High Efficiency furnace

Can someone with strong HVAC knowledge please advise if a two-pipe 90+ furnace has benefits over a one-pipe system? I know the second pipe is used to intake fresh air directly into the heat chamber.


One pipe draws warm air from the home depressurizing the home consequently cold air from outside is drawn in to balance the pressure.
Two pipe uses outside air and does not lower the pressure inside It uses out side air for the furnace.
A bathroom fan does the same thing draws air in to make up for the air discharged via the fan .
But the bath room discharge has removed damp air…
Roy Cooke

I don’t have a strong background in HVAC but I can tell you that general rule of thumb is that you need about a 2,000 square foot home to get your pay back from a high-efficiency unit.

Increasing the efficiency of your furnace doesn’t always equate to instant savings. For example, if you increase the efficiency of a your furnace from a mid-efficiency at 84% AFUE to a high efficiency at 92% AFUE your percentage savings on fuel is 8.6% (12% (92-84) divided by 92%) a year. While a nice savings, this must be compared with the increased purchase price and maintenance costs if any of a high-efficiency unit. If you spent $600 in fuel with the a mid-efficiency unit then you’ll save $52 (8.6% X $600) a year in fuel switching to high-efficiency. If the unit costs $500 more it would take about 10 years ($500 divided by $52) to re-coup your costs or before you’d start to save.

A high-efficiency unit is not unlike a finely tuned racing car. For best performance, regular maintenance (tune-ups) are required which can be more costly than non-high performance units. This, coupled with higher costing parts and labour to repair same, may defer or increase the pay-back period as well.

Here’s a good link…

In my opinion, if your furnace has two pipes, both should be used (often they are not). As posted above; if you burn air, you must replace it by drawing unconditioned air from around window and door openings ect…

Negative pressures cause mold spore migration from crawlspace/basements as well as Radon gas.

If the furnace is installed in a garage, attic or crawlspace, this is not an issue and two pipes are not needed. It actually increases ventilation in these areas which may be desirable.

If you are selecting a furnace to buy, if you are putting it indoors, get and use the two pipe system, there is a very large difference in efficiency.

That’s about it!

Two pipe system- one is intake one is exhaust.

Thanks for your replies…I believe the principle reason the intake pipe was stubbed out and not extended to the exterior of the building was to save money on the cost of installation…:shock:

No matter what the efficiency of your furnace, it is more important to make sure it is sized properly. 95% of the ones I see are over sized. You don’t gain any benefit from a high efficient furnace unless if when it runs it cycles for a long period. If the unit is too big it will short cycle and your efficiency will not be realized.

That’s usually the reason, and it is wrong.

Yea, and what is that astronomical cost?
No where near the Loss concerned!
Any contractor that can’t afford the cost of a few feet of PVC needs to find another job.

Several years ago, I did a home inspection on new construction fora high end contractor who was in business with his daughter. Nobody really wanted me to be there. I placed a radon test prior to the inspection so that I could read the test at the day of inspection. Radon level was extremely excessive. The basement of the house was unfinished at the time and there were two high-efficiency furnaces and a 75 gallon hot water heater installed in the basement(no combustion air ventilators installed). Before recommending a radon mitigation system ,I suggested to the contractor that his contractor install 5 feet of 4 inch PVC and reduce the vacuum pressure in the house. He got on his cell phone and contacted his HVAC contractor who began arguing with him over the phone. Due to lack of knowledge he asked if I would talk to the contractor. I took his cell phone, advised the contractor of the circumstances involved, to which he said he doesn’t put the intake pipe on furnace is if they’re in an unfinished basement. I advised him that besides of the fact that he installed flexible duct between the fire wall of the garage in the main house (which he would have to come back and fix any way) that the condition would only get worse when the contractor finished basement when the HVAC equipment was not piped to the outdoors. He stood his ground! I asked the contractor (in front of the general contractor), “how many HVAC systems do you install for this contractor”? He responded with something. I simply asked “do you ever want to do another one”? He advised that he would be there to correct the situation in five minutes.
The contractor, who really didn’t want me there in the first place, bought me coffee and became my best of friends.