How much condensate debris/deposit is "OK"?

[ejager]

I've certainly seen older systems with NO staining, or deposits at the flue collector gaskets. (maybe they were changed....)

These were two Lennox 80% furnaces, one 44,000BTU for the basement, and one 88,00 for the main floor. Vented together with a water heater. Plenty of combustion air available in the furnace room. No other signs of problems.

What could be happening? How much is too much? What consequences/worse case scenarios can be imagined?

[Jim Katen]

When the furnace is running, the inside of that collector box is normally dripping with condensate. Some of that condensate wicks through the gasket and evaporates at the surface, leaving behind a small accretion of minerals. 

The fact that you can see these minerals is unimportant. If the gasket were better, you wouldn't see them, but they'd still be there. Eventually, I guess, the collector box will corrode through. (Which is why some manufacturers use plastic ones.) In my experience, the furnace fails before that happens. 

I'd brush it away every so often to make me feel better. 

 

 

[ejager]

Thanks for the reply Jim,

I am quite surprised to hear that there would be so much condensate on a mid efficiency furnace. I can certainly understand that a high efficiency furnace with secondary heat exchanger might have condensation, and can certainly understand (and have seen) where a plastic flue collection box would work. 

 

[Jim Katen]

4 hours ago, ejager said:

Thanks for the reply Jim,

I am quite surprised to hear that there would be so much condensate on a mid efficiency furnace. I can certainly understand that a high efficiency furnace with secondary heat exchanger might have condensation, and can certainly understand (and have seen) where a plastic flue collection box would work. 

 

The classic answer is that this is an indication of short cycling.  Without a long enough cycle, the system doesn't warm up enough to "re-evaporate" the condensate and you end up with liquid in places where it might not otherwise be if the cycles were longer. 

I'm not entirely convinced of that scenario, but, heck, your observations mesh nicely with it. Were they short cycling? Were the thermostats in the air flow from the registers? Near the front door? Were the filters clogged? Lots of stuff could cause short cycling. . . 

[ejager]

Now that's an interesting avenue of thought, Jim.

It was a relatively modest sized (less than 1200 sq ft on main floor + same in basement), 14 yr old, semi detached "villa", with a fully developed walk-out basement.

So I was surprised to find two furnaces; there is certainly more than enough heating capacity. Without doing calculations, I typically see larger two story detached homes, with many more sq ft of exterior wall, having less total heating capacity. This home also had two thermostatically controlled gas-fireplaces. The furnace thermostats were appropriately located.

I can see where between the sizing and the fireplaces there could be issues with short cycling. The homeowner was diligent about clean filters and what she thought was maintenance (by a duct cleaning company). The furnaces didn't short cycle in the time I was there but off-season tests are not always indicative of everything that can/does go wrong.

Thanks!

[inspector57]

Oversized units can cause short cycling and that was my thought as I was getting to Katen's post. That would be my guess but I'm in a mild heating climate where furnaces are generally replaced long before they are used up and are almost always oversized.