# Boiler aqustat settings.



## nrp3 (Jan 24, 2009)

This for the coil in the boiler itself or an external storage tank?


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## junkcollector (Dec 30, 2007)

Hi,
Long time lurker first time poster... My parents used to have a system like you describe when I was a kid. My dad would work on the boiler, and I learned a lot about boilers and controls as a kid. I remember that he needed the aquastat set at least 175 degrees off in order to have adequate hot water out of the tankless. It turned on at about 145. After some time, the tankless coil would lime up and then the mixing valve would get turned up. At some points the water was nearly 175 degrees coming out of the tap. Ouch. I would say your mileage would very based on the age of the tankless, how finicky the mixing valve is, and what temp you need to do your space heating. 140 might not be enough on the really cold days. My thermometer has seen -35.


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## kb1jb1 (Nov 11, 2017)

nrp3 said:


> This for the coil in the boiler itself or an external storage tank?


Coil in boiler. 6 gallons per minute rated. No indirect.


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## CAUSA (Apr 3, 2013)

Keep the temp above 140F keeps the bugs out.

mixing around 120F keeps the burns down.

link for the things to be cognizant of.






Controlling Legionella in Potable Water Systems | CDC


Learn about the routine maintenance of building water and recreational water systems for the prevention of disease.




www.cdc.gov


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## SWDweller (Dec 9, 2020)

Guessing I think your 180 is a bit high, unless your on the arctic circle and a large system.

I would try 150- 160 for the high depending on your feelings about how the system works.


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## hbiss (Mar 1, 2008)

A million years ago, back when hydronic heat was taking over from steam, the boiler manufacturers got together to standardize ratings. They determined that 180 Deg was optimum and thus they all use it to determine their capacities on the nameplate. That's why everybody sets the max temp to 180. If you didn't have a tankless for domestic hot water, I would say knock it down to 160 and see what happens. It depends on your heat emitters (baseboard or cast iron radiators or fan coils.) But with the boiler supplying hot water, I think you are stuck with 180. And that's year round at $6/gal. 

My boiler is set up to be maintained temperature, it sits at 180 Deg all the time. That was because of the tankless. Thermostats just control the circulators. Aquastat (actually in the tapping on the old tankless coil), is set at 180 and just turns the burner on and off. The low limit, set at 160, keeps the circulators from operating until the boiler gets up to 160 if there is a heavy load and the temperature drops below that. Gives it a chance to catch up if all zones were calling at once. 

The other option, which most boilers are now is cold start- boiler fires when a thermostat is calling for heat. Can go to stone cold until then. If that boiler has a tankless there has to be a control to maintain a minimum temperature much like a maintained temperature boiler, but usually set to below the heating temperature. 

I like to also always have a high limit aquastat set to around 200. I don't like to have a boiler cycle on the same aquastat that controls the temperature that acts as a high limit safety. Many jurisdictions also require a low water cut off if for some reason the boiler runs out of water. 

Years ago I got smart and brought gas into the house, ditched the tankless and now use a regular 50 Gal gas fired water heater. Boiler gets shut down from March to November. 

As my electric dryer and range died, I replaced them with gas appliances. Can't stand cooking with electric.

-Hal


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## joe-nwt (Mar 28, 2019)

180-185 common around here.


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## Going_Commando (Oct 1, 2011)

You definitely don't want to run a boiler at 120-140 degree water temperature, if it's a conventional boiler and not a high efficiency condensing gas boiler (those fancy wall hungs). If it vents into a chimney or outside with metal pipe, if you go below a certain temp based on a combustion analyzer (measuring stack temp) you will get condensation forming in the chimney and it will corrode. You should probably go 160-180 or 170-190 with a tankless coil, and install a mixing valve to drop the domestic to 120 degrees to prevent scalding. You are probably best off calling a plumber/boiler guy to do it, because every boiler is a bit different and needs to be tweaked for best performance. 

Your brother's temp recommendation is for water heater temperature, which is completely different. That you want above 120 degrees to prevent legionella bacteria from growing in the tank which can make people sick. You also don't want more than 120 degree water at faucets if you have kids or whatever so they don't get scalded.

Disclaimer: I'm not a heating pro, other than mini splits but we do mess with oil boilers sometimes as my old man has been playing with heating systems for a long time. I'm also a nerd so I read a lot.


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