New Furnace installed using existing flu and now condensate problem

[JohnnyJack]

I had a new furnace installed last year, now we are getting condensate dripping and also some rust spots on top of the furnace and inside as well. I wanted to nip this issue in the bud so I called the installers. They said there was nothing they could do - that all 80% efficiency furnaces have this issue. I replied that my previous 30 year old 80% furnace didn't have this issue - what's changed?

We have a flat roof.
A 5 foot section of 4" B-Vent goes through ceiling/roof with 3 feet above the roof and a cap.
There is a galvanized single wall elbow, 2 foot section at a slight angle, another elbow, and a 1.5 foot section into the top of the furnace.

Lennox
66,000 BTU in

Previous furnace was:
Trane
Model:TUSO60A936A1
60,000 BTU Input

The installer was suggesting I replace this furnace with a high efficiency model and that would solve the problem. Otherwise, there was nothing he could do to fix the problem. I found that to be very odd. It's one year old. He just spent an hour trying to convince me this was normal and didn't touch the unit or check it's settings.

I would like to hear recommendations and also best practices for a HVAC installer in a situation like this. Isn't there some testing that can be done, measurements of airflow, etc?

Thanks

 

[Stuff]

Are you finding the condensate on the outside or inside of the pipe? Pictures would help.

Ask installer for a statement from Lennox that dripping and rust is normal.

 

[Sponsor]

 

[Dana]

I had a new furnace installed last year, now we are getting condensate dripping and also some rust spots on top of the furnace and inside as well. I wanted to nip this issue in the bud so I called the installers. They said there was nothing they could do - that all 80% efficiency furnaces have this issue. I replied that my previous 30 year old 80% furnace didn't have this issue - what's changed?

We have a flat roof.
A 5 foot section of 4" B-Vent goes through ceiling/roof with 3 feet above the roof and a cap.
There is a galvanized single wall elbow, 2 foot section at a slight angle, another elbow, and a 1.5 foot section into the top of the furnace.

Lennox
66,000 BTU in

Previous furnace was:
Trane
Model:TUSO60A936A1
60,000 BTU Input

The installer was suggesting I replace this furnace with a high efficiency model and that would solve the problem. Otherwise, there was nothing he could do to fix the problem. I found that to be very odd. It's one year old. He just spent an hour trying to convince me this was normal and didn't touch the unit or check it's settings.

I would like to hear recommendations and also best practices for a HVAC installer in a situation like this. Isn't there some testing that can be done, measurements of airflow, etc?

Thanks

There is no such thing as an 80% furnace anymore- residential furnaces need to be at least 82% (by regulation)

Even an 82-85% mid-efficiency furnace has lower exhaust temperatures than an 80% furnace. With only 8' of stack height above the unit, 3' of which is exposed to the cold outdoor air isn't going to drive much stack velocity with a 66K burner, especially if it's a pretty tight house.

Take a look at the BTU min-max ratings in Table 1 on page 12. You may have just barely enough stack height for a 66K burner- lateral runs and ells cut it down by quite a bit.

Does it backdraft at all? Turn on all your kitchen & exhaust fans, and the clothes dryer too, thenfire up the furnace and test at the draft hood intake with a smoke pencil or incense stick, blown out match, something to see whether dilution air is being drawn up the stack or whether it's all coming back at you.

The model number of the current furnace would be more useful than the older one. Both are probably sub-optimally oversized for comfort, though probably not tragically so. If you end up replacing it, don't just replace it with another 50K+ output furnace (condensing or otherwise)- right size it. Most Chicago-land homes built since 1940 would do better with a 30-45K-in furnace, from a creature comfort point of view, unless it's a large "mid century modern" house with a lot of glass.

To get a handle on what would be a right size for your house, run this math on last winter's (wintertime only) gas bills. If your 99% heat load as measured by fuel use is something like 28,000 BTU/hr, the biggest furnace that makes sense would be (28K x 1.4=) ~40K-out, but even a 95% efficiency 40K-in would be enough to cover you for Polar Vortex disturbance coolth. If the heat load numbers come in under 25K a modulating cold climate heat pump can be more comfortable than a 2-stage 30K condensing furnace. Another good low heat-load solution is a properly sized hydro-air handler running off the (preferably condensing) water heater.

For the reasoning behind not oversizing see Nate Adams' " House Whisperer" blog pieces & short topical videos:

Home Comfort 101

HVAC 101

HVAC 102