Mechanically driven air changes in existing home
Hello,
I recently performed an energy audit on a home in North Jersey. The home, approximately 5500 sq ft, has baseboard heat, 2 air handlers in the attic for AC, and was built in 2005.
During the process of the audit, I noticed that the bedrooms pressurize to approximately 15 pascals with the doors closed and the air handlers running. I understand that this pressurization forces air out of the envelope and induces air changes; I also grasp the related building durability concerns that go along with that. I don’t, however, understand the energy penalty involved with this pressurization.
Is there data available on exactly how significant these mechanically driven air changes are on whole house energy efficiency? Is this customer paying a huge energy penalty for this? Would relieving the pressure in these bedrooms cause noticeable energy savings? How many undue air changes are caused by this situation?
Also, the home has 10 tons of AC, whereas an accurate load calculation would call for approximately 7 tons. Would swapping out 5 year old units be cost effective in this situation in order to “right size” the AC’s?
Any help is appreciated. Thanks!
-Art
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Replies
My first thought, wouldn't a home 1/3 as large built correctly to start with be the smart move for this customer if what they want is to lower their costs?
Seems anyone with a 5,500sqft home , well, the last thing they must be concerned with is how they spend money, no?
Art,
Q. "I don't understand the energy penalty involved with this [bedroom] pressurization."
A. When a bedroom is pressurized by a forced air system, conditioned air from the bedroom is forced through cracks in the ceiling into the attic and through cracks in the wall to the outdoors. That's why the air leakage to the outdoors is much higher when the HVAC system is operating. This air leakage to the outdoors is the cause of the energy penalty.
A knowledgeable blower door operator should be able to perform the testing and calculations to quantify this energy penalty, if it is important to you to quantify it. However, a 15-pascal pressure difference between the bedroom and common spaces is considerable and should be fixed.
For more information on this situation, see Return-Air Problems.
Q. "Would swapping out 5-year-old units be cost-effective in this situation in order to ‘right size’ the ACs?"
A. No. Researchers have found that swapping oversized AC units for right-sized units is NOT a cost-effective retrofit measure, and in many cases it doesn't even save energy.
Here's what I wrote on the topic in a recent blog: "Increasing evidence shows that energy experts have exaggerated the negative effects of equipment oversizing, however. Studies have confirmed that oversized furnaces don't use any more energy than right-sized furnaces. Moreover, newer modulating or two-speed furnaces operate efficiently under part-load conditions, solving any possible problems from furnace oversizing.
"Although there are ample reasons to believe that oversized air conditioners are less effective than right-sized equipment at dehumidification, at least one field study was unable to measure any performance improvements or energy savings after replacing an existing oversized air conditioner with a new right-sized unit."
Here's the link to the whole blog: Saving Energy With Manual J and Manual D .
"A knowledgeable blower door operator should be able to perform the testing and calculations to quantify this energy penalty, if it is important to you to quantify it. However, a 15-pascal pressure difference between the bedroom and common spaces is considerable and should be fixed."
Martin,
I've run a few hundred blower door tests, and I think I'm at least fairly handy with the tool. Are you referring to any specific tests that can be run with the blower door to account for these mechanically driven air changes? I don't think I have that test in my bag of tricks yet....
thanks.
-Art
Art,
Good question, and I may have misspoken. I invite blower-door technicians to comment, since I'm unsure of whether it is possible to quantify the energy penalty we are talking about.
I know that it's possible to use a Duct Blaster to quantify duct leakage to the outdoors. (See Duct Leakage Testing.)
How about it, fellow nerds: Any good ways to quantify the energy penalty of the problem under discussion?
Why not place the blower door in the doorway of the bedroom and pressurize to 15pa. Then get the air leakage and turn that into energy lost. I know the total energy lost is much more complicated than that but it will give you a rough estimate.
Matt,
I think that your testing method won't distinguish between leaks from the bedroom into common areas of the house and leaks from the bedroom to the outdoors.
Yeah sounds like two blower doors would be needed. One for the entire home as well as one for the bedroom. Same principle as testing duct leakage to the outside.
Two blower doors makes sense. Pressurize house to +15, pressurize bedroom separately to +15, measure CFM used to pressurize bedroom, calculate BTU per cubic foot.
Ultimately it just wants to be balanced. Add jumper grilles/ducts or return air ducts, (or remove supply ducts, I suppose) to make the air handler pressure-neutral in rooms with closed doors. In a 5500 foot house I would think there are huge opportunities to find duct improvements.
just a guess but how about measuring the leakage with the door closed and then opening the door and record the new reading. The difference is what is lost to outside
2 blower doors makes sense. That will give envelope leakage in CFM based on the operational pressure induced by the AH. Perfect.
David, any further insight on the formula you gave to calculate BTU/cubic foot? Wouldn't that formula give us total BTU consumed in the room? How does that correlate to BTU per tested fan flow in CFM? Do we just multiply it out by 60 min/hr?
Thanks!
A comment on the "right sizing" line of discussion. My understanding is that oversized equipment cycles quicker than "right sized" or even "under sized" equipment which allows for the removal of heat, but not humidity (as was alluded to). When you add mechanical pressurization to any given room (especially a high humidity space such as a kitchen or bathroom) aren't we concerned with moisture condensing in the walls? Won't this cause deterioration of the envelope? I agree that it might be difficult to quantify an "energy penalty" regarding this condition (sorry guys I don't carry around 2 blower doors with me, but I applaud those of you that do...energy geekdom at its finest) but there certainly is a "penalty" that needs to be addressed for the long term comfort and safety of the homeowner. I would also argue that there is an inverse problem assuming that this pressurization occurs due to a lack of a return in the space. My experience is that newer homes opt for a central return and when those "pressurized" rooms are closed they starve the return, depressurize the rest of the house and can cause the HVAC system to look for air in some unfortunate places, namely flues or other "path of least resistance" infiltration points into the home. I would think that would have to be calculated into any "energy penalty" equation as well. I'm fairly young in this industry and from what I've read there are some really smart people in this forum. I'm curious to know how my analysis holds up!
Hmmm... I just read the article you linked to and everything I said was addressed, but I stand by the fact I came by it honestly!
Art, I'll be rising to the level of my own incompetence here shortly, but let me hazard a guess. Suppose you measure leakage to the outside from the bedroom as created by the air handler. For the sake of example let's say that's 100 CFM.
Convert 100 CFM to pounds of air/minute. Air apparently weighs .075 pounds per CF, so 100 CF weighs 7.5 pounds and at 100 CFM you are losing 7.5 pounds of air per minute.
Factor in the specific heat of air, which is apparently .24 BTU/lb/degree F.
So.... multiply .24 x 7.5 and you get 1.8 BTU needed to raise 100 CF of air by one degree F
Factor in the temperature rise, i.e. air being forced out is 70 degrees, replacement air is 30 degrees, for a delta T of 40.
Now you need 40 x 1.8 BTU to raise 100 CF of air by 40 degrees, or 72 BTU.
If I'm not mistaken (and I sure as hell could be!), 100 CFM of air leakage is costing you 72 BTU/minute assuming the 40-degree delta T.
Taking it a bit further, if the air handler is running 12 hours per day, 51,840BTU would be lost. Locally the owner might be paying about 2.5 cents per 1000 BTU, or about $1.30 per day for that heat leak.
I think this stuff is fascinating, and am fully in favor of you acquiring a second blower door posthaste and returning shortly with some actual numbers we can play with. I also will be unsurprised if someone with a better grasp comes along momentarily and points out flaws in my thinking.
Seems to me the fact that there is a positive pressure in a room does not necessarily mean that there are leaks but maybe that the room is actually very tight and not allowing the room to balance out.
To measure the air leakage to the outside of a particular room - you can use the "Add a Hole" method - Saturn books have a good description of this and energy conservatory.
Easy solution - add transfer grills or jumper ducts and your problem should be resolved.
A few comments
1) The 2 blower door method will be time consuming and may not be very accurate because you will need to mount a blower door (more like duct blaster) to control the pressure in the room which means it is either in a door or window, which changes the room leakage in either case.
2) You could use zonal pressure diagnostics -- the "open a door" method is quick and easy to use. You may find that the room to exterior leakage is so low that it's hard to get a good measurement here, but the dP measured would give a good indication of the fraction of leakage to the exterior.
3) The biggest energy penalty would likely be related to the change in pressure in the main body of the house rather than just in the bedrooms. Changing the main house pressure by 3 Pa would likely cause more air leakage than changing bedroom pressures by 15 Pa.
4) A tracer gas test is the best way to measure the change in air leakage rate of the building from the air handler operation. Researchers have used SF6 decay for this and found a doubling or more of natural infiltration from air handler operation often. You might be able to rig up something cheaper using CO2.
5) The specific house in question only uses the ducts for cooling and is located in NJ where the cooling season is not very long. Any savings calculations would need to include an estimate of the air conditioner run time, which is likely to be only a few hundred hours. You should start by analyzing the electric bills to estimate the cooling load -- which may also help you keep the savings estimate within the realm of reason (much less than 100% of the load).
6) The most likely impacts of having 15 Pa of pressurization in the bedrooms in this home are a not-very-large energy penalty (unless they have large cooling loads), the bedrooms may not be adequately cooled due to the pressurization reducing supply flow, if carpeted with a light color carpet then the carpet under the door may get stained from the dust left behind by air flow and you may even see dark lines on the carpet in a 4x8 grid that coincides with the subfloor underneath.
Great response, Michael. Thanks for the input!
I think this thread has become a theoretical discussion moreso than a practical one. In real life, add some jumper ducts to relieve the pressure and call it a day. Interesting to hear the different thought processes at play here, though....
-Art
Art -- I agree. 15 Pa is quite high and jumper ducts would solve the problem -- and anyone living in a 5500 sq.ft. house can afford this solution. In some cases, the cost of performing more detailed diagnostics and calculations to get a slightly better answer is as large as the cost of just fixing the problem.