Water Temp Differential

[Eagleye]

OK here goes.  We heat a very large farmhouse using a combination Taylor wood fired boiler (T-750), heat pump and propane when needed.  Over the past four years the Taylor has become less and less efficient so we decided that we needed to fix it.  Those not familiar with these units it has an aquastat that controlls a blower to aid combustion, there is a thermometer to indicate water temp.  What has happened is the water temp reaches about 150 and the acquastat cuts off the fan thinking that the water temp has reached 180.
We pulled the acquastat and the thermometer and put them on the stove in a pot of water along with a digital thermometer and an ohm meter across the acquastat terminals.  After several travels up and back down we confirmed that the thermometer and the acquastat are functioning correctly, fan is turned on at 160 and off at 180.
This led us back to the boiler and an inspection of the internals.  Can't see much because there is limited access but we pulled the aquastat well and found it clean.  Measuring the actual well temp it really is 180+.  So we attempted to figure out what was causing that part of the boiler to be so hot, so far that explaination has eluded us.  We have a Taylor rep locally and he has been conferring with the factory, they have the thought that it is stratifaction (sp?).  Our problem with this is that the thermometer and the acquastat are at the same height in the reseviour and are about 3 feet apart.
So we thought that there might be some sort of build up inside so we took our pressure washer to the area around the controller well and everywhere else we could point the red streamer, no change.  Our next move is to install a circulator that will move water top to bottom.
Any hints? or suggestions>

[TomW]

Your Photo:

[wpowokal]

Never seen one but it once worked OK so installing a circulating pump does not make sense, have you cleaned the flue? Has the top (outside) of the flue corroded away allowing the wind to affect draw?

By less efficient you mean you no longer get the same heating for a given amount of wood, you say the temperature probe and switch are functioning correctly so one must look at the combustion side.

allan

[Eagleye]

So the firebox is about 4 by 4 feet square, at the back of it there are 6 tubes that lead back to a collector point that then connects to a return box that runs the full length of the unit terminating at the junction with the flue.  I keep all the tubes, return box and chimney flue cleaned by running the wire brush and a custom set of rams to reduce the creosote tar stuff from accumulating.  Pretty near monthly task.  The water reservoir is 750 gallons and is static, we have a heat exchanger that sends domestic water into that water jacket to harvest the heat returning it to a Polaris propane 80 gallon unit located inside the garage.  Water from that unit is sent to the heat exchanger for forced air heat.  The current configuration has one heat exchanger for the first floor and we want to add a second for the 2nd floor but want this thing working before we do that.
I agree that the circulator is suspect but we really don't have any other ideas.  Your assumption is correct, when this unit worked it would boil water when there was no load, now it only gets to an indicated 160 or so in that same condition.
I was not sure how to get the photo posted with the message, thanks.
Need to think about this some more.

[wpowokal]

If I understand you correctly it is essentially a fire tube boiler, so what is the quality of the feed water particularly with regard to minerals, I am thinking here of scale build up on the water side of the tubes that will interfere with heat transfer, monitoring  flue gas temperature through the life of the unit would highlight this problem.

I would be doing an acid clean on the water side to remove this scale, I am sure there will be some, no I can't recommend which acid, the unit makers should be able to advise you.

You clean the fire tubes√
The forced air fan is being triggered at correct temperatures and has no obstructions or impaired performance√
You have no extra load on the unit√
You are using the same wood grade and dryness as always√
Then the water side is not receiving the same amount of heat as previous, and performance has declined slowly.

allan

[Eagleye]

Thanks for the advice.  We are looking into what might be the appropriate cleaner for the water reservoir.  I did find a diagram of the unit on the web while searching, will attach it.  The water is from our well that is filtered, water softener and sanitizer in that order.  The boiler itself uses an anode rod and we pour an anti corrosion material in it once per year usually at the beginning of the heating season.
Maybe to support your theory, when the weather turned warm here in VA the past few days we did not have a call for heat, thus there was no load on the boiler and the temp sat almost steady at 140 indicated and the blower was not running meaning the area near the aquastat was above the 180 point.
We have a call into the rep but will continue to search elsewhere for a cleaner.