Thread: 95% Eff Furnace Draws Combustion Air From Inside

I ran across this arrangement while doing a home energy assessment - not a home inspection.

This 95% efficient gas furnace was installed one year ago. The owner stated he was not pleased with the installers' work and that the furnace failed the first inspection (joints in the exhaust pipe were not properly glued).

The first thing that caught my eye was while walking the exterior of the house I saw a single PVC pipe venting to the exterior. I asked the owner what the pipe was for and he said it was for his new 95% efficient furnace. When I looked at the furnace itself I confirmed that combustion air was being drawn from inside the mechanical closet, not the exterior.

Pikes Peak Regional Building Department approved this installation but it does not look right to me. For one thing, part of the advantage of a high-efficiency furnace is that you are not using room air (air that you have paid to heat) as combustion air. The way this furnace is set up it is room air for combustion air just like a conventional furnace.

What are your thoughts on this arrangement?

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Bruce - Drawing combustion air from the interior with a Category 4 furnace may be allowed by the manufacturer, but isn't ideal for a number of reasons. I see this same setup a lot.

I'm not crazy about these types of installations when I see them Bruce but from my understanding, there isn't anything out there that says the combustion air intake pipe can not be done this way. I run across them as well. If the manufacturer allows it, not much you can do but make a recommendation. It just seems lazy to me.

I see the same set-up frequently too. However one advantage is that this will help with air exchange within the house, dependent on how tight the house is, and whether there is a enough make up air.

It would not be costly to extend the pipe to the exterior.

Is it still operating at 95% eff. as set up by not pulling outside air for combustion is the question.

Cheers,

I would call this out and recommend that a licensed HVAC contractor review the installation.

Many manufacturers of these high efficienies furnaces require that the vent pipe and combustion air pipe are BOTH vented outside AND:

1. Require a minimum length and size of pipe to prevent condensation and water droplets. (5 feet is required for Bryant)
2. Require that both terminations are within the same atmospheric the pressure zone. (ie: placed together and on the same side of the house)
3. Do not recommend cementing the combusion air pipe to the inlet pipe for access and repair of the flame sensor.

Drawing combustion air from the Combustion Appliance Zone in a Cat.IV furnace can depressurize the CAZ and backdraft a draft hood equipped water heater. I"ve seen it more than once.

I would not get into specifying the glue on the PVC joints or not. That is a function of the appliance listing. You could test the joints with a chemical smoke puffer or soap bubble solution to detect leaks. It must be air and liquid tight.


FYI, that 95% efficiency would not the overall efficiency since removing indoor air lowers overall efficiency and must suck in cold air from the path of least resistance.
HTH,
Bob

As a practical matter when I see this setup in new construction or rehabs (I see this a lot in older condo conversions here in Chicago) the furnace has often been used as a "construction heater", and especially if there is no protective elbow as recommended by some manufacturers debris has fallen into the intake - in extreme cases the stuff bouncing around in the inducer fan housing can sound like loose change in a dryer.

Thanks for the input. We are going to check with the manufacturer and find out if this arrangement meets their specifications.

I understand that this setup reduces the overall efficiency of the furnace. (That's why I posed the question.) While the furnace itself may operate at 95% efficiency (i.e., only 5% of the heat is exhausted) it is using room air for combustion air - air that is replaced by cold outside air and that must be heated. If the combustion air was drawn from outside there would be less cold air in the house to heat.

Do you think backdrafting of the water heater is a real concern with this setup? I did not check the pressure in the CAZ (that's Combustion Appliance Zone) with the furnace running. It is obvious that drawing combustion air from the CAZ would reduce the pressure in that room but would it reduce it enough to backdraft that appliance?

It would depend on the combustion air vents to the outside, if there are any.

Originally Posted by Bruce Breedlove

Pikes Peak Regional Building Department approved this installation but it does not look right to me. For one thing, part of the advantage of a high-efficiency furnace is that you are not using room air (air that you have paid to heat) as combustion air. The way this furnace is set up it is room air for combustion air just like a conventional furnace.

What are your thoughts on this arrangement?

I'm not sure the efficiency calculation is quite that straight forward. Feeding the flame with sub-zero air from an exterior intake may result in a lower flame temperature and a lower temperature at the registers. The final answer can only be determined by doing a detailed heat balance calculation across the heat exchanger for both scenarios.

There is another reason in favour of an outdoor intake though. Builders in my area commonly use an indoor intake in the basement - the reason, it's cheaper. My house was this way when we moved in and we found the air extremely dry in the winter from all the air exchanges. With the manufacturer's blessing, I drilled a hole in my foundation and plumbed the intake line outdoors. What a difference - the air in the house is no longer dry in the winter.

I still get air exchanges, although not so frequently, because my power vented water heater draws air from inside.

Pulling combustion air from the interior space is allowed by our City code. It is a poor installation but acceptable. That having been said. You should consider the following points.
- The space has to be of sufficient volume to handle the BTU needs of all units combined within that space. If the space is an enclosed room the cubic volume is even more important. Transfer grilles, either fire rated or not, depending on occupancy also need to be installed to allow air changes to the room and allow combustion air to enter the room.
- More inportantly however in this situation and many I have seen, is the potential for negative pressure within the 'room' that can affect the draft of the hot water tank flue.
- The best option is to run the combustion air pipe to the exterior
Good luck, Markus

I don't think I'm understanding you on inside air vs. outside in terms of 95% efficiency.
I ran accross a photo this weekend of the first Rheem 95% furnace I installed (back in the 90's). I recall reading the specs thoroughly because I was running a PVC flu which seemed insane until start up.
Anyway my photo showed the same set up as the one in your pic.
It is my understanding that the 95% efficiency comes from the furnace's ability to condense heat.
I could be wrong but the intake or combustion air is simply suppling air to the flame and would not be a significant part of the efficiency variable.
The set up is critical however in terms of safety and proper combustion.
As far as air that has been already heated or not I think return air would be more important.

As for back drafting, that should certainly be considered if a water heater is near by, however; I think backdrafting would be more likely if the vents were joined and not run independently.

Kevin, to clarify what I meant, I agree with you take exterior air piping to the furnace is intended to supply dedicated air to the burners for flame. It does not have anything to do with the units efficiency. Unless of course the unit gets starved for air in a confined space and cannot continue to operate properly.
I seen this very rarely, when called out for a service call and no one can figure out why the furnace and/or hot water tank shut down intermittently. Opening up a closed off room has solved that problem.
The problem I see so often is when unskilled workers install, developers demand and owners desire that all mechanicals are crammed into the smallest space possible, without of course giving consideration to the consequences.
I looked at a new construction recently where a 120,000 90+ furnace, a 170,000 tankless unit, a 180,000 boiler for radiant in the concrete and a 40 gallon back-up tank were all pulling combustion air from a 6x8 room.

It has to do more with pulling warmed inside air from the house and exhausting it out the flue. The warm inside air will be replaced by cold outside air through infiltration caused by negative pressure inside the house. Thus, the furnace will run more to warm up the air inside the house. This uses more gas to keep the house warm than if the combustion air was pulled from outside, lowering the efficiency. The actual efficiency of the furnace itself probably doesn't change much, but the efficiency of the heating system certainly will.