Cracked heat X ????

[Gross]

ok here's my analogy. iv'e just completed a clean & service to an old natural draft furnace with a lot of rust in the firebox. i run the furnace 3 different times for 10 minutes allowing it to cool to ambient temp. before restarting. one cycle each at low. med. & high speeds. would you hold that the same amount of moisture passed through the flue cap to atmosphere on each of those cycles?

Depends, did the draft change? If not, yes. Even with the different fan speeds, that usually only correlates to a 100-175 degree flue gas difference... But it's still going to be over 270 easily, usually much higher. Water doesn't condense, at this pressure, until 212 degrees so it isn't condensing in the hx. UNLESS there is insufficient venting of the flue gases allowing the to cool in the flue and hx after shut down... This is also why you get rusty and soft flues

The fix for ND furnaces is slightly different than what I listed before

[stonefly]

sure it changes but its not the on cycle draft i'm concerned with but what happens between the time W terminates and stack effect no longer takes place. i've removed different amounts of heat from the heat exchanger at the end of the heat cycle by virtue of moving different amounts of air across it.

[Gross]

sure it changes but its not the on cycle draft i'm concerned with but what happens between the time W terminates and stack effect no longer takes place. i've removed different amounts of heat from the heat exchanger at the end of the heat cycle by virtue of moving different amounts of air across it.

....so youre saying the flue stops drafting immediately after w no longer receives a signal?

your thought process isnt complete. the flue will still be warm for some time and there for stack effect will continue..again, you have to make a repair, (connecting the flue to the furnace) but that will stop it. take a look, when you see a rusted hx youll also usually notice a replaced flue or rusted flue.

they are hand in hand

[stonefly]

no, just the opposite. i'm saying the flue will draft differently at the end of the heat cycle in each of those three conditions. if you agree what would be the effect on moisture retained in the heat exchanger?

connect the flue to the furnace? um.....ok but thats not a variable in my analogy.

[chuckcrj]

no, just the opposite. i'm saying the flue will draft differently at the end of the heat cycle in each of those three conditions. if you agree what would be the effect on moisture retained in the heat exchanger?

connect the flue to the furnace? um.....ok but thats not a variable in my analogy.

If this is a natural draft furnace with a draft diverter/draft hood then no the flue is not connected to the furnace and has little to no effect on the draft through the HX.

[Gross]

no, just the opposite. i'm saying the flue will draft differently at the end of the heat cycle in each of those three conditions. if you agree what would be the effect on moisture retained in the heat exchanger?

connect the flue to the furnace? um.....ok but thats not a variable in my analogy.

it should be, thats why they rust out, natural draft furnaces have unconnected flues...while draft will decrease as the flue cools, it will still continue to draft for some time. this will remove any gaseous moisture in the hx...before it has time to condense...but you have to have a connected flue to do that...something ND furnaces dont have without modification

[ch4man]

Depends, did the draft change? If not, yes. Even with the different fan speeds, that usually only correlates to a 100-175 degree flue gas difference... But it's still going to be over 270 easily, usually much higher. Water doesn't condense, at this pressure, until 212 degrees so it isn't condensing in the hx. UNLESS there is insufficient venting of the flue gases allowing the to cool in the flue and hx after shut down... This is also why you get rusty and soft flues

The fix for ND furnaces is slightly different than what I listed before

the dewpoint of flue gasses from natural gas is i believe 128 deg.

im a big believer in post purge. my amana came with one from the factory, goin on 20 yrs old and no rust

[stonefly]

in one condition "low speed" as compared to high speed. the heat exchanger is nominally hotter at the time "w" terminates (its been running at a higher heat rise). the airflow across the heat exchanger is removing less heat from the heat exchanger during the time the fan runs before cycling off because less air has moved across it and at a lower difference in temperature. the stack effect is in effect longer and at a higher temp before cooling in low speed.does this have no effect on the moisture retained in the heat exchanger?

[Gross]

in one condition "low speed" as compared to high speed. the heat exchanger is nominally hotter at the time "w" terminates (its been running at a higher heat rise). the airflow across the heat exchanger is removing less heat from the heat exchanger during the time the fan runs before cycling off because less air has moved across it and at a lower difference in temperature. the stack effect is in effect longer and at a higher temp before cooling in low speed.does this have no effect on the moisture retained in the heat exchanger?

With a connected flue on a natural draft furnace, not really. The temperature variance isnt that great and the flue will draft afterwards and pull spent gases out of the hx regardless

[stonefly]

well i can only defend what i said. i declined to go off topic on the furnaces i've seen vented to brick chimneys, the use of a draft gauge, curtain effect, barometric dampers, blocking off the draft hood (whatever phrase for that you prefer) etc,etc. my point was on eggshell metal, my observation about heat rise "often" is valid and i'll continue to correct it as needed when i pull cold sweaty furnaces into the maintenance pool. profound venting problems not withstanding. i have you down as a no to my question.

[ironpit]

heil is ICP. Look for little round metal rings that might be laying in the furnace. These pop off sometimes, usually due to overheated hx.

[CAtech1986]

The best way to identify a cracked heat exchanger is with a combustion analyzer. with gas pressure set at 3.5 for natural and 10-11" w.c for lp. you'll notice a rise in co when the blower comes on. a drastic rise. you don't need to pull it out or anything. if it is rusting that indicates there has been a problem for a long time. What type of furnace is it?

[chuckcrj]

The best way to identify a cracked heat exchanger is with a combustion analyzer. with gas pressure set at 3.5 for natural and 10-11" w.c for lp. you'll notice a rise in co when the blower comes on. a drastic rise. you don't need to pull it out or anything. if it is rusting that indicates there has been a problem for a long time. What type of furnace is it?

Most cracks will show no changes on the CA when the blower comes on. Only the big ones.