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AC installation causing furnace to hit high limit

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AC installation causing furnace to hit high limit Jorge 11-13-2007
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Posted by Tony Hwang on November 13, 2007, 10:48 pm
Jorge wrote:
> 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
>
Hi,
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)

Posted by HVAC Guy on November 14, 2007, 9:47 am
Jorge wrote:

> The company suggests the following options:

> 2) modify ductwork by adding a bypass around the coil for winter usage

> Does anyone have an opinion about the best option.

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.

> Also, when scoping out a house for AC, is it standard
> procedure to look at the size of the duct work?

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.

Posted by kool on November 14, 2007, 11:43 am

> Jorge wrote:
>
>> The company suggests the following options:
>
>> 2) modify ductwork by adding a bypass around the coil for winter usage
>
>> Does anyone have an opinion about the best option.
>
> 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

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.



Posted by Jorge on November 14, 2007, 1:19 pm
> > Jorge wrote:
>
> >> The company suggests the following options:
>
> >> 2) modify ductwork by adding a bypass around the coil for winter usage
>
> >> Does anyone have an opinion about the best option.
>
> > 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
>
> 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
furnace performance?

Thanks,
Jorge






Posted by Zyp on November 14, 2007, 1:29 pm
Jorge wrote:
>>> Jorge wrote:
>>
>>>> The company suggests the following options:
>>
>>>> 2) modify ductwork by adding a bypass around the coil for winter
>>>> usage
>>
>>>> Does anyone have an opinion about the best option.
>>
>>> 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
>>
>> 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
> furnace performance?
>
> Thanks,
> Jorge

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.

--
Zyp



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