Weather-Responsive Controls (Outdoor Reset Controls)


WEATHER-RESPONSIVE CONTROLS AND LOW-TEMPERATURE BOILERS

Heat Loss

Total heat loss is calculated in British thermal units per hour (Btuh). As an example, let’s assume that a 3,800 square foot house has a heat loss of 150,000 Btuh. That means that on the coldest night of the year, with a temperature outside of either zero or minus ten degrees, you will need a boiler that will produce 150,000 Btuh at 180 degree boiler water to make up for the heat that leaks out of your home.

But, what happens on all the other nights that are not the coldest night? The average outdoor temperature through the entire heating season is 48 degrees F. When you turn up your thermostat on one of these average days, that 150,000 Btuh boiler kicks on and starts pumping 180 degree water around. Two minutes later, just as you feel that first wave of heat rolling toward you, the boiler shuts off. Because the boiler is sized for the worst case day (and then some), it is way too big for the building at 48 degrees outside. It heats up much faster than the baseboard or radiators can distribute the heat to the house. This causes ‘short-cycling’ which wastes fuel and leads to unnecessary wear and tear on your heating equipment. Rooms can overheat and radiators and baseboard can make strange ticking or moaning noises. It is not the best situation.

As you can see from the chart below, the vast majority of our heating hours have temperatures well above freezing. The design conditions occur less than .3 percent of the heating season (approximately 20 hours out of 7,300). This chart is based on actual weather data over the last 65 years and reminds us that the vast majority of our heating hours are at temperatures well above what your boiler was sized for.

Now everyone wants a boiler that can heat up the house fast. Right? Not really. If fuel were free and people didn’t mind large temperature swings every time the thermostat came on it would be OK. But fuel is more and more expensive and people are always more comfortable with an even temperature. The problem is that your boiler is actually too big for your house 99.7% of the time. The warmer it is outdoors, the more oversized your boiler becomes. Like a car with a way too big engine, an oversized boiler wastes a lot of fuel dollars.

If you are interested in saving a sizable portion of your heating budget every year and be more comfortable, there is a solution to this problem.

The Solution – Weather-Responsive Controls

The easiest and most cost-effective way to reduce fuel is to use a weather-responsive control, also known as an outdoor reset control.  We install a variety of weather responsive controls.  Visit our Boiler Technology page to learn more.

An outdoor reset control essentially adjusts the size of your boiler to match the necessary heating output based on outside temperature. It doesn’t physically change the boiler size, of course. It does adjust the boiler’s output by changing the boiler’s water temperature. The direct relation between the outdoor temperature and how hot your boiler needs to be is illustrated below.

Weather responsive controls (reset controls) are an electronic energy management system that uses a tiny computer chip to balance boiler water temperature with outdoor temperature. By constantly measuring outside temperature, they determine the optimum temperature needed to heat your home. It is a brilliant yet simple control strategy. These controls have a sensor on the North side of your home and a sensor mounted at the boiler to sense water temperature. Some use a third internal room sensor to provide additional information to the control. This room sensor can detect inside heat gains from solar, fireplaces, cooking or people or extreme heat losses from wind chill.

The Benefits of Outdoor Reset Controls

Energy Savings

For every three degrees that the building’s water temperature can be lowered, there is a 1% fuel reduction. If the system was firing at 180 degrees and now is averaging 130 degree water, the savings will be at least 15% (180 ­ 130=50 divided by 3 = 16% savings).

Comfort

Because the output of the radiators or baseboard is now precisely matched to your home’s heat loss, there is always just enough heat being radiated to keep your house warm. You keep up with heat loss, versus trying to catch up. Radiators don’t get hot and then cold, they always give off just the right amount of heat. Weather responsive controls leads to long on cycles and long off cycles on the boiler. This provides constant room temperature, extremely quiet operation and reduced wear and tear on the heating system components.

Equipment Stress

Without reset controls, your radiation cycles on and off constantly. Just as your baseboard has cooled down, the boiler kicks on and pumps scalding hot water back through it. This can cause ticking or banging as everything expands from sudden temperature change. These ‘on and offs’ are rough on every part of the system: boiler sections, pumps, burner, motors and piping. Imagine a car that did not have a gas pedal but only had an on switch and a brake. You would constantly have to switch from full speed to brakes to achieve a safe speed. Imagine what that would do to your car and its efficiency and you can see the stress that a high temperature on/off system endures.


Weather-Responsive Controls (Outdoor Reset Controls)

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