DIY Home Improvement, Remodeling & Repair Forum > Appliances > HVAC > Is outside air really needed for a furnace?

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Old 02-11-2008, 09:26 PM  
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Hmm. Outside air is required to feed the furnace. I understand that. However, does the requirement for ducting outside air into the space around the furnace change when there is also a heat recovery ventilator (HRV) in the house? The HRV brings the fresh air into the house from outdoors, but warms, or tempers it before ducting into the supply side of the furnace?
This seems intelligent, whereas bringing cold air into the house and simply dumping it (untempered) into the basement seems counter productive especially at -20C! Would the continued use for this be simply for the domestic water heater?
My suburban house is only 15 years old, but since tract housing is done on the cheap, it doesn't surprise me much.

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Old 01-28-2013, 03:13 PM  
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Does a high eff. furnace need air ducted from outside to the return air plenum?

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Old 01-28-2013, 04:33 PM  
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The make-up air is for combustion and not to fill the plenum with cold air.

Cold air into a siphon break (pail) is very common and required by code in many areas.

I have one in my townhouse and keep the door to the utility room closed to prevent the furnace from sucking out the warm air from the house and burning it up. - That area of the house has several cold air returns drawing cool floor air into the furnace/plenum to be heated and recirculated.

The idea is to separate the fresh air intake for combustion from the interior air that is circulated through the furnace/duct system. Cold exterior air is not all that cold and it works quite well to provide oxygen for combustion in a "controlled" way.

High efficiency (90+%) is different and the outdoor intakes are piped directly to the furnace. I chose to put is an 80% furnace to avoid all the problems (exterior appearance, destruction of interior ceilings, etc.) that come with a pair of plastic pipes and could not justify the cost with a high efficiency furnace. - My heating costs with an 80% furnace in MN are much lower than my AC costs annually.

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Old 03-01-2014, 05:09 PM  
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we have a large 8" fresh air intake in our furnace room in the garage. we also have a tankless water heater in the same room. this is the 2nd time my water pipes have frozen this winter. It is -12F outside. what the heck can i do to keep my pipes from freezing? We have a cistern and the water supply comes into the same room to the pump.
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Old 03-01-2014, 05:42 PM  
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Until the weather breaks, cap off the 8" intake and leave the furnace closet door open to use the air in the garage. Wrap your pipes with heat tape.
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Old 06-04-2014, 09:01 PM  
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Install a gas-fuelled garage heater.

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