Vent hoods/makeup air
Posted on Jul 23rd, 2009 by Doug Marsh VP, CR, CAPS, GBP | 0 Comments
There I was sitting in another Green Builder seminar and the presenter, a Green Builder from Fredericksburg, started talking about vent hoods and make up air. I had never really got it about how these sometimes huge CFM vent hoods can radically affect the air inside a building. Since appliances go in last on a project the client is often running behind or late in the game in selecting their appliance package. Well the HVAC manual J calculations have been run months ago possibly with all the ducting and fresh air intakes and exhaust fans already installed and then along comes the vent hood selection: ack 1200 CFM. This is equivalent to 2 tons of air conditioning. This means that if your whole house is on 4 tons then this sucker will exhaust the ENTIRE house in two minutes at high speed! This means, as builders, we have to push for the vent hood selection in the beginning, soon after the start of the project so that the size of the unit can go into the Manual J calcs. If the Client is using a commercial size range , which is very common these days, they may want a matching brand with, possibly, a remote blower really moving some air. Here are some possibilities from David Fransik:
You can add a vent and duct to the hood area, leave it closed off unless they select one of the larger hoods and then introduce it with a control damper near the unit.
You could also just add to your return air system and oversized pressure balance damper tied to a duct for intake air that opens when the house goes negative.
Talk it over with your HVAC trade and see if they can come up with some ideas.
Keeping the pressure in the house balanced is the name of the game to keep negative pressure from sucking in air through every crack available: in a remodel these cracks could be considerable
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