I'm considering adding a Heat Recovery Ventilator to my 1200 sq.ft.
house heated by solar thermal with a wood stove backup. However, most
of these devices that I've found seem suited only for larger houses
(>2500 sq.ft.). Can anybody recommend a good brand of 'mini' heat
I have a Venmar HEPA 3000 model, which is rated for up to 3,000 sq.
ft. I'm very pleased with the performance of this product. I run
mine mostly in "recirc" mode as a way to distribute heat from my
ductless heat pump (located in the living room) throughout my home;
you could do the same with your wood stove.
If you don't require this much capacity (it provides 105 cfm of fresh
air for a total of 270 cfm of filtered air), you could operate it on a
timer so that it runs only at set times of the day, e.g., three or
four hours in the morning and likewise in the early evening. Since it
is plugs into a standard wall outlet, an inexpensive lamp timer (e.g.,
the ones with the multiple pin settings) is all you need.
Any of the HRV models will work in your home.
Run the calculations for volume of the air in your house and divide that
by the amount of fresh air you want brought into the house. Divide the
answer by 60 to get hours to exchange rate
Answers of less than 3 hours and you may want to cycle the unit on a timer.
For example 1200sqft with 8 ft ceilings is 9600 CU FT.
Using Paul's installation as an example above, it brings in 105CFM
A constant 105CFM does a complete change out of the air in your home in
So a reasonable approach is to use a timer that will allow you to run it
20 minutes out of every hour for a complete change out in 4.5 hours
Paul's lamp timer idea is useful, but you may want a slightly more
flexible timer for this application.
For example, this timer has a remote control and allows 20 on/off
cycles. Not enough to cover all 24 hours, but close and very inexpensive.
Some good points. A fully programmable timer with a remote control
would be a smart way to go and at under $25.00, the price is
I should add this particular Venmar operates at two speeds ("normal"
and "boost") and at this lower setting the outside air portion is 70
CFM. As mentioned, I run mine in "recirc" mode most of the time and
only use the fresh air option when showering and at other times as
required for humidity and odour control (a simple wall control located
in the main bathroom allows me to change these modes and to turn the
unit on and off).
Depending upon occupancy load and living habits, showering and cooking
can be major sources of indoor humidity. Here in Nova Scotia, it's
not uncommon for people who heat with wood to store a portion of this
wood inside their basement; as you can imagine, the amount of moisture
given off by this wood as it dries can be significant. I would
closely monitor indoor humidity levels and adjust run times
accordingly. With the remote control timer you've recommended, that's
a fairly simple matter.
Thanks everyone for the information. I hadn't considered running the
HRV on a timer but it sounds like a reasonable approach to scaling down
the oversized equipment.
As for whether or not I actually need an HRV, I'm far from an HVAC
expert so I cannot tell for sure. The house however, located on Cape
Cod, is (by far) in the most humid environment I've ever lived. Its
rare, even in the winter with the wood stove running, that the indoor
humidity level will drop below 60%RH (in the summer if floats around
70-80%). The house is relatively tight, but could be better (I haven't
yet had a blower door test). In the past I've run a bathroom
ventilation fan for an hour or so each day to circulate the air.
Is there a difference between so called HRV and ERVs? Does one not
deal with humidity?
Hi again Greg,
I live on the Atlantic coast as well (Halifax, N.S.) and at this
moment the relatively humidity stands at 97 per cent. I run my
dehumidifier virtually non-stop nine months of the year and try to
minimize any indoor sources of moisture, e.g., wiping down the walls
after showering, cooking with tightly covered pots, etc. I'm not
convinced exhaust fans are appropriate in our climate given that the
makeup air (i.e., the air drawn back inside the house) is likely to be
equally humid. I tend to believe a dehumidifier and HRV would be the
better option, especially if mould and mildew are of concern.
Although I'm not absolutely positive about this, I believe EVRs
transfer at least some humidity between the incoming and outgoing air
streams, so a HVR would be the better choice.
ERVs transfer heat and humidity. HRVs only transfer heat.
The price difference is not extreme.
HRVs gain favor in Canada in particular as cold air entering the home is
a bigger problem for them than for most of us, and electric resistance
heating seems to dominate the heatings systems there.
Humidity management suggests running an ERV so long as outside temps are
in the range 20F to 100F, and RH indoors is above RH outdoors. Got to
worry about DewPoint and condensation.
But a typical 2400 ft^2 US house leaks 0.7 ACH, ie 0.7x2400x8/60 = 224 cfm,
so an HRV saves almost no energy, compared to a less-expensive exhaust fan
which might only run when the outdoor temp is close to the indoor temp.
Way too much. Enough for 7 full-time occupants, by ASHRAE standards.
Still too much. Andersen says an average family of 4 evaporates 2 gallons
per day of water, ie 16.66lb/24h = 0.694 lb/h. In a mythical airtight US
house with 15 cfm of fresh air with an outdoor humidity ratio wo = 0.0025
pounds of water per pound of dry air (Phila in January) and wi indoors,
15x60x0.075(wi-wo) = 0.694 makes wi = 0.0128, so Pi = 29.921/(1+0.62198/wi)
= 0.603 "Hg, vs 0.748 for 70 F at 100% RH, for an 81% indoor RH.
But that's high enough to allow mold and mildew, so it might be reasonable
to run an HRV when the indoor RH rises to 60% in wintertime, with a timer
that only allows it to run in the afternoon, when outdoor air is warmer.
16 Oct 2006
Typically, HRVs are useful for very tight, extremely well insulated
A lot of conventionally built houses are sufficiently leaky / drafty,
so as not to require much more than exhaust fans in the bathrooms.
About 18 years ago, I made a homemade HRV, which I used in my house for
the next 16 years (then I moved). In my current house, I use prolonged
use of exhaust fans, and opening windows (just a crack) to feed the
exhaust fans. That seems to provide a fairly continuous supply of
fresh air, but is a bit drafty in the winter. But it rarely gets below
+15 degrees F. where I currently live, so it is tolerable.
For a source of information on infiltration, HRVs and ventilation, I
have some documents you can access on-line.
All the text data is also on a Google site (with no diagrams), but it
has the links to the Yahoo site and two alternative MSN sites, with
much of the same data.
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