燃氣熱水系統的排氣連接管嚴重腐蝕


1

我有一個組合式輻射采暖系統/家用熱水系統。該裝置是額定效率為87%的中效裝置。鍋爐通過單壁通風孔連接器連接到B排氣煙囪。鍋爐還配備有電動風門,以減少不運行時向上流向煙囪的空氣。

B型通風孔已於15年前更換,並且狀態良好。通風口接頭腐蝕得很厲害,以至於昨晚它崩潰了。同時更換了它。

我已關閉設備,直到我可以再次正確放氣。

A:為什麼我的排氣口連接管腐蝕得如此嚴重?

B:如何防止這種情況再次發生?


我的懷疑是鍋爐尺寸過大(90,000 BTU),循環時間短。即使在寒冷的冬季,也很少一次運行超過2-3分鐘,而關閉時間卻只有10-20分鐘。這會在通風孔和煙囪中產生潮濕的熱空氣,當電動風門關閉時,通風孔和煙囪會保留在那裡。水在排氣系統中凝結,從不真正乾燥。

我的想法:

  • 在打開位置禁用風門。

  • 在風門上打一個孔,以便在關閉位置不會關閉所有空氣。

1

I would start with a couple of small 1/2” holes if you disable the damper totally you loose efficiency quickly where several smaller holes will allow a draft and the combustion moisture to dry 15 years is a fair amount of time but I have had systems last much longer in some cases when doing annual inspections I have recoated single wall with zinc rich paint it could have the last piece dipped in a batch but excess moisture is probably the cause so start small and see if that works without totally disabling the energy saving feature of your damper.


1

On replacing the pipe, I found that the damper had a 3/4" knockout apparently for this purpose. I've removed it, and we'll see what happens in the coming winter. This amounts of a tiny air leak, but with a 20 foot stack effect should help to keep the exhaust system dry. Since the house is an older one, built in the mid 80's, and we heat with wood a lot, this is an inconsequential air leak.

The pipe was inexpensive -- four 3 foot sections, two adjustable corners, and a T ran me $80. Replacement was straight forward, with the usual minor problems of working with sheet metal. An extra pair of hands holding one end is useful. I used 26 ga pipe this time. This is heavier, I think, than the last time I replaced it.

Guys at the Heating and gas company say that this is a common problem in our climate especially with medium efficiency units vented through older stacks. The units with their higher efficiency produce some NOx gasses which combine with water vapour to produce acid that chews on the pipe. Anyway the guy claimed that the vent pipes are good for only about 15-20 years.

One comment suggested swapping the unit for a smaller one. While this is doable, it would cost thousands of dollars. We do a lot of heating with wood, and keep unused parts of the house cold. The house is well designed for winter solar gain. On a sunny day even at temps of -20 C (roughly 0F) we build a morning fire, let it go out, and build another when the sun goes over the hill in late afternoon. The unit is properly sized for worst weather scenarios, and no auxiliary heat.

At present the indirect hot water tank is used as a buffer. This increased the cycle time from 2 minutes on/15 off to a cycle about 2.5 times as long during cold weather. This decreased but did not eliminate condensation in the stack.

A better solution to this would be a variable fire water heater where the blower and gas mixture are adjusted according to need. W. O. Stone makes a series of units that have a 4:1 fire range. E.g. It fires at 25% of full when the water temp falls below 140, increases to 50% if after two minutes the temp falls below 137, increases to 75% if it falls below 135, and increases to 100% if it falls below 132

Other solutions:

  • adding an additional buffer tank. Space makes this difficult.
  • adding an on-demand 20,000 BTU water heater. These are usually not designed as water reheaters, and a supply of not quite hot enough water may result in boiling in the heater.