Aquastat / Thermostat Question

Backround information: My house has an oil boiler and baseboard hot water heat. With outside temperatures of 20F or above I can keep the house comfortable with my coal stove alone. However, the boiler will run occasionally to maintain its temperature even if the thermostat is not calling for heat. It doesn't run very long, or that often , but it still runs... With the price of fuel oil at $3.12 per gallon, it pains me to see the boiler kick on, and the circulator stay off.

I have never messed with an aquastat before and don't even know if that is the place to start. I did check the temperature guage on the boiler the last time it started, it appeared to be at 160 degrees. In my opinion, if the thermostat isn't calling for heat the boiler should remain off.

I'm looking for advice on what to check or change to see why the boiler behaves this way.

-Robert

The aquastat runs the burner based on the water temperature in the boiler. It will start at a fall to the low limit and run to the high limit and shut off. If there is a call for heat from the thermostat it may run during that call depending on your particular aquastat, most do but the details vary.

The thermostat will just start the circulator pump and/or open zone valves to supply heat to the home. It doesn't directly relate to the burner firing though, which may or may not run during the heat call.

If the burner is short cycling (and above the low limit setpoint) it is probably doing so on a call for heat. While the pump is running, the burner will too to keep the temperature maintained in the boiler.

As long as your boiler is powered up, it will run on occasion as there are heat loses through the chimney. Its just the nature of the beast. An electrically operated stack damper that opens when fired will reduce this loss.

True in almost all cases. The house I grew up in has a gas-fired boiler and radiators. There is a line-voltage thermostat that controls the circulator pump as well as the gas controls. When there is no call for heat, everything is shut off. Upon a call for heat the circulator pump starts, and a step-down transformer is energized, sending power to the aquastat and on to the gas control valve. The circulator pump continues to run until the thermostat setting is satisfied. The aquastat cycles the gas burner according to it's settings. Some of the other houses in the neighborhood have been converted to low voltage t-stats, line voltage t-stats are a little hard to come by.

coaledsweat wrote:As long as your boiler is powered up, it will run on occasion as there are heat loses through the chimney. Its just the nature of the beast. An electricly operated stack damper that opens when fired will reduce this loss.

Alright, I guess I will have to live with it the way it is. On weekends I shut the boiler off and enjoy the coal stove. During week days I leave the boiler on, that way if the the temperature or wind changes signifcantly the boiler will ensure a resonable temperature in the house. If I upgrade to a coal stove that is thermostat controlled I may get brave and just leave the boiler off.

Thanks for the quick replies.

-Robert

I have an WeilMclain oil fired boiler for heat and hot water. Since putting in my coal stove, the boiler is only used for hot water ( I run a superstor off of one of the boiler zones). To reduce my oil consumption, I dropped my boiler's low set point down to 125F - which is a bit above my superstor temp. Durring the day my boiler cycles once or twice on its own (to maintian itself or the superstor) When there is a call for hot water, the boiler kicks in. I think my original setup had 170F or so for the low set point. The longer the boiler stays at the low point, the more the savings (lower temp difference between boiler and ambient - just like a set back thermostat)

I manually control boiler makeup water - if on auto fill, the valve would will fill the boiler to pressure @ 125F & the pressure relief would blow around 180. Some older/ or other boilers may leak when their temp drops too much. Mine does not, but my low set is determined by my DMHW requirements.

I could be wrong, but I was under the impression that any setpoints below 175-180 degrees F, will cause condensate build up in your fire box, leading to early rusting of your boiler tubes.

I've used lower than 180* water temps for years and never had a condesation problems or premature rusting. 125* is still pretty warm, I can't imagine any condesation on a surface that hot.

Greg L

Hi Greg. I checked the aquastat settings on my boiler, it was set to at 160/180 degrees. I changed the low limit to 140, the boiler runs much less since I made the change. I can keep the house warm with the coal stove and the boiler runs 2-3 times per day to maintain it's jacket temp, I can live with that.

-Robert