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direct exhaust-vented


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No,

I think you're confusing terms.

The Categories 1 to 4 only deal with the characteristics of exhaust temperature and positive or non-positive static exhaust gas pressure. You can have a direct vent category 1 or a non-direct vent category 4.

A water heater that has an exhaust fan mounted at the end of the exhaust vent or inline with the exhaust on top of the tank is a "power-vented" water heater whether it takes it's combustion air from inside or outside.

A direct-vent appliance has either a combination intake-exhaust vent or a two-pipe system - one for intake air from outside and another for exhaust air. There are direct vent water heaters that produce exhaust that's as hot as what you'd have going out the top of a conventional gravity stack and there are those that produce exhaust that's body temperature warm.

You can have a condensing furnace that's a Category IV that can be configured as a direct vent or it can be configured to get it's combustion air from the interior - in which case it is no longer considered a direct vent appliance.

A category 1 appliance operates with non-positive static vent pressure and temperatures that will prevent condensation in the vent. Translation - gets it's air from the area around it and the exhaust is too hot to touch.

A category 2 appliance operates with non-positive static vent pressure and temperatures that will cause condensation in the vent. Translation - gets it's air from the area around it and the exhaust is not much hotter than body temperature and the exhaust system collects a lot of condensation.

A category 3 appliance operates with positive static vent pressure and temperatures that will not cause condensation in the vent. Translation - it's got a high volume exhaust gas inducer and a closed burner. It can get it's air from outside or from around it but the exhaust is too hot to touch.

A category 4 appliance operates with positive static vent pressure and an exhaust temperature that will cause condensation in the vent. Translation - it's getting it's air from inside or outside it has a high-volume inducer and a staged heat exchanger that extracts heat even from the condensate produced by the exhaust. The exhaust gas is barely body temp.

OT - OF!!!

M.

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Now I'm confused. Mike's response is too lengthy. Bill's is too brief.

How about this:

Cat I thru IV refer to whether the exhaust vent is at a positive pressure or not and whether the exhaust is condensing or not. There's 4 permutations and they're called Cat I thru Cat IV.

Direct vent is when the combustion air comes from outside the house. I've no idea why they gave it this name. Doesn't make sense.

Direct exhaust - I yield. Sounds like another lousy use of the word 'direct'.

Can anyone explain why the use of the word 'direct'?

Marc

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Well, here's why I asked.....

I got Joe L's latest ASHRAE thing, he has a glossary of terms, and he lists this as the term applying to any fan vented appliance where the vented air is from inside the pressure barrier, i.e., any Cat IV appliance that uses interior combustion air or a power vent water heater.

He lists direct vent as any appliance where the vented air is from outside the pressure barrier, which is what I understand to be correct.

Because before, I agreed with you guys. Now, I don't know.

Kibbel's explanation would be in line with Joe's, near as I can figure.

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Direct vent simply means it takes 100% of combustion air directly from outside and exhaust 100% of the flue gases and stack loss heat. This may be two co-linear pipes, a co-axial double walled pipe within a pipe or sometimes combinations of both.

You can have a CAT IV furnace that exhausts with pvc but draws combustion air from indoors so it would not be "direct vent" in that case. Now, take the same unit and two-pipe it and it becomes "direct vent".

Most modern gas fireplaces are now "direct vent" but almost none are condensing units. Some, however, are power vented.

HTH,

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