When installing a new boiler, furnace, air conditioning system or a replacement appliance it must never be oversized. With the growing popularity of mod/con boilers which modulate the fuel input up and down as determined by the supply and return water sensors, setpoint temperature and software in the control. Most of the circulators used today are a single speed circulator which will always flow the same amount of water through the boiler at all times. If the flow through the boiler is the same all the time the only difference in heat output is the fuel input. With these boilers the fuel input will change as outdoor temperature changes requiring different system outputs based on water temperatures. The amount of modulation is dependent on supply and return temperatures and control software which will calculate the water temperature to send to the radiation to match the heat loss. The more heat emitters you have and the larger the water volume the better the application of a mod/con boiler.

These boilers should operate with an outdoor reset (ODR) control that includes ODR will reduce the system water temperature as the outside air temperature increases. These boilers are normally piped primary/secondary (p/s). As the fuel input changes the temperature of water out changes. For example, if you are inputting 80,000 btu's into 8 gpm each gallon will absorb 10,000 btu's per gallon per hour. If the boiler has a 4:1 turn down, which means divide the input by the turn down (80,000/4=20,000), while the flow is still 8 gpm each gallon now absorbs 2500 btu's per gallon per hour. This is great if all the water carrying these btu's would all go out to the system and dissipate these btu's in the home. With multiple zones in the home each zone is smaller than the boiler size. A small zone would not have a problem getting rid of these btu's.

Let's oversize the boiler due to not doing a proper heat loss. Let's say the new boiler is sized off the amount of radiation load or installing the same size boiler that will be removed. The same system as above now has a 150,000 btu boiler with a 5:1 turn down (150000/5=30,000) instead of the 80,000 btu boiler. This boiler requires somewhere around 15 gpm of flow. On low fire each gallon absorbs about 2000 btu's as it passes through the boiler at the lower input. This sounds like there is no problem. There is due to high flow needed by the boiler. The higher flow through the boiler and lower flow in the system causes the boiler to short cycle thus reducing the operating AFUE efficiency.

We will end up with reverse flow between the closely spaced tees on the primary loop. This means we pre-heat the return water. As warmer water enters the boiler internal water temperature goes up quicker and shuts the boiler down quicker. If the water temperature changes fast enough the fan is searching all the time for the proper RPM. It will cool rapidly and run for a short cycle again. The shorter the cycle the less efficient the operation becomes. As the boilers increase in size the required flow through the boiler increases. As the flow required for larger boilers is applied to smaller systems or worse yet oversized boilers on a zoned system to reduce the flow even further the possibility of short cycling becomes imminent. I have seen mod/con boilers hitting high limit so fast they do not have a chance to modulate. I also see many units installed without connecting the outdoor reset control.

With primary/secondary piping water can flow either direction between the tees or have no flow between the tees. The diagrams below show what happens with flow and temperature in the boiler piping as boilers become more oversized. The numbers in are increased as the boiler continues to run. We are assuming a heat loss of 60,000 btu's. The system requirement is 6 gpm. We are assuming all zones are running or there is a single zone in the home. If the home is zoned and only one zone is running the conditions get worse. As we approach the setpoint temperature and the boiler stars modulating back the conditions get better. Most if not all mod/con boilers you can access a display that shows number of cycles and runtime hours. If you have this menu option you can determine the average run time. To calculate the average runtime, us this formula.

Average runtime minutes = (runtime hours x 60) /# cycles

A good time would be 20 minutes or more. If you are too short of a run time the boiler efficiency is affected and you may see pre-mature parts failure. To change this runtime, check the control to see if you can change the maximum fan speed. if so it can be reduced to improve modulation rate and produce slower temperature rise. Unfortunately, you probably will not be able to reduce the minimum fan speed which is the lower end of modulation.

The following diagrams show boiler at maximum fan speed which is where you will be with larger delta-T's normally at start-up on each thermostat demand.