Gas and Oil Home Heating Furnaces - Furnace return air

Just a question
In the US are you allowed to have the return air sucked from the same area as the furnace is located or do you have to have the return taken from an area outside of were the frunace is?
Is there any difference in oil furnaces to gas furnaces ?
Just wonder that is all

As per the national gas code for the US if the furnace is located outside the airflow (which I imagine is the case in most installations) then the return air must also be from this space and not the space in which the furnace is installed.

However, since in many parts of the US the national codes are modified to suit state and local laws this requirement may be different in various places.

This applies to gas furnaces - cannot comment on the oil furnaces.

As far as we go as long as there is no down draft at flue on gas and with the oil we have a 2 over the fire its ok. In fact lots of times we put a small register in the cold air drop. For return in like an open basement that the unit is in. The main thing we run into and have to look out for is a fireplace in the home that dont have its own outside air intake. Now thats the killer of all killers if you want to call it that.;) ED

Here in NZ we are not permitted to have the return taken from the same area that the gas furnace gets its air from, This prevents the back draft from the fire (if the flue got blocked for example) getting into the occupied areas.
With oil there was not this requirement but this does cause a problem when one does a oil to gas converstion as we have to sometimes put a remote return system in.
I am abit puzzled as to why a cracked H/E is all that much of a worry as the main fan always blows at a much higher pressure than the combustion fan so any leakage is from the main airways into the combustion area. I don't see this as a great problem appart from the fact that the combustion fire will then have a lot of dilution air and you won't be able to get a good efficency. But I don't see it as dangerous. Sure there may be a slight leakage the other way in the period between the fire starting and the main fan starting but this will soon be diluted once the fan starts.
However this is my opinion what does every one else think? Opinions please

Merry Christmas every one
Santa has already been to our place.

I am a little unclear, I was in a new house the other day where they were DIY finishing a basement (which contains their furnace). They were adding cold and also warm air returns :confused:

So, if I understand it right you are saying they should not be installing cold air returns, right? And what about the warm returns then?

Hank

I am a little unclear, I was in a new house the other day where they were DIY finishing a basement (which contains their furnace). They were adding cold and also warm air returns :confused:

So, if I understand it right you are saying they should not be installing cold air returns, right? And what about the warm returns then?

Hank

No What I'm saying is the furnace MUST have a return from another area than were the furnace is. Be it outside cold or inside the home. As I said before if the furnace had say a blocked flue and the products were pouring out the furnace then the products could NOT be drawn into the house via the return air and ducting.
These rules were made back in the days of peramant pilots and DDDs. Not so important now with induction fans and rollover switches but it is still the regs.
As Ed says fireplace heater are more dangerous. We had an example not long ago on TV how this clown of a gasfitter a heater in the fireplace and didn't do a good job of the flue resulting in the products coming back into the room and made the occupants very sick luckily no deaths.

This is a good point Narroc. My view is that you are probably right - it is not that much of a worry in normal circumstances. I think that the worry stems from situations where many things may be going bad all at the same time. For example if the furnace has already got poor combustion and producing lots of CO if you then have a cracked exchanger with a large opening and the furnace has been running all day with very little air from outside coming in to the home then this may well pose a danger.

Under normal circumstances very little CO should be produced anyway. Coupling this to your argument I gues the risk is low.

I guess for you Santa arrives almost 24 hours before us.