High efficiency furnace burning room air. How much efficiency is gained by adding a proper outside air intake?

[soparklion11]

This is how my high efficiency furnace was installed... the combustion air intake is on the left side and is using room air. The exhaust pipe travels a circuitous route to the wall. Water often accumulates on the top of the furnace at the origin of the PVC pipe vent, even though it seems to be caulked appropriately.

How much am I losing by using room air for combustion? What are the regulations around intake piping? Does it have to be 2" solid PVC or is there a more flexible alternative?

NOTE: cannot repipe intake or exhaust to the left of the unit, as there is a chimney there. To the left of the chimney is a hill and I don't want the pipes to enter/exit below ground level.

 

[user 4883]

You will need to pitch the exhaust towards the exit to prevent condensate from running back into the unit.
Also check outside, could be terminated in such a way that heavy rain and wind will drive water into the exhaust.
Plumb the combustion intake to the outside or you'll not burn efficiently and create a negative pressure situation in the home.
All installs I've seen are rigid pipe, 3" diameter. Check the installation guide that came with the unit.
Really can't tell how much efficiency is being lost but, your not getting what you paid for, that's for sure.

 

[Sparky617]

You will need to pitch the exhaust towards the exit to prevent condensate from running back into the unit.
Also check outside, could be terminated in such a way that heavy rain and wind will drive water into the exhaust.
Plumb the combustion intake to the outside or you'll not burn efficiently and create a negative pressure situation in the home.
All installs I've seen are rigid pipe, 3" diameter. Check the installation guide that came with the unit.
Really can't tell how much efficiency is being lost but, your not getting what you paid for, that's for sure.

With mine, we couldn't slope it out, so it slopes in and has a drain line that flows into the condensate pump's reservoir. It can't have a dip in it that will hold water, it needs to positively flow one way or the other with no spots to hold water.

 

[soparklion11]

You will need to pitch the exhaust towards the exit to prevent condensate from running back into the unit.

They used to call them condensing furnaces. I thought that they were manufactured to handle condensate.

 

[Steve123]

That is the way the furnace on a previous house of mine was installed. The furnace was rated to use either indoor air for combustion, or connected to outdoors via pipe. My furnace was even right up against an exterior wall. Installer saved maybe an hour by not running the combustion air to outdoors.
I can't put a number on it, but consider that, in the winter, for every cubic foot of air that the furnace exhausts, another cubic foot of cold, dry winter air will be sucked into your house.
Go online and find the installation manual for your furnace. It will have all the specs for pipe size, max length, length correction for elbows, etc.
Required pipe diameter probably depends on length of the pipe.

 

[Sparky617]

They used to call them condensing furnaces. I thought that they were manufactured to handle condensate.

The water needs to be handled, either by draining to the outdoors by gravity or indoors and pumped out. On the combustion side, they create a lot of water vapor burning the natural gas. On the intake side, if the intake doesn't drain any water that collects in the pipe, it will eventually block the pipe or certainly restrict the air flow.

 

[Steve123]

Water often accumulates on the top of the furnace at the origin of the PVC pipe vent, even though it seems to be caulked appropriately.

Your furnace should be connected to a floor drain. If water is collecting and not running into the floor drain, something is wrong with the slope or the line running to the drain is clogged (very possible).