The state of HRVs

archibald tuttle

Never installed an air to air heat exchanger and must not have any buildings so tight as to demand it. never had any notable combustion problems with atmospheric heating units (or forced room air combustion typical of most oil and gas conversion burners) in basements or they had provided air in the boiler room (and now these larger units are all on sealed combustion piped outdoor air anyway, but I'm considering an HRV for my own home. New bathroom.

Thinking globally, i.e. reversing field this raises the question of supplying air for gas dryer, kitchen hood and typical oil boiler as well, but you got to start somewhere.

So on the bathroom side, best install given the size and approach of various HRV units I've seen (airiva[suncourt], fantech, and nutech[lifebreath] would be hanging from the 10' basement ceiling close to the outdoor wall. easy details for wall pentrations there. what kind of separation is generally needed between inflow and outflow. Is vertical separation preferable. this will be a moist outflow, i always believed moisture goes up so thought the exhaust should be higher than intake but???

With some engineering, i.e. piping, I could actually send the exhaust up next to the chimney and exhaust through the roof peak. But this raises my next question which is pipe friction and vertical integration.

The Bathroom is on the first floor so my intake from the bathroom for exhaust air would be at the 1st floor ceiling and if my HRV is installed at the basement ceiling, air would have to be drawn down ab out 10 feet vertically and then about 15 feet horizonitally. Could keep it to 1 elbow.

I'm thinking of using light schedule PVC for piping, I hate metal ductwork, most especially the joining and elbows. And since this is a moisture laden stream I figure the PVC could handle some condensing on the walls. The units that seem to fall withing a manageable price range are nominally 150cfm.
($400 for in line, $600 for vertical in/out).

I have normally looked to take advantage of house leaks for makeup air by using a stronger exhaust fan (maybe 300-400 cfm nom. -- I know all this cfm stuff actually depends on pressure difference inlet and outlet including system pressure drop from the piping and heat exhanger itself, but I'm generally taking the 0" water column static pressure as a comparison if not actually detailing performance) and letting nature take its course, since the bathroom fan, classically speaking is not a constant source of air exchange but is meant to exhaust excess moisture on a few occasions during the day.

So the question becomes, is all the piping and the 10' drop in exhaust air defeating my purpose or does the fact that there is direct make up air supplied in modest proximity (coming in at bathroom floor 8 feet down and 6 feet sideways in the same room through a passage in radiant heated floor to make final comfort) still give me some hope of accomplishing something. I do assume that at least the airflow constriction of the heat exchanger itself is considered in the CFM ratings for these HRVs although they may favor themselves by allowing some factor for the make up air creating less pressure difference. I don't see anything in most literature on HRVs addressing these comparisons.

As to the payback of these installation considerations and the $300 or more added cost for the HRV over exhaust fan, the Airiva has a plastic heat exchanger but claims about 70% recovery (I can't find the table in their literature that explains what the relative temperatures of intake and exhaust air are that lead to that efficiency figure but I'm assuming they are picking their best efficiency to promote rather than average). I like the idea of poly pro meaning I'd have virtually an all plastic airflow system, but it strikes me as likely not as efficient as aluminum at the actual heat exchange.

Any personal experience or technical thoughts would be appreciated.

Also haven't been able to find product literature as readily on the lifebreath.nutech site. I did find a store where they offer a single model for $1200 in the 150cfm range but they have all these testimonials about how they saved the known world in various vast commercial applications and I can't believe they did it by lining up 150cfm units next to each other, maybe I'm wrong.

Anyway, I can't see any reason to dump $1200 on a nutech compared to $600 or less for a fantech aluminum heat exchanger or $400 for a poly-pro airiva. Please set me right if I'm missing something although if the answer is that only the $1200 unit is useful, its probably back to the $125 fantech inline 6" exhaust fan anyway.

Finally (everyone mops brow) Assuming I'm moving this way because I'm tightening house and energy budget, I've seen outdoor air supply kits on oil burners although not often and never had a chance to really get close to them to see whether these are just boot adapters and piping to regular burners where the air shutters are set to reflect the friction in the piping (which if open to the outdoors may not be so much more than trying to draw air in through house leaks anyway depending on the house) or are these burners specially manufactured -, e.g. bigger supply air fans and motors to handle running them ,etc.

I haven't seen a supplied air to air heat exhanger for a dryer but I could set up a relatively short piped space for that.

On the kitchen side you have grease instead of lint to complicate matters. But if you run an air to air heat exchanger independent of the range hood, this doesn't really give you make up air for the kitchen hood itself. I can wrap an intake duct in 3/8 or 1/2 pex and heat the air on the way in, but obviously I'm not recovering exhaust heat by that methodology. Still, if greasy cooking is 1/2 hour a day, maybe it is over the top.

I do have buried intake air for russian furnace and fireplace yet to be built.

Sorry about the novel, but I have to pull the trigger one way or another within a couple weeks so I thought I'd bare my soul.

Brian