How Outdoor Reset Works in Heating Systems

\What is Outdoor Reset?

Outdoor reset is a feature of heating controls that optimizes the efficiency of steam and hot water boilers used for building heat based on the temperature outside the building. For hot water boilers, it will raise the boiler water temperature as the outside temperature drops, for steam boilers it will adjust the on-off cycling length of the boiler.

Outdoor Reset Can Help Prevent Overheated Buildings

Building codes require that the heating systems in residential and commercial buildings be sized to keep the interior warm at the location’s “Design Temperature.” Design Temperature is defined by ASHRAE as the minimum outdoor temperature that the location is statistically expected to experience for 99% of the year based on a 30-year average. This means that 1% of the time that the outdoor temperature is above the Design Temperature, the building heating system is oversized for the conditions.

In a typical single-family home this is not an issue. On mild winter days the thermostat continually cycles the heat from full on to full off. Because the home heating system is small, it responds in only a minute or two when the thermostat calls for heat, and when the thermostat is satisfied the boiler shuts off and the residual heat in the system dissipates quickly, causing a only small temperature overshoot.

A commercial building is very different. The hundreds of feet of pipe and hundreds of gallons of water in a commercial heating system have a lot of thermal mass that takes a long time to heat up and cool down. If a thermostat calls for heat in a commercial steam building, it can sometimes take up to 20 minutes just to make steam in the boiler. Then due to the size of the building and the condition of steam traps, air vents , etc. it can sometimes take an “additional” 10 to 20 minutes to fill the system with steam. During this time, the building is still cooling down. Once all that mass is hot and the thermostat is satisfied, the steam or hot water remaining in the system will continue to give off residual heat into the radiators, significantly overheating many rooms in the building. Because of this lag in heating the thermal mass, and then having it give up its residual heat, the thermostat is always playing catch-up, overheating the building, and wasting energy.

Outdoor reset solves this problem by allowing the boiler controller to predict how much heat the building requires based on the actual outdoor temperature. During the 99% of the time that the outside temperature is above the Design Temperature, the boiler controller adjusts the boiler output to match the anticipated need.

How Outdoor Reset works with Hydronic Heating

The advantage of outdoor reset is easiest to understand in a hydronic heating system. A properly sized hydronic system will keep the building warm at the Outdoor Design Temperature when the circulating water is maintained at the boiler’s maximum operating temperature. However, this is too much heat for the building 99% of the time. With outdoor reset, the boiler controller will lower the operating temperature of the boiler in a ratio with the outside temperature. This ratio, called the reset ratio, varies from building to building based on various factors. For example, if the boiler operates at 180° at the Design Temperature, the controller will use the reset ratio to reduce the boiler temperature to a lower temperature on a mild winter day. This reduces the amount of residual heat in the system and eliminates overshoot when the thermostat calls for heat. With a condensing boiler, the controller can lower the water temperature much lower in the spring and fall, delivering exactly the amount of heat the building needs.

The savings of lowering boiler temperature are significant. For every 4°F the boiler water temperature is reduced, there is 1% energy savings. This alone can result in a cost savings of 15% or more during the heating season.

How Outdoor Reset Works with Steam Heat

For a steam system, outdoor reset is even more important, because condensing steam can release almost 7 times the energy into the building as an equivalent weight of hot water, leading to large temperature overshoots and overheated rooms. Outdoor reset allows the boiler controller to predict ahead of time how much heat is needed in the building and how long the boiler’s on-off cycle needs to be.

When the outdoor air temperature drops, the boiler controller turns on the heat source. During the first part of the logic the boiler controller fires the boiler until the heating system sensor, a temperature sensor located at the far end of the heating system, indicates that steam has reached the entire system. This is often called “establishing heat.” Once heat has been established in the system, the boiler controller will calculate how long the boiler needs to stay on based on the outside temperature. At the end of the calculated time period the boiler will shut off, and the residual heat in the system will heat the building.

Because of the energy savings from outdoor reset, basic outdoor reset functionality on new commercial construction and renovations has been required by the International Energy Conservation Code (IECC) and IRC since 2015. To comply with these requirements, basic outdoor reset is now included standard with many new boilers.

Basic Outdoor-Reset is Only One Factor in Building Energy Conservation

While basic outdoor reset saves energy, it is only a first step in reducing a building’s energy costs. A full-featured premium boiler controller like the Heat-Timer® Internet series boiler controller with wireless space sensors, offers several other energy saving features, which working together have been shown to save an additional 20% of energy costs over outdoor reset alone.

It turns out that the most important feature of a premium boiler controller is the ability of the controller to use the information from multiple space sensors placed throughout the building to optimize the boiler response. Buildings have different heat loss characteristics due to age quality of insulation, quality of windows, etc. So the Heat-Timer control offers various reset ratio options, to try and match the heat loss of the various types of buildings. The difficulty with tweaking selecting the correct reset ratio is that you need to get accurate feedback from the building. This is usually accomplished by asking tenants or occupants. So now you are left to make energy saving and comfort decisions based on the personal preferences of a few.

When using a Heat-Timer® control with space feedback, the control uses an additional piece of information. It is a reset control, but now you can give the control a desired space temperature target that you are trying to maintain.  The control analyzes the actual space temperature vs the desired target space temperature and automatically adjusts the reset curves to try and attain your desired space temperature. It does not just do this at one point in time, but is constantly adjusting to maintain your desired space target. This not only saves energy , but also dramatically increases building comfort.

All outdoor reset systems include a single heating system sensor either on the condensate return line in a steam system or on the supply in a hydronic system. That single sensor doesn’t tell a basic system what is happening in the rest of the building. A Heat-Timer internet control with wireless space sensors can use the temperature data throughout the building to optimize its heating curves. Monitoring heat in multiple areas of the building has been shown in DOE studies to significantly increase resident comfort and local heating code compliance.

Heat-Timer makes Energy Savings Easy

It has been reported that 53% of people with programmable thermostats don’t use the programmable features, which makes it clear that even if a full-featured boiler controller can reduce energy costs, the controller can only do so if the energy saving features are actually used. That is why Heat-Timer® makes system set-up and maintenance easy, with simple, easy to follow display menus, detailed, easy to find online manuals, and outstanding technical support.

Best of all, many of these energy saving features can be monitored and optimized from anywhere with BuildingNet, Heat-Timer’s exclusive remote management website and mobile app that allows building managers and HVAC professions to access the Heat-Timer controllers from any device with internet access. Why just settle for basic outdoor reset, when you can have the energy saving features and remote connectivity of a Heat-Timer® Internet boiler controller?

For more information, visit https://www.heat-timer.com/

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