I was called out to a MasterTemp pool heater that was knocking and shutting off after running for just a couple of minutes. I assumed I might have to replace the unitherm governor, but found that the header was plugged with flakes of white scale which seemed to have a rounded green side like it used to be coating the inside of the copper heat exchanger. There were 1 to 2 cups of this scale. After flushing it out, the heater worked fine again and the heat exchanger looked to be in good condition.
Any ideas as to why it formed in the heater and what caused it to break free (seemingly in a short time) from the tubes and move into the header?
It is possible that the scale was simply the result of scaling water (high pH, Alkalinity, etc.), but, I also wonder if venting is an issue. The vent starts small, and increases (see picture). Notice that there is corrosion the elbow that is right below the increaser indicating that condensation formed there. Since exhaust gases cool when the pipe volume is increased, this cool / condensing exhaust gas might have sat in the pipe (rather than rising) and created a "thermal plug" that obstructed the flow of the exhaust out of the heater. This would have caused more heat to remain in the heater and be transferred to the water. This would inadvertently raise the efficiency of the heater beyond its normal level. I know that an efficiency greater than 84% would result in the formation of acidic condensate on the outside of the exchanger, but would it also promote the formation of scale in the exchanger? If so, why?
Comments
See link to article below. Could be calcium phosphate scale. If phosphate levels are running high and calcium hardness is high! test phosphate and calcium hardness. If phosphate is high use a phosphate remover. Prevention keep phosphate levels down, avoid using phosphate based chemicals like metal removers that contain phosphate. You could have the flakes analyzed at a analytical lab and they can determine the make up of the scale. Good Luck!
http://www.poolspanews.com/2012/011/011tech.html
Clint
The scale build up is from high pH or alaklinity levels. There also may be a water flow issue allowing the scale to build up.
I'm not overly familiar with the Master Temp but, like any other heater there is a bypass to regulate flow. Too much flow and a manual bypass via a couple of valves must be installed. Auto bypass in this heater may not be functioning properly. Check the filter to make sure it's clean and moving the water properly.
As far as that venting-Ouch!
Too many 90's. I would come out the heater with a horizontal vent pipe as long as possible, and then turn a 90 to a 45 as they do swivel and head up towards the main trunk at a 45 degree angle.
The vent pipe may be cooling too quickly creating condesation and then rusting. Visible in photo.
If the vent pipe was plugged, it would and it should set off any safety switches in the heater from excessive heat and shut it off immediately.