We are planning on a new gas furnace next month. We have gotten
several estimates and some conflicting advice. One person told us to
get a 90% efficient furnace. It will require venting from the side of
the house. The second person said that these vents can freeze up in
the winter and cause flooding in the basement and possible carbon
monoxide troubles. He suggested the older type, an 80% efficient
furnace which can use our existing roof vent.
Apparently the water being sent out by the newer types is from
condensation? This surprised me since our house is always SO dry in
the winter that we need to use a humidifier.
As for the problem with freezing - if we went away for a month could we
expect to come back to a basement full of water? Naturally we would
want to set the heat down low (65 or so).
The 80% furnace is cheaper but will require some type of air exchange
installed. Guess what we have doesn't meet code. That means that
getting the 80% won't really save us much money over the 90%. On the
other hand I don't want to have to worry about problems. That's why we
are getting rid of our old furnace in the first place.
Advice and/or experiences would be much appreciated.
90% furnace vents freezing up and flooding the basement? Right. I'd
suggest you get another estimate to compare with the one from the first
person, and don't bother calling back the second person.
I've lived with a 90% furnace since 1985 here in St. Paul, MN, where it does
get rather cold in the winter. Never had the intake or exhaust pipes on the
furnace freeze up. I have had some neat looking icicles form around the
exhaust pipe in really cold weather from time to time, but it never blocked
Only time a 90% furnace could freeze up is if it's installed in an unheated
The 90% furnaces use a sealed combustion chamber; outside air is brought in
via one PVC pipe, heated and exhausted out the second pipe. The heat
exchangers pull so much heat out that the exhaust leaving the furnace is
only warm and it forms condensation, which is drained out via a hose. By
the way, the stuff that comes out is acidic, so don't drink it.
One advantage to the separate air and exhaust on the furnace is when it's
running it's not pulling air in around doors, windows and other leaks in
your house. Oh and the reason they call them 90% efficient is 90% of the
heat created is used to warm your house, with the remaining 10% going out
the exhaust vent.
A basement full of water? Naaa. Maybe a few gallons if the drain hose on
the furnace was disconnected. Now if the furnace were turned off in
sub-zero weather, well, then you are looking at burst pipes.
When I had my furnace put in back in '85, I also looked at 80% and 90%
furnaces. Yes, there was a large price difference between the two, until I
factored in that would need a flue liner installed in my chimney with the
80% furnace. Seems one contractor forgot to include that in his estimate,
but the company I went with pointed that out when I got quotes from them on
both an 80 and 90% furnace. Also, you may need a fresh air intake for the
80% furnace so it can draw outside air when it's running. The air exchanger
that you were quoted may take care of that, but they are expensive.
said may be possible, but unlikely! The venting on an 80% can have problems
too, and kill you! Flooding? Maybe, but just a small amount of water
certainly not a "flood"! The chances of getting CO problems with a 90% are
less than the are with a 80%.
Get the higher efficiency and be done with it! I just pulled a 90+ furnace
out that was 15 yeas old. It cost me $5 in parts in that time. It only let
me down twice, one when a motor capacitor blew, and once when the condensate
drain plugged up. The only reason I replaced it is because it was too large
so I installed a smaller BTU Ruud modulating furnace. hoping to reduce gas
bills and increase comfort, so far it has been more comfortable than the old
Thanks all for the responses. We decide to go with the high efficiency
unit. Rebates from Carrier are incredible right now and it's cheaper
to go with a 96% furnace than an 80% - a real no brainer.
wall. I just had a new high efficiency furnace put in
and it uses a vertacle flue that goes through the roof
the same place the old one did. It has a special
concentric flue pipe. There are 2 pipes, one inside the
other. The flue gas goes up through the inside one and
combustion air comes down through the outside one.
As far as freezing up is concerned, in colder climates
the drain line should be connected into your sanitary
sewer. If it runs outside the drips may freeze up and
back up water into the furnace. In mine there is a
float switch that detects a backup and turns everything
off. The company that installed it tells me that they
have never had any problem around here, so they ran it
through the wall outside. Someplaces there is
apparently a code requirement for the sanitary sewer
because of a pump failure. If you're away for any length of time in the
winter (we head to Florida for a month) why in the world would I want to
shut down the furnace because of a condensate pump problem? What's water on
the floor as compared to freezing all the pipes in the house?
When I install equipment I will connect the air conditioning to the safety
shut off on the condensate pump, but not the heat.
No AC in North Dakota = no big problem.
No heat in ND in January = big problem!
A little condensate on the floor is much better than frozen pipes, and allot
In my case I don't have a condensate pump, there is a
gravity feed to the outside, so I don't have to worry
about that. The main thing I would have to worry about
is the drain freezing up where it goes out through the
foundation. That isn't too likely, since it has an
elbow right outside the foundation and is right down
against the ground. However, in the unlikely event that
it does happen they put in a tee pointing up inside the
foundation and it will just drain into the crawl space.
It is something I will keep an eye on this winter,
just to be sure.
Of course if you have a slab foundation you can't manage
that very well. That's one more draw back to a slab.
I'd be very reluctant to have your condensate drain to the outside. Unless
you're very lucky, sooner or later you're going to experience a frozen line.
A freeze up happens when the drain just drips and then it freezes at the end
of the tube (same way an icicle forms). The process continues until the line
becomes blocked. In your case, maybe what you have is OK but if the crawl
space is cold is an ice back up to the Tee a possibility? Why not find a
place on the inside to dump the condensate---In my setup, a plastic tube
runs from the pump, up and along the wall ceiling to the laundry room and
then down to the washing machine drain. You could even gravity feed to a
condensate pump (inexpensive ones at Home Depot or Lowes) and then pump to
your inside drain
This article talks about carbon monoxide poisoning. It mentions a few
cases where snow covered a house vent and caused problems.
Yup, I now about those cases. We live in MA.
In fact the guy installing our new system talked about this.
Apparently the vents were installed too low to the ground. Ours will
be about five feet above grade which should be fine. The snow is not
plowed against that side of the house. Also there are lots of high
bushes protecting that side. Drifting snow tends to get stopped by the
bushes. In addition there is a new law that requires a carbon monoxide
alarm in the furnace room. We also have one on each floor.
We were more worried about the water issue. As it turns out we
misunderstood. The condensation gets pumped out of our existing drain.
Yes, if the pump goes there can be a problem. Same thing with the air
conditioner. We had that happen to our central air. We had water but
not a flood. A flood is when your hot water heater leaks for a month
while you're in Florida. Now that was a REAL nightmare.
Don't you shut your water off when you're gone? The gas furnace can operate
very well with your water off. Just going away for a month or so there is a
minimum level of protection that you can provide yourself. I shut off the
water just in front of the water meter (ball valve) and also at the hot
water heater (a bit of redundancy). Get a few gallons of RV Anti freeze at
Home Depot and put some in all the house drains (bath tub, shower, washing
machine, sinks, dishwasher, toilet, toilet tank (flush first after water is
off). Thermostat at 60F. At least I go away feeling that I've done the best
under the circumstances.
Yes, we shut off the main valve. Unfortunately we started doing that
AFTER our hot water heater flooded.
Of course this would not save us from a furnace or air conditioning
problem since that water is from condensation. We had problems with
that this summer while we gone two weeks. Fortunately the amount of
water was minimal.
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