Stack Effect vs. Warm Basement

MarkReynolds1

Hi all;
First, thanks for all the great info here, it is greatly appreciated. But, I searched the Wall and can't find an answer to this question.
I have a 1900, 4800 square foot, three story home in Detroit, with a one pipe gas steam system and a new-ish Burnham boiler.
Thanks to help on the site and several of Dan's books, I have added a few big Gorton tuna can main vents, replaced all radiator vents, and added 1 to 2 inch fiberglass insulation to all of the bare basement steam mains. I had a blower door energy efficiency test done a few years ago, and went after all of the low hanging fruit the test reported; added attic and wall insulation, storm windows, fixed leaks, etc. My gas-plus-electric bills dropped from $900+ in winter months to about $500+ now. I would like to get it lower.
The efficiency test contractor recommended I weatherstrip seal the basement door, to counteract the stack effect. OK, but the basement is almost uncomfortably warm (my workshop is there). Temps are in the mid 70's, while the thermostat is set at 66, which is pretty consistent through the house.
So, what is better - open the basement door and let the excess warm air heat the first floor, or keep it closed? Am I missing something here? Suggestions welcome!
Mark

nicholas bonham-carter

You need some outside air coming in for combustion, especially if there is some sort of exhaust fan/clothes drier in the house.
Have you got the equivalent of one Gorton #2 for each 20 feet of 2 inch pipe? The faster the steam fills the mains, rises up, heats the rads, and satisfies the thermostat, the less time there is to lose heat in the basement.
What pressure is the system attaining when steaming?--NBC

MarkReynolds1

I have a Model 216 Skuttle piped to a basement window that exits next to the boiler. There is a Gorton #2 plus Hoffman 4A at the end of the three mains, I added the Gortons. Two mains are 30' or so, one is 40'. I have the standard pressure gauge, so it reads zero when steaming, but I have the pressuretrol set to the lowest indicated settings.

vaporvac

I would weather seal and insulate the rim joists before anything else. I can't imagine they didn't suggest this in lieu of sealing the basement door as its the major contributor to the "stack effect".

SWEI

MarkReynolds1 said:

I have a Model 216 Skuttle piped to a basement window that exits next to the boiler.

Is there a fan along with the 216? 28 square inches of area plus the duct resistance is not a lot of air.

Bob Harper

Sealing lower parts of the house does not minimize the Stack Effect--it exacerbates it. The Stack Effect is simply making the house into a chimney. Warm air rises, which draws in cold air at the base. The more warm air exhausting at the top, the more cold air drawn in below. if you want to reduce the cold air infiltration, you don't close the door at the base, you reduce the exfiltration at the top and keep the heated air inside the thermal envelope of the house. You need fresh air infiltration or makeup air to feed any atmospherically vented combustion appliances. The top of your house should be sealed as tight as Tupperware then work your way down. Above all else, you need combustion analysis done with a Worst Case Depressurization Test to ensure your combustion appliances won't backdraft.
HTH

ChrisJ

Top of house tight as tupperware.

Reminds me of all the pictures I've seen of severe mold problems.

Canucker

Bob Harper said:

Sealing lower parts of the house does not minimize the Stack Effect--it exacerbates it.
HTH

Do you mean from a combustion point of view? Sealing up holes in your building envelope will make your stack effect worse doesn't intuitively make sense to me. I would think you're making your house shorter, thereby lowering the stack effect, regardless of which end you start on, no? Whether it's more effective to start at the bottom or top of your would depend on the target you're aiming for, yes? It would seem there is a gap in my knowledge on this, so I would appreciate any information you can send my way.

ChrisJ

Canucker said:

Bob Harper said:

Sealing lower parts of the house does not minimize the Stack Effect--it exacerbates it.
HTH

Do you mean from a combustion point of view? Sealing up holes in your building envelope will make your stack effect worse doesn't intuitively make sense to me. I would think you're making your house shorter, thereby lowering the stack effect, regardless of which end you start on, no? Whether it's more effective to start at the bottom or top of your would depend on the target you're aiming for, yes? It would seem there is a gap in my knowledge on this, so I would appreciate any information you can send my way.

I would think in order for the stack effect to work, that you need a stack that is open on top and bottom or somewhere in the middle.

Although, if you have an igloo it'll hold some heat but if you flip it upside down so the bottom is sealed but top is open, it won't.

Gordy

http://www.finehomebuilding.com/how-to/articles/how-it-works-the-stack-effect.aspx

Bob is correct. In order for stack effect to work air needs to escape to the highest point, and enter in the lower points of a structure. In summer the stack effect is the opposite of winter.

Stack effect is caused by pressure differences from bottom to top of structure. In winter it is more prominent because of the greater temperature differences from inside to outside......delta t.

If air can't escape then it is harder for air to enter at the lowest points in the home.

Take a straw put your thumb over the end, and stick it in a glass of water open end down. No water, or very little enters the bottom of the straw. The deeper you put the straw in the water the higher the water level in the straw becomes. .....pressure. Now release your thumb off the straw, and what happens? The straw fills up.

Now of course we want to seal the lower infiltration because of wind etc.

As for the energy auditors recommendation of keeping the basement sealed off. The intent probably to decrease the size of the pressure zone. Also sealing off a basement door opening may be cheaper than sealing, and insulating, and finding points of infiltration in a basement. Also the need for fuel burning appliances to have that make up air.

Canucker

Maybe I'm confused because to me, to use an actual chimney as an example, wouldn't different chimney heights affect how large the draw of air through them is if all other points were equal? Perhaps a chimney is a bad comparison, that's partly why I'm asking. In my mind an old house may have a lot of leak points, but if I sealed up the basement first, effectively making the houses' leak points closer together(from ground floor to upper level instead of basement to upper level), wouldn't that lessen the stack effect?

Gordy

Canucker said:

Maybe I'm confused because to me, to use an actual chimney as an example, wouldn't different chimney heights affect how large the draw of air through them is if all other points were equal? Perhaps a chimney is a bad comparison, that's partly why I'm asking. In my mind an old house may have a lot of leak points, but if I sealed up the basement first, effectively making the houses' leak points closer together(from ground floor to upper level instead of basement to upper level), wouldn't that lessen the stack effect?

If you put a lid on top of the chimney what happens to the fire, and smoke?

Yes the higher the chimney the greater the draw because the lower the air pressure at the top. Mother Nature likes to equalize, and high pressure goes to low.

In a way it's 6 of one 1/2 dozen of the other. However starting at the top will give faster results with out working to the bottom. If anything is missed at the bottom putting a lid on it all does not make it as severe.
Really in the summer everything is reversed as far as stack effect the leak points come in the top, and out the bottom. Depending on the delta of inside to outside. That delta is never as great as a winter time delta. Remember the greater that delt the more severe stack effect becomes.

Canucker

It was probably the statement that it would exacerbate the stack effect that threw me off. I couldn't see how sealing the lower portion of a structure would make the stack effect worse. I am just looking at it from an air exchange point of view. I didn't want to get into the ROI as there are too many variables and goals, as far as energy conservation is concerned, to say one starting point is better than the other.

Gordy

If you seal at the bottom. It could exasperate entry points until the top is sealed. Think the chimney flue is open the fire is roaring wide open. Now close the doors which are not air tight. The air entering will have a higher velocity through smaller entry points cracks, and gaps in the doors etc. verses a wide open opening.trying to achieve the same volume of air from the delta of a roaring fire. Until the fire burns down then the velocity decreases.

Your house is always usually the same temp so the velocity control is the outside temp. The lower the outside temp the higher the delta to the inside temp.

As far as ROI of only a set amount of dollars are available to only be able to do a certain amount of weatherization then start at the top. This will decrease infiltration with greater success until the rest can be finished the goal being to do everything.

ChrisJ

Ok.

Seal the bottom of the straw and try to get water to enter with the top open. I'm betting less will enter than if you leave the bottom open and close the top.

Hmmm?

Canucker

Ok, let's see if I have it now. Would a bathroom fan be a good analogy? Let's say I have a bathroom fan that is exhausting 100 cfm with the door to the room open. The fan has a constant speed. So in this scenario the exhaust ducting is the top of your house and the bathroom door is the basement, the fan itself is representing the delta P of your house based on a constant temp.(stack effect) So if I have this correct, reducing the exhaust opening(sealing the top floor) will slow the air exchange but closing the door(sealing the basement) will pull air under the door faster resulting in the same flow of air? That seems counter-intuitive to me. I would expect the blocking of the exhaust to be a more effective way to slow the flow but closing the door should reduce it too? So much to learn but I thank you for taking the time to walk me through it if I'm getting it wrong @Gordy I've never liked just know how to do something, I always want to know why

Gordy

Your bath fan anology is a good one @Canucker. Except I will add when you close the door with out blocking the fan exhaust while the velocity under the door increases, and may not be enough to keep the 100cfm going. Other leaks in the walls outlets, switches, etc may start to leak air where before they did not with the door open. Trying to get the 100cfm the fan wants to exhaust.

However of you block the exhaust duct 100 % then there will be absence of delta p to equalize. So no air flow with door open, or shut.

So back to the original scenario of I start out sealing the basement, and get it 100%. That may induce air flow that did not exist at the first floor level, and at higher velocity because now the top is still leaking at the same rate, and delta p is trying desperately to equalize even more aggressively.

In reality we would want to do an excellent job sealing from top to bottom in one improvement. Or bottom to top.

Bob Harper

Part of the confusion on Stack Effect comes from the existence of a pressure gradient versus mass flow. You can have the pressure differential without flow as with my Tupperware analogy and it does not cause cold air infiltration in the basement as badly as if there is exfiltration up top. Cold air infiltrates because there is a depressurization in the basement With Respect To (WRT) outdoors AND an orifice meaning an opening to allow air to flow.

If you open a window does the warm air exhaust or cold air come in and how does it know which way to go? The answer is pressure gradient. Typically you have negative pressure down low and positive pressure up high or above the Neutral Pressure Zone/ Plane.

Fans may have a rating but they can meet that only if there is an adeaquately sized duct that is smooth enough, straight enough, and short enough without restrictions such as a screened termination AND sufficient air available.
HTH

Gordy

ChrisJ said:

Ok.

Seal the bottom of the straw and try to get water to enter with the top open. I'm betting less will enter than if you leave the bottom open and close the top.

Hmmm?

Obviously the straw example is a sealed vessel with two open ends. A structure has various holes through out its elevation.

vaporvac

However, the OP already stated that he started sealing and insulating in the attic, installed good storms and sealed around his windows, insulated his steam mains, added venting, etc., resulting in $400 per month Savings! He wanted to know the NEXT best steps to take. MY vote is for sealing the rim joists. Am I missing something?

ChrisJ

Gordy said:

ChrisJ said:

Ok.

Seal the bottom of the straw and try to get water to enter with the top open. I'm betting less will enter than if you leave the bottom open and close the top.

Hmmm?

Obviously the straw example is a sealed vessel with two open ends. A structure has various holes through out its elevation.

There aren't any holes in my air tight fortress!!!!!

Fred

Quick next step, in my mind is to:
- make sure the Pressuretrol is actually above the boiler water line (looks suspect but hard to tell from this picture). If it is below the water line, it is possible the pressure runs higher than the settings, especially on colder days/nights.
- Add a 0 - 3 PSI gauge on that same pigtail so you can actually see what the pressuretrol sees. That 0 - 30 PSI gauge is just there because code requires it. Not useful for actually monitoring system.
- Make sure the Pressuretrol is set for a cut-out of around 1.5PSI.
- Make sure the thermostat is set for steam at 1 or 2 cycles per hour. The factory default on most thermostats is 5 CPH, for forced air. At 5, the cycles are too short for the boiler to get steam throughout the system but will obviously keep the mains in the basement hot.
All of the above, if not correct can cause unnecessary fuel consumption. Also, you said you had a blower door test done and that you addressed the "low Hanging fruit". What were some of the "higher hanging fruit" that needs to be addressed?

Gordy

ChrisJ said:

Gordy said:

ChrisJ said:

Ok.

Seal the bottom of the straw and try to get water to enter with the top open. I'm betting less will enter than if you leave the bottom open and close the top.

Hmmm?

Obviously the straw example is a sealed vessel with two open ends. A structure has various holes through out its elevation.

There aren't any holes in my air tight fortress!!!!!

So then your humidity issues must be from with in..........

MarkReynolds1

Wow! Thanks for the great discussion, it is helpful.
To answer some of the recent questions..
- The pressuretrol is above the water line; on this boiler it is half way up the sight glass.
- Pressuretrol settings are a 2PSI Main, and 2PSI Diff. There are no lower marked settings. It is a Honeywell unit.
- The thermostat is set for steam. The boiler seems to run a lot less frequently since I installed the Gorton #2's, but I have not timed it, but it is definitely not 4 or 5 times an hour. More like 2.
- The higher hanging fruit was to insulate the rim joists. I did a partial foam on the basement joists (closed cell foamed the basement wall and joists inside a crawl space, and the floor there too). I can take down the plaster ceiling in the rest of the basement and do those joists too (messy, but..). The tough one is to do the joists between the first and second and third floors, that requires a lot of demolition. I could also get another blower test to see if I missed anything. BTW, the third floor ceiling has no ceiling lights, and I sealed the pipe chases. There is about 1.5 feet of cellulose in the attic. the only ceiling penetrations are the Spacepack A.C. ducts (caulked) and the A.C. air intake, for which I made a foam board barrier that is a force fit where the air filter resides.
- Again, thanks for the comments. I will look at getting a low pressure gauge too.
- Mark
-

Fred

With the work/cost that you will incur insulating the perimeter of the house on the second and third floors, between the joists, it is not likely you will recover that in fuel savings in your lifetime. I'd only do that at a time when I were replacing ceilings at some point in time, for other reasons. You certainly don't need to do the entire underside of those floors either. The goal is to keep the heat inside the structure, not necessarily on each floor. You can balance the amount of heat by floor through your radiator venting.

vaporvac

I agree with Fred, but I only meant the basement in any case I can see why you haven't done it yet if the ceilings are plastered. I have the same situation and don't want to take it down. It's very messy and one never knows of what that ceiling is made. It's really only necessary to do the perimeter area.