Draft door placement??

[DetroiTug]

Something I've been wondering about for sometime on many boilers I've looked at.

The boiler I have and many others have the draft control in the fuel door - above the grate.

I have a fair bit of experience with woodstoves and remember that draft doors above the grate are not very efficient, it is like trying to feed oxygen to a blacksmith's forge, above the fire.

The best models (in my experience) had the draft below the grate in the ashpan door. They would feed oxygen below the fire.

I guess the question is: Is there a reason for positioning the draft control above the grate? Like not pulling ash into the firebox/flues or something similar?

-Ron

[dhic001]

There are two aspects of the air needed for a good fire on natural drafting solid fuel boilers, primary and secondary air. Primary air is the air needed for the fire to burn, this should come from below the grate via the ashpan area. If you are going for real efficency it will be preheated. The secondary air comes in above the grate, and this air helps with combustion, allowing the fire to have the extra air it can't pull through the grate. Lack of primary air will mean a very ineffective fire, if much of a fire at all, and I doubt you'd make much steam without it. Lack of secondary air means poor combustion, particularly when there is a good spread of new fuel on the fire, which will result in excessive smoke, and hence soot.

In running Zeltic I need to make use of both primary and secondary air. Secondary air is allowed into the box by having the door open slightly, usually about half an inch at the opening side. Primary air is controlled by the ashpit door, generally its either open or closed, but occasionally its put partly in place. On lighting the fire I use initially only primary air, and once the fire is roaring I put the door on the notch to allow secondary air in. After each shovel of coal is added, I close the door completely, allowing the new coal to catch on primary air, and once the smoke starts appearing at the funnel, I open the door on to the notch. Towards the end of the run, while trying to thin the fire right out, I stop firing, and as the fire thins I close the secondary air off so only primary air is being used. This prevents cool air coming in and cooling the boiler before the last of the fire is burnt. If I intend to run again later, I will build a bit of a fire up, push it all into a pile and then close both primary and secondary air. That shuts the air off completely to the fire, meaning it sits hot, maintaining steam but hopefully not raising the pressure enough to blow the safety valve. A good airtight furnace with a good banked fire will keep the boiler in steam all night, ready to cook breakfast on it in the morning, prior to fire cleaning, raising the anchor and getting underway again for another days run.

Hope that all helps.
Daniel

[daysaver1]

Could it be that your are seeing the small door associated with the oil attomizer? Since this type of burner needs some amount of air pulled along with the steam injector generally an adjustable opening door is fabricated associated with this appurtanence. This would true whether oil was the secondary or primary fuel. Either way, then there could very well be a larger draft door down lower that you may not notice.