Sooting is caused by either not enough air or too much gas creating an inefficient and incomplete burn.

Two things to look for are cob webs blocking incoming air and too much gas being sent,

A manometer will need to be used to check the static incoming, under load incoming and the outgoing pressures.

Also check the burner orifices to make sure none are clogged.

Scott

What size is your circulation pump and does the heater have a manual by-pass?

I'll have the gas line tested, but shouldn't the gas valve adjust the pressure? Or am I giving it too much credit?

Cascade--I'm not sure what size the pump is, or whether there is a by-pass. I'll have to check when I get home. But the setup has been the same for 30 years, and the existing heater has been working for 3 or 4 years (when it was new). If the water flow was too high, wouldn't there have been a problem before this?

BTW, I looked at the flames again last night. The bottom half of each flame is blue, and the top half is orange. Is that normal?

The internal by-pass could be faulty.

That would cause the 1st hi-limit to trip and reset.

There really shouldn't be a lot of orange in the flame but without seeing it for myself, this is subjective.

The manifold pressure sounds a touch high.

Find out, when the gas man is there what the pressures are I mentioned earlier as tested with a manometer.

Scott

A sooted heat exchanger can be caused by a low temp rise situation. This may be due to:
a) improper (generally low) gas pressure or
b) excessive water flow, i.e. over sized pump/internal bypass not opening

or a combination of a and b.

Another possibility may be that the sooting was caused by the leaking heat exchanger. A pinhole leak spraying on the burners can cause a bad burn.

I've never hear of b causing sooting. Internal erosion or failing gaskets, yes.

I can see condensation induced dripping causing a similar effect to the pinhole scenario though.

Scott

I looked again, and I don't believe there is any bypass. The heater manual references the location for a manual bypass, and I don't have one there. I don't see a reference to an internal bypass, but there is a water pressure switch (which maybe does the same thing?) But I've had no problem with this for a number of years, and I recently cleaned out the filter. The water pressure has been consistently around 18-20, so I wouldn't think I have excessive water flow.

The pinhole leak is a possibility, although the problem with blowing fusible links has been going on for some months, and I haven't seen a noticeable drop in water level until a couple of weeks ago. At that point I opened up the heater and saw the water leaking. I suppose it's possible I had a pinhole leak for a while, and it just recently expanded into a real leak.

I'll try to get the gas company out next week, so I should have more info at that point.

How about poor ventilation? The heater casing has vents starting about halfway up, all venting upwards (from outside to in). There really aren't any lower vents, although there seems to be enough space at each of the four corners to allow air in. But until I dug it out, there was dirt covering the base about halfway around. Could that have reduced the airflow enough to weaken the flame?