MOD/CON

Proper mod/con boiler sizing

Hats off to Dan H for such a great site and all the other extremely knowledgeable and kind people devoting their valuable time this cause. I am by no means an expert so I'm hoping those in-the-know will educate others like myself about how to deal with a minimum sizing problem I see with mod/con boilers which is likely to affect quite a few installations.

While the virtues of a proper heat loss calculation are espoused repeatedly here for maximum boiler output sizing, one thing not frequently mentioned is that minimum output is important too when multiple zones are used.

Here is an example of the mod/con minimum sizing problem:

Design day heat loss: 120,000 BTU/hr

Number of zones: 6

Turndown ration: 5:1

Boiler input: 30,000-150,000 BTU/hr

Boiler output: 24,000-120,000 BTU/hr

Heat emitters: copper/aluminum fin baseboard

Emitter output per zone: 20,000 BTU/hr @ 180 degrees.


If only one baseboard zone calls for heat, it can only give off only 20,000 BTU/hr. The boiler can only modulate down to 24,000 BTU/hr so it is going to quickly overshoot the desired 180 degree supply temp. This occurs repeatedly until the heating call ends or more than one zone calls for heat.

Although this boiler is perfectly sized for total heat loss it still short cycles. Outdoor reset will make things worse since even less heat is emitted per zone and the cycles will become even shorter.

While I don't have quite this situation, I do have one zone like this and another on the borderline. There are more problems with my situation, but I want to stick to this aspect of boiler sizing so others avoid this problem. This leads to the following questions:

1) What can be done to address short cycling due to boiler output being greater than emitter output in severe cases and borderline cases?
2) Would higher output emitters help the problem or make new problems?
3) What solutions will work with outdoor reset?

Thanks.