Condensation in furnaces

[RandomNick]

I'm trying to understand the issue of condensation in gas furnaces (furnai?). My understanding is represented by this simplified image and description:



Non-Condensing Furnace: Cold air from intake vent is fed to the blue burner which produces hot gases that travel through heat exchanger tubes and exit at high temperature from the exhaust vent. Although not clearly shown, there is no place for the acidic condensation to drain and thus it is imperative to minimize/eliminate it. Condensation is minimized due to higher temperatures throughout the path. The heat exchanger shape/pitch is such that should droplets of condensation form inside the tube, they would slide back towards the hotter burner region to be evaporated.

Condensing Furnace: Cold air from intake vent is fed to the blue burner which produces hot gases that travel through heat exchanger tubes and exit at warm temperature from the exhaust vent. There is a drain for condensation, and condensation is always expected in the final segments. The heat exchanger(s) shape/pitch is such that droplets of condensation slide away from the burner and towards the condensate drain.

I don't know if any non-condensing furnaces have "primary" and "secondary" heat exchangers, but I've noticed that condensing furnaces do (with the hotter early segments comprising the primary, and the final segments comprising the often physically different, larger surface area secondary).

Based on this it is much easier to imagine a condensation problem in low-firing non-condensing furnaces. In the condensing furnace, should condensation form it has a path through the "absolutely must be condensation resistant" secondary and out through the drain. The only potential points of concern, that I can think of, would be a) improperly shaped or pitched heat exchanger tubes, b) bad heat exchanger tube junctions which allow the condensation to collect, and c) surface condensation which isn't sufficient to form droplets/streams that move towards the drain. I don't know that any of those actually pose a real world problem though (?).

One thing that came to me as I was preparing this: IF I have a reasonably correct grasp of this subject (I certainly may not), then at least until the furnace condensate drain trap fills, there is a path for hazardous exhaust gases to go right out the condensate drain line. I have seen several instances where the furnace condensate drain line and evaporator coil drain line were tied together (directly, or indirectly through a condensate pump). Thus it appears to me that in certain, perhaps code-breaking, installs there is a [temporary] threat of furnace exhaust making its way up the evaporator coil drain line and out the open evaporator coil pan into the air stream blowing into the home. If there is something I'm overlooking... something besides a potentially dry trap... that would prevent said exhaust from taking that route I'd be interested to know what it is.

Comments on the above welcome and much appreciated!

[beenthere]

Condensing furnaces have 2 heat exchangers.
NO condensate is suppose to be in the primary HE, only the secondary HE can condensate.

[beenthere]

When teh A/C and furnace drain lines are piped together. the A/c line is suppose to have its own trap to prevent exhaust from entering the air stream.

[RandomNick]

When teh A/C and furnace drain lines are piped together. the A/c line is suppose to have its own trap to prevent exhaust from entering the air stream.

Uh... what keeps that evaporator coil condensate trap full of water during the months when the air conditioner isn't running? In every install I've looked at, said trap was up at evaporator coil level and thus up hill from where the furnace line and ac line met.

[beenthere]

By the time the trap is empty. The furnace trap has water in it, and can't exhaust vapors into the drain line.

[RandomNick]

By the time the trap is empty. The furnace trap has water in it, and can't exhaust vapors into the drain line.

Sounds like a bad assumption in there... that the air conditioner had been run recently enough that the evaporator drain trap still had water in it. I know two people in our general neck of the woods who have gone some summers without ever using their AC.

Just how quickly does a condensing furnace trap fill up when dry?

Is the furnace trap usually internal or external? I ask because I *think* in the case of one of the installs I looked at there was no external trap on the furnace line. IIRC, it just came out the side of the furnace and ran straight down into a condensate pump (how far from the bottom I don't know).

[fearlessfurnace]

exhaust gasses can get out till the furnace trap has water in it. every condensating manual I have read says to fill the trap before firing the furnace. In fact it takes like 5 minutes for the trap to fill if you just fire the furnace with a dry trap.

[fearlessfurnace]

most brands the 90% primary heat exchanger is the same as the 80% heat exchanger and the secondary heat exchanger is added betwean it and the inducer in the 90%

[fearlessfurnace]

and some funaces will not make the pressure switch untill the trap is full

[b26440510]

from what I have gathered, the two condensate systems shouldnt share a common drain.

[beenthere]

The furnace trap fills the first time it runs.

[tinmarine]

Alot of condensing furnaces have an internal trap and it states in the IO manual not to put an external trap on it. It's amazing how long water can stand in internal traps. I just checked one last week and it was still full from last season.

[fearlessfurnace]

duhhhhh
almost all furnace traps are on the low pressure (sucky) side of the inducer. before they fill up the furnace sucks in room air not blow out vent gasses. old goodman 90's and lennox pulses notwithstanding. that is kinda why we use inducers and not combustion blowers. boy do i feel DUMB!!!!!!!

[Airmechanical]

highly efficient furnaces= condensing furnaces

the secondary HX on all high efficiency furnaces do require their own independant trap

you can drain that down into some other drain line, but it must be trapped first


.

[beenthere]

Many furnaces have a drain line from the positive pressure side also.
It prevents teh inducer from being over whelmed with condensate from the flue pipe.

[fearlessfurnace]

The positive pressure inducer drain almost always runs to the same origonal equipment furnace trap. But some furnaces have had retrofits that can get kinda ugly. The collector box drain almost always sucks more than the inducer drain blows.

The manuals show an open "t" after the furnace trap. The drain might be trapped but the furnace trap outlet is not trapped. That is clumsy but you see what I'm saying?

[fearlessfurnace]

Any way a 2" vent has fifteen-sixteen times the area of a 1/2" drain so most of the exhaust is going outside even in the worst case. The manual still says prime the trap before you fire it. I did not say anyone does.

[genduct]

How much flue gas goes into the space before the vent pipe warms up and allows that warmer flue gas to rise out of a conventional 80% unit compared to the condensate of a 90% before the trap seal is established?
Me thinks it is, at worse, the same

[RandomNick]

Thanks for the replies. To be honest, I wasn't sure if air was pulled through the heat exchanger or pushed and thus didn't consider how that could affect things.

I now have an application specific question, if someone would be willing to comment. I believe my upstairs unit and the neighbor's downstairs unit SHARE the one condensate drain line that protrudes from the house. It runs in the walls and thus this can not be verified without cutting into the wall or double checking it via experiment. Lets say it is true... common condensate drain line. Running a new line for my unit would be an option, but there is an approval process which could possibly result in my contractor having to run a new line down the outside of the building where it would be exposed to prolonged periods of freezing temperatures. Thus I'm inclined to think that utilizing the existing, shared, drain line would be "preferred". I don't know enough about proper trapping, condensate drain line pressures, etc... is there a way to safely/correctly drain my new furnace & coil into an existing shared drain line?

[b26440510]

well, first thing I'd do is check the local building code and see if they permit sharing of the line. If not allowed then they'll have to fix it, which means they'll also have to take the appropriate measures to ensure against freezing.