*
The house is sealed high(ceiling/attic)and low(crawl space/basement), quality windows have been installed with great care taken to seal to the sheathing, fireplace/heatstove breaths outside air, caulk or foam occupies every crack and the doors are sealed with referigerator type seals. This house is probably very warm, but do the people inside need some fresh air?
Heat Recovery Ventilators (HRV’s), how necessary are these devices? How important is it to adhere to the suggested .35 ACH (Air Changes per Hour)? Where is the best location to exhaust the interior air? Best place to supply the incoming fresh air? Who manufacturers a good quality unit? (or a poor quality one?) How best to control? (Speed control, timer?)
Thank You in advance for any info.
Replies
*
You don't really give enough info. for a quantitative answer. But no matter what make sure you get enough fresh air in the house! The chemicals outgassed from furniture, carpets, cooking, etc will quickly make the house the most toxic place you visit during the day if you don't insure adequate air exchange.
What is the most economical can only be determined by a thorough examination of the house. This should include a blower door test for air infiltration, a physical examination and a cost/savings analysis using data for your geographical area.
Then you can make a decision. If its' not new construction often times Tight sealing/more insulation together with the HRV just don't pay off in the homeowner's lifetime.
*After rereading your question I think I might have jumped on the wrong track in my first answer. It looks like you already have a tightly sealed house. Probably you also have plenty of insulation but you could check to be sure.When you get through all the ventilation snake oil the remaining question is: How best to get fresh air in and the stale, possibly toxic air out?In a recent remodel in a very cold climate we decided after a blower door test and climate appropriate calculations to seal some cracks, leave others open, and add a continuous, quiet fan in one bathroom. This will keep the air as fresh as outside at least, minimize energy consumption(we think), and the total cost should pay back fairly quickly.Maybe a few decades downstream we'll have test data on just how bad air can be and still not kill us, etc. But for now; take your best crack at the problem, relax and enjoy your house.
*Fred, I've heard you mention the amount of leakage you usually find in "well sealed houses" and now my curiosty is killing me! Where is it that you most commonly find this leakage? and secondly I haven't seen you comment too much on actual air or thermal leakage past new windows or doors. Could you share your thoughts on this with us? Thanks again......
*
Gary, Fred et al
HRV's are required by code in all new construction here in Ontario, along with housewrap (with a few exceptions), and attic ventilation (with one notable exception). The general theory of the HRV, recapturing some heat from the exhausted air and preheating the incoming air, makes a certain amount of sense but as Fred ponts out doesn't stand up to scrutiny for cost effectiveness, and is probably overkill in most new 'pretend tight' construction.Unfortunatly the design/layout part of the code is lagging behind.
I was doing a big job in an unfinished basement in a new condo rowhouse a couple of winters ago and kept smelling 'funny' odours. One day the unmistakable stench of PVC cement came swirling around me, followed by the equally unmistakable scent of 'old stogie'.I wasn't producing either of these smells so I went upstairs to check it out and upon looking out the front window saw a workman gluing up a PVC drain fitting next door while chomping on a stogie. The intake for the HRV was near ground level, on the street side of the house, not 5 feet from the neighbours garage and drive. The other odours I had been smelling were probably stale car exhaust from the neighbours morning car warmup ritual. The only code requirement at the time had been adhered to; the intake and exhaust had to be seperated by 3 feet, and they were, exactly. They could have taken their incoming air from the back of the house, and exhasted out the front, but they saved themselves 10' of duct and created a serious hazard which they weren't libel for! Needless to say it was repeated in every dwelling in the development. I've met a lot of bozos in this business but none more than in the HVAC trade, and that includes their designers, and inspecters.
*The sort of system Fred L outlines in his reply is provided by the Aldes central exhaust (only) system, suitable for mounting in the attic. It kinda looks like a big udder with variable length, uh, teats (ducts). It provides boost-able local venting in noxious areas (e.g. baths), and general low-level venting in broader common areas. Fresh air make-up comes through adjustable intentional fresh air vents usually installed in bedrooms and living rooms. I have no financial interest in the Aldes system, but years of research in this area points to this system as a sound strategy. This is the "ventilate right" part of the "build tight, then ventilate right" mantra. And of course, the "build tight, ventilate right" approach requires that combustion appliances are all sealed and use direct outside air for combustion.
*
Gary, Fred, et al. HRVs are necessary because the HRV manufacturers say they are. Indeed, their sole goal is to equip every house in America with one, or two if necessary.
The Northwest Power Planning Council (Sduper Good Cents Program)found that air-to-air-heat exchangers (AAHE) as they where then called, found them generally not cost effective.Higher costs and less tight houses spelled the end of the widespread use of AAHEs, particularly in the milder climates of Washington and Oregon.
Engineers on state funded housing projects in South Dakota also found AAHEs to be not cost effective.Installed costs, at the time, of HRVs ranged from $500.00 to $2000.00, and their efficiencies ranged from 55 to 75 percent. The most effective units had incremental savings of only $1800 per year.
The need for am HRV depends on: (1) the energy cost of ventilation; (2) the cost of savings from heat recovery; (3)the cost of a HRV. The cost of ventilation depends on (a)the rate of ventilation; (b)the coldness and duration of the winter; (C)fuel costs; (d)furnace efficiency.
The energy cost of ventialtion over a heating season is computed from the formula: Q = 0.018 x ACH x V x 24 x DD. Where Q = heat flow in Btus, 0.018 is the heat capacity of air, ACH is the ventilation rate in air changes per hour, V is house volume, and DD is the number of degree days per year.For a 2500 sq.ft. house with a volume of 17,400 cubic feetin a 7000 DD year, heat with propane costing $.99 a gallon, and an air change rate of 05 ACH, what is the annual energy consumption?
Q= 0.018x0.5x17,400x24x7000 = 26,308,800 Btus per year.
The cost of this energy is found from C=[q/(Uxe)]x P. where C is the yearly cost in dollars, U is the number of Btus in the purchased fuel,E is the furnace efficiency and P is the cost of the purchased energy.
C=[26,308,800/(95,000x.75)]x0.98= $365.55 per year. The ventilation costs of this house are high and a HRV might make sense. Reducing the ACH to 0.25 ACH reduces the cost to $182.78. Changing to natural gas and a 95 percent efficient furnace reduces costs to (assuming an ACH of 0.25 and fuel costs of $.65 per therm) C = 13,154, 400/(100,000 x.95)] x 0,65 = $92.77. NOTE. the cost of operating the fan is not included in these calculations. Here ventilation costs are low and heat recovery is less likely.
Clearly then, there are a number of factors to consider before buying an HRV. In Saskatchewan, Canada with a 12,000 DD year, a HRV provides substantial savings. But in milder climates savings are marginal. The 1977 Leger house in a 6700 DD year,an ACH of 0.25 and using natural gas achieved savings of only $25.00 annually. In this instance the HRV is and extremely expensive and unnecessary method of ventilation.
Fred as usual gives good advice. Hope this helps.
*In my 12/31 post thee is a mistake in the 3rd paragraph. Change $1800 to read $18.00. Gene L.
*
The Humidity in my house is about 35%, and yet I still get condinsation on some of the windows, especially the bay windows. It seems to dry during the day and then reapears in the morning. Could this be from showering? We always use the exhaust fan. Dave Magliacane
*
It seems logical to me that bay windows are cold sinks, not benefitting from air circulation as much as other windows.
*Dave. Having a bathroom fan means only that you have one. If the bathroom door is too tight to the floor the best bathroom exhaust fan is useless: no makeup air. The fan may be undersized for the bathroom. The bay window is too deply recesswed to benefit from the warm air. Are you certain that the RH is only 35 percent? Is your glass single pane? Gene l.
*
The house is sealed high(ceiling/attic)and low(crawl space/basement), quality windows have been installed with great care taken to seal to the sheathing, fireplace/heatstove breaths outside air, caulk or foam occupies every crack and the doors are sealed with referigerator type seals. This house is probably very warm, but do the people inside need some fresh air?
Heat Recovery Ventilators (HRV's), how necessary are these devices? How important is it to adhere to the suggested .35 ACH (Air Changes per Hour)? Where is the best location to exhaust the interior air? Best place to supply the incoming fresh air? Who manufacturers a good quality unit? (or a poor quality one?) How best to control? (Speed control, timer?)
Thank You in advance for any info.