Diesel Heater: Flue Damper or Heat Exchanger

[Martkimwat]

Originally Posted by Jim Cate

Since the combustion chamber is not open to the interior of the boat, and is vented through the stack to outside the boat, what calamitous consequences are you worried about?

Jim

Maybe I'm missing something here Jim but where does the combustion air come from, surely it's from inside the boat, in which case there is a path for poisonous gasses to enter the boat...?

[Jim Cate]

Originally Posted by Martkimwat

Maybe I'm missing something here Jim but where does the combustion air come from, surely it's from inside the boat, in which case there is a path for poisonous gasses to enter the boat...?

Some models have the combustion air supplied by a duct directly to outside air supply. This is truly isolated combustion, and there is essentially zero chance of anything gettingback into the cabin.

But most installations do draw directly from ambient air in the cabin, and there is theoretically a path where combustion products could return to that area. However, in the normal operation process, the draft which is created by the heat in the burner causes cool air to come into the bottom of the burner and the hot exhaust gasses to rise out the stack. There is the possibility of reversing that flow (big gust hitting the stack, for instance), and this could and occasionally does cause brief gusts of combustion products to reenter the cabin. Not good! But, these events are very brief, so little contamination occurs. In the unlikely event that it perseveres, the flame goes out due to lack of oxygen. With the flame out, there is no CO produced. There can be diesel vapours, soot, and unpleasant odors dissipated in the cabin, and it can be a mess to clean up. But the chance for a continuous stream of CO somehow sneaking back out through the air inlet is slim indeed.

When we first started using our diesel heater on board we had a couple of sooty back flow events, and one where the flame was extinguished, diesel vapour blew back into the cabin and then ignited. That was an attention getter!!
Subsequently I fitted a small muffin fan below the inlet, acting to increase the draft through the heater. Since then we've had zero backflow events in 14 years of usage. We don't leave the heater running unattended or when we are sleeping, but feel quite happy the rest of the time.

Others are free to doubt the safety of such installations, but I'm not too worried.

Jim

[Martkimwat]

Originally Posted by Jim Cate

Some models have the combustion air supplied by a duct directly to outside air supply. This is truly isolated combustion, and there is essentially zero chance of anything gettingback into the cabin.

But most installations do draw directly from ambient air in the cabin, and there is theoretically a path where combustion products could return to that area. However, in the normal operation process, the draft which is created by the heat in the burner causes cool air to come into the bottom of the burner and the hot exhaust gasses to rise out the stack. There is the possibility of reversing that flow (big gust hitting the stack, for instance), and this could and occasionally does cause brief gusts of combustion products to reenter the cabin. Not good! But, these events are very brief, so little contamination occurs. In the unlikely event that it perseveres, the flame goes out due to lack of oxygen. With the flame out, there is no CO produced. There can be diesel vapours, soot, and unpleasant odors dissipated in the cabin, and it can be a mess to clean up. But the chance for a continuous stream of CO somehow sneaking back out through the air inlet is slim indeed.

When we first started using our diesel heater on board we had a couple of sooty back flow events, and one where the flame was extinguished, diesel vapour blew back into the cabin and then ignited. That was an attention getter!!
Subsequently I fitted a small muffin fan below the inlet, acting to increase the draft through the heater. Since then we've had zero backflow events in 14 years of usage. We don't leave the heater running unattended or when we are sleeping, but feel quite happy the rest of the time.

Others are free to doubt the safety of such installations, but I'm not too worried.

Jim

Many years ago, I nearly lost my first wife and three daughters to carbon monoxide poisoning because this is exactly what happenend.We had a LNG fuelled central heating boiler that drew air from inside the house but had a flue running outside. The LNG tank regulator developed a fault and allowed too much gas to enter the boiler and it did not combust properly causing large amounts of CO which fed back into the house and one by one my family succumbed to the fumes in different parts of the house (note house which is a lot larger space than most boats). Fortunately my wife phoned her sister who lived nearby to say she felt weird and all the kids were 'asleep' having complained of headaches. Her sister rushed round, found them all unconsious and called an ambulance. They were extremely lucky and survived despite having to spend two days on pure oxygen at the hospital. So you can see why I think your hypothesis doesn't add up. It's one of the reasons balanced flue boilers were developed.

[Jim Cate]

Originally Posted by Martkimwat

Many years ago, I nearly lost my first wife and three daughters to carbon monoxide poisoning because this is exactly what happenend.We had a LNG fuelled central heating boiler that drew air from inside the house but had a flue running outside. The LNG tank regulator developed a fault and allowed too much gas to enter the boiler and it did not combust properly causing large amounts of CO which fed back into the house and one by one my family succumbed to the fumes in different parts of the house (note house which is a lot larger space than most boats). Fortunately my wife phoned her sister who lived nearby to say she felt weird and all the kids were 'asleep' having complained of headaches. Her sister rushed round, found them all unconsious and called an ambulance. They were extremely lucky and survived despite having to spend two days on pure oxygen at the hospital. So you can see why I think your hypothesis doesn't add up. It's one of the reasons balanced flue boilers were developed.

Frightening story for sure, but I'm not sure that they are analogous situations. There are significant differences between a LNG fueled home furnace in the basement and our diesel burner which sits in plain view a few feet from us at all times. At any rate, you are entitled to your views, and with a scare like you had they are understandable. My personal experience and my observation of a lot of other yacht installations lead to my acceptance of our risks. If I was worried about it, I could (and perhaps should) install a CO monitor.

Meanwhile, as the summer wanes here in Tasmania, I'll be lighting up the Fab-All heater with care and enjoying the warmth it provides.

Jim