Thread: 90% Efficiency Furnace

I'm having a furnace replaced. The HVAC guy told me that he can't put in a 90% efficiency because he cannot use the metal exhaust vent that the current unit uses. He says that he must use 3/4" PVC. The current furnace and the water heater share a metal exhaust vent that goes up the chimney. The closest wall is not an external wall because an addition was put on. The only other wall has an exhaust vent for the dryer, but a rafter is very close (will a picture here help?) and the vent is small. There is a window on that wall. It is not an escape window. He says he cannot put the exhaust out that window because "it is not right." He says his only option is to put in an 80% efficiency furnace.

So, here are my two questions:

1. Is what he is saying correct? If you cannot tell, what more information or pictures do you need?
2. How much more in heating will an 80% efficiency furnace over a 90% cost me? Will it cost 10% more (the mathematician in me says to say, "11.1%")

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High efficiency furnaces use PVC flue piping; however, it is not 3/4 inch, it is more like 2 inch. If there is no way to run the PVC, then he is correct. However, the PVC can be run in many ways that a metal b-vent cannot. I would ask another heating contractor to take a look before you give up.

I do not have the specific information on operational cost difference, but I would certainly try to get the high efficiency.

Originally Posted by Dave Angulo

I'm having a furnace replaced. The HVAC guy told me that he can't put in a 90% efficiency because he cannot use the metal exhaust vent that the current unit uses. He says that he must use 3/4" PVC. The current furnace and the water heater share a metal exhaust vent that goes up the chimney. The closest wall is not an external wall because an addition was put on. The only other wall has an exhaust vent for the dryer, but a rafter is very close (will a picture here help?) and the vent is small. There is a window on that wall. It is not an escape window. He says he cannot put the exhaust out that window because "it is not right." He says his only option is to put in an 80% efficiency furnace.

So, here are my two questions:

1. Is what he is saying correct? If you cannot tell, what more information or pictures do you need?
2. How much more in heating will an 80% efficiency furnace over a 90% cost me? Will it cost 10% more (the mathematician in me says to say, "11.1%")

Your installer is correct in that some times trying to put new systems in an old hole will not work. You should contact the Manufacturer and ask these question to them.

Best

Ron

P.S. Is the a home or a condo? If a condo. check your CC&Rs

I faced a similar problem recently... One option may be to go with an electric water heater and use the existing flue as a sleeve to run the newer (smaller) pvc exhaust from the hi-efficiency furnace. Of course, the elecric vs. gas water heater may eat up some/all of the potential cost savings of the newer furance but it's an option.

In the end I found a way to run my vent and kept the gas water heater but your situation may be different.

As always, it's a good idea to get several bids and plans for the new equipment. Fwiw... nothing your contractor has said sounds too far off base (except it's likely 3 or 4" exhaust, not 3/4").

There must be some means of getting outside with either a 2" or 3" PVC vent. It sounds like the contractor is just looking for the easy way out. Keep in mind that if you install a 95% furnace you will qualify for up to $1500 in tax savings. This alone may offset any price increase to upgrade equipment from 80% and save you money long term in fuel bills.

Dave,

One other thought. I sounds to me like you are saying the furnace and water heater share a metal b-vent flue pipe that is routed through a masonry chimney. Is that correct?

If so, then it might be possible to route the PVC exhaust for the new furnace through that chimney as well, assuming there is enough room to provide the necessary clearance. If the water heater is actually vented through a masonry chimney (the metal flue pipe would terminate at the masonry chimney), then it would not be possible unless a b-vent can be installed. There might be enough room as long as the chimney was of sufficient size. Difficult to say without actually seeing the current installation.

As an aside, I prefer to see a metal b-vent for any natural draft gas appliance (water heater or 80% furnace) rather than actually using a masonry chimney for the flue pipe.

One other thing, depending on the manufacturer's installation instructions for the furnace, some 90% furnaces are allowed to terminate on a wall that is common with a window. Specific clearances must be adhered to.

Once again, I would consider talking to another heating contractor. It may be that a different perspective is needed.

Hope this helps.

It is my understanding that once you use a masonry chimney as a chase for a direct vent appliance that chimney can no longer be used as a natural vent for say a water heater due to the flue temps. A concentric system for a high eff. furnace needs little space and should be easily vented in almost all applications.