Outdoor Reset Explained

What is Outdoor Reset? (ODR)

How Does ODR Work?

Outdoor reset controls will sense outside air temperature and re-adjust the heating system water temperature to a minimum and still satisfy the heat loss of the home. The heat loss of a home changes as outside air temperature changes. As the outside air temperature gets colder, the heating water gets warmer, 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 within the home. While running at lower water temperature in the radiation than we were used to, the radiation will feel cooler, but the comfort level is increased and balanced, the cost of operation will decrease. We are running cooler water around us for a longer period of time. It becomes a trickle of warmth all the time which tries to mimic constant heat. We do not feel temperature changes within the structure. To maximize the fuel savings, one needs to properly adjust the reset curve which changes from application to application. Outdoor reset controls have many settings which let you dial in a maximum fuel savings, but many are left at factory defaults which work but not as efficiently as changing the parameters to match your specific system. We will need to gather certain information to adjust the curve properly. Here is a list of information that is handy to have when setting up an outdoor reset control.

1. Heat Loss - Perform a Manual-J room to room (new or retrofit installation) or envelope (if a retrofit boiler) heat loss.
2. Measure the 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 or MATCH THE OLD BOILER SIZE! Measuring radiation has nothing at all to do with the size of the new boiler. If you size the boiler from the amount of radiation in the home the boiler will be oversized or grossly oversized most of the time. Oversizing the boiler will always waste fuel. The boiler will short cycle and decrease the efficiency of the boiler. If you are installing a high efficiency boiler and the boiler I oversized you will lose the most efficient part of the operation, minimum fire will not go low enough. See Don"t Oversize HE boilers under the High Efficiency Boiler Tab
3. Boiler design temperature - maximum boiler temperature required to heat the home at the lowest OD air 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
Note: Not all ODR controls will have all the parameters listed above.

Let's determine how to get the above information

1. We can help you with the heat loss. You can contact us on the home page or heat loss tab
2. Measure the amount of radiation. You will need to calculate the heat output from your system. Measure only the element of the copper tube baseboard not all the piping within the cover. 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 in our CHARTS & GRAPHS tab to help calculate the water temperature required for your system. We can help with this also if purchase a PREMIUM heat loss. That will give btu output per sq. ft. of radiation. We than use another set of charts to calculate water temperature.
2a. Hydro air systems the minimum water temperature should be 130º to 135º f and the maximum will normally be about 160f - 180f.
2b. If the system is coppertube baseboard I would normally suggest a minimum water temperature of 120f to 125f and a maximum of 160f - 180f
2c. Cast iron radiation systems the minimum should be 100f to 110f and a maximum of 160f.
2d. All the above water temperatures are general settings. Actual and more fuel savings is dependent on the building heat loss and amount of radiation in the system.
3. Boiler design temperature varies by boiler build. You have cast iron equipment which requires warmer water temperature or less flow due to condensing boilers unless you have a mod/con boiler which are designed to condense. The lower the water temperature required for your system the better choice is the newer mod/con boilerse because the cooler the water temperature the more efficient the boiler operates.
4. The boiler start temperature will normally be the same as the thermostat setting although it may not be. This sets the bottom parameter for the heating curve.
5. The outdoor start temperature will normally be set 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. The outdoor design temperature is the average coldest outdoor temperature 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 and your heat loss.
8. The differential above and below may be adjustable but I normally suggest the automatic mode setting. Not all outdoor reset controls have this parameter
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 more depending on the manufacturer. Not all outdoor reset controls have this option.
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 the outdoor air temperature warms up above this setting. If there is a heat demand from the thermostat the boiler will ignore it. It is usually set between 65f and 68f. If the thermostat is set at 70º and the WWSD is 68º and the outside air temperature is 71º the boiler(s) will not turn on. Not all outdoor reset controls have this option.

Now that we have the information from above we can set the functions in the outdoor reset control and use a reset curve. The following reset curve is set to the examples listed below the curve. We calculated all the information and came up with those values. 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 180f water temperature all the time with a 225,000 btu boiler. The heat loss was about 222,000 btu with no attic insulation. I suggested add 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. Heat loss of 144,235/1600 sq. ft. of cast iron radiation = 90 btu's per square foot of radiation. We calculated the water temperature from this chart.

Also have charts for older (standard) copper tube baseboard.

We removed the old boiler, reduced the boiler size from 225k to 160 btu's to match the new heat loss. We used two 80,000btu New Yorker oil fired boilers and a Taco SR503-EXP multi-zone relay including a Taco PC-702 outdoor reset control.

This is how we plotted the heating curve to set our control.

At the bottom of the above curve is all the information from the calculations. You will see from the example the water temperature at 70f outdoor air temperature is 95f. If the outdoor air temperature dropped to 0f the system water temperature will be 140f. The outdoor air temperatures and water temperatures can all be changed to fit any scenario.

If the temperature today was 25f the reset control would use a target temperature of about 123f for the system supply water 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 without the proper boiler protection

Let's take a look at the outdoor curve from the way it was shipped (factory default) and what system water temps we would have been operating at. 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 as explained above.


Here is a Blank ODR Worksheet (pictured below) you can print out and play with 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.

Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website as it should be reviewed by a professional.