oversizing a modulating boiler?

lolo

Hi everybody,
I would like to find some technical answers.
If a modulating boiler has better efficiencies when it is not running at maximum. Shouldn't it be size to be running normally at much less than its maximum capacity?

What would it happen if a modulating boiler with a minimum of, let's say 45k btu, gets a call for heat from one floor that is only 25k btu?
Wouldn't it just switch on and off every ten minutes or so?? and what's the problem about that? I mean, this doesn't seem to be the same as a hughe cast iron boiler short cycling (loosing energy) problem.
It seems to me that with a modulating boiler it is a better idea to oversize a bit so you get better efficiencies, and have a reserve of btu for an extremly cold day, or to add an small snowmelt in the future (both of them are not use everyday).

Greg Swob

Recall that when sizing

you are sizing for coldest design temperatures. Unless your region gets to design temp. early on in the winter and remains there perpetually, you have in effect oversized for all the remainder of the heating season. Here in my corner of the world, we saw -11F last monday night, today our highs should be in the low 60's. We probably (hopefully!) won't see that -11 again this year, according to the law of averages. Even the higher efficiency condensing boilers take a little while to get to their design efficiency following start up. Albeit, this time frame is immensely shorter than high mass boilers, short cycling is not a good idea. Perhaps not a fair comparison, but example: an automobile with 100K highway miles vs. one with 75K city-only miles- which one is likely to be in better overall condition? Start ups are what really cause wear and tear on most any mechanical device. My $.02 take on the issue of oversizing. Greg

jeff_25

here in wisconsin we heat more days from 60 to 30 than at disign of 0-10 so thats why you use modulating control and or staged boilers or look at the outdoor resets and why we use them jeff

heretic

Modulation

Not sure that your initial premise is correct.

It seems unlikely that a small boiler operating at 100% is [theoretically] going to have a lower operating efficiency than a larger boiler modulated down to operate at [say] 50% to produce the same output. Could be, but not something I have heard about.

You may be confusing modulation factors with efficiencies that come from operating at lower [water] temperatures with condensing appliances. It is generally reported that output temps of [somewhere around] 130 or less are optimal with condensing appliances, though return temps are also a significant factor. It is somewhat difficult to get any good data on this subject. AFUE is not very helpful for condensing appliances.

Guy_5

sizing

Really, you want to size your equipment to fit your highest demand. Think like you are buying a truck- if you will need to occasionally carry 1 ton of equipment, you will need a one ton truck. Anything less either will not fill the need, or will fail prematurely. Either way you won't be happy.
The beauty of modulating is that the boiler will run like a smaller boiler when there is a smaller demand put on to it.

heretic

Guy

Hey, just the man to ask!
To the poster's point, is there any significant difference in operating efficiency depending upon where a Munchkin is operating within its modulation range?

Dave Yates (PAH)

fun stuff

It's been a hoot to observe my Munch as the crunch of bitter cold weather swept over our area. We more than hit design temps last weekend and this coming weekend promises to take us beyond design temps.

Think of it like an auto-pilot that comes with cruise control. My reset curve had to be adjusted a tad, but the Vision 1 control makes doing so almost a no-brainer. With data logging going on at the same time, I can see the results immediately. Turns out my reset ratio used with the Tekmar was close, but off by a few degrees.

Because the modulation means the boiler almost never shuts off, our comfort level has risen and room temperatures don't vary the degree or two they used to when the thermostat was calling all the shots.

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Contractor"

Jim Davis

Modulating Boiler

Modulating boilers are not more efficient, they are 25-35% less efficient when it comes to actual fuel usage. They only produce fictious high efficiency calculations on combustion analyzers because of low flame temperatures and low flue temperatures.. In the 1980's when working with energy auditors and studying fuel bills it was found that modulating equipment always used 25-35% more fuel. Modular will work but not modulating. To get certain large boilers(over 4,000,000 BTU) to lightlow -run high -shutdown low, modulating controls work the best.