Best High setting on boiler aquastat

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Fitter30

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All boilers have two temperature controls. One is the hi limit set at 190* this is a safety in case the operating control fails. Operating control has two settings on and off. Neither change the efficiency of the boiler. Cast iron, copper fin 80-85% made in the last 20 years. Condensing boilers mid 90% unless return water is over 130* then 85%. Outdoor reset control will save fuel they vary boiler water temp on outdoors temp. To get the most out of the system boiler not short cycling and lower water temp that keeps house warm. Cast iron and copper fin return water temp minimum 135*.
 

Sylvan

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"All boilers have two temperature controls. " <<< ONLY hydronic boilers



Steam boilers have a high limit "PRESSURE" controllers 1 PSI of steam = 213 deg F

Temperature is not taken into consideration on vapor boilers


According to ASME and NBBI a Low-pressure hot water boiler 160 PSI or 250 DEG See section IV ASME code



 

Jadnashua

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A heating device will always take some time to come up to full efficiency, so, ideally, it would run constantly, supplying the exact amount of energy needed to maintain your desired set point so that ramp up to full efficiency only happens very seldom.

But, that ideal is generally not attainable. Short cycling of the boiler can easily happen when you oversize it for the heat loss of the building, or, as common, you're trying to heat it on a barely cold day and only need a little heat. One of the better choices today is a condensing boiler that has an adjustable burner output, ideally, with a large turn-down ratio to handle those mild days. As said, though, a lot of boilers out there require the return water to be above a certain point, or they can be damaged.

So, with the tendency to put in a big boiler 'just in case' or using inadequate square footage seat of the pants calculations, a lot of boilers are not very efficient, or not as efficient as they could be once installed.

An outdoor reset controller that can adjust the set point to the load can be a big aid in efficiency because there is no one ideal boiler outlet temperature that works generically. It depends on the available radiation, the insulation, the outdoor temperature, and the actual boiler in use.

You should measure the outlet and return temperatures of the heating water. If the return is still quite high, you can safely lower the outlet temperature. Now, consider that the actual radiation is not linear, meaning that a smaller drop in outlet temperature may mean a bigger loss in the actual heat energy radiated but running longer at a lower temperature is likely going to be more comfortable with fewer temperature swings.
 
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Fitter30

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"All boilers have two temperature controls. " <<< ONLY hydronic boilers



Steam boilers have a high limit "PRESSURE" controllers 1 PSI of steam = 213 deg F
Homeowner asking about a temperature limit isn't thinking of steam. When working on process steam, temperature sometimes has to be takin into the setup of the boiler. A pasta plant wanted 235* water for process and control humidity in the plant. Boilers, pumps,heat exchangers and system water pressure were all design. Stay warm.

Temperature is not taken into consideration on vapor boilers


According to ASME and NBBI a Low-pressure hot water boiler 160 PSI or 250 DEG See section IV ASME code


 

Dana

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My boiler Aquastat is set to a High Temp limit of 180 F. Some folks say you get better efficiency by running at a slightly higher temp, like 190 F. What does your experience tell you?

https://inspectapedia.com/heat/Aquastat_Settings.php
___ "Generally a heating boiler is more efficient if we operate it at a higher temperature."

The full quote from that link is:

"Generally a heating boiler is more efficient if we operate it at a higher temperature.

The thermal conductivity of heating water inside of finned copper tubing baseboards or through radiator surfaces is exponentially greater at higher temperatures.

In other words, hotter water actually transfers heat into the occupied space more efficiently than cooler water.
"

What they are a calling "efficient" isn't fuel-efficiency, but rather it delivers heat rate coming off the heat emitters. They are only saying that higher temp means the heat rate is higher (DUH!), and says nothing about fuel-economy or comfort.

Higher boiler temperatures are the opposite of both higher comfort, and higher fuel efficiency. For comfort the LOWEST water temperature and SLOWEST heat rate that still satisfies the thermostat is the most effective since it delivers steadier longer on-cycles. That also improves the overall system, so long as there is enough radiation to suppress short-cycling at the lower water temperature.

The raw steady state combustion efficiency will actually FALL (slightly) at a higher temperature. At the same time the standby losses from the boiler to the boiler room will increase as well. A lower average water temperature will increase the average combustion efficiency and reduce standby losses, for an overall net increase in system efficiency, provided the boiler isn't short cycling. The hit in combustion efficiency is small when going from 180F to 190F about 0.1-0.2% at most.

Boiler2.gif


The biggest efficiency problems with cast iron boiler systems are short burn time (short cycle) and low duty cycle (= more standby time, more standby loss.) There isn't much improvement to be made on duty cycle with fixed rate non-modulating burners, but the burn times can be improved by maximizing the differential between high and low limit settings.

The greater the temperature difference the system has to slew through, the longer it takes to heat up the system's thermal mass. For the average efficiency to hit closed the steady state (continuous burn) combustion efficiency usually takes ~10 minutes per burn or more, but for most boilers there's only about a 1% hit if the burns are only 5 minutes. But with burn times significantly shorter than that the average efficiency starts to fall off a cliff.

The relatively safe low limit for most oil boilers is 140F for oil boilers, 130F for most gas burners. These low limits are necessary to limit the amount of acidic flue condensation, and to avoid excessive corrosion on the fire side of the heat exchanger plates. With condensing gas burners this isn't a problem- condensation reaps significant efficiency improvements when the entering water temp at the boiler drops further below ~125F. But cast iron can't take the acidity and can be destroyed in one heating season if allowed to chronically condense, which can be an issue if there is too much radiation, or too much thermal mass in the radiation. There are plumbing solutions to protect against low-temp entering the cast iron boiler, but unless you have that issue I won't go into it here.

Replacing aquatstats with heat purging economizer controls such as the Beckett 7512 HeatManager, Beckett AquaSmart, IntelliCon HW+ 3250, HydroStat 3200/3250-Plus, etc. will increase burn times by "exercising" the thermal mass, suppressing the burner on new calls of heat until it hits the low limit, pre-purging heat from the boiler into the zone. Some of them it "learn" the system's behavior and cuts the burner a bit before the thermostat is satisfied, finishing the call for heat with the heat already in the boiler, lowering it's average temperature, which lowers the standby and distribution losses. When the systems are significantly oversized (2-3x or more) with very low average duty cycle this reduction in idling loss has a dramatic effect on the as-used efficiency and fuel use. Most new cast iron boilers now come fitted with (or can be ordered with) this type of control already installed.

So before screwing around with aquastat controls, measure the burn times. If you have a heating history on the place, run a fuel-use based heat load calculation (wintertime fuel use only) and compare it to the BTU output (the raw DOE BTU numbers, not the net-water IBR output) on the boiler's nameplate. AFUE measurements assume a 1.7x oversize factor, and are usually pretty close to the steady-state efficiency of the boiler. But per ASHRAE recommendations the optimal oversize factor for both comfort & efficiency is 1.4x- which is enough to cover Polar Vortex disturbance cold snaps well below the 99% outside design temp, while still maintaining a high duty cycle during normal cold weather.

The amount of radiation also affects the optimal boiler size and may affect the near-boiler plumbing if there is so much radiation that the return temps are too cool. But in general the smallest boiler that actually covers the peak load is the most comfortable, since it's delivering steadier heat and long burns, not cycling on/off giving you the hot flash followed by the extended chill when it's truly cold outside.
 
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Tangent

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Replacing aquatstats with heat purging economizer controls such as the Beckett 7512 HeatManager, Beckett AquaSmart, IntelliCon HW+ 3250, HydroStat 3200/3250-Plus, etc. will increase burn times by "exercising" the thermal mass

Thanks. From a computation you had me do several years ago, it is clear that the Utica boiler is way too big (2x +), and it has short cycles.

In the list of economizers, I believe the IntelliCon is discontinued. The Becket 7512 is an option -- I guess the 24V control output would need to be adapted to work with my millivolt system. And leaving the High temp limit where it has always been -- at 180F.
 
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Dana

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Thanks. From a computation you had me do several years ago, it is clear that the Utica boiler is way too big (2x +), and it has short cycles.

In the list of economizers, I believe the IntelliCon is discontinued. The Becket 7512 is an option -- I guess the 24V control output would need to be adapted to work with my millivolt system. And leaving the High temp limit where it has always been -- at 180F.

With heat purging economizers it's usually better to set the aquastat high limit to the maximum allowed by the boiler manufacturer, and let the economizer decide how high it really goes. If the boiler is rated for 200F or 210F, that gives it another 20-30F of potential differential to work with to extend burn times. The economizer would then be better able to skip entire burn cycles on it's way down to the low limit.

If the low limit is set to 140F and the high is 180F that's a max differential of 40F. If the high is allowed to hit 200F it's a differential of 60F, for roughly a 50% increase in potential burn time for a short-cycling boiler.

In most systems it would rarely actually hit the 200F high limit before a call for heat from a zone is satisfied, especially if starting from under 150F. But on extended calls for heat the difference between 180F and 200F could be the difference between cycling the burner multiple times during the call vs. satisfying the thermostat in a single burn. Even though on some cycles the boiler might be idled at a higher temp, that also delivers more heat purge BTUs before it drops to the low limit.
 

Tangent

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Becket7512 can be used with a millivolt system with these changes. Gold plated contact relay with 120vac coil

Thanks for the link to the relay.

Your diagram is very neat, but I think I might need a photo with higher magnification.
 
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