Heat Exchanger sweating

[t527ed]

does not need the chimney back drafting to cause the condensation.

the heat exchanger is after the evap coil so it is cold, it would probably sweat even if the chimney were disconnected.
if its a horizontal system in a damp crawl space it would even be worse.

only solution is to reinstall it properly, probably with a new furnace at this point.

[nehvacman]

The reason I am confused is that there is about 10 foot of uninsulated duct between the evap coil and the furnace and it is not sweating. I thought that would rule out high humidity in the furnace room being the cause.

That is why I was wondering if the outdoor air from the flue would be related to the issue.

[BPToilman]

Like someone said the hx is prob acting as another coil. The hx is prob slowing the air down enough that it will start to condense on the warmer metal. How hot is it in the basement?

[syndicated]

The HX has a lot more surface area than duct work does, obviously to promote heat transfer. It's now working in reverse, and is probably garbage.
I don't know what code you have where you're at, but you can't generally add a damper to a flue, if the appliance isnt approved for one. If you can, you better make damn sure it proves open before the gas valve opens. Guaranteed this won't solve your problem though.

[energy star]

If hot outdoor air travels back down the flue - that goes against the laws of physics! What next? Will the law of gravity be repealed?

Exhaust fan in the attic/basement pulling in air. Pilot now off, did it stop? Where is the furnace installed at?

[energy star]

The reason I am confused is that there is about 10 foot of uninsulated duct between the evap coil and the furnace and it is not sweating. I thought that would rule out high humidity in the furnace room being the cause.

That is why I was wondering if the outdoor air from the flue would be related to the issue.

Is it lined?

[nehvacman]

I will check back with them tomorrow to see if the pilot was causing any problems.

\

[ch4man]

we can discuss the hows and whys of dewpoint and condensation all you'd like... we're all here to learn.


but the reality is that this is a red tag that turned itself into a sales opportunity... that way everyone wins.

nows the time before it gets cold

[Eric S]

My comany has put in furnaces with the evaporator upstream of the Heat exchanger. It was for dehumidifying a swimming pool room. Had to have a stainless heat exchanger though.

[Kevin Weaver]

Went to a house today that complained of water dripping from the furnace. The setup is an older horizontal gas furnace with a evap coil on the return side. The water is dripping from the heat exchanger chambers. There are 6 vertical chambers in the furnace and water is running down the insides of all the chambers. I haven't ran across this before, but my first guess is that warm outside air (90-100 degrees for the past three weeks) is coming down the flue pipe and condensing as the heat exchanger is in the cold air stream and around 55 degrees.

Does this sound reasonable to you guys? Would it help to add a damper to the flue to stop the warm outside air from traveling into the heat exchanger? Or do you think I have something else causing the condensation problem?

Thanks
Josh

warm air in the furnaces heat exchanger and cold air on outside of heat exchanger equals sweat.

Saw this once before in the 80's at a automobile repair shop. also once on residential and it rotted the heat exchanger out and about killed a lady, found this one in winter time.

But you know what, doesn't rtu's have their evaporator coils upstream of the heat exchanger, just saying.

[7X]

It is not uncommon to have pressures in the building that are negative with reference to the outside when an air handler is operating. Could be due to duct leaks or door closure issues. So it is entirely possible to be sucking outdoor air down the vent pipe with during A/C operation. This could cause the large amount of condensate in all of the cells. Can be verified by puffing a little smoke at the draft hood with the air handler operating.

[ICanHas]

It is not uncommon to have pressures in the building that are negative with reference to the outside when an air handler is operating.

It's a serious life & safety concern if the said house has a gas water heater or gas fire place. The flue gas will get drawn inside whenever the airhandler is running unless other appliances have a power vent that can overcome the negative pressure.