On the topic of whether or not you need the preheater, it's there like you said because of the capacity of your hoodfan.
When the capacity of your hoodfan exhaust exceeds over 600cfms, you need to balance the air being exhausted with adequate fresh air otherwise you end up with negative air pressure inside the house which can draw the exhaust air from your furnace exhaust back into the house (ie carbon monoxide). To do this the hoodfan is wired so the furnace kicks in to draw in the air. This process is known as a makeup air setup. When it's bitterly cold out like -30C, you don't want to circulate cold air through the house, so the heat exchanger on the furnace will kick in to do what it's supposed to do, heat the air. However, in order to burn natural gas, you need combustion air intake (separate system from the fresh air that circulates through the house). If the combustion air temperature is cold enough, it can cause damage to the heat exchanger in the form of condensation, which then in turns rusts out and could cause combustion issues such as O2. The reason for a preheater is to do exactly as it says, preheat the combustion air so that it is warmed up before hitting the heat exchanger on the furnace.
There are a few options you can go with:
1) Ask for a different brand/kind of preheater/set up if you keep your current hoodfan.
2) Remove the preheater, but along with that you'd have to downgrade your hoodfan to a smaller capacity.
Calgary is so much like an iphone: iCalgary - There's a bylaw for that.
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