Boiler Size vs. Efficiency

[ToddinMaine]

In general, and all other things being equal, is a 100,000 btu mod-con firing at 50% power more efficient than a comparable 50,000 btu unit at full power?

[Dana]

In general, and all other things being equal, is a 100,000 btu mod-con firing at 50% power more efficient than a comparable 50,000 btu unit at full power?

In general, yes.

The primary factor in raw combustion efficiency is the return water temp, but firing rate is a strong secondary factor. The sweet-spot for most mod-cons is about ~1/4-1/3 of full-fire. Below about 1/4-fire the turbulence on the fire side of the heat exchanger drops dramatically & quickly, leading to a laminar-flow, increasing the stack temp. At full-max the turbulence is good, but the velocity is higher, with greater volume of gases blowing by more quickly than the water can condense out. Different heat exchanger designs will have different firing ranges where the efficiency peaks, but no mod-con heat exchangers are designed for peak efficiency at full-fire, since the design-presumption is that it will be modulating at part load the vast majority of the time.

It looks roughly like this:



With suitably low return water temps for condensing that difference is less than 5% though, and with return water higher than condensing temps that difference is under 1%.

[jadnashua]

Keep in mind, though, that most places do not need the max very often, and you want to select the boiler that will be in the condensing phase most of the time. A big boiler that can't fire down low enough, which is likely the majority of the time it's running, won't be as efficient overall as a smaller one that only occasionally needs to run at max, but can better meet the more moderate needs the majority of the time.

[Dana]

Keep in mind, though, that most places do not need the max very often, and you want to select the boiler that will be in the condensing phase most of the time. A big boiler that can't fire down low enough, which is likely the majority of the time it's running, won't be as efficient overall as a smaller one that only occasionally needs to run at max, but can better meet the more moderate needs the majority of the time.

Jim, he already understand (or should already understand) that from previous discussions. He currently has a boiler 3-4x oversized for his whole-house load, and GROTESQUELY oversized for the radiation on his smaller zones. He's currently programmed that boiler to limit to 50% fire.

The question he maybe WANTED to ask was whether running this boiler programmed that way would be more efficient than using a boiler with half the max output, the answer to which is "ABSOLUTELY NOT".

But the question he actually asked in this post is predicated on "...all other things being equal...". The steady-state combustion efficiency at equal water temps, radiation, return water temps tips in favor of the larger boiler. But short-cycling the larger boiler rather than modulating a smaller boiler eats up WAY more efficiency than can ever be gained (except with a fairly large buffering thermal mass.)

With his current radiation and method of operating he can never run in condensing mode, so the combustion efficiency difference is essentially the 0.5-1% range somewhere a bit below 90% raw combustion efficiency. But short-cycling the larger boiler throws away more heat per cycle than with the smaller boiler (larger heat exchanger= larger losses during flue-purges, larger burner = larger losses during ignition cycles.)

[BadgerBoilerMN]

It depends.

If the unit is full out at acceptable return water temperatures I like the certainty of sealed combustion, direct vent combustion, as conditioned air is not be ejected nor free outside air introduce to the living space. The ModCon will normally operate in the 88% combustion range while working at the hottest design temperature around 180°F. This is better than the best atmospheric boiler and the difference in stack temperature is beyond debate 450° vs. 150°F conservatively.

However, if the unit does modulate and shutdown in cycles below 5 minutes, combustion efficiency will suffer as DANA suggests. Electrically draw increases as well and the cycle life of the various system components will be shortened.

The worst mistake one can make when specifying a condensing boiler is to assume modulation gives you a pass on doing a proper heat load. As is born out by Dana's graph the real savings are in those low output, low temperature modes.

A boiler too big for the space, is more bitter than death.

A condensing boiler is properly sized for maximum output and minimum output according the the block heat load of the structure it will serve and the minimum output for the zone that is both the coldest (highest heat load per square foot) and the smallest relative to other rooms. For instance, on a recent remodel an expensive sun room was installed some ten years ago and the owner-in Woodbury, MN-found the new "sun" room quite cold in all but sunny weather. In fact, for three months out of the year no one could be in the 62°F room.

Since the system was forced air and three registers, improved sliding glass doors and extra insulation in the crawl space and sub-floor had no effect drastic measures had to be taken. The problem of course was that the sun room presented the largest load per square foot, needing more heat that the rest of the house in all weather conditions, save sunny mild days-not common in Minnesota. The answer was to radiate the floor beneath and using the existing water heater as heat source. The water heater serves this micro-zone best by virtue of its relatively high mass allowing 3800btuh to be delivered to the space without short-cycling even the first 75 m stage of the 150 mbtuh furnace.

In the perfect hydronic design a modulating, condensing boiler with properly tuned out door reset would never shut down on any limit and easily reach its 98% potential combustion efficiency with no penalty on the system side, save circulator operations and pipe loss.