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TJeepman 05-14-2010 03:19 PM

Oil Furnace - Hot Water Heating - Aquastat Settings
Setting for aquastats seem to be all over the place, depending on whose furnace one looks at. Is there rhyme or reason to the differences? Maybe somebody on the Wrangler Forum can shed a bit of light into what is going on and what settings are optimal.

I had my furnace serviced in March and the Techician raised the high setting to 210F for some reason, the low at 160F and a differential of 15 degrees. The hot setting was too hot for my domestic hot water (no mixing valve) and started some leaks with plastic tubing under a couple of sinks. I have since backed the high setting down to 200F. I have compared three different furnaces.
Mine: High @ 200F, Low at 160F, differential at 15 degees.
Friend 1: High @ 180F, Low at 150F, differential at 15 degees.
Friend 2: High @ 190F, Low at 160F, differential at 25 degees.

All three of these aquastats are similar Honeywell models.

For those wondering:
- aquastat is a control box on the front of the furnace that controls when the furnace cuts in or cuts out. It has three different settings in my case.
- high temperature setting is the setting that the furnace cuts out at and is based on the temperature of the water in the boiler located within the furnace. This setting only comes into play when heat is required to warm the house through the hot water radiators (ie heating season).
- low temperature setting is a two purpose setting. For the provision of heat to warm the house, it is the temperature at which the water in the boiler does not go below. The furnace will cut in and heat it up.
For heating domestic hot water, during the non house heating season, it is the high temperature cut off point for the water in the domestic hot water heating coil within the furnace.
- differential setting is the amount the temperature in the domestic hot water heating coil is allowed to go down by before the furnace cuts in to heat it back up.

So with my settings of 200F, 160F and a differential of 15 degrees:
(a) during the cold days of Winter, the water in the boiler for heating is controlling and operating between 160F and 200F.
(b) during the hot days of summer, there is no demand for house heating, so the domestic hot water coil "rules" and operates between 160F and 145F, ie a differential of 15 degrees.

Some say any temperatures below 140F are considered "not good" because condensation becomes a problem for cast iron heat exchangers (versus stainless steel) leading to corrosion. I don't know how true that is or what materials are commonly used for the heat exchangers.

Any thoughts of what aquastat settings are most beneficial?

TJeepman 10-07-2010 06:27 PM

Didn't get a response to this at the end of the last heating season, so I will try again with this year's heating system fastly approaching.
Basically I would like to know what the typical aquastat settings are for the boiler (not domestic hot water) and why would one ever go as high as 210F.

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