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What is Outdoor Reset? (ODR)
How Does IT Work?
Outdoor reset or sometimes called outside reset is when we have a control on the boiler that will sense outside air temperature and readjust the heating system water to a minimum water temperature to satisfy the heat loss of the home. As the outside temperature gets colder the heating water gets hotter and as the outside temperature gets warmer the system water temperature gets cooler. This does two major things for us. One it will save fuel and second it creates more comfort. While running at lower water temperature than we were used to the radiation will feel cooler but the comfort level is increased. We are running cooler water around us for a longer period of time. It becomes almost a trickle of warmth all the time which tries to mimic constant heat. We do not feel temperature changes. To maximize the fuel savings one needs to properly adjust the reset curve which changes from application to application.
We will need to know certain information to adjust the curve properly. Here is a list of information that is handy to have when setting a reset curve.
1. Do a Heat Loss
2. Amount of Radiation. This is the only time you need to measure radiation on hot water boilers. DO NOT SIZE THE BOILER FROM THE AMOUNT OF RADIATION! Oversizing the boiler when doing ODR will waste fuel. The boiler will short cycle.
3. Boiler design temperature
4. Boiler start temperature
5. Outdoor Start Temperature
6. Outdoor design temperature
7. Boiler minimum temperature
8. Differential required
9. Boiler delay
10. Warm Weather Shut Down temperature
Let's determine how to get the above information and what the functions do. They will be numbered as above.
1. Do a complete heat loss. We can help you with that.
2. Measure the amount of radiation. You will need to determine the heat output from your system. Measure only the element of the copper tube baseboard. Measure all the cabinet convector's width, depth and height. Determine the sq ft of cast iron radiation. When you have this information there are charts available here to help in determining the water temperature required. We can help with this also if we do your heat loss. When you have the total radiation output we will divide it into the heat loss. That will give btu output per sq ft of radiation. We than use another set of charts to determine water temperature. With hydro air and copper tube baseboard the minimum should normally be 120º.
3. Boiler design temperature varies by boiler build. You have cast iron equipment which requires warmer water temperature or less flow due to condensing and you have the mod/con boilers which are designed to condense. The lower the water temperature required the better design for the newer mod/con boilers.
4. The boiler start temperature will normally be the same as the thermostat setting.
5. The outdoor start temperature will normally be the same as the thermostat setting. This is the outdoor air temperature where the heating curve starts. This is normally the same as the building temperature.
6. The outdoor design temperature is the temperature we used to do the heat loss. We use the ASHRAE design temperature recommendations for your area.
7. Boiler design temperature is the highest water temperature you want to see in the system at the coldest outside temperature. This setting is dependent on the amount and type of radiation.
8. The differential as adjustable but I normally suggest the automatic mode setting.
9. Boiler delay is how long the control will wait until the second boiler is started. This is normally adjustable from 30 seconds to 4 minutes or ore depending on the manufacturer.
10. Warm weather shut down (WWSD) is an option which is nice for commercial buildings and apartments. It will turn the boiler(s) off when this temperature is set even if there is a call for heat from the thermostat. It is usually set at the building thermostat setting. If the thermostat is set at 70º and the WWSD is 70º and the outside air temperature is 71º the boiler(s) will not turn on.
Now that we have the information from above we can set the functions in the outdoor reset control and use a reset curve. Below is a reset curve set to the examples listed below the curve. We calculated all the information and came up with those values. This is not very hard to do.
Here is an example of a reset curve. Water temperature is on the left side and air temperature is on the bottom. This is one I actually did for a 4 unit apartment house that used to maintain 180º water all the time in the 225,000 btu boiler. The heat loss was about 222,000 btu with no attic insulation. He added R-19 to the attic and the heat loss was reduced to 144,235. The building had 1600 sq ft of cast iron radiation. With a heat loss of 144,235 and 1600 sq ft of radiation we just need to do some simple math. 144,235/1600 = 90 btu's per square foot of radiation. We acquired the water temperature from this chart. Find the closest btu per square ft in the second row and look directly up to the top row. See cast iron radiation chart chart.
Also have charts for older (standard) copper tube baseboard chart
and newer lower water temp baseboard from Slantfin chart
We removed the old boiler, reduced the boiler size from 225k to 160 btu's and than installed two smaller boilers to match the heat loss. In other words we installed two 80,000 boilers with a Taco SR503-EXP and PC-702 outdoor reset control.
This is how we plotted the heating curve to see our control by.
At the bottom is all the information from the calculations. You will see from the example the water temperature at 70º outdoor air temperature is 110º. If the outdoor air temperature dropped to 0º the water temperature will be 145º The outdoor air temperatures and water temperatures can all be changed to fit any scenario.
How to read the chart
If the temperature today was 25º the reset control would use a target temperature of 123º for the system supply temperature. If you raised the water temperature on the left, and redrew the line to the new maximum temperature, the water temperature would go higher. The lower the water temperature setting the more fuel is saved. I must repeat myself here and remind you to be careful of lower water settings with cast iron boilers with out the proper boiler protection.
Let's take a look at the pump curve from the way it was shipped (Original Curve) and what water temps we are operating at after doing our calculation. You can see on a 20 degree day the system water temperature is almost 30 degrees cooler. This will use less fuel. We need to calculate the heating curves by doing the heat losses and measuring the radiation and readjust the curves for each job.
Here is a blank heating curve sheet you can print out and play with by changing outdoor air temps and water temps. If this is in your own home you can change the settings and fine tune your system. Prints best in Landscape Mode.
