return should Equal the air coming back through the vents, hence a balanced housed that is not negative or positive. I know I am missing something. Just can't find it....

tcleve4911 01-23-2011 09:44 AM

Where's the cat?:jester:

yuri 01-23-2011 10:01 AM

It can go negative and this may be a long winded explanation but I will try. I have some leaky poorly built and insulated 2 story military houses about 40-50 yrs old. Heat flows from hot to cold from inside your house to outside at a certain rate/speed CFM thru the cracks in the walls and siding. Most homes do not have sealed vapor barrier in the walls or well sealed so air is continuosly passing thru the walls and not just the floor and attic and around windows. In a 2 story house thise creates large convection currents and changes in pressure. Basically I have problems with chimneys downdrafting in these homes that I don't have in better insulated and sealed houses. Bungalows or ranchers as you call them (I believe) don't seem to have these problems as they are 1 story. Stack effect is not a numbers equation where minus = plus. The air handler adds pressure from the energy created by the horsepower of the fan. My Bud Beenthere knows a lot about this and duct sizing and may chime in later. Doubt that most homes have professionally sized and installed ductwork so yours may be under or oversized and also has leaks. Sealing all the joints with mastic helps. All you can do is try sealing the house better and see what happens. There is no scientific exact way to know what happens in every house as they are all built differently.

Superrick 01-24-2011 09:13 AM

Describe your system to us! Do you have returns in all rooms or just one in the hallway. If only one is there at least a 3/4" gap under the doors. It sounds like an unbalanced system. Is there a partial blockage of any of the return ductwork (furnature, boxes, ect). If not you may need to add duct work and or slow down the blower (if it is a mullti speed). Has this only been noticeable since the unit was replaced.

Jackofall1 01-24-2011 10:07 AM

You have the system understanding exactly correct.

Where is the system located? In the crawl space?

If it is in the crawl space, my first thought would be that you have a supply duct open to the space, which is fine, as long as you balance the return air to match what you are putting into the crawl space.

Jguerriere 01-24-2011 07:17 PM

The unit is outside not in the crawl. There is one return in the hallway and all doors have about an inch of space. I fact I have tested with the doors opened and closed and it is still the same.

I also checked all the vents and they are all flowing at about the same rate. Not obstructions as far as I can tell.

One thing I did to test was to seal off the upstairs (sep unit for upstairs) at the top of the stairway with a 5mil painters plastic drop cloth and tape. I pretty much created a huge diaphragm.

I turned on the downstairs unit and it sucked the drop cloth into a big curved shaped until the tape pulled off. So I retaped it, opened the front door all the way, turned the unit on and the drop cloth did not move. Then as soon as I closed the front door (with the hvac running) it broke the seal on the drop cloth.

So, must be an imbalance, right?

If the leak is not in the duct in the crawl, where else can it be? Maybe where the unit meets the side of the house?

Jackofall1 01-24-2011 07:19 PM

That would be where I would be looking, crawl and outside, definately on the supply side.

beenthere 01-24-2011 08:11 PM

The return being loud is an indication that either the return grille is too small, or the return duct is too small, or both.

As for duct leaks. Take an incense candle/stick. Turn on your blower, go into your crawlspace, and light the stick, and take it along all seams and joints of your supply. If you see it blowing the smoke, you found a leak. it you see it start to glow brighter, you found a leak. Also check all of your registers, to make sure the boots are sealed to the floor. If they aren't, then air can be blown into the crawl putting your house into a negative pressure.

A quick simple hard easy way to find supply leaks. Is to remove the return duct from the unit. Seal the end of the return duct. Turn on the blower, and let it pressurize your house with outside air. Your supply duct and return duct will both be at a high pressure. And it makes leak finding easier.

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