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Does it matter if the newer Modulating/Condensing (Mod/Con) boilers are oversized?

When installing a new boiler, furnace or air conditioning system it must never be oversized. With the coming of mod/con boilers which will modulate the fuel input as determined by the supply and return water sensors and software in the control. The majority of the circulators used today are a single speed circulator which will move the same amount of water all the time. If the flow through the boiler is the same all the time the only difference in output is the fuel input. With these boilers the fuel input will change as outdoor temperature changes requiring different system water temperatures. The amount of modulation is dependent on supply and return temperatures and control software which is dependent on how many btu's the heat emitters can get rid of. 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 control which will reduce the system water temperature as the outside air temperature increases. These boilers are normally piped primary/secondary (p/s). The boiler circulators are always moving the same amount of water. As the fuel input changes the temperature of water out changes. For example, if you are inputing 80,000 btu's into 8 gpm each gallon will absorb 10,000 btu's per gallon. If the boiler has a 4:1 turn down, which means divide the input by the turn down (80,000/4=20,000), now the flow is still 8 gpm each gallons absorbs 2500 btu's. This is great if all the water carrying these btu's would all go out to the system and dissipate these btu's. 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 heat loss, sizing 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. It sounds like there is no problem. There is due to flow required 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 warmer water will reenter the boiler so the boiler water temp goes up quicker and shuts the boiler down quicker. It will cool rapidly and run for a short cycle again. The shorter the cycle the less efficient the operation becomes. As the  boilers increases in size the flow through the boiler increases. As the flow required for larger boilers is applied to smaller systems or worse yet zoned systems to reduce the flow even further the possibility of short cycling becomes imminent. I have seen mod/con boilers hitting limit so fast they do not have  a chance to modulate. I also see many units installed without connecting the outdoor reset control.

In this drawing with proper sized boiler with lower flow required the flow all the hot water flow is leaving the boiler. If there is more flow in the system than boiler requires some system flow moves between the tees. If the flow is equal between the boiler and the system there would be no flow between the tees.

This drawing represents an oversized boiler and the system flow is less than the boiler required flow. Some of the hot supply water is going back between the tees, blending with the return water temperature and the returning water temperature is increased.

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