On Saturday, February 8, 2014 9:44:11 AM UTC-5, philo* wrote:
On 02/08/2014 08:19 AM, jamesgang wrote:

On Thursday, February 6, 2014 6:04:08 PM UTC-5, wrote:

I'm installing a 35'x5" round aluminum chimney liner because my exterior chimney (terracotta inside block) is showing wet spots in the mortar joints. I have a 36,000 btu gas fired hot water heater that has a 3" gravity flue piped to my chimney and also a 100,000 btu input 80% power vented heater with a 3" flue that bumps up to 5" before it enters my chimney. I'm a pipe fitter by trade so I deal with installing these all the time but never deal with the flue side. My question is when I bring the 5" through my chimney and tie in a 5" wye, which appliance should go on the bull of the T? I've been told both ways, just curious what some thoughts are. Thanks.





I'm not an expert but I've owned homes with the gas hw heater and gas T'd together before myself. I've always wondered how the waste heat from the hw was enough to get out the vent without condensing when the furnace was not on.



One consideration is that your next gas furnace will probably be a condensing one. How old is your furnace and hw heater now?









It's interesting that you mentioned that...because that was the very

thing I did not bring up before. The building inspector I talked to told

me that the water heater alone is /not/ sufficient without the furnace

also running.





The reason I did not mention that was because it did not seem right...

but then realized that in the summer, when the furnace is not

running...since it's warmer, there of course would not be a condensation

problem. In my situation it needs to get to -10F before there's a problem..

That's the essence of the condensation issue. Condensation becomes
a problem when a chimney is sized for a furnace plus WH and then the
furnace is converted to direct vent. You then have a chimney that is
too large for just the water heater, making it very easy for condensation
to occur. With the large chimney and the furnace, when the weather was
cold enough for condensation to occur, the furnace was running enough
of the time to keep the chimney free of condensation. Also, with a
chimney that is too large, you won't get proper drafting. So, if you
go with a direct vent furnace and keep an existing WH,
you probably need a flue liner to reduce the
size. I guess at some point, if the run is long enough and the climate
cold enough, you're going to get condensation anyway, because the WH
doesn't generate enough heat. But with a liner, any condensation is
going to stay in the liner, not screw up the flue.