We had our house environmentally tested a few months ago. One thing they look for is air exchange between the inside and outside. This confused me. How is it efficient for the warm air heated by my furnace to now flow outside. Isn’t this the same as having crappy windows?
Also, my gas fireplace is more effecient if the source of air is from outside. How is it cheaper to heat cool air than air that is already warmed?
Your fireplace is designed to burn things. Presumably, outside air is higher in oxygen than the inside air and so burns more efficiently. Also, fireplaces heat by radiation, not by convection. Some fireplaces have convection passageways but they circulate room air, not combustion air. If the hot air in a fireplace isn’t going up the flue, your room fills with smoke and CO2 and you die.
You need fresh air coming into the house, but you want to control the temperature of that air, so your heater pulls in fresh air and heats it by running it through a heat exchanger and then expelling it into the house. Leaky windows are pulling in cold air and not heating it so you run the heater more than needed because of the uncontrolled cold air coming in.
>How is it efficient for the warm air heated by my furnace to now flow outside.
It isn’t “efficient”, not in the energy savings sense. But you want oxygen to breathe, don’t you?
There’s a balancing act between energy conservation and ventillation, and as energy prices went up over the years and houses got tightened sometimes this balance went too far in the other direction. There are also HVAC systems that force some air in and out, and include heat exchangers or even enthalpy exchangers to take heat and moisture out of the exhaust and put it into the fresh intake; these systems give the best efficiency/ventillation compromise, but also cost the most.
Sometimes, when your house is nearly airtight, you breathe more of the radon from your crawl space than you should. The formaldehyde from your fresh plywood or new carpet may rise to unfortunate levels. If you lock in all the moisture, some dormant mold might find your house a friendly place. It doesn’t happen often, but there’s that chance.
Remember that an ‘Environmentally Friendly” house is not just efficient, it is also a good place to live.
So allowing outside air in doesn’t necessarily improve efficiency (although in some cases it can, such as combustion air for specific types of stoves/furnaces), but can dramatically affect safety and comfort.
So typically when doing these kinds of evaluations, they check to see if you haven’t gone overboard on the efficiency part and created a risk by sealing yourself in too tightly.
It’s not. How badly do you want your corpse to be efficiently warmed?
As others have said, efficiency takes second place to safety. You need a certain amount of fresh air to get into the house so you can be alive and well to enjoy the warmth inside.
For combustion you usually have intake air and exhaust air.
That exhaust air is warm but has products of combustion in them and lower levels of O2, so it is generally good to vent this outside.
The intake air needed for combustion can come from 2 places, inside the house, or outside. If it is taken from inside that means air you paid good money to heat will be vented to the outside and that air will be replaced by outside air creeping into you home through cracks and the like. If taken from the outside the warm air inside your house is not vented to the outside. In some setups that use outside air, the intake air is preheated by the exhaust air.
The combustion appliance will normally exchange heat with another seperate airstream, or hot water system to deliver heat to the house.