Re: It's said that induction fans (draft inducers) increase efficiency

	Re: It's said that induction fans (draft inducers) increase efficiency - how?

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Re: It's said that induction fans (draft inducers) increase efficiency - how?

nicksanspam

12-29-2007

Re: It's said that induction fans (draft inducers)...

Zyp

12-29-2007

If you were Registered and logged in, you could reply and use other advanced thread options

Posted by on December 29, 2007, 8:57 am

>FYI - I didn't see Nick's post until Roger quoted it. Nick's post

>didn't show up on the NNTP server I use.

Maybe Paul's cancelling posts again.

>Nick Wrote:

>> That might reduce the cold air leakage into the house by a few cfm,

>> since the furnace would not pull combustion air out of the house, but

>> how would it increase efficiency? Starting with outdoor air at say,

>> 30 F, we can either heat it to 70 F in the house, then heat it up

>> to the flue gas temp, or heat it from 30 to the flue gas temp

>> in one step, with the same energy, no?

>I'm thinking that AFUE ratings include or take into account the degree

>to which a furnace prevents outside air leakage into a house, hence

>any furnace that is designed for a closed intake and combustion path

>would result in a higher AFUE rating.

But the combustion-air-heating energy wouldn't change, and the furnace
would only pull its combustion air into the house. It wouldn't increase
the total house air leakage more than that.

>Roger wrote:

>> If you use a mechanism to stop the air up the flue during the

>> off cycle, it can still be more efficient...

>I don't think that any such mechanism is necessary.

>If you have a furnace where the combustion intake has been sealed such

>that it can only draw air from the outside, and naturally the flue is

>sealed so that it exhausts out the chimney, then you've got no reason

>to close that pathway when the furnace is off.

Hot thermal mass, esp in a boiler vs a furnace...

>You've got no reason to close it because the only reason to close it

>is if you have a high, natural ambient airflow through the intake and

>out the flue. The distance from the exterior opening of the intake

>and the top of the chimney is probably no more than 15 or 20 feet, and

>they would be on the same side of the house. Unless the geometry of

>the house is very unusual, they would both experience the same ambient

>air pressure, so you'd expect to get little or no net airflow from

>intake to exhaust.

A 6" pipe with A = 0.79 ft^2 and H = 20' and dT = (300-30) F might have
an initial 16.6Asqrt(HdT) = 958 cfm stack-effect and lose roughly cfmdT
= 259K Btu/h until it cools off, even with no wind pressure difference.

Nick

Posted by Zyp on December 29, 2007, 2:46 pm

nicksanspam@ece.villanova.edu wrote:

>> FYI - I didn't see Nick's post until Roger quoted it. Nick's post

>> didn't show up on the NNTP server I use.

> Maybe Paul's cancelling posts again.

>> Nick Wrote:

>>> That might reduce the cold air leakage into the house by a few cfm,

>>> since the furnace would not pull combustion air out of the house,

>>> but how would it increase efficiency? Starting with outdoor air at

>>> say, 30 F, we can either heat it to 70 F in the house, then heat it

>>> up

>>> to the flue gas temp, or heat it from 30 to the flue gas temp

>>> in one step, with the same energy, no?

>> I'm thinking that AFUE ratings include or take into account the

>> degree to which a furnace prevents outside air leakage into a house,

>> hence any furnace that is designed for a closed intake and

>> combustion path would result in a higher AFUE rating.

> But the combustion-air-heating energy wouldn't change, and the furnace

> would only pull its combustion air into the house. It wouldn't

> increase the total house air leakage more than that.

>> Roger wrote:

>>> If you use a mechanism to stop the air up the flue during the

>>> off cycle, it can still be more efficient...

>> I don't think that any such mechanism is necessary.

>> If you have a furnace where the combustion intake has been sealed

>> such that it can only draw air from the outside, and naturally the

>> flue is sealed so that it exhausts out the chimney, then you've got

>> no reason to close that pathway when the furnace is off.

> Hot thermal mass, esp in a boiler vs a furnace...

>> You've got no reason to close it because the only reason to close it

>> is if you have a high, natural ambient airflow through the intake and

>> out the flue. The distance from the exterior opening of the intake

>> and the top of the chimney is probably no more than 15 or 20 feet,

>> and they would be on the same side of the house. Unless the

>> geometry of the house is very unusual, they would both experience

>> the same ambient air pressure, so you'd expect to get little or no

>> net airflow from intake to exhaust.

> A 6" pipe with A = 0.79 ft^2 and H = 20' and dT = (300-30) F might

> have an initial 16.6Asqrt(HdT) = 958 cfm stack-effect and lose

> roughly cfmdT = 259K Btu/h until it cools off, even with no wind

> pressure difference.

> Nick

You guys are funny! :) I love reading this group.

The answer to your question of "why induced draft" is simple. Hey Paul, can
you enlighten?

--
Zyp

Posted by on December 29, 2007, 3:52 pm

>nicksanspam@ece.villanova.edu wrote:

>>> FYI - I didn't see Nick's post until Roger quoted it. Nick's post

>>> didn't show up on the NNTP server I use.

>> Maybe Paul's cancelling posts again.

>>> Nick Wrote:

>>>

>>>> That might reduce the cold air leakage into the house by a few cfm,

>>>> since the furnace would not pull combustion air out of the house,

>>>> but how would it increase efficiency? Starting with outdoor air at

>>>> say, 30 F, we can either heat it to 70 F in the house, then heat it

>>>> up

>>>> to the flue gas temp, or heat it from 30 to the flue gas temp

>>>> in one step, with the same energy, no?

>>>

>>> I'm thinking that AFUE ratings include or take into account the

>>> degree to which a furnace prevents outside air leakage into a house,

>>> hence any furnace that is designed for a closed intake and

>>> combustion path would result in a higher AFUE rating.

>> But the combustion-air-heating energy wouldn't change, and the furnace

>> would only pull its combustion air into the house. It wouldn't

>> increase the total house air leakage more than that.

>>> Roger wrote:

>>>

>>>> If you use a mechanism to stop the air up the flue during the

>>>> off cycle, it can still be more efficient...

>>> I don't think that any such mechanism is necessary.

>>>

>>> If you have a furnace where the combustion intake has been sealed

>>> such that it can only draw air from the outside, and naturally the

>>> flue is sealed so that it exhausts out the chimney, then you've got

>>> no reason to close that pathway when the furnace is off.

>> Hot thermal mass, esp in a boiler vs a furnace...

>>> You've got no reason to close it because the only reason to close it

>>> is if you have a high, natural ambient airflow through the intake and

>>> out the flue. The distance from the exterior opening of the intake

>>> and the top of the chimney is probably no more than 15 or 20 feet,

>>> and they would be on the same side of the house. Unless the

>>> geometry of the house is very unusual, they would both experience

>>> the same ambient air pressure, so you'd expect to get little or no

>>> net airflow from intake to exhaust.

>> A 6" pipe with A = 0.79 ft^2 and H = 20' and dT = (300-30) F might

>> have an initial 16.6Asqrt(HdT) = 958 cfm stack-effect and lose

>> roughly cfmdT = 259K Btu/h until it cools off, even with no wind

>> pressure difference.

>> Nick

>You guys are funny! :) I love reading this group.

>The answer to your question of "why induced draft" is simple. Hey Paul, can

>you enlighten?

Not until your minion dues are caught up :-)

--
Click here every day to feed an animal that needs you today !!!
http://www.theanimalrescuesite.com/

Paul ( pjm @ pobox . com ) - remove spaces to email me
'Some days, it's just not worth chewing through the restraints.'
'With sufficient thrust, pigs fly just fine.'
HVAC/R program for Palm PDA's
Free demo now available online http://pmilligan.net/palm/

Posted by Zyp on December 29, 2007, 4:51 pm

.p.jm@see_my_sig_for_address.com wrote:

>> nicksanspam@ece.villanova.edu wrote:

>>>

>>>> FYI - I didn't see Nick's post until Roger quoted it. Nick's post

>>>> didn't show up on the NNTP server I use.

>>>

>>> Maybe Paul's cancelling posts again.

>>>

>>>> Nick Wrote:

>>>>

>>>>> That might reduce the cold air leakage into the house by a few

>>>>> cfm, since the furnace would not pull combustion air out of the

>>>>> house, but how would it increase efficiency? Starting with

>>>>> outdoor air at say, 30 F, we can either heat it to 70 F in the

>>>>> house, then heat it up

>>>>> to the flue gas temp, or heat it from 30 to the flue gas temp

>>>>> in one step, with the same energy, no?

>>>>

>>>> I'm thinking that AFUE ratings include or take into account the

>>>> degree to which a furnace prevents outside air leakage into a

>>>> house, hence any furnace that is designed for a closed intake and

>>>> combustion path would result in a higher AFUE rating.

>>>

>>> But the combustion-air-heating energy wouldn't change, and the

>>> furnace would only pull its combustion air into the house. It

>>> wouldn't increase the total house air leakage more than that.

>>>

>>>> Roger wrote:

>>>>

>>>>> If you use a mechanism to stop the air up the flue during the

>>>>> off cycle, it can still be more efficient...

>>>

>>>> I don't think that any such mechanism is necessary.

>>>>

>>>> If you have a furnace where the combustion intake has been sealed

>>>> such that it can only draw air from the outside, and naturally the

>>>> flue is sealed so that it exhausts out the chimney, then you've got

>>>> no reason to close that pathway when the furnace is off.

>>>

>>> Hot thermal mass, esp in a boiler vs a furnace...

>>>

>>>> You've got no reason to close it because the only reason to close

>>>> it is if you have a high, natural ambient airflow through the

>>>> intake and out the flue. The distance from the exterior opening

>>>> of the intake and the top of the chimney is probably no more than

>>>> 15 or 20 feet, and they would be on the same side of the house.

>>>> Unless the geometry of the house is very unusual, they would both

>>>> experience the same ambient air pressure, so you'd expect to get

>>>> little or no net airflow from intake to exhaust.

>>>

>>> A 6" pipe with A = 0.79 ft^2 and H = 20' and dT = (300-30) F might

>>> have an initial 16.6Asqrt(HdT) = 958 cfm stack-effect and lose

>>> roughly cfmdT = 259K Btu/h until it cools off, even with no wind

>>> pressure difference.

>>>

>>> Nick

>> You guys are funny! :) I love reading this group.

>> The answer to your question of "why induced draft" is simple. Hey

>> Paul, can you enlighten?

> Not until your minion dues are caught up :-)

> --

> Click here every day to feed an animal that needs you today !!!

> http://www.theanimalrescuesite.com/

> Paul ( pjm @ pobox . com ) - remove spaces to email me

> 'Some days, it's just not worth chewing through the restraints.'

> 'With sufficient thrust, pigs fly just fine.'

> HVAC/R program for Palm PDA's

> Free demo now available online http://pmilligan.net/palm/

Sorry Nick;

Paul doesn't want to 'xplain.

The truth to the matter is the inducer itself isn't what increases the
efficiency. Actually the cost of running the inducer motor decreases the
efficiency a bit. The answer lies in the flue gas temperature.

On older natural draft furnaces, the flue gas temperature is about 450º or
more. To increase the effective efficency of the furance, you would have to
reduce the flue gas temperature [that which goes up the flue stack to the
outdoors]. The manufacturer's reduced internal spacing of the heat
exchanger's to increase the transfer of flue gas heat to the conditioned air
[that which heats your home.] The flue gas termperature of the induced
draft furnace is now 350º or there abouts. This heat exchanger space
reduction doesn't allow for "natural draft" and hence the need for "induced
draft." There is a trade off for the cost of the inducer verus the
increased heat transfer. Also, the induction of the induced draft decreased
the need for "dilution air" and less secondary air. [For the older furnaces
approximately 30 cu. foot of flue gas per one cubic foot of fuel burned.
The newer furnaces only create / need 13 - 15 cu. foot of flue gas for the
same cubic foot of fuel burned.] No dilution air needed.

A new problem though also came with the reduction of the heat exchanger
sizing. The manufacturer had to increase the airflow. This was to decrease
the heat exchanger temperture and prolong it's service life. So the older
natural draft furnaces had a higher temperture rise than the newer induced
draft furnaces. This resulted [at least in California] in a new problem.

Many trac homes were built with furnaces that received it's majority of
return air from under the house and a little from inside the house. [Many
California homes built after WW II didn't have air conditioning, but had
forced air heating.] Replacement of these furnaces meant that a new return
had to be installed and the draw from under the house closed off. Most
contracdtors did, and some didn't.

I hope this helps you understand why the induced draft blower on the newer
80% furnaces is needed.

One other note is the understanding of the inshot burners and how that
increased the effective efficiency of today's mid-efficiency furances.

Ok Paul, did I pay enough?

--
Zyp

Posted by on December 29, 2007, 5:12 pm

>.p.jm@see_my_sig_for_address.com wrote:

>>> nicksanspam@ece.villanova.edu wrote:

>>>>

>>>>> FYI - I didn't see Nick's post until Roger quoted it. Nick's post

>>>>> didn't show up on the NNTP server I use.

>>>>

>>>> Maybe Paul's cancelling posts again.

>>>>

>>>>> Nick Wrote:

>>>>>

>>>>>> That might reduce the cold air leakage into the house by a few

>>>>>> cfm, since the furnace would not pull combustion air out of the

>>>>>> house, but how would it increase efficiency? Starting with

>>>>>> outdoor air at say, 30 F, we can either heat it to 70 F in the

>>>>>> house, then heat it up

>>>>>> to the flue gas temp, or heat it from 30 to the flue gas temp

>>>>>> in one step, with the same energy, no?

>>>>>

>>>>> I'm thinking that AFUE ratings include or take into account the

>>>>> degree to which a furnace prevents outside air leakage into a

>>>>> house, hence any furnace that is designed for a closed intake and

>>>>> combustion path would result in a higher AFUE rating.

>>>>

>>>> But the combustion-air-heating energy wouldn't change, and the

>>>> furnace would only pull its combustion air into the house. It

>>>> wouldn't increase the total house air leakage more than that.

>>>>

>>>>> Roger wrote:

>>>>>

>>>>>> If you use a mechanism to stop the air up the flue during the

>>>>>> off cycle, it can still be more efficient...

>>>>

>>>>> I don't think that any such mechanism is necessary.

>>>>>

>>>>> If you have a furnace where the combustion intake has been sealed

>>>>> such that it can only draw air from the outside, and naturally the

>>>>> flue is sealed so that it exhausts out the chimney, then you've got

>>>>> no reason to close that pathway when the furnace is off.

>>>>

>>>> Hot thermal mass, esp in a boiler vs a furnace...

>>>>

>>>>> You've got no reason to close it because the only reason to close

>>>>> it is if you have a high, natural ambient airflow through the

>>>>> intake and out the flue. The distance from the exterior opening

>>>>> of the intake and the top of the chimney is probably no more than

>>>>> 15 or 20 feet, and they would be on the same side of the house.

>>>>> Unless the geometry of the house is very unusual, they would both

>>>>> experience the same ambient air pressure, so you'd expect to get

>>>>> little or no net airflow from intake to exhaust.

>>>>

>>>> A 6" pipe with A = 0.79 ft^2 and H = 20' and dT = (300-30) F might

>>>> have an initial 16.6Asqrt(HdT) = 958 cfm stack-effect and lose

>>>> roughly cfmdT = 259K Btu/h until it cools off, even with no wind

>>>> pressure difference.

>>>>

>>>> Nick

>>>

>>> You guys are funny! :) I love reading this group.

>>>

>>> The answer to your question of "why induced draft" is simple. Hey

>>> Paul, can you enlighten?

>> Not until your minion dues are caught up :-)

>> --

>> Click here every day to feed an animal that needs you today !!!

>> http://www.theanimalrescuesite.com/

>> Paul ( pjm @ pobox . com ) - remove spaces to email me

>> 'Some days, it's just not worth chewing through the restraints.'

>> 'With sufficient thrust, pigs fly just fine.'

>> HVAC/R program for Palm PDA's

>> Free demo now available online http://pmilligan.net/palm/

>Sorry Nick;

>Paul doesn't want to 'xplain.

>The truth to the matter is the inducer itself isn't what increases the

>efficiency. Actually the cost of running the inducer motor decreases the

>efficiency a bit. The answer lies in the flue gas temperature.

>On older natural draft furnaces, the flue gas temperature is about 450º or

>more. To increase the effective efficency of the furance, you would have to

>reduce the flue gas temperature [that which goes up the flue stack to the

>outdoors]. The manufacturer's reduced internal spacing of the heat

>exchanger's to increase the transfer of flue gas heat to the conditioned air

>[that which heats your home.] The flue gas termperature of the induced

>draft furnace is now 350º or there abouts. This heat exchanger space

>reduction doesn't allow for "natural draft" and hence the need for "induced

>draft." There is a trade off for the cost of the inducer verus the

>increased heat transfer. Also, the induction of the induced draft decreased

>the need for "dilution air" and less secondary air. [For the older furnaces

>approximately 30 cu. foot of flue gas per one cubic foot of fuel burned.

>The newer furnaces only create / need 13 - 15 cu. foot of flue gas for the

>same cubic foot of fuel burned.] No dilution air needed.

>A new problem though also came with the reduction of the heat exchanger

>sizing. The manufacturer had to increase the airflow. This was to decrease

>the heat exchanger temperture and prolong it's service life. So the older

>natural draft furnaces had a higher temperture rise than the newer induced

>draft furnaces. This resulted [at least in California] in a new problem.

>Many trac homes were built with furnaces that received it's majority of

>return air from under the house and a little from inside the house. [Many

>California homes built after WW II didn't have air conditioning, but had

>forced air heating.] Replacement of these furnaces meant that a new return

>had to be installed and the draw from under the house closed off. Most

>contracdtors did, and some didn't.

>I hope this helps you understand why the induced draft blower on the newer

>80% furnaces is needed.

>One other note is the understanding of the inshot burners and how that

>increased the effective efficiency of today's mid-efficiency furances.

>Ok Paul, did I pay enough?

Nice discussion, but I ain't seen the $$$ yet .....

BTW, Nick is the proverbial 'horse you can lead to water'... I
gave up on trying to educate him about a decade ago.

--
Click here every day to feed an animal that needs you today !!!
http://www.theanimalrescuesite.com/

Paul ( pjm @ pobox . com ) - remove spaces to email me
'Some days, it's just not worth chewing through the restraints.'
'With sufficient thrust, pigs fly just fine.'
HVAC/R program for Palm PDA's
Free demo now available online http://pmilligan.net/palm/

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