I have a 25 yr old Olson gas furnace. Our house has never had AC until
this summer, when I had one installed.
Now, the furnace is hitting the high limit and cycling off. The tech
said that my outflow duct is quite small, and that
the AC coil has reduced air flow even further, causing the furnace to
overheat. The company suggests the following options:
1) new furnace: higher blower pressure would solve overheating issue
2) modify ductwork by adding a bypass around the coil for winter usage
3) increase return air flow somehow
I was not informed of any of these issues before the installation.
Does anyone have an opinion about the best option. Also, when scoping
out a house for AC, is it standard procedure
to look at the size of the duct work?
First consider whether the blower is on low speed in heating mode.
Often they're on low for furnace and high for AC. It might be very
simple to rewire it so that it runs on high speed in heating mode, too.
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25 year old, Jurassic furnace?? Why are you even screwing with it?? Call a
*competent* contractor, get a complete "room-by-room" Manual J heat
load/lloss calculation, and a Manual D, ductwork calculation done to find
out exactly what your home *NEEDS*, then go from there.
Right now all your doing is putting band-aids on the system. Get it done
right, and the savings on your utility bills alone could pay for the new
system in as little as 5 years. The lowest bidder is *NOT* the way to go....
nor is the cheapest equipment. You *WILL* get what you pay for, likewise,
your not gonna get what you don't pay for.
Before you ask about brands, the best brand is the one that is correctly
sized and installed. Brand names?? try consumer reports.
Since the furnace worked for however many years without the AC coil, the
bypass makes a lot of sense.
As another poster suggested, check and see if the blower has multiple
speeds. Typically high for AC, and medium or low for heat. His suggestion
was workable, use high fan for heat, also.
Third option. Duct in another heat run, and some more return duct. Increase
the physical size of, or numbers of ducts and vents. Since there's not
enough duct, put more in.
Christopher A. Young;
"Jorge" < firstname.lastname@example.org> wrote in message
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The contractor &/or tech should have evaluated the ductwork & airflow
delivery conditions & informed you of the apparent problems prior to the
installation of the AC coil. It might require all three options above &
Look at the furnace info data tag for the Blower horse power & the ESP
static pressure rating.
It might have a mere 1/6th HP blower motor & the ESP Rating of the
airhandler may be well below 0.50" WC.
If the above is the case, I would replace the furnace because when
operated in the cooling mode it will NOT perform well & could possibly
result in damage to the compressor.
Also, the ductwork may be totally inadequate for air conditioning mode.
The ductwork system must be designed to provide ample airflow in "the
cooling mode which normally requires considerable more airflow (esp. in
southern hot climate states) than the heating mode." Follow manual D
for proper ductwork design to match the airflow requirements of your AC
system & proper air delivery to each room!
Study these pages & then find a contractor that can do the job right!
The link below is for those with similar "Oil furnaces with added air
conditioning." - Do it right!
Check the blower curve graph or chart!
These are only suggestions, you are responsible for what you do or have
done, NOT me!
WISDOM PRINCIPLED EMPOWERMENT COMMUNICATIONS -
THE REAL POLITICAL ISSUES & WISDOM Principled PEOPLE EMPOWERMENT
Did you just ask the tech to install a/c only or include all necessary
additional work to make the heating/cooling system work properly.
Now you have a problem at hand which will have to be solved depending on
original contract(work order)
Change the size of the pulley wheel on your motor (make it a little
larger) and get a new belt if necessary. This will increase the air
flow speed through your furnace and *might* prevent it from hitting
the high limit.
The other option is turn down the setting on your gas valve to reduce
the flame size. Your furnace will run a little cooler, and you might
realize some fuel savings.
Also, are all of your vents in the house open and unobstructed?
Your last resort is to build a bypass duct around the coil, but that
is definately better than replacing your furnace for such a trivial
No. The idiots who claim to be experts don't waste their time with
that. They assume that if your furnace works, and if you're warm
enough in the winter, then the AC evap coil will work just fine with
your existing ducting.
Ya, overamp your motor and burn it out...good suggestion. Seriously Jorge,
don't try this without using an Amp-meter. Be prepared to upsize your motor
horsepower so as not to overamp the motor.I am assuming it is belt drive
because of the age so changing speed taps is probably not an option.
Thanks for all of the suggestions. I think I have a better
understanding of the problem now.
Can anyone comment on the urgency of the situation... how soon do I
need to fix this problem?
If the high limit switch burns out, then I am probably in serious
trouble. How likely is that to happen?
Also, I looked at the return duct system in the furnace room and it
looks strange: the two ceiling joists directly
over the furnace are walled of with galvanized metal to form a duct.
On the left side of the room, where
the furnace is, this ceiling duct is connected to another duct leading
down into the furnace. On the right wall,
this duct connects to the return duct system leading to the rest of
the house. But, there is a sizeable
gap between the ceiling duct and the return duct, about 6 inches wide
and 4 inches deep. So, the
return is sucking some air from the furnace room back into the
furnace. Would filling this gap improve
Was the blower motor replaced when the air conditioning was added?
Your installer may not have sized the blower motor correctly on the lower
speed for heating.
Have him / her come back out, explain the problem, and see if he can't
rectify the problem.
Your average 1/4 or 1/3 hp blower motor with a 1.15 sf rating can
easily handle a pulley change from 3" to 4" or 4" to 5".
Something you bone-heads forget is that the furnace blower motor is
helped by the fact that it's always being cooled by the return air, so
it's thermally well protected.
Jorge, increasing the pulley size by 1" is a perfectly good and
perfectly reliable solution. A year ago I did the same thing on my
furnace to increase airflow to the second story of my house.
If you're really concerned about reducing the life of your motor, then
oil the motor and the blower bearings when you change the pulley (I
bet they haven't been oiled for years). The motors they put in the
old furnaces like you and I have are not very efficient (no small
electric motor is) but they're durable.
No furnace should be hitting it's high limit. It's not a situation
you want to happen. Normally it happens because your fan belt breaks,
or your motor dies. And no, the high-limit switch doesn't "burn out"
because of repeated use. It's usually part of the furnace thermostat
that starts your blower anyways and stops your motor when the furnace
has cooled down at the end of a heating cycle.
You'd only be in serious trouble if hi-limit switch failed AND your
fan stopped turning at the same time.
And in spite of what other fools here have said, it's perfectly safe
to turn down your burners a little to compensate for insufficient
airflow. There is nothing magical or critical about the burners that
require a precisely controlled and specified NG input pressure. They
can operate over a wide range of reduced NG pressure and flow rates.
Turning your burners down is safer than having your furnace run and
always be hitting the high limit. That is a fact.
Do you have a humidifier attached to the furnace? Humidifiers are
ducted to take air from the supply duct, force it through a rotating
drum or stationary sponge, and let it re-enter the furnace through the
cold air return. You might have too much air flowing through the
humidifier circuit. See if there's a gate or baffle in the humidifer
circuit and close it down and see if that helps.
Something else you can do if you're really in a bind is to open a hole
(or maybe you already have a vent) directly on the return air plenum.
If you have insufficient cold-air return then opening a hole will
allow the fan to "suck" more air through the furnace, thereby making
it less likely it will hit the hi-limit.
Taking the cover off your humidifier (if you have one) would suffice.
"ding, ding, ding" We have a winner.
The newest Stormy has just spoken.
"notAhvacGuy"....you are absolutely clueless and NO, there is no way I
would tell you what you said is wrong. You arent worth the effort.
Dude give it up. Those that know sure as hell aren't interested in
teaching you combustion basics.
You've already exposed yourself for a clueless asshole. How about you
take your expert ass over to alt.homemoaner.repair where you can brag
how you know more than the assholes at alt.hvac and they can actually
I bet when you get a flat tire you adjust the engine air intake.
That you haven't the brains to study what we do, shows you for the we
todd did sob you are.
Don't bother with a smart-ass retort. You'd just fuck it up. Just move
Sorry junior. Im "full-quoting" you because I, as others, love a great
laugh at how stupid someone such as yourself can be.
I think you have already proven who the moron is and had you been here
long enough you would know that embarrasment is not in my vocabulary.
How about I take bets on how long your keyboard (mouth) keeps running
Looks like I've found another highway sitting diapered window licker.
I didn't forget crap. Just because the service factor is +15% doesn't mean
you still can't over amp the motor. You need to check it after adjusting
the sheeve ass hole. Otherwise you'll be back with complaints. How the
f*** do you know? Can you see his house from your computer?
My blower doesn't have a belt, should I just blank off 1/2 the squirrel cage
w/duct tape? That's gotta change the airflow.
And when I adjust my burners, do I want that bright blue flame, or can milk
it down to that pretty orange & yellow w/ the black stream of soot?
Hope your buying life insurance on all the folks your killing.
BTW: It's never a good idea to adjust the appliance gas pressure regulator
to anything but 3.5" w.c. for natural gas. [Unless it's other than a
*common* forced air furnace. The the manifold pressure recommended by the
Mfg. could be different.] The size of the burner spuds, verus the burner
primare air are set for that pressure. Anything lower would result in less
than complete combustion.
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