Aquastat or thermometer?

Old 12-25-06, 07:44 PM
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Aquastat or thermometer?

Given an older oil-fired hot water boiler, which is more likely to accurately determine the water temperature within the boiler, the aquastat or the thermometer?

My aquastat is set at 180 Hi, 160 Lo, 15 Diff yet the thermometer routinely kicks up as high as 215 deg F.

What would be safe and reasonable values for actual water temperature and pressure within the system? FWIW, this is an 80 year-old house with cast iron radiators and a mix of old steel pipe/cast iron fittings along with some newer copper pipe in the basement.
Old 12-26-06, 09:32 AM
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You mean which would I trust ? ummmm, I'd probably get a third opinion, meaning a known good thermometer, and compare that to the others.

What does the temperature read when the aquastat actually shuts the burner off ? It's possible that after the burner goes down the heat in the cast iron boiler transfers to the water inside which is no longer circulating and heats it a bit further. (your automobile engine does the same thing, ever notice the electric cooling fan go on minutes after you shut the car off?)

What pressure are you seeing on your gauge ?
Old 12-26-06, 09:57 AM
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A few things come to mind, and I am no pro.. First, the aquastat has a bulb in an immersion well that relies on the heat transfer between the water, well and bulb.. There's usually some thermal compound between the well and bulb, if it is missing the acquastat might not sense the correct temps. Or, maybe the bulb isn't fully inserted? I'd guess it's working as intended though. The water temps across the boiler could be different--if the temp gauge is in a front tapping, and the acquastat in a rear, the temp spread could be several degrees. Just some guess on my part.

Old 12-26-06, 05:57 PM
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Aquastat or Thermometer

If the thermometer you are reading is the boiler gauge, they are notoriously inaccurate. Both Radioconnection & NJ Trooper are correct in their statements. I often use a digital thermometer with a well insulated surface mountable sensor to check the accuracy of the aquastat & gauge.

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