I’ve never had or seen an air-to-air heat exchanger system and I can’t say for sure I really understand how they work. But I’m contemplating building a very tight home and I’ve heard that the exchangers are becoming commonplace, escpecially in tight homes.
My problem is that my limited reading on the subject suggests that these miracle appliances completely “exchange” the heat between incoming and outgoing air, but this hardly seems possible to me. Logic would suggest that, at best, the heat between the two is “averaged” – say at 40 degrees when inside temp is 70 and outside is 10. And that, more likely, it would be a bit less than average – say 35 incoming & 45 outgoing.
If this is correct, it hardly seems like a great advantage when it comes to energy savings – some advantage, yes, but great advantage? no. This would seem especially true when the rate of air exchange is taken into account. If the exchanger is set to exchange say, 100 cu ft of air per minute at my imagined 50% efficiency then wouldn’t that be the energy saving equivalent of simply drilling a hole in your house that exchanged 50 cu ft of air per minute?
I know that there are other issues to consider like venting “bad” air from your house. But there are other ways to avoid “bad” air in the first place. Is there something here that I’m missing? I’m certainly willing to have one installed if it is the miracle innovation it seems to be.
I need some educating on this subject…
Replies
I think you are confusing two different beasts. An air EXCHANGER exhausts household air one-to-one for the air it brings in. The air usually passes through a heat exhanger, so that the outgoing air preheats the incoming air. However, it is not a 100% sytem; there is always a temperature differential ("approach") between the outgoing and incoming air streams. The heat recovery is sort of an extra; the whole point is to get fresh air into the house, and stale, humid air out. Having an air exhanger, or opening windows, is critical in a "tight" house, or else you wind up with stale air and humidity problems. The unit is really a glorified ventilation fan.
An air to air heat exchanger, on the other hand, is similar to an air conditioning system. In this case, there is a unit that sits outdoors; it uses a refrigerant system to pull heat out of the ambient outdoor air. Then, on the inside of the house, it takes teh heat out of the refrigerant and uses it to heat the indoor air. Like a fridge running in reverse (a fridge takes the heat out of its interior, and dumps it into your kitchen; an air to air heat exchanger takes heat out of the outdoor air, and dumps it into your house). Currently, I think the typical crossover point (the temperature at which an air to air system won't pull any more heat out of the ambient air) is in the -10 to -15 C range.
"I think you are confusing two different beasts. An air EXCHANGER...."
You may well be correct. I was referring to the "EXCHANGER" you mentioned first - certainly not a compressor driven unit.
I'm still confused by a couple of things though;
Where I'm located, winters are extremely dry and humidity in the house is desireable - in fact many people run humidifiers. In the summer, windows would be open. I don't see humidity as a reason to install such a unit.
If heat recovery really doesn't amount to much, why install the system? Crack open a window. I don't know how much these units cost, but it seems like a perfect example of a low-tech solution trumping a high-tech one to me.
By "stale air" are you referring to air that is just musty and too full of BO or are you referring to air too saturated with carbon dioxide from breathing and a danger of suffocation. Is suffocation ever really a legitimate concern?
Thank you!
The air-to-air exchangers in the application you are thinking about is also referred to as a heat recovery ventilator (HRV). The highest claimed efficiencies I have seen are a little more than 80%. Most are less and some considerably less.
At first glance, it would seem that if you put heated air and colder air in contact, they would end up at the average of the two temperatures. But to think about it a bit more, if you had a long duct with a divider down the middle, and had hot air flowing on one side and cold air flowing the opposite direction down the other, things would be a little different. Using totally hypothetical figures (because I really don't know what I am talking about here...) At one end, you would have the air coming from inside the house that started out at, say, 70 degrees, and exited at say, 40 degrees because it had passed heat energy to the cold air. The incoming air would be starting out at 30 degrees and would be gaining heat from the now 40 degree air flowing out the adjacent channel. As the cold air passed through the duct, it would be warming up but is also meeting air from the house that had not cooled down yet. So at the last few inches, you would have outside air that is now warmed up to nearly 60 degrees gaining heat from the outgoing house heated air that is almost 70 degrees. Or something like that.
A diagram would make more sense, but I'm too lazy to try to draw in "Paint" and upload it...
The famous LMTD (log mean temperature differential) and counter-current flow in a single pass heat exchanger....oh, to go back to my thermodynamics and heat transfer courses. On second thought, no thanks.
nannygee, the point of an air exchanger is this: in theory, you build a well insulated and tight house for energy efficiency. however, cooking, breathing, showers, etc etc, all generate a lot of moisture in the air. on top of the moisture, you have co2 from breathing, etc etc. if you had a tight house with no air exchange, you would gradually become extremely wet and also suffocate. so, you put in an air exchange system. yes, it functions like opening a window. but - it does have some heat recovery potential, so it is better than an open window; you don't lose all of the heat.
in northern climates, even though the outside humidity is very low, if the house is well sealed, you will find high humidity levels inside depending on how much cooking you do, how many people are in the house and for how long, and how many baths/showers you have.
Thanks for the info.
I guess I'm still not totally convinced of the necessity of such a device. Even in cold weather, exterior doors will be used frequently. Even very tight houses leak a little around windows & doors at least.
But that impression could change in a flash if anyone has any horror storries to tell about tight houses without special ventilation. I don't have a death wish, after all.
Thanks again.
Not so much horror story as practicality.
Suppose that the house is closed up tight for three strtaight months of winter. At some point it will want newer, outside (call it fresh, if nothing else) air inside (if only to take away "mustiness").
If it's 11° outside, you are not very likely to crack open a window for an hour or two (or overnight). So, a way to bring in air makes sense that way. Also, in the heating design, the engineering idea is that no structure is 100% "tight." Rather than guess, or "force" infiltration some place in the structure--it is simpler to design one.
Let's now combine those two items. The HRV is connected to the "return" side of the furnace in a forced air system. Feeding 11° air into the heater will cure it of being cold. However, the furnace will run better if we can use a bit of the existing room temperature air to warm that incoming air. Then, it's circulated in the structure.Occupational hazard of my occupation not being around (sorry Bubba)
I thought the HRV is usually NOT connected to the return air of the furnace ... it is most often a separate system. Exhaust air and fresh air intake separate from heating/cooling system.
We install to the furnace return. Probably the most cost effective way to get the air back into the house.
I have an extreme aversion to interlocking the HRV to the furnace. Had a furnace go out on high limit and did not reset. But the HRV kept running along with the furnace fan. At -40, it does not take long to cool off a house and make a VERY unhappy customer.
Yeah, the furnace return is a reasonable place to put it. Sound like a control issue, though, too. Maybe you can avoid that by interlocking the HRV [better] w/ the furnace controls? At minus 40, not sure why the furnace went out on high limit if your return air temp was low ... admitedly, I'm not acutely aware of how that furnace control specifically works.
Doesn't matter what the return temp is if the filters don't get changed.
Yeah ... you're right. Was that the problem you had w/ one? We always miss the easy stuff. I swear ... need a big sign on the furnace ... check filter here often! change regularly! put one on the frig, too (that's where people ALWAYS look for info, isn't it?).
NannyGee, Most everything in your house is out gassing, and you need some air changes for healthy air. We preach tight houses, and you can have yours tested. The points are:
A. control when and where to bring in the fresh air. It will get in, but down a chimney isn't a great path...
B. the heat recovery systems allow some heat recuperation as well as a filtering opportunity.
PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
I think I read somewhere that HRVs/ERVs are installed in most all commercial buildings, and I also think they may be required by code in many parts of Canada? Can anyone verify this (or not)?
HRVs recover heat. ERVs recover cool air too for climates with heat and A/C and are more expensive. ERVs also deal with moisture.
I also believe that the further north you live, the more cost effective they are. They can be expensive (up to $2000), although I haven't priced them lately.
It would seem like, at some point HVAC manafactures would start building these units into their HVAC systems.Matt
No, they aren't installed in 'most' commercial buildings. HRV heat recovery ventilation and air to air heat exchangers are the same thing ... terms used primarily in residential systems. In commercial systems it is called exhaust air heat recovery (EAHR). Some commercial applications running large quantities of exhaust for long periods of time find a good application ... from primarily an energy perspective. Hospitals are a good example as they run exhaust 24/7 usually and it has always made sense to recover some of the energy. Other applications can include shop exhaust. They use various methods to recover the heat including hydronic coils in the exhaust and ventilation air streams.
CaseyR, Cairo, and Paul are right.
An HRV recovers much of the heat from the exhausted air and that beats opening a window. They also have automatic controls based on outside temperatues and indoor humidity. Like having a servent open and close windows at just the right time. The one in my 1700-square-foot house cost $1100 plus $700 for ALL the ductwork (HRV, range hood, dryer vent).
Without an HRV, I'd have to open a window. At -20F/-30C, that is wicked cold on your feet, kills the adjacent house plants and wastes energy. With the HRV, the incoming air is about 45F*. 45F still feels a bit cold but a LOT less cold (65F) than -20F.
If I didn't run an HRV or open an window, body odors, cooking smells, CO2 and humidity would build up. Therefore, my wife would move out. And while the humidity might finally equilibrate at 70-80% R.H., that would cause massive condensation on the windows - huge puddles on teh sills and the floors below (it's -3F right as I type this). And any leaks into the building envelope would carry lots of moisture into the wall cavities.
By code, I'm required to have a certain air exchanger per hour (and it's just good building practice). Since my house was the tightest ever tested in my town, it's good that it has an HRV.
*45F incoming air when -20F outside the first year. Then I shoved a double length of 4' fin tube in the HRV duct. Now if the HRV is on and the thermostat calls for heat, it comes in 90-95F.
Excellent explanation and example David.
Thanks all.
I guess these units are more efficient than I thought, which is good to hear. I don't have a lot of experience in super tight houses so I shouldn't be surprised if it's quite different than I'm used to. My current home is a 1924 bungalow which is the epitome of a "breathing" house.
Have a great Thanksgiving!
By code, I'm required to have a certain air exchanger per hour (and it's just good building practice).
I was just this morning discussing this with my MD/PhD physician. Code here is the same, but interpreted such that an operable window satisfies the requirement. However, we have 2 seasons, total 6 months, where open windows would be uncommon. As we're just now into another closed window season, he was extremely interested in alternatives to drugs for solving respiratory problems in his patients. He was most interested in my observations on the availability of low cost systems.
I built my own, which has historically been called an air-to-air heat exchanger, now HRV. It is similar to Lossnay but with different sized fans such that I pressurize the house. The reason for pressurizing is to ensure that all leaks go out. Material cost was well under $100. Temperature testing showed efficiency right at 80%. I followed Canadian code of the time to give us a 2 hr total airchange for our 20,000 cu ft. These are very small fans. I included ductwork to closets, bringing in continuous fresh air. As we also have a dehumidifier in the system, used seasonally it completes our needs as long as I remember to change the external filter, which collects pollens and smoke admirably. It is not unusual for guests to remark on our indoor air quality.
Almost anybody with existing ductwork could add an appropriate-sized system easily. Nanny Gee's house may be leaky enough to benefit little (other than making the leaks go out), but modern houses are a very different matter. We live in a moderate-risk radon area which is now a moot point.PAHS Designer/Builder- Bury it!
I have been thinking of building my own HRV. Any additional details would be appreciated - particularly on dealing with freezing of the condensate in the exhausted air duct.
Maybe we ought to exchange phone numbers for a quicker exchange of ideas but my thoughts on DIY HRVs are broadly:
85-90% is easy. 97-98% is also easy and cheap (but it weighs more).
Compact, light, and cheap is more elusive.
The first of two general approachs I have: BIG, BIG surface area. Tight spacing on the plates. LONG, really LONG for high efficiency. But sized to fit within a joist bay (like 9" x 14" by 10 or 15 feet long). Built up out of either the thinnest possible Al sheet metal. Or the heaviest Al foil stretched across the frame. spaced about 1/4" apart would give 600 square feet of HX surface. Vastly more than commercial HRVs. And would be truly, purely countercurrent. Some are a hybrid countercurrent/cross current to make the units more compact.
The other general approach: Hot rocks. I've done this at 1100F for really high efficiency (95-99%) catalytic oxidation pollution control equipment. Blow out through one bed of pea gravel while blowing in through another. Switch every 10 or 60 minutes, depending on the mass of rocks. The rocks have such huge surface area, turbulent flow over them and high mass, they capture almost all the heat.
Toddler's up. Gotta go.David Thomas Overlooking Cook Inlet in Kenai, Alaska
You are a genius. I'm putting one of these in my new home and never would have thought of running thin tube in the duct work to boost up the temp. of the incoming air.
Quick question: hrv or erv in the northeast.I keep getting different answers from the manufacturer's.
Thanks and Happy Thanksgiving to All.
"hrv or erv?"
You say Toe-May-Toe and I say Toe-Mot-Toe. You say Po-Tay-Toe and I say Po-Tot-Toe. :-)
Happy T-Day.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
Hi David
did you hook up your hrv to your range hood? I thought that was a bad idea due to all the grease.
Saw a real cool idea a few monthes back in JLC. Instead of a hrv or erv or any such thing, just use good bath exhaust fans (panasonic) that are on a goodly amount of the time and route your incoming fresh (and cold) air to wash across the back of your refridgerator. I think I am going to try that on my next house. But the climate here in Sitka is a lot warmer than where you are. Heck if you did it when it was 40 below your fridge might end up being your freezer.
25degrees ,first snow of the year 4", and the turkey is soon to go into the oven
You're right. Greasy range hood air goes out its own duct. The passages in the HRV HX's are about 1/8" x 1/8" That would clog up in a hurry.
I like blowing cool air over the fridge coils. Here's one I've always wanted to do: Frame a exterior doorway on the north side of the house, sized for the fridge (or fish freezer). Have the coils OUTSIDE the house. Have one or two inflatable gaskets (bicycle intertubes?) to seal the gap.
I think of Sitka, climatically, as North Seattle. :-)
-8F here now. Happy T-day everyone!
David Thomas Overlooking Cook Inlet in Kenai, Alaska
I can top that. My plan is to add another set of coils on the exterior and a valve, or pair of valves, so I can send the heat outside when I don't want it or keep it inside when I do want it. I figure a little extra plumbing is less complex than sealing a refrigerator size hole in the wall. But I've been wrong before.
"My plan is to add another set of coils on the exterior and a valve, or pair of valves, so I can send the heat outside when I don't want it or keep it inside when I do want it."
Agreed, that is superior. Then in conditins when you want the heat - input in to the house,. But when heat pumping to the inside isn't that different than heating pumping to the outside (outdoor temp 50-60F?); you can turn those electrons into interior BTU's. And, of course, dump heat outside if outside temps exceed 70F. More $ to run the fridge but less $ to run the A/C.
But as it gets cold, a KWH is so much more expensive than buying the same 3,413 BTUs as natural gas, that it makes sense to improve the efficiecy of the refrigeration cycle.
Now, if you house is electrically heated (what a thermodynamic heresy!), sure, then it is a easy decision - dump heat inside during your heating season and outside in your cooling season.
Better yet, burn some kind of dinosaur juice for heat instead of making the electric meter spin so fast.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Another thought I had was to dump the refrigerator heat into a preheat tank for the domestic hot water. I haven't tried to do the calculations, but I figure cool water, ~45F in winter to ~70F in summer, would have to cool the coils more effectively than air at 70 or 80F. Might even be able to get by with a much smaller condenser. And the heat would be doing something useful all year round.
Not only would the incoming water be cooler, but the water's conductivitiy so much better. So, yes, you would use fewer KWHs in the fridge, a couple less BTUs in the HWH and/or could use a small condenser coil.
A modern fridge is such a strange thing. A small cool box (butter trays are heated a bit on upper end models) in a cold box (the fridge) inside ####warm box (the house) with is (for me) inside of lots of cold air.
There was a time when people would dig a hole in the ground and call it root cellar. And use no freon, urethane foam, electricity, fiberglass, or compressors.David Thomas Overlooking Cook Inlet in Kenai, Alaska
David,RE the things we do with a fridge...we also condense moisture out
of the air, collect it in the freezer, auto defrost it to drain into a pan where we evaporate it back into the house air (except for the globs that turn red).That is OK for your climate, but here in the A/C world, moisture is the bad guy. A number of years ago, I put a small floor drain under the fridge to a basement laundry tub. I ran a vinyl hose from the catch pan under the fridge to the drain. Now the condensate drains away in the a/c season, a few quarts per day that the central air does not have to deal with. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
And, of course, a frost-free fridge/freezer has to cool to a lower temperature in order to drop out the moisture. Hence the heat pump is working is working against a larger delta T. Sure is nice not to have defrost once a year though!
At least up here, you just leave the ice cream on the porch for a day as you defrost and it's easy. Unless a bear comes by!David Thomas Overlooking Cook Inlet in Kenai, Alaska
Just picked up the thread and have a question. I have a sealed furnace room with a gas furnace and gas water heater. I want to add an HRV kit as my home in Seattle gets stale, and when I add new windows, it will be worse. Any thoughts on using the sealed room to locate the HRV and tie into the ducting? Any CO worries from the burners? The HRV should be a sealed unit, no?
Injecting HRV incoming air into most forced-air furnace systems would be fine. The HRV air is low flow compared to the forced air system. But have a good furnace tech consult on it. Maybe an air check valve would be needed to prevent backdraft (carbon monoxide hazard or blowing out the pilot light). I'd be inclined to run separate ducts for outgoing HRV air. Take it from the stinky rooms - bathroom, kitchens. Put fresh air into the bedrooms (which out to be cooler anyway) and the living areas.David Thomas Overlooking Cook Inlet in Kenai, Alaska
I think a big 6 inch intake at the litter box would be just right...............P.U.!
David -
Unfortunately, my 80 db hearing loss precludes telephone conversations - not particularly handy in a society obsessed with telephones... E-mail and fax are my long distance communications media of choice.
Your #1 method sounds very much like what I had in mind. Very long adjacent ducts, or possibly long thin tubes running through a long duct. The local paper advertises thin sheets of aluminum for cheap, but a multi-layered exchanger would require a lot of welding.
I have always had this dream of having a heat exchanger for summer which had the out going air go up a chimney which had one side of glass oriented toward the sun and the inside of the opposite wall painted black and having the air get heated enough to create an out going air current flow. Hopefully it would eliminate the need for a fan - but then it might not and in summer a PV panel could be used to power a blower motor when the sun was at max.
My other pipe dream is that since my area of Oregon has hot dry summers (around 15% RH not unusual) was to use a swamp cooler to cool air on one side of a heat exchanger which would cool the incoming air without getting the air too muggy (although introducing a little extra humidity would be too bad). Of course, you aren't quite so in need of air conditioning up there - at least until global warming brings you up even with Miami or such...
(Actually, what I have read is that the winter maximum lows - max mins? - have gotten higher by as much as 6 degrees over the past couple of decades while the summer highs haven't changed all that much. Still, it is going to prove interesting when the North Slope turns into a giant marsh most of the year and all the folks whose homes are resting on perma frost find out they need house boats instead...)
Some of the commercial HRVs have (or at least to have) a revolving plate of some type that transferred heat and some moisture (and I assume some polutants as well) between the warm and cold sides. The ones I read about didn't have all that high of an efficiency rating, however, if I remember correctly. I think the one I read the details about was from a Japanese company.
"Some of the commercial HRVs have a revolving plate of some type that transferred heat and some moisture (and I assume some pollutants as well) between the warm and cold sides. The ones I read about didn't have all that high of an efficiency rating, however, if I remember correctly."
Yeah, like a disk of metal tubes parallel to the air flow (or rocks). Can be like 8 slices of a pie, that remove the heat from the top position where the air is going out. Then rotate to the bottom position where the air is coming in the reverse direction. At 90 and 270 degrees, it is blinded off to prevent cross connections, Has the advantage of never having to reverse a valve back and forth or reverse blower direction. Just keep rotating slowly.
If they aren't getting great efficiency it is either because there is leakage from one side to the other or (more likely) because the rotating disk is very shallow. It needs to be long so the gradient is very gradual. Think reversible processes - those that do not increase entrophy. Everything done in infinistially small steps. That's where you would achieve perfect heat recovery.
The same thing is done in modern jet engines. They, however, have excuse for keeping things small and light. But in a house? At the cost of pea gravel? Why scrimp?
What did you study/teach at UCB? Chem Eng for me.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Much of my study was one of perpetual studentdom... M.B.A., M.L.I.S., Almost PH.D. in Higher Education Administration but spent most of my time auditing courses in City and Regional Planning (they wouldn't let me register for any courses in the School of Architecture) and in Anthropology. Taught courses in school audio-visual methods and then Environmental Studies ("Man and the Environment" was the title back in the pre PC days) and taught workshops in database management and scripting for Computing Services in Evans Hall. My largest chunk of time was spent trying to create an interdisciplinary ecology program, but we only got parts of it - lots of turf vigorously held at Berkeley... My M.B.A. advisor was Wes Churchman who wrote the original textbook on operations research, he might have given some courses in Engineering.
After I couldn't afford to be a student any longer, I went to work as the Manager of Computing for the Berkeley Business School and then when they decided they needed a fund raiser rather than a computer manager, I went to work as the Assistant Registrar for Systems, so I might have been riding herd on all your course records at one point. (One of the ladies I worked with got fired for forging higher grades on student transcripts. Unfortunately, I only found out about it after the fact or maybe I could have had a real PH.D. on the transcript...)
Anyway, what sort of worries, if any, are they expressing in Juneau over the possibility of some/most of the permafrost melting over the next several decades? Is politics in Alaska still the same type of small club that it was when I was up there in the mid 70's? (I hear they cut your government oil dole this year with more cuts to come, I assume there were more than a few howls about that...)
"Anyway, what sort of worries, if any, are they expressing in Juneau over the possibility of some/most of the permafrost melting over the next several decades? Is politics in Alaska still the same type of small club that it was when I was up there in the mid 70's? (I hear they cut your government oil dole this year with more cuts to come, I assume there were more than a few howls about that...)"
Global warming: We get it a lot more up here than you do. Like +5F instead of +2F on average. Glaciers retreating many miles, loss of permafrost, eroding coastlines due to rising sealevel. In 4 year, I've lost 1/6 acre of my land into Cook Inlet. But Alaska will never say, "Don't buy oil." Or "don't burn fossil fuel." So Senator Ted $tevens ("Uncle Ted") just grabs a few more billion (about $2 billion/year more than our share by most estimates) and coastly villages get relocated, seawalls get builts, etc.
Governent Oil Dole: The PFD (not personal floation device, but permanent fund dividend) is down from a high of about $1,900 in 2001 (high Clinton-era stockmarket) to about $1,300 now. Yes, people are upset. A family with 4 kids went from an check of almost $11,400 to one of $7,800. That two less snowmobiles or 6 less rifles/shotguns each year. The majority of the population is adament that free money should continue (just for taking up space), yet continue with no income tax, no sales tax, and the budget problems should be solved by vague "limiting government spending". But maintaining police, fire, schools, and road maintenance. Those people have always seemed a few olives short of a Greek salad in the arithematic and logic departments, IMO.David Thomas Overlooking Cook Inlet in Kenai, Alaska
I lived in Fairbanks for many years and watched this house slowly deteriorate because of the permafrost it was built on. Thankful it wasn't mine!
pete....i thought this was a current thread... was remarking to myself that i hadn't seen dave thomas in quite a while... then started looking at the previous datesMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
I have been thinking of building my own HRV. Any additional details would be appreciated - particularly on dealing with freezing of the condensate in the exhausted air duct.
Happy to oblige. I probably know less than David. My original inspiration came from a 1986 Popular Science article which I'd be happy to post if somebody can tell me how to get a reasonably small jpg out of Photoshop. It scanned great on my new scanner but I'm having trouble getting a small image that's readable.
They used Coroplast double wall plastic panels for the core, which made little sense to me. I used corrugated aluminum roofing cut into 2'x2' sizes for the core instead. The freezing of condensate shouldn't be a problem as long as it's collected inside the heated space. Well, maybe in Alaska, but in Virginia (down to teens) it's not an issue. Actually I found that the way we run our house, I had almost zero condensation and ended up disconnecting the drain.PAHS Designer/Builder- Bury it!
You said " if somebody can tell me how to get a reasonably small jpg out of Photoshop. "
There is a "Save for Web" option under the "File" pulldown. Ir makes pics quite compact (kb wise).
Matt
There is a "Save for Web" option under the "File" pulldown.
Thank you very much. And similar to Uncle Dunc's suggestion, it automatically turned into a gif. Aren't they still bigger than they need to be?
Anybody getting eyestrain from reading the text can give me a tutorial. The Help pulldowns aren't very.PAHS Designer/Builder- Bury it!
Tom, Just printed pic . Sister had left photo paper in printer, beautiful results of article. Boxy looking device. Believe I will make one about 8' long using 6 -8" dia pvc or duct. Thanks, PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
>> ... how to get a reasonably small jpg out of Photoshop.
JPEG isn't well suited to text. When you reduce size you lose resolution, and it's a lot more noticeable on text, which has to have fairly sharp edges, than it is on most photographs. Can you manipulate/save it as a GIF? I can't promise that'll help, but it might.
VaTom, I have built some ultra filtered intake systems, but not an HRV.
I was considering building one, and have some finned aluminum coil stock. If you have a diagram, I would be interested in looking it over.
With a 28 year old house, it is not as tight as I could build today, but an HRV would be a nice addition to my fresh air non-recovery system. Believe a few others would benefit from your diagram as well.
PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
No problem if you can direct me through Photoshop to post the PS article.PAHS Designer/Builder- Bury it!
Tom, when I get home,(Richmond) I'll get with you. I may have the PS from the past. Fax is a good method as well. Where in Va are you?
I would love to see David's set up, but I don't get to drive that route often,(like twice in my life). I, like many on here, like to invent, design, build my own systems.
Enjoy your Turkey Day. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
David:
Very interesting... and by your locale, I'd bet money you know exactly what your talking about. Makes sense that if you want to know about building tight houses, talk to someone in Alaska!!
You said: 'Since my house was the tightest ever tested in my town, it's good that it has an HRV." What, was it a blower door test? Not to be nosey, but what was the number CFM of leakage? Also, is the air leakage testing done on requirement of the building dept, code, or just good building practice? More being nosey... did you pressure test your ducts - is that common up there?
You also said: "Then I shoved a double length of 4' fin tube in the HRV duct." I think I know what fin tube is... pretty much like what is in a hotwater baseboard unit? How does that help? Not scrutinizing... just trying to learn something.
Thanks,Matt
I'll try to dig up the blower door test numbers. The private tester (also the city building inspector 9 to 5) almost couldn't read the delta P. He had all three of his insert rings in the blower door and thought he might have to order the smallest, fourth ring that he'd never had need for before. Tested 5 times tighter than any other he'd done. But my house has no attached garage (common up here). By test protocol, he'd have to have left the door to a attched garage open hence would really be testing the leakiness of the garage vehicle doors.
I'm sure it is not super-tight compared to some of the tightest that have been nationally. Structurally, my house is stick-built 2x6s on 24-inch centers. But 5 inches of spray-in-place urethane mean that accoustically, thermally, and airtightness-wise, I'm living in a big styrofoam cup. And I paid attention to all the openings and penetrations. And have a direct-vent HWH (potable + space heating) as the only stack in the house.
I didn't pressure test the ducts but visually inspected the installation and workmanship. Except the range hood that the carpenters put in. I should have. I may have a little leak there. I definitely have one somewhere near but it has to be -10F or less to thermally image it.
Yep. I bought two 4-foot baseboard units. Like $24 each? Aluminium fins on 3/4 copper pipe. Tossed away the sheetmetal covers and sweated them into a U-tube that fit within my HRV ductwork. The duct carrying outside air, after the HRV on the way to the house. Don't be worried about stealing my idea. I'm thinking of sending Andy Engel a one page article on it. I figure I'm in the business of coming up with NEW clever ideas (about toxic wastes). Not trying to profit from old ones. Sitting on the couch eating bon-bons while the royality checks roll in is both a fantasy and undesirable, IMO.
At -20F outside temp, the incoming air went from 40-45F to 85-90F. In moderate weather, the incoming air runs 95-105 instead of 50-55F before. But only if the thermostat wants heat. I've got a high mass system so I can overshoot if, like 8 days ago, the outside temps come up 50F in 18 hours. Then cool air coming in is good.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Sorry I guess I'm a little dense. You are saying that there is hot water running through the fin tube that is in the HRV ducts?.. Essentially preheating the incoming air...
BTW - you don't have to look up the #s in the blower door test. I was just curious.
ThanksMatt
I didn't know you were allowed to run your range hood through the system. That's always been a contentious issue in the lower 48. Maybe since you can remove the heat exchanger to clean, it is OK. Grease build up can deter both efficiency and promote a fire hazard.
Everyone has given their two cents ... now mine.
Air to air heat exchangers, heat recovery ventilators, air exchangers ... pretty much all the same thing. While you can get a heat pump style exhaust heat recovery device, most of these systems are not refrigerant based devices. They are simple devices that recover wasted heat from your exhaust air and put it back in your house.
A tight house has a tendency to retain moisture (showers, cooking, plants, people breathing, washing clothes, aquariums, etc.). Also as some mentioned you have other 'bad' things like CO, VOCs, and plain everyday odor. I've seen significant moisture damage from failure to ventilate a tight house enough to rid it of moisture. So ... you need to either open some windows and/or turn on an exhaust fan ... but remember in a tight house ... what you take out HAS TO come in some where. A real tight house w/ an exhaust fan on may remove very little air if it can't bring that air in some how (usually small openings in the construction, around window sashes and around doors).
The HRV allows you to recover some of the energy it takes to recondition the air normally coming in through the window or other openings. If you run an exhaust fan a lot because you have a tight house, the HRV gives you an opprotunity to recapture the energy in a controlled fashion. The colder the climate the more potential.
There are various technologies used to recover heat. A common one is a box made of plastic panels with channels in them (a thick panel w/ a series of holes running the length). Each panel is oriented perpenicular to each other ... one direction for exhaust, one for fresh air. This allows the transfer of heat through the material to the other air stream.
Efficiency (it's actually called effectiveness) ranges from 50% and higher ... one guy mentioned 80% ... maybe under theoretical/ideal conditions. I think the normal range is 50-70% ... that is, you will recover 50-70% of the otherwise energy required to reheat that air that you exhaust.
At 0.35 air changes per hour ... the typical average for a well constructed house (lower if it is 'tight') for air leakage ... I think this can be anywhere from 1-4 times the heat loss through walls/roofs/windows. It is significant and the answer generally is NOT to turn off your bath fan. The house has to breath ... you need air exchange ... do you want to pay for that or do you want to recover some of that. I can't answer that for you ... if you have an old house .... there isn't much need to run your fans as much as in a tight house. You really can't recover heat in an uncontrolled heat exchange situation (i.e. an old, leaky house).