Energy Kinetics System 2000 boiler (C) Daniel FriedmanHeating Boiler Economizers
Boiler aquastat adjusters, outdoor reset modules, boiler operating efficiency

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Heating System Controls to improve hydronic heating boiler operating efficiency & economy.

The tradeoffs beween higher boiler temperatures and longer boiler on-time runs are discussed here.

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Heating Boiler Energy Savings Devices & Control Manuals

Honeywell R8182D internal (C) Honeywell Outdoor Reset Module Boiler Aquastat Adjustment for Outdoor Temperature

Question: boiler not heating house when an economizer or ORM module is in use

2017/04/23 Steve Carlson said:

I have a brand new Monitron Slant Fin electric boiler with floor heat in the basement and baseboard heat on the main floor. I have 5 zones (installed 20 years ago). I added the Outdoor Rest Module (ORM) to this system when the new boiler was installed. I live in northern Minnesota and when it's cold outside the system works very well. I have 2 questions:

1) The boiler is set for HL 180 degrees and LL 150 degrees. It stays within those parameters even when the thermostats are not calling for heat. Is that normal?

The issue is that the hot water "gravity feeds" the hot water through the upstairs baseboards even though the thermostats are not calling for heat. So this means if the outside temperature is 35 degrees the upstairs rooms get too warm (around 75 degrees). Shouldn't the boiler shut down if there is no call for heat?

2) If the outside temperature is 42 degrees and cloudy for several days, the ORM shuts off at 40 degrees (default setting).

Then, even if the thermostats call for heat, the boiler does not activate. Shouldn't the boiler kick in even if the outside temp is above the default temp setting? If I increase the temperature setting I get the problem stated in question #1.

This question & reply were posted originally at AQUASTAT SETTINGS vs HEATING COST



In Minnesota we prefer to see the boiler run a bit hotter for efficiency and thus lower heating cost. You can set the HI limit to 200 as long as you don't see spillage at the relief valve. LO needs to be at least 20F lower than HI - so LO can go up to just under 180. The thermal conductivity of hot water in baseboards and radiators is exponentially greater at higher temperatures, so you in effect get more efficient heat transfer into the living area at higher settings.

It's normal for the boiler to remain between the HI and LO if your system has a tankless coil installed, since the boiler keeps water hot in case needed for the coil. Most aquastats are set up assuming that's the case. So IF your boiler doesn't include a tankless coil for making domestic hot water (washing and bathing) then you could change the aquastat settings and even disable the LO.


Independent of the ORM module, when the room temperature is below the thermostat setting the circulator pump should turn on to send hot heating water to the radiating devices in the room. But the boiler's (gas or oil fired) burner won't turn on until the temperature drops about 15 degF below the HI setting. That may take a bit longer.

About the "Outdoor Rest Module (ORM)" you cite, I think you meant to say "Outdoor RESET Module " such as the unit produced by Honeywell.
An ORM in essence re-programs the HI setting or boiler aquastat to what Honeywell thinks is the most efficient heat setting depending on outdoor temperature.

Using a "boiler reset" module or ORM is an old, well-established concept, idea, and device, though my point about thermal heat transfer being more efficient at higher remains true. There are two different economies in play:

1. Setting the boiler temperature HIGHER gives more efficient heat transfer from boiler and radiators into the heated space

2. Setting the boiler temperature LOWER causes the boiler to run for longer intervals. Keeping the boiler on longer burns fuel, particularly oil but also gas, more efficiently in that it takes a heating boiler as much as 5 minutes to get up to full operating temperature from when the burner first turns on.

How would I decide the balance of these two economies?

1. For a boiler without an ORM I would prefer the higher temperature settings

2. For a boiler at which I observe that in normal heating use under most conditions the boiler is staying on for 5 minutes or less there would probably be a significant improvement in boiler economy by using a boiler reset module or outdoor reset module to lengthen the boiler on-time runs.

3. For newer boilers that tend to be smaller in size and higher in efficiency, the boiler heats up quickly. For virtually all such boilers, a reset module or ORM should result in a good savings in heating costs.

4. Older heating boilers may not benefit or not benefit so much from a boiler reset module or ORM because at low outdoor temperatures the boiler may begin to run for too-brief intervals (short cycling); As I explained, a boiler whose burner on for just brief intervals will run inefficiently because at cooler boiler temperatures the fuel combustion is incomplete - we're wasting fuel. That's why oil heat service techs are taught not to measure boiler efficiency until the boiler is up to full temperature: about 5 minutes or so on older systems.

Watch out: running a heating boiler consistently at too low a temperature - say under 140F - risks formation of acidic condensate inside the unit, a source of corrosion and boiler leaks or damage.

A little alternative that gained some heating boiler efficiency on older heating systems using a thermostat like the Honeywell T87 that included a heat anticipator (search InspectApedia for HEAT ANTICIPATOR SETTINGS) would turn off the boiler a bit early at the end of a heating cycle, allowing the thermal mass of the boiler to provide heat to finish satisfying the room thermostat and to avoid overshooting the desired room temperature. Cast iron boiler heating systems and cast iron radiators or baseboards with lots of thermal mass particularly benefited from a heat anticipator or other devices to turn off the boiler's burner a bit early at the end of a heating cycle.

An alternative (or possibly supplement) to an ORM or boiler reset module that makes sense for older boilers that transfer heat less efficiently such as a fin tube boiler, a mixing valve on the heating loop that keeps the heating loop piping temperature down a bit (and keeps boiler temperature up a bit) will also produce heating cost savings.

Here is how Honeywell describes that device

Outdoor Reset Controls help save energy by operating a boiler at the most efficient setpoint based on outdoor temperature. As the outdoor temperature changes, the control adjusts the boiler to the optimum temperature that will ensure the homeowner stays comfortable. Some advanced reset controls also monitor indoor temperature when adjusting the boiler, to account for fluctuations within the home. The result:

• Heating costs reduced by up to 15 percent*
• Improved indoor comfort with fewer temperature fluctuations
• Reduced standby losses from the boiler and stack losses
from the exhaust flue
• Reduced equipment cycling for longer equipment life and
more continuous, comfortable space heating

Here is information from Honeywell [PDF] describing the various models and options for their ORM

And here is the installation manual with more detailed instructions for the ORM

5. Boilers that were over-sized for their heating load will benefit from any economizer device that uses a "heat purging" approach such as we described above.

Studies, Manuals & Specifications for Heat Managers, Boiler Reset Modules, Outdoor Reset Modules, ORMs & Boiler Economizers

Energy Kinetics 2000 High Efficiency Boiler (C) Daniel Friedman


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