Adjusting the aquastat--compare setting to what?

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Old 01-16-13, 10:19 AM
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Adjusting the aquastat--compare setting to what?

The dial on my cottage aquastat (L8148E) is set to about 195F according to the dial. I had thought the previous owners did this so the system would heat the place up quicker on the rare winter trips up for the weekend but I measure 180F on the hot pipe just above the boiler, using an IR thermometer. The temp dial on my T&P gauge is broke and there's currently no other place to check water temperature without opening something & introducing air.
Return pipe reading was about 165.

So my question is where does the discrepancy come from? Is my reading probably accurate and the aquastat is that inaccurate...or is it typical to lose 15 from inside the boiler to 2 feet above (or outside pipe vs. inside)?


BTW I do understand that IR thermometers can have reading errors due to how shiny the object is so I took several readings on the pipe, dark cast iron manifold, etc and they all agreed very closely.

TIA
 
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Old 01-16-13, 01:51 PM
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Before merely hazarding a guess, please replace the gauge.
 
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Old 01-16-13, 05:35 PM
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I would have to say that the aquastat setting rarely will match the actual output temp of the boiler. It simply isn't all that critical. As long as the shutoff temp is REPEATABLE every single time, simply adjust the aquastat to match whatever temp you want, where ever you want, and ignore the dial markings on the aquastat.

Temps inside a boiler will vary a lot from one position inside the boiler to another. This is because of the turbulent flow patterns inside. Where the aquastat is located could well be that much hotter than the outlet pipe.

I do understand that IR thermometers can have reading errors due to how shiny the object
More specifically than how 'shiny' (although that's a very good clue) the object is is the EMISSIVITY of the object.

If you want to compare apples to apples, place a piece of black electrical tape on, or paint the area you want to measure with flat black paint.

Understand the 'pattern spread' of the IR tool you are using in order that you measure ONLY the object and not partially the surrounding area.

I have found that rusty old cast iron measures VERY close to actual with an IR thermometer. Copper, WHETHER SHINY OR NOT!, is very inaccurate with IR gun. Even oxidized old red copper with no shine at all will read poorly.
 
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Old 01-16-13, 06:58 PM
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I have an IR gun, and readings never seem to correlate with one another.

Replace the gauge!
 
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Old 01-16-13, 07:45 PM
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So the typical setting of 180 is arbitrary and not a desired temperature in some specific place?
 
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Old 01-16-13, 08:29 PM
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180 is a 'standard' that is used.

The BTU rating of baseboards and radiators is usually spec'd at this temp, and enough heat emission is installed based on the rated BTU output of said heat emitters.

That's more or less the reason for the 'standard'.

If one designed a system that was able to supply enough BTU at outdoor design at a lower temp, say 160, then there would be no reason to NOT lower the high limit.

If a home happened to be UNDER radiated, then naturally the temp would have to be run hotter...
 
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Old 01-17-13, 11:21 AM
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I need to be more specific. I don't mean to say 180 is arbitrary & meaningless, I mean is there a particular spot in the system that's supposed to be 180? As has been pointed out the inside of the boiler is a jumble of temperatures and the typical placement of the dual gauge might not be ideal (and they can't be relied on anyways I hear). It makes sense to me that what I or a tech might aim for is 180 at the outlet of the boiler, maybe some slight distance away to insure non-turbulent flow & a stable reading. Am I over-thinking this?

After the heating season is over & I start tearing into this system I plan to add a proper thermocouple. I'm looking for a suggestion on the best location for it.
 
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Old 01-17-13, 03:57 PM
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I mean is there a particular spot in the system that's supposed to be 180?
I think I would answer that with: No, not really. It's really not all that critical. If the supply out was 170 or 190, you probably wouldn't even notice a difference in heat output. The only time you might notice would be if ... say your baseboards actually put out the rated BTU output at 180 ENTERING water.. (and remember that the water cools as it traverses the loop), and your home heat loss calculation was exactly correct, and the exact amount of baseboard was installed to match the heat loss on a design day running with 180 water, and one night it dipped down below the design temperature...

In this case, your home would slowly cool off because your heat emitters wouldn't be able to keep up with the heat loss. In this case, if the boiler was also exactly matched, the burner would be firing the entire time. If oversized, it would still cycle somewhat...

It makes sense to me that what I or a tech might aim for is 180 at the outlet of the boiler, maybe some slight distance away to insure non-turbulent flow & a stable reading. Am I over-thinking this?
You might aim for that. A tech wouldn't even notice... or care, sad to say.

Don't ask me about over-thinking! I would repeat what Fibonacci said (and David Mustaine repeated) ... "Spiral out, keep going!"

Reality is that it's not all that critical. Set it up so that the supply water is '~around~' 180 and leave it at that.
 
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Old 01-17-13, 05:11 PM
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After the heating season is over & I start tearing into this system I plan to add a proper thermocouple.
What do you mean "proper thermocouple"? To measure what temperature and why? Don't worry about installing a thermocouple when your boiler gauge is broken. Replace your gauge.
 
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Old 01-17-13, 08:30 PM
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"Proper thermocouple" meaning a standard type J or K probe to connect to a good meter. They last forever, are accurate, cheap...far better than some crusty bimetal mechanical gauge.
 
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Old 01-17-13, 08:33 PM
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Can you explain your persistence with the gauge?
 
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Old 01-18-13, 08:17 AM
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An operating boiler pressure and temperature gauge is important for knowing the perfomance of the boiler and for safety. Replacement should be easy - cool down and depressurize the system, but don't drain it, and work fast.
 
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Old 01-18-13, 10:29 AM
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Now that I think back on it it's pressure portion of the gauge that doesn't work but I have a temporary gauge on the drain spigot. I have other options for monitoring so it's not a priority right now. I do plan to replace the gauge after the heating season when I need to move some baseboards.
 
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