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sgbotsford 12-01-2007 02:25 PM

Corrosion problem in vent stack.
Several years ago I replaced the 40 year old boiler and 20 year old gas heater, and 20 year old forced air furnace in our house with a Lars Mini-Therm model JVS.

The house is in two parts: The old part has a basement and main floor. The addition has a crawlspace, main floor and 2nd floor. The boiler is in the basement. It's stack is above the old part of the house, and so the top is about 12 feet below the peak of the addition. This results in a more or less constant negative pressure in the boiler room. (I created a manometer with some scrap clear tubing: The pressure differential between the cleanout port on the B vent and the room is pretty constant at -4 mm water pressure whenever the boiler is not operating. When the boiler fires this drops to 2 mm. Boiler room is negative compared to stack.) We have a CO detector on the main floor of the house. It has never gone off except for tests.

The boiler vent system goes up about 4 feet, then gradually up traversing a horizontal distance of about 6 feet. This is all single layer sheet metal. It connects with a T to the side of a B vent stack, with a cleanout/inspection port on the bottom side of the T. The B vent stack is about 15 feet, passing through the main floor and attic.

The house is divided into 6 zones:
1. Indirect DHW heater
2. Main Bathroom
3. Crawl Space
4. Fan coil air heater
5. Upstairs addition
6. Downstairs addition.
The last three zones almost never operate, as our house is well designed for solar heating, and we also have a wood heater and a wood cooking range. The total area of the house is about 2400 square feet.
We live in a climate with about 10,000 degree days, with coldest temperatures around -40C

This year in my fall inspection I noticed that there was a lot of corrosion on the vent stack, both the single layer piping, and the B-vent itself. This water has been flowing down the pipe and has turned the top of the boiler white.

The single metal pipe has corrosion on the entire joint, powdery white residue, about an inch wide. The metal is solid underneath this. I don't understand how this powder accumulates on the outside, and why it is evenly distributed around the entire joint, even on the horizontal sections.

The B-vent is corrode with a combination of white powder and rust stains.

In hind sight the reasons for this are fairly clear.

In operation the boiler almost never has demand for more than one zone at a time. This means that the unit short cycles. Even when it is bitter cold, the unit will come on for about two minutes, then go off for about 15. This means that the piping never really gets hot. Then as soon as the unit shuts off, the damper plate closes. Some of the flue gases will diffuse out, but cold air will come back down, mix with the exhaust gases, and condense.

I have contacted the installer (Academy Mechanical of Spruce Grove & Edmonton). Darrel has inspected it, but wanted to talk it over with his peers before recommending a course of action beyond replacing the B vent.

I can see several possible solutions to this problem, but lack the expertise to know what is practical:

Category one: Changes that will increase the cycle time.
1. Replace the aquastat with one that has a larger differential.
2. Create a new zone and use a large insulated tank for this zone. This zone is always on, and so acts as a thermal buffer for the system. This may also require a larger expansion tank. (I was considering using an old electric water heater for this.) To work well, this requires a change in programming of the sequencer in the boiler so that there is a longer delay between the the circulation pump turning on, and the boiler firing. (Much of the time the buffer tank can provide enough heat to delay the firing, but the piping to the buffer tank won't be as hot so the boiler will get a brief input of cooler water.)
3. Consolidate two zones. Unfortunately the DHW and the bathroom are the two that are responsible for 90% of the boiler use, and during the three warmest months in the summer, only the DHW is used. When hot water is used in the bathroom the bathroom warms up, so that it is rare for the bathroom and DHW zone to come on together. It would also mean I'm heating the bathroom when I'm making hot water even in the summer.
4. De-rate the boiler by putting in smaller jets, and adjusting the shutters accordingly. (I would get a gas fitter to do this.) Don't know how much a unit can be de-rated
5. Replace the burner unit with one that has two stage firing.

Category two: changes that will reduce the condensation:
1. Reprogram the sequencer so that the damper plate stays open for a while after firing so that the hot flue gasses can get out of the stack before the bottom end closes. I don't think this is possible.

2. Install an auxiliary small fan to blow air up the pipe, either constantly, or for a period of time after the boiler fires. This unit would need a damper so that if the fan failed, it would not allow flue gases into the house. I think it would need to be installed on a side pipe so that the fan wasn't cooked by the flue gases.

What is your recommendation?

HvacWiz 12-01-2007 02:28 PM

:eek: wow!! that was long

sgbotsford 12-01-2007 02:47 PM


Originally Posted by HvacWiz (Post 77761)
:eek: wow!! that was long

Didn't see a lot of point in having a ping-pong match as people asked a bunch of questions about it. And without the full explanation, it seemed to me that I'd get a bunch of answers that would only work for what I told them.

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