Figuring ductwork, CFM, for HRV
I’ve decided to take the plunge and DIY an HRV system. I mastered and DIY’d radiant floor and super insulation/sealing, but now obviously need some air changes (moisture on windows, last night’s dinner still providing aroma in the morning). Trouble is there is no ductwork, as I have (and don’t need) no AC.
7000+ HDD; <1000 CDD climate
I’ve been looking at some of the instruction sheets from “Renewaire” brand HRVs, and it looks like I can run a fresh air supply duct and supply grill on the floor of a central hallway, and a stale intake/grill on the kitchen floor, but the way the floor plan and access areas work out, the HRV unit itself needs to be about 4 ft from the stale air intake to allow basement through-the-wall mounting, and the fresh air supply duct will be about 50 ft long. This is a 1600 sq foot house with typical 8′ ceilings through most, but the 24 X 20 living room/great room does have a cathedral ceiling at a 4-12 pitch.
There is a standard hieght basement under the kitchen, with the rest of the foundation being sealed and insulated crawl space.
The Renewaire techs say this would be an out of balance situation, but I might get adequate air flow if I go to 8″ duct for the long run, and a slightly over-sized unit (130 CFM). I thought I would use flexible insualted duct for ease of install. though I know I would be paying a friction penalty with flex duct. How much I don’t know.
I do have good exit fans already, one in master bath off the central hallway, and one in the laundry/bath off the kitchen.
What do you HVAC guys say? Will my plan work…if not what would you suggest?
Thanks.
Edited 10/17/2005 12:02 pm ET by johnnyd
Replies
maybe I missed it, but what type of heating system do you have in this home ?
It is radiant floor. Very successful DIY sandwich with modulating electric boiler at .037/kWH, propane backup, five zones. Works great except the lack of scorched air leaves me with humidity in the high 40s or low 50s. Even low-E windows mist up in cold weather.
Talk to VaTom - he built his own HRV...
A truly good HRV system has a supply grille/register in every room in which we spend good chunks of our interior time- bedrooms, den, family room, living room, dining room. Exhaust ports are placed kitchens, bath or washrooms and possibly the laundry. The idea is to supply fresh air to the areas where we spend time and then it moves through the house to the exhaust ports in the rooms where we create moisture and odours.
Just supplying to the hall outside bedrooms with closed doors for 8-10 hours overnight does not ensure good air quality and moisture control in those rooms.
I am not familiar with "Renewaire". Are their systems sold over the counter at hardware stores? If they are, they may be offering the "handyman special" and not an HVAC contractor's version with better controls and more options/modes of operation. These are features that most people don't know they wanted/needed until they see a better system installed and running. I have seen this time and time again in my home inspection business as I explain the lesser systems installed in most "spec" homes to a potential home buyer. "Why wouldn't they put in a better system??" the buyer asks. After all, it's about good air quality and your health, why scrimp?
I installed my first HRV in 1981 and have done about 450 since and inspected about 1300-1500 for various purposes.
Experienced, you have quite possibly installed more systems then there are in my entire State of Maryland. No one I have met knows of HRVs. (except you genius BTers :-)
What brand do you recommend?
Since I have a very open floor plan, I'll probably keep the ducting simple...
Treat every person you meet like you will know them the rest of your life - you just might!
The Venmar brand (a Canadian co. now owned by Broan of Rhode Island) makes some of the units I would install in my home today (even after fighting with them all throughout the 1980's). See their website at:
http://www.venmar.ca
They have heat wheels and plate type heat exchange mechanisms. The heat wheel is more effective if you air condition a lot. Install the "Ultima" control.
Thanks much for your comments so far.
The idea is not to DIY the HRV unit itself, I think the "Breeze" unit itself will be satisfactory. I'm still wondering about my ducting scheme, which would have a really long duct supplying fresh air to a central hallway, with adjoining rooms recieving fresh air from that grill when the doors are open, which is nearly always.
That duct will travel through the crawl space to the unit which will be mounted on the interior basement wall, have its inlet and outlet going through the basement wall (PTW foundation easy to cut holes), and recieve exhaust air from a grill directly above it through the kitchen floor.
There would be other ways to route the ducting, all of them comparatively labor and disruption intensive, which is why I'm willing to compromise a bit on the effectivness and over-size the unit. But, will it work at all with that much difference between the exhaust and fresh air duct lengths, or will that fan just be churning away, unable to overcome the inherently un-balanced ducting?.
A similar situation I suppose would be where a house would have one exhaust fan unit in the attic serving multiple bathrooms and a kitchen, with widely variable duct lengths. Does the room farthest from the fan unit get the same CFM volume as the room closest to it?
From having designed and built my own RFH, I know there are certain design parameters (tube length, water temperature, near-boiler piping etc) that if stretched too far from the norm, render the system nearly non-functional, which is why I'm proceeding with caution on this HRV system.
Digressions, as you know, are common. <G>
a really long duct supplying fresh air to a central hallway, with adjoining rooms recieving fresh air from that grill when the doors are open, which is nearly always.
Why would the air enter those rooms? You planning an exhaust duct in each one? I pressurized all my rooms and closets with a fresh air supply. It pushes air out the doorways to the returns located in our large open space. You need to think about air movement, it doesn't happen by chance. Pressure (+ and -) and convection both work well.
I won't pretend to fully understand your plan. We have one fresh air run that's 80' long. It works, but the closer vents get more air, as I planned.
If you're hoping for someone to give you static pressures and fan capacities, I don't think you have the right forum. Certainly I'm no expert. I based our air system on my knowledge of shop dust systems. Works pretty much the same way except it's much easier when you have 2 movement sources (push and pull) in the house air system.PAHS Designer/Builder- Bury it!
I second the choice of a Venmar unit; great fans, we use them all the time. they have balancing dampers, with a magnahelic gauge you can balance the situation as needed.I would wonder about a floor register for supply air though. Supply air will necessarily be cooler than room air, thus having it high off the floor would be best.. even with radiant, there is *some* hot air stratification and this will help prevent any drafty sensations. Don't you dare use ceiling fans with radiant!!!! That's like peeing in the punch at a party!!!Long flex runs should be upsized, yes. Pull the duct tight to minimize frictional loss. If it's a straight run, then perhaps using galvanized wouldn't be much harder though.I would take a hard look at the floor plan and see if there are any places where you can do some creative carpentry to distribute the air a bit better than what you are describing. Perhaps building out the side of a closet wall, for example.-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
A Google search brought me here:
http://www.positive-energy.com/pages/VentilationWH3.html
From there you get this:
http://www.breeze.renewaire.com/ventilation.htm
I understand the best system would be one which has a supply to every room and and an exhaust from moisture producing rooms. Fairly easily done where the HRV was a factor during frame-up, or where there's an existing forced air system, but in my case, with radiant floors and no AC, like it or not, the HRV is an after thought.
I'm just wondering if a single inlet and outlet would still be signifigantly better than cracking a window and leaving the bath fans on. Especially given that I would like to run one 50' 8" flex duct for the supply and ~ 5' 6" for the exhaust. I mean, would that kind of a duct setup even work?
I know it's fairly common to duct a forced air funace with several supplys in each room and ONE cold air return to the furnace. But there you're delivering heat, and the HRV is simply exchanging air. I also would run ceiling fans on low speed to disturb stratification and spread the fresh air around.
My plan is to pull air from 2 baths and a kitchen on the main level, and return air to the basement, so both levels will be cleared. My floorplan is rather open, so I think it would be too much to duct every room.
Treat every person you meet like you will know them the rest of your life - you just might!
Yes, my plan is open as well, but the basement has an exterior entrance, so is not included in the envelope. The basement/crawl space is well sealed and insulated as well, though, and since the HRV unit itself is located there, I may be able to get some moderate circulation there as well.
What has me flumoxed is that with so many forced air systems all over the country, why an HVAC guy that does alot of ductwork doesn't respond to this thread...if only to say that my design will not work and why?
What has me flumoxed is that with so many forced air systems all over the country, why an HVAC guy that does alot of ductwork doesn't respond to this thread...if only to say that my design will not work and why?
In my area, HVAC guys are only HACs. I've met exactly one who knew anything about ventilation, and that was because his daughter was asthmatic and had to learn. Operable windows are considered to have satisfied ventilation code requirements, even though at least half the year most never crack a window.
As Brian mentioned, I DIYed. We have 1600 sq ft, but 20,000 cu ft (average ceiling height of 12.5'), which is the far more important number. What you're looking for is airchanges, preferably at least .5 ACH (air changes/hr). These will be tiny fans, requiring small ductwork. I used 5" round (flex) duct and supply fresh air to all rooms and all closets. It's working just fine, thank you.
Controlling humidity is another matter. We monitor rh constantly and act accordingly. For the remainder of the winter, adding humidity is in order here. Roughly half the year we dehumidify at least occasionally. I haven't looked at the climate differences between Va and Se Minnesota in terms of rh, but I don't think we're far apart. Obviously heating and, possibly, cooling degree-days will differ greatly.
Phone service, and dialup, was disrupted here for a couple of days, so I haven't looked at the link. I used aluminum roofing for the heat exchanger core and got nearly 90% efficiency, based on a Popular Science article for an air-to-air heat exchanger. Our plan is extremely open, which makes air circulation very simple. I planned for a convection current that worked out even better than I'd expected. We have 6 fresh air sources, 2 returns, in a 1 BR, 1 BA house. Guests have often remarked on the quality of our indoor air.
Hope this helped.
PAHS Designer/Builder- Bury it!
I have long thought about building my own aluminum HRV. I originally thought I might TIG weld the thing. However, given my TIG welding speed (and current quality), I think I will go a different route...
A couple of questions, if I might, about your DIY HRV - How many partitions of the aluminum do you have in yours. When you mention "aluminum roofing" did you use the standing seam stuff, if so, did you include the "V's" in your design or just use the flat sections. Have you had any problem with moisture freeze ups (or anticipate any)?
Back in my alternative energy days in college (long, long, ago), some friends tried burying long runs of fairly large diameter pipe in the earth to try and get some cooling of the incoming air from the soil. They ended up getting a fair amount of condensation and a lot of mold in the pipes so they recommended against others doing this. Have you checked for mold? Did you use marine paint on all surfaces that might have some antifungal properties?
Thanks, Casey
Hi Casey. Sounds like you've been looking into this for awhile. Congrats.
Textured corrugated aluminum roofing was what I used, with weather-stripping intended for use with Onduline roofing, which I had around. Probably still have enough for yours if you're interested. Then I used sheet metal screws to hold it together. IIRC, I made a 2'x'2x~8" core. It was whatever 1 sheet of roofing yielded. Other than the core and pressurizing the house as Brian mentioned, our airflow is very similar to the below article.
On the advice of the article, I made a drain pan plumbed into a house drain. It never saw any flow so I disconnected it when changing fans awhile back. My guess is that the only time we'd get inner condensation, summers, is when I was already drying the air with a dehumidifier before the incoming air (now dry) hit the exchanger. Winters, we add humidity (whenever we fire up the woodstove) and never run the shower exhaust fan. Icing hasn't been a problem, but this isn't exactly the frozen north. I also have a few feet of duct in our heated space before the exchanger.
We have very humid sunmmers, don't use AC, and are serious about dehumidification. Monitoring indoor rh tells us when to turn on shower exhaust fan, dehumidifier, whatever's necessary to correct the imbalance. Automation we happily avoid. One word of advice, make it convenient to change fans. They don't last forever. We got upwards of a decade of non-stop use. That was muffin, the squirrelcage ones I tried were worthless. Surplus Center in Nebraska usually has a good selection. http://surpluscenter.com/community.asp?UID=2003031823141350&catname=electric
The earth tubes you mention are often a subject on the alternative architecture lists. Nobody, myself included, has had the temerity to use them in the SE that I'm aware of. The problem, as you identified, is condensation which will provide an excellent habitat for mold growth. We get no mold with our simple system.
Hait, the guy who wrote the book I followed for our passive annual heat storage (PAHS), believes earth tubes to be an important integral part of the plan. While we don't get his performance, this is by far the best performing house I've lived in. His demonstration house was in Missoula. If we lived in an arid region, I'd have installed earth tubes. There's a guy in Vermont who's planning on using them, currently trying to figure out flow rate. Another says he's going to in Missouri but he's not there yet.
I believe the answer in a humid climate is to either pre-dry the air, use a marine coating inside the tubes, or let the mold grow and treat the air with UV. The latter is what I'd like to try, but no way am I going to be the canary. And adding yet another active system is counter to my housing goal.
By the way, a good friend here is having a small elaborate (expensive) house built. One HVAC guy told him that all he needed was normal opening and closing of outside doors to ventilate. He's been through several firms trying to find somebody who knows something about ventilation. I warned him about talking to somebody (me) who had a little knowledge and wasn't involved with his house. Can create all sorts of problems. But he's not DIY, and that's the rub.
Anyhow, enough of this tome. And I have a hydraulic line that lost over $100 worth of oil yesterday awaiting me. Here's the article:
PAHS Designer/Builder- Bury it!
I cannot speak for ductwork, but I'd take the cubic footage of your house (probably s.f. x 8 or 9 feet - conditioned area only) and figure how many air changes/hour (ACH) you desire, do the math, and this will give you cfm requirements.
VaTom, another poster here, recommends a push/pull arrangement on fans with different ratings creating a slight positive pressure in the house. (e.g. 500 cfm output 650 cfm input)
My thoughts on ductwork are oversize them so you won't stress out fans or have noise at your registers. I am in the dark on particulars.
edit - thanks VaTom - I was typing while you posted
Treat every person you meet like you will know them the rest of your life - you just might!
Edited 10/18/2005 11:03 pm ET by Brian