AC installation causing furnace to hit high limit

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Hello, 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 or 2) modify ductwork by adding a bypass around the coil for winter usage or 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?
Thanks!!
Jorge
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Jorge wrote:

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.
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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.
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This is a multi-part message in MIME format. --------------090505020104050303050705 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit
Jorge wrote:

delivery conditions & informed you of the apparent problems prior to the installation of the AC coil. It might require all three options above & more! (?)
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! http://www.udarrell.com/proper_cfm_btuh_duct_sizing_air_conditioning_systems.html
http://www.udarrell.com/external_static_pressure_readings.html The link below is for those with similar "Oil furnaces with added air conditioning." - Do it right! Check the blower curve graph or chart! http://www.udarrell.com/oil_furnace_heating.html These are only suggestions, you are responsible for what you do or have done, NOT me! - udarrell
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Jorge wrote:

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)
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Jorge wrote:

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 reason.

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.
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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.
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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 furnace performance?
Thanks, Jorge
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Jorge wrote:

George;
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.
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kool wrote:

Oh pulleeeze.
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 wrote:

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.
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"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. Bubba
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Full-quoter Bubba wrote:
(thanks for full-quoting me. You're repeating my advice, which is giving it a wider audience - you fucking stupid moron)

Because you're a chicken-shit moron who doesn't want to embarras himself by getting into an argument you're going to lose. You know you can't win this argument because you know I'm right.
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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 believe you. 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 along boy.
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actually Moron , i think he refuses to have a battle of wits with an unarmed person ;-)

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HVAC Guy wrote:

HVAC Gay Guy hasn't been right yet. What' makes you *think* your right is your gay boy friend said so.
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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 in here? Looks like I've found another highway sitting diapered window licker. Bubba
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HVAC Guy wrote:

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?
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HVAC Guy...
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. geothermaljones

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HVAC Guy wrote:

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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