trader_4 wrote:
On Monday, June 9, 2014 7:41:56 AM UTC-4, philo wrote:
On 06/08/2014 11:37 PM, Bob F wrote:

philo wrote:

We just got done with a very cold winter.



On some of the coldest days there was a lot of condensation in the

house right near the ceiling adjacent to the chimney.



I have a standard gas furnace that's about 22 years old and there is

NO liner in the chimney. Various chimney repair companies have told

me I should get a liner installed to avoid condensation.





I was about to do so then realized that I'll probably need a new

furnace within the next few years anyway and now might be the time to

not only get a high efficiency furnace but to get central air instead.





The 100 amp service in my 116 year old house is maxed out so I have

hired a contractor to upgrade the service to 200 amps.





Now, my question: I still have a gas hot water heater using that

chimney which will be otherwise unused. Is it likely I'll still get

condensation on cold winter days?



I can't say for dure about the condensation, but the chimney will likely not

operate correctly with just the gas water heater. When I converted to a

condensing gas furnace with PVC pipe out the wall, I, by code, had to install 4"

"B" vent in the chimney for the water heater. The chimney is designed for the

larger heat of the gas furnace. I would imagine, condensation might be one of

the problems with only the water heater especially if it is already a problem in

cold weather.









I've heard that said but to me it did not make sense because the furnace

is of course not used in the summer and even in the winter the furnace

and the hot water heater would rarely be on at the same time.


Regarding the sizing, I've often wondered about that myself. The chimney
is supposed to be correctly sized to the appliances and not over-sized to
draft properly. But as you say, there are plenty of chimneys sized for
both a large furnace and a WH, where only the WH is on during summer.

When you go to a direct vent furnace, the chimney should have a liner
installed because when it's the only appliance on that larger chimney,
in winter in cold climates you will get condensation inside the chimney.
With the furnace, it kept the chimney warm in winter, the WH will not.
The condensate is acidic and over time, it will damage the mortar joints.
Hi,
Installing liner is code here. Inspector won't pass the install. if
liner is not installed.