Can central air-filtration include furnace output?

[jcoronat]

I’d like an opinion on the following scheme to incorporate my forced-air heating into a whole-house air-filtration system.

I have a house with 10,000 cu. ft. of living space, which I’d like to fit with a high-efficiency ventilating and air-purifying system. The objective is to obtain hospital-quality air. My wife suffers from severe asthmatic allergies, and the house is beset with various molds, dusts, and other pollutants that would require too much at this time to remediate at their source.

My plan includes an adjustable air-handling system that will at its maximum provide an air change every 1.5 minutes (say, 6700 cfm), and filter the entire output of the forced-air heating system before it’s distributed throughout the house. The heating plant is an 80,000 btu/hr. downdraft furnace, which I’d rate at about 6.5 (out of 10) for overall efficiency, capacity, and balance.

The idea is to construct a filtration unit about 2 ft. x 2 ft. x 5 ft. long (I’m guessing here) that will house a blower and filters. Since the filtration system will move more air than the furnace, I’ll route the furnace plenum directly into the filtration module, and add additional return-air ducts to feed the filtration blower from a central location in the house.

The house is small and space is at a premium, so the ducts and filter module are likely to go in the crawl space.

I figure I’ll run the system year-round, even though it draws through the furnace, and put prefilters on the furnace return and the other return-air intakes, and particulate and carbon filters in the filter module downstream of the blower.

My questions: (1) Is 6700 cfm a reasonable size to shoot for? (2) If the furnace is included in the filtration system, will its already-mediocre performance be reduced any further? (3) Do you see any problems with this idea?

Many thanks,
James.

 

[quark]

Jcoronat!

As far as the size of the Air Handling System is concerned the size would be redundant. Generally for sterile room applications US Pharmacoepea stipulates not less than 20 air changes per hour. 40 Air changes per hour as you think is generally considered for industrial application where particle generation is high.

As far as filtration system is concerned use HEPA filters terminally installed for better results. This will prevent both viable and nonviable micro organism.

Regards,

 

[jcoronat]

quark,

Many thanks for your reply. Regarding the HEPA filters, what does "terminally installed" mean?

Thanks again.

 

[quark]

Jcoronat!

Terminally installed means no duct work after the filter installation. Exit of air occurs from filter face itself. This prevents any particle pickup in the duct work.

Regards,

 

[jcoronat]

quark,

I see your point about picking up dust in the ductwork; but wouldn't that require every register in the house to be fitted with its own HEPA filter? Is that what is normally done when very clean air is needed?

Regards,
jcoronat

 

[stevenw]

For your house, I would go with a central HEPA filter system. The positive pressure in the duct will keep the dirt out. With all of the additional filters be sure to check the motor amp draw after the filters are added.

 

[jcoronat]

Central would be easier.

I'm thinking of using plywood to construct the box that houses the blower and filters. That way it could be assembled in the crawl space (there are no openings in the floor big enough to accommodate a pre-assembled unit).

What problems do you see with this scheme? Does the plywood need to be sealed so that it isn't dried out by the air flow, or does it need to be lined with duct material (duct board, sheet metal, etc.)?

 

[ChasBean1]

James, a couple of comments:

1) 6,700 cfm is a pretty large quantity of air for a house that I'm estimating to be 1,180 ft2. This will give 5.7 cfm per ft2, where ASHRAE recommends 1 cfm/ft2 for space conditioning, and as Quark mentioned, you don't generally see greater than 20 ACH needed for contaminant control. Also, I can't really see how you'll put that kind of volume of air into the house and not create thermal comfort problems (i.e., always feeling a rush of air on the skin at any location).

2) The filter assembly you mention will pass air at about 1,675 feet per minte accross the filter media. Typically HEPA and carbon filtration beds are applied in applications of ~500 fpm maximum. There probably are models available capable of maintaining efficiency at this velocity but they might be more costly and have a higher pressure loss.

My recommendations:

• Keep total air supply at 1,200 - 1,800 cfm.
• Keep the living space significantly positive by using greater make-up than exhaust air to prevent infiltration (approx 400-500 cfm positive). Use the filtration device you mention - this will be nicely sized for this flow range. Be careful of temp limits imposed by the filter and charcoal manufacturers.
• Shop around for reduced lint materials and replace any old dust sources. De-carpet. Replace cloth furniture with leather/vinyl, etc.
• Shave Fluffy.
• Replace your wife with a less allergic model - this will save in both fan power and filter maintenance.

Good luck with this project! -CB

 

[jcoronat]

ChasBean1,

I had wondered about the uncomfortable indoor wind speed. Suppose I used 20 ACH. That would imply 3300 cfm. Do you think that would yield too high an air movement?

Excess capacity seems attractive in the case in which I need to clean out the house in a matter of minutes (perfumes, a cleaning-product spill, etc.), and so I'm a little wary about sizing the blower so that it would take (at 1800 cfm) almost 6 minutes for a complete air change. Are these concerns ill-informed?

Suppose I put the system on a fan speed control so that I could run it at lower speed when possible, and at full power when the seasons or contaminants made it necessary? Is that normally done?

Your point about the filters is well-taken. My plan was to insert them in a fan-fold configuration so as to get greater surface area in a smaller volume--say four 1 ft. x 2 ft. filters arranged horizontally in a double-V formation facing into the flow from the blower. In my understanding that would yield 8 sq. feet of filter, which at 500 fpm would allow 4,000 cfm through the filter assembly without exceeding that 500 fpm standard. What do you think?

By the way, Fluffy is a distant memory. And as for the allergic model...well, love charges in where even engineers sometimes hesitate to go.

Many thanks for all your great information. Any feedback is greatly appreciated.

jcoronat1

 

[quark]

jcoronat!

Your answer to chasbean's 5th point is perfect in all senses, as I always dislike Ayn Rand. You can have plenum made of plywood if you take care of fire hazards. You should put decolam or sunmica or anything which is smooth from inside.

I have worked in areas with as high as 60 air changes per hour but never felt the rush of air. One reason may be I was wearing coveralls.

Duct surfaces are possible areas for growth of molds, mildew and bacterial colonies which may dislodge from the surfaces and flow into the air stream. You terminally install HEPA filter atleast in your Wife's room. Try to recirculate as much air as possible. (16 cfm fresh air is sufficient per person to take care of CO2) so that energy cost can be reduced.

You can go for speed control and the logic you are thinking. That is a general practice.

Note: I don't know what a fluffy is

Regards,

 

[jcoronat]

The air-exchange rate did concern me, since we live in northern Minnesota (very cold and very hot). My strategy is usually to build a lot of adjustments into a system so that it can be tuned to a wide range of conditions.

My wife likes the strong air movement, anyway. If I install a beefy enough system, maybe we can lose a few of our oscillating fans.

“Fluffy” - what the family pet might be named if it were a dog, cat, rabbit, or the like. In other words, anything that sheds, has hair, or generates dander.

Ayn Rand? Yes, I’d say capitalism can be a powerful force for good; but, perhaps like you, I’ve also learned there is more than one market where the heart is concerned. Engineering and life are both disciplines in which the envelope is pushed outward by those with true love for their work.

You've been a great help. If you think of anything else that you think might be useful, I hope you'll let me know.

My best to you all,
jcoronat.

 

[quark]

Jcoronat!

Pardon me if I am off the track. Capitalism with hypocrisy is what I hate most. I prefer O'Henry's "Makes the whole world kin" mostly.

If you don't want to cough up more, 10% fresh air is sufficient to keep the area fresh. 90% recirculation will optimize your heating as well as cooling loads. You can maintain the critical area just positive. (say 5 pascals)Just by manual control. (auto control will be redundant)

Note:1. Suggesting things discovered/invented by others cannot be a great help.
2. Conversion of high grade to low grade energy is a loss.
(electrical = high grade, heat = low grade)

Regards,

 

[ChasBean1]

jcoronat, It sounds like you've put some thought into it. Yes, 3,300 cfm can probably be put into your house without thermal comfort problems. Just be careful how and where the air is diffused.

 

[jcoronat]

Does keeping a positive air pressure inside the house preclude having an air-to-air heat exchanger (where usually exhaust = intake), or are exchangers made so that you can vary the ratio of intake vs. exhaust air?

 

[Islander]

....if you dump 3300 cfm in a 1000 sqft house you are looking at humidity problems that will increase/cause the mold problem.

ie...the temp will satisfy too quickly..meaning the cooling coil won't stay on long enough to remove enough moisture from the air.

if it was me i would go for the 1 or 1.1cfm per sqft of usable living space and not a single bit more.

go ahead and run an outside air duct and set to maintain 20 cfm per person.(keeps the air from feeling stale).

You could add more outside air to create a higher positive pressure inside...a little smoke with front door cracked open 1/4" will show if you have a positive.

a good hepa filter regularly changed will do a lot of good.
end result will be dry, fresh and fairly clean air.

you could get hospital grade hepa filters..i've seen isolation ones a foot thick..if you went that route you have to account for expense..need a seperate filter containor box..and account for the 1/2" or so static pressure loss across the filter.

Oscilliating fans...are bad for people with allergies and the like...blows dustmites around.

Lastly..consider a fully ducted return to each room balanced appropriatley...else undercut the doors..or you won't be getting adequate air changes in each room.


PC
Test/Balance

 

[ChasBean1]

j - I think keeping your house positive is a main key to your goal. Filtration as you mention is certainly necessary, but if you flip on the bathroom exhaust switch for maybe 20 minutes per day total, or having certain air diffusion patterns creating the venturi effect in small areas, or opening and closing doors could lead to infiltration from spaces you describe as "beset with various molds, dusts, and other pollutants." Your design sounds good, but incorporate some volume of make-up air prior to the filter bank. This make-up air volume should significantly exceed the total possible exhaust volume (bathroom, stovetop, etc.). In my opinion, this is absolutely necessary.

 

[jcoronat]

ChasBean - What you're saying is starting to sink in. We do get a lot of infiltration from crawl and attic spaces, and other areas. The positive pressure makes a lot of sense, although I'd like to be able to reclaim some of that heat during the winter (routine winter temps are -20F)

Islander - I'm not sure I understood your first comment. The 3300 cfm is intended to be the amount I'm continuously circulating through my filters. We do have a through-wall A/C unit, but it's not connected with the furnace and the proposed air-handling system. Do your comments take this into account?

 

[Islander]

A window unit for cooling..did not see that...assumed your "airhandler/filtration unit had a cooling coil and you would dump all of that air into the house.

if you are going to run the furnace/filtration unit year round..what will control the humidity=mold during warm weather? especially if you will be adding outside air.

 

[jcoronat]

I don't know. I plan to power-vent the crawl space, but aside from what the air conditioner and the (proposed) bath exhaust would remove, no other plan existed.

This house is pretty leaky; what other options should be considered to remove moisture?

 

[quark]

Jcoronat!

One conventional practice is to go for desiccant dehumidifiers. There is one very good dehumidifier (based on Cathabar dehumidifier) which uses liquid desiccant as dehumidifying media(Lithium Chloride). The manufacturer is DryKor, basically it's an Israeli Company but they have offices and marketing setup is USA also. I stuied this system for 3 months (being in a Pharmaceutical Company have to go through lot of details which are unnecessary to a mechanical engineer:-() Lithium Chloride is a disinfectant and can reduce bacteria and fungi to 3 log reduction as per M/S Drykor. They can provide you lot of documentation on this as they already prepared for USFDA.

One problem is Lithium Chloride carryover caused if proper balancing is not done. (You can get documentation on this too)

One beauty of this system is desiccant is regenerated using refrigerant itself, thus saving you energy. As the desiccant is liquid, it filters particulate matter too. (again you can get documentation on that) I have lot of data collected on that but not possible to send to you because of geographical problems.

Regards,