Well, one of the things we didn't compromise on was our range. We chose a professional grade LP range that has 4 high-output (13,000 BTU each) burners and an electric griddle.. The total output of the range mandates a high-volume vent hood of 720 CFM (4 x 13,000 BTU at 1 cfm per 100 BTU and adding 200 cfm for the griddle). We chose a range hood with 715 advertised CFM with 6 speeds and external venting (required due to the combustion appliance). While not the monstrous 1100-plus CFM required by some ranges this clearly causes some concern in the tightly constructed PassivHaus design and mandates make-up air to replace that being sucked out at high rates.
To afford the required make-up air we are investigating a number of options. One option that exists is a make-up air damper made by Broan for use with specific Broan and Best hoods. The unit connects to the switching mechanism of those hoods so a motorized damper opens when the fan is powered on. This allows for the draw of fresh make-up air into the house to avoid the significant negative pressure that would otherwise occur. It is interesting to note that the range hood manufacturer does include a warning that make-up air may be required but they do not have any integrated solutions to control make-up air supplies with the vent hood controls.
Since the vent-hood we've chosen is not one of the recommended models, we are researching the use of a current-sensing switch to control the powered damper. These switches are used for many applications but one in particular is booster fans for clothes dryers. When the dryer is drawing power, the switch senses the current flow (by means of a clamp-on sensor) and activates the boost-fan and possibly the motorized damper.
Another option for switching is the use of a pressure-differential switch that senses the difference between outside barometric pressure and a pre-set inside pressure. These are made for use in mechanical rooms where heaters and other devices require make-up air. One commercially available unit appears to be a supplemental heat supply with a blower that has the current sensor in a single zero-clearance box (meaning it can be installed between rafters). We are putting that brochure in our "in case we need it" file but it is not currently planned for our installation because we anticipate no need for supplemental heating in our climate zone.
Reading through other blogs (including Green Building Advisor) , the placement of the make-up air supply vent is critical to avoid drafts while ensuring minimum conditioned air is drawn out by the range hood. Several placements are currently being considered but the one that we favor currently is the location either under or behind the refrigerator (this was suggested on other blogs). This affords the 'pre-heating' of the air entering during cold months while increasing the efficiency of the "fire under the ice-box" (quoting Anchorage Builder Corp's Chris Senior) effect of current refrigerator designs. Clearly there is some concern about hot moist air from the outside during summer months so we're still reviewing options. The placement near the refrigerator also avoids the penalties for supplemental heating elements for the supply air. Under no circumstances will the vent hood be connected to the ERV but we have not ruled out connecting the make-up air to the ERV system although that could mandate changes to the ERV design. We definitely don't want to create that infamous "ductopus" that robs efficiencies from any of the units so this design is still far from complete.
Below you'll find my crude drawing of the solution for make-up air. One caution is that this is a home-gronn solution using off-the-shelf parts. It isn't something that has an engineers stamp of approval but since it is low voltage and very simple construction it should not be an issue. My recommendation is you check with local building codes to ensure it is within code. Our local code does not address make-up air but with a very tight house we really need a solution like this. It is much less costly than the self-contained powered units. It is a "passive" source since there is no fan driving the air into the house. The grate for the supply of air in our case is a floor-vent that is under the refrigerator which serves several purposes; the pre-conditioning of the air by passing it over the refrigerator coils, the increased efficiency of the refrigerator and finally, the cleaning effect of air blowing dust and pet fur off the coils on a regular basis preventing the normal build-up from occuring.
The Broan MD6T is a powered damper that fits on a 6' duct which is controlled by the Skuttle A50 current sensing relay. The relay is attached to the common wire of the range hood and senses when power is drawn by the an. The 24v transformer is supplied with the MD6T. I purchased the MD6T through Lowes Home Improvement and the relay through Bel-Air (www.electronicaircleaners.com/a50.aspx). Total cost of he parts was about $175.00. The Broan instructions say it is only compatible with their range hoods but it's a simple 24v motorized damper with no electronic controls so there does not appear to be any reason it will not work with other control sources.
Another option for switching is the use of a pressure-differential switch that senses the difference between outside barometric pressure and a pre-set inside pressure. These are made for use in mechanical rooms where heaters and other devices require make-up air. One commercially available unit appears to be a supplemental heat supply with a blower that has the current sensor in a single zero-clearance box (meaning it can be installed between rafters). We are putting that brochure in our "in case we need it" file but it is not currently planned for our installation because we anticipate no need for supplemental heating in our climate zone.
Reading through other blogs (including Green Building Advisor) , the placement of the make-up air supply vent is critical to avoid drafts while ensuring minimum conditioned air is drawn out by the range hood. Several placements are currently being considered but the one that we favor currently is the location either under or behind the refrigerator (this was suggested on other blogs). This affords the 'pre-heating' of the air entering during cold months while increasing the efficiency of the "fire under the ice-box" (quoting Anchorage Builder Corp's Chris Senior) effect of current refrigerator designs. Clearly there is some concern about hot moist air from the outside during summer months so we're still reviewing options. The placement near the refrigerator also avoids the penalties for supplemental heating elements for the supply air. Under no circumstances will the vent hood be connected to the ERV but we have not ruled out connecting the make-up air to the ERV system although that could mandate changes to the ERV design. We definitely don't want to create that infamous "ductopus" that robs efficiencies from any of the units so this design is still far from complete.
