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Run time vs right-sizing

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Run time vs right-sizing Vance 12-07-2006
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Posted by Steve Scott on December 7, 2006, 11:44 pm
You've forgotten the fan laws. A smaller furnace will move less air.
If you take a given motor and move 1/2 the air that motor will use 1/4
of the electricity. Since you're putting less heat into the air it
takes fewer CFM to move the heat.

If you move to a modulating furnace with a variable speed the numbers
drop further. Or better yet move to a mod/con boiler with constant
circulation and the electricity consumed is less yet.


>t me expand on "Those would be electric numbers":
>
>KWH is a standard symbol representing power usage usually in the form kWh
>which is 1000 watts used for a period of 1 hour. If the smaller furnace has
>a 1/2 HP motor drawing 4A it will consume 1452 kWh in a 6 month heating
>season (4*120*24*30*6*.7/1000) with a 70% duty cycle whereas the larger
>furnace will consume only 207 kWh with a 10% duty cycle. At 20cents/kWh the
>difference will heat my home for 4 months. This does not even take into
>consideration the inducer fan or increased wear and maintenance costs.


--
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Posted by Bubba on December 8, 2006, 8:03 pm

>
>>
>>>
>>>> wrote:
>>>>
>>>>>At -10C, which is also my design temp, my condensing gas furnace runs
>>>>>for
>>>>>20
>>>>>mins every 2 hours to maintain a setpoint of 18C with a spread of .5C,
>>>>>obviously it is theoretically oversized, however, if sized 'correctly'
>>>>>it
>>>>>would run continuously at -10C and use significantly more KWH's. Which
>>>>>would
>>>>>be the better scenario, almost continuous run times to improve comfort
>>>>>at
>>>>>the expense of higher electricity usage or the shorter cycles with less
>>>>>wear
>>>>>on the blower and draft inducer fan?
>>>>>
>>>>
>>>> WRONG! If it were sized correctly, it would have longer run times
>>>> using a smaller KW heater. Thus balancing out the electric
>>>> consumption. It is always better to properly size a furnace.
>>>> Bubba
>>>
>>>There are no heat strips in this application - gas only. A smaller furnace
>>>would cost more to operate and maintain due to much longer run times. 20
>>>min
>>>run times are certainly not short cycling nor reducing the efficiency of
>>>the
>>>furnace.
>>>
>> Interesting how you "use significantly more KWH's" as you point out.
>> KWH's would be Kilo-Watt-hours. Those would be electric numbers
>> junior.
>> A smaller furnace would NOT cost more to operated and maintain.
>> 20 min run times per 2 HOURS at design temperature IS definately
>> over-sized.
>> You're an IDIOT and/or TROLL.
>
>
>You are either uneducated, misinformed or both.
>
>Let me expand on "Those would be electric numbers":
>
>KWH is a standard symbol representing power usage usually in the form kWh
>which is 1000 watts used for a period of 1 hour. If the smaller furnace has
>a 1/2 HP motor drawing 4A it will consume 1452 kWh in a 6 month heating
>season (4*120*24*30*6*.7/1000) with a 70% duty cycle whereas the larger
>furnace will consume only 207 kWh with a 10% duty cycle. At 20cents/kWh the
>difference will heat my home for 4 months. This does not even take into
>consideration the inducer fan or increased wear and maintenance costs.
>

Nope. Neither. Let me help you out just a bit.
In the world of heating and cooling there are two different worlds:
One, happens to be the world you live in. Its called the "Labratory
World".
The other world happens to be called "The real world". That happens to
be the world I live in.
If you even remotely think the difference in those two furnaces will
heat your home for 4 months for free (compared to the smaller one)
then you need to put your tinfoil hat on and go out in the middle of a
busy highway and lick a window in your diapers.
You can quote all the numbers you want. It just "aint" goina happen.
Get it sized right and get it done the first time.
Bubba

Posted by Stormin Mormon on December 7, 2006, 9:33 am
I'd prefer a longer run time. So the heat doesn't all dump into the
home at once. As you have only a half degree temp swing, you've
aparently got good insulation, though. You could replace the furnace,
and see if it's more comfortable next year. And then consider put the
other one back.

Let me guess... you're an engineer?

--

Christopher A. Young
You can't shout down a troll.
You have to starve them.
.

At -10C, which is also my design temp, my condensing gas furnace runs
for 20
mins every 2 hours to maintain a setpoint of 18C with a spread of .5C,
obviously it is theoretically oversized, however, if sized 'correctly'
it
would run continuously at -10C and use significantly more KWH's. Which
would
be the better scenario, almost continuous run times to improve comfort
at
the expense of higher electricity usage or the shorter cycles with
less wear
on the blower and draft inducer fan?




Posted by DIMwit on December 7, 2006, 11:37 am
Stormy, try to be original


> I'd prefer a longer run time. So the heat doesn't all dump into the
> home at once. As you have only a half degree temp swing, you've
> aparently got good insulation, though. You could replace the furnace,
> and see if it's more comfortable next year. And then consider put the
> other one back.
>
> Let me guess... you're an engineer?
>
> --
>
> Christopher A. Young
> You can't shout down a troll.
> You have to starve them.
> .
>
> At -10C, which is also my design temp, my condensing gas furnace runs
> for 20
> mins every 2 hours to maintain a setpoint of 18C with a spread of .5C,
> obviously it is theoretically oversized, however, if sized 'correctly'
> it
> would run continuously at -10C and use significantly more KWH's. Which
> would
> be the better scenario, almost continuous run times to improve comfort
> at
> the expense of higher electricity usage or the shorter cycles with
> less wear
> on the blower and draft inducer fan?
>
>
>



Posted by Stormin Mormon on December 7, 2006, 2:10 pm
I don't remember ever hearing that advice. Please remind me.

--

Christopher A. Young
You can't shout down a troll.
You have to starve them.
.

Stormy, try to be original


> I'd prefer a longer run time. So the heat doesn't all dump into the
> home at once. As you have only a half degree temp swing, you've
> aparently got good insulation, though. You could replace the
furnace,
> and see if it's more comfortable next year. And then consider put
the
> other one back.
>
> Let me guess... you're an engineer?
>
> --
>
> Christopher A. Young
> You can't shout down a troll.
> You have to starve them.
> .
>
> At -10C, which is also my design temp, my condensing gas furnace
runs
> for 20
> mins every 2 hours to maintain a setpoint of 18C with a spread of
.5C,
> obviously it is theoretically oversized, however, if sized
'correctly'
> it
> would run continuously at -10C and use significantly more KWH's.
Which
> would
> be the better scenario, almost continuous run times to improve
comfort
> at
> the expense of higher electricity usage or the shorter cycles with
> less wear
> on the blower and draft inducer fan?
>
>
>




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