My wife and I acquired a 46 year old home. The furnace flue is located in a 6′-0″ wide chimney next to a fireplace flue. All winter we have had an ice build-up problem on the rain hood, along with water leaking out of the flue cleanout in the basement.
After having had two chimney contractors look at problem, they both suggested that the existing 5″ flue was to big and that a 4″ replacement would be adequate. We proceeded to have the work done. A 4″ V-vent was installed. This vent looked somewhat like a one-piece clothes dryer vent pipe all the way to the top. Now that all is said and done, I still have an ice build-up problem on the new rain hood and water leakage through the new vent cleanout in the basement.
I had an inspector look at the installation. He suggested that a B-vent might have solved the problem. In mentioning this to the contractor, I was advised that a B-vent could not be installed due to a curve the vent has to accommodate inside the chimney.
Any suggestions you may have, particularly towards solving the water problem out of the vent cleanout would be much appreciated. Also, what changes are you suggesting in rain hood and flashing design?
Being an owner of an older home myself, I am not immune to the same problems experienced by readers like you. Yes, I too have had ice building up on my rain hood and chimney liner. This has occurred after replacing my 30 + year old furnace with a mid-efficient model last year. I suspect that the furnace in question has also been upgraded to a mid-efficient model.
The reason that ice is forming near the top of the chimney with the new furnace is the nature of the exhaust emitted by the furnace. The older standard natural draft furnaces dumped large amounts of very warm exhaust up the chimney, which was diluted with house air. This warm air made it to the top of the chimney and escaped into the air above the home. With this exhaust went many of the heating dollars used to heat the home. Newer mid-efficient furnaces have heat-saving features to prevent much of this wasted heat from escaping up the chimney. Because of this, the exhaust is much cooler and a fan is required to force this cool exhaust up the chimney, rather that rising by convection, like the old furnace. This cool exhaust may not make it to the top of a tall older chimney without cooling dramatically. When this cool, moist exhaust hits the –20 air outside the chimney liner, it can freeze quickly and create ice on the chimney. This normally has little effect on the metal chimney liner or hood, but in some cases like yours, the ice build-up will be excessive. This ice may partially block the flue and make the situation worse. This ice may melt with warmer weather, and run back down the flue and into the home through the cleanout. In this extreme case, similar to the situation here, remedial action should be taken.
The first obvious response is to check the size and style of chimney liner installed. As was the case in your chimney, the old liner was too large for the newer furnace. Installation of a smaller inner liner or a double walled B-vent in the chimney will often remedy the situation. The space in between the two walls of this style of vent will help insulate the exhaust and keep it warmer until it reaches the top of the vent. Repairs in this manner eliminated this same condensation and leakage problem recently for one of my neighbours. I also had a double-walled B-vent installed with my new furnace, and ice build-up is still a problem, but is not excessive enough to run back in through the cleanout in the basement.
In the instance in your question, this solution was not possible because the design of the older chimney would not accommodate the rigid B-venting. The flexible liner installed was obviously not a complete solution. Other remedial actions should be attempted at this time.
Reducing the amount of moisture in the house air that goes up the chimney with the furnace exhaust may be one possible solution. Installing a fresh-air intake next to the furnace for combustion air should be attempted. Removing or shutting off the water supply to older humidifiers on the furnace may also reduce the relative humidity in the house air in the heating season.