Black Soot on Rads, Walls, Ceilings

[accumech]

Hi,
I've encountered a problem that I need help with, here's what I've got: a duplex, built in the 50's. In the basement of one of the duplex's, there are two gas-fired hot water heaters servicing both unit's domestic hot water, and one gas-fired boiler servicing both unit's hot water radiators. About a year ago, one of the heaters was replaced, and soon thereafter there was a black, sooty residue collecting on the rads, walls, and ceilings in the duplex that is above the appliances (the other duplex is experiencing none of the problems described here). My initial thought was that the breeching and chimney stack were undersized as perhaps a larger HW heater was installed, but that's not the case, the heater was simply changed out, same model went in. I then thought perhaps there was not enough combustion air entering the building. However, there have been no renovations, no weatherstripping added, no improvements done to decrease the amount of infiltration into the building. So, how is the black soot forming? The local Gas Company went down and investigated, possibly products of combustion weren't venting properly, but they say everything is in working order, only that the boiler needs a cleaning. The people living there are complaining of respiratory problems and headaches. The gas company did CO detection testing throughout the space and came up with nothing. Has anyone any thoughts as to the formation of the black soot? We're stumped and concerned for our clients health. Any information or comments would be greatly appreciated. Thanks.

 

[ChrisConley]

You say that nothing has been done to affect combustion air, such as new weather-stripping, but there are many other things that can affect combustion air. Has any modification been done to the units like a new exhaust fan? If the pressure in the space is negative relative to the outside the chimneys will back-draft. How exactly is combustion air being supplied to the room? Is there a combustion air duct? If so, has the flow in this duct become impeded? I would really focus on airflow into the space and try to ensure that the room is not at a negative pressure relative to the outside. I'd also check the vents to ensure that nothing has gotten into them.

Could you post some numbers on the domestics hot water heaters, boiler and any other gas burning appliances in the space: how many BTUs from the nameplate.

 

[TBP]

I'm not aware of any cases like this that the source of the problem wasn't decorative candles.

 

[pennpoint]

accumech,
Maybe you have a humidity problem. Check out the black sooty stuff for microbes, like moldspores,etc.
Just a stab in the dark!

pennpoint

 

[accumech]

Hey, thanks for the responses. ChrisConley, to answer some of your questions, there have been no exhaust fans added anywhere in the building. Absolutely nothing was changed/added/improved other than a hot water heater was changed out. The two hot water heaters are rated at 38,000 Btu/h input, and the boiler is rated at 140,000 Bty/h input. I'm not sure if the vents were inspected to check for any blockages. I'll check that out. There is no combustion air duct. The house was built late 1940's/early 1950's, and combustion air has been relying on infiltration all this time. I should reiterate that the system worked fine for all these years up until recently, when one of the hot water heaters was replaced. Anyways, hope this helps.

Pennpoint, there is no humidity problem, we're up in Winnipeg Canada, and we are dry and cold in the winter, especially in older houses that experience high infiltration rates. Thanks for the input though.

 

[DLANDISSR]

Accumech:
If the black sooty material is produts of combustion they must be coming from the stack. Is the BTU input rating of the new heaterthe same as the old one? If so the stack may indeed be too small? A gas flame if properly adjusted should not give off sooty smoke or particles. Check the flame for the appropriate amount of combustion air. Is soot accumulating in the combustion chamber of the heater?

When installing the new heater the stack could have been moved and disconnected inside the wall. I am assuming it is a double or tripple wall metal stack.

 

[accumech]

Dlandissr, the btu input rating of the new heater is exactly the same as the old one. I suspected that during the removal, that the stack could have been damaged/not connected back up, but that is not the case. Not sure if soot is accumulating in the combustion chamber of the heater, I'll check that out.

Thanks for your input.

 

[walkes]

There have been instances where the byproducts from burning decorative candles have been deposited throughout the space, becoming visible as black soot with added heat. Changing the boilers out, cleaning piping, making radiators more effective etc may have been a catalyst.

 

[DouginMB]

Is there a fireplace in the space? It could be that a fire in the fireplace could cause enough of a stack effect draft to pull a negative pressure through the boiler flue and pull soot into the space. The soot dust could have been knocked loose during the boiler installation. This might be far-fetched but it could happen.

 

[accumech]

DouginMB, there's no fireplace. That was one of the first thoughts, that perhaps there was a fireplace and the problems we're seeing were associated with it, but that's not the case. Good idea, though, thanks.

 

[bruiser]

It is possible that you are burning rich. Check your flame color it should be light blue. Check that the fuel supply pressure to the burner is per the manufacture specification. Ensure that the heat input to the burner (volume flow rate) in BTU per hr matches the manufacturer spec. ie 38,000 BTU/hr is approximately 38 SCFH for natural gas and roughly half that for LP. This can be determined at the gas meter with a stop watch

 

[pmover]

DLANDISSR comments regarding the flame are the first matter to be investigated! having been a boilermaker for a quite a few years, i have first hand knowledge of soot and its causes-improper air/fuel ratio; thus bad combustion. i cannot tell you the number of occasions in which boiler flues / combustion air were not properly established or adjusted, resulting in soot deposits everywhere!
definitely check exhaust gas paths for any obstructions, check combustion air supply, the flame itself (should be a nice blue color with slightly yellow tips. note the shape of the flame.), and check the fireside of the boiler itself for soot deposits. lastly, you might want to contact a reputable service representative to investigate. soot deposits represent a potentially hazardous situation.
-pmover

 

[khal200ca]

Don't know if the problem is solved by the time I am writing this thought.....hope so.....it is a dangerous problem and should be solved asap.

My guess is, something happened during the replacement of the the new unit.....like there must be something blocking the common vent...is there a barometric damper on the chimney?

The other possibility is the new unit might not be exactly the same type as the old one....is it power assissted or natural draft.....you should check and make sure the new and old units are TYPICALLY the same type.....