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

>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.

>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.

>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.

>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.

>> 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

>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?

>> 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/

>.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?