Modulating - Is a 80Mb burner at 100% burner less efficient than a 100Mb at 80% ?

Dave Carpentier

I've seen some graphs that show an increased efficiency % as the burner modulates downward. If you compared an 80Mb unit burning at 100% fire, to a 100Mb unit that was software-capped at 80% burn, would there be a difference in fuel used if your heating system was steadily consuming that 80Mb of heat ?

If they were both 10:1 turndown units, the capped one would only have a reduced turndown to play with (80% to 10%) whereas the 80Mb unit would have its full range (100% down to 10%).

In_New_England

Theoretically, all other things the same, yes, that is what typical graphs show e.g (from here):




But I note that efficiency is dominated by the return water temperature, i.e. if the boiler can condense or not. So firing rate is efficiency icing, condensing (or not) is the cake.

mattmia2

It has a whole lot more to do with your return water temp. The lower you can get the return water temp, the more water vapor it will condense out of the products of combustion and the more heat of vaporization that will transfer to your house. Condensing is considered to start around 130 f return water temp.

There is more surface area in the larger boiler so you get a little more transfer, but if you end up cycling more on small loads you will lose what little difference in efficiency you gained with using the larger hx.

hot_rod

Viessmann has some great data on that topic. Generally a larger HX with a small firing will be most efficient as you expose more surface area to a smaller flame and should see more condensation, . Assuming you are running in condensing temperatures. 
Check the Coffee with Caleffi archive on you tube, Jody Samuls did a webinar on this topic a number of years back. I think you will find the Viessmann slides there.

Hot_water_fan

How much time will it ever spend at 80kBtu? A day/year if sized for exact load? 

Dave Carpentier

Wow, Caleffi is such a good resource.
I didnt find the exact video that mentions the issue, with Jody Samuell yet, but I did learn some other stuff..

-When I have my boiler setup this summer, the tech is going to have to come back with the combustion analyser in the winter.
-Dew point for condensing changes based on multiple things including the temperature and humidity of the outside air for combustion.

@Hot_water_fan : The design temp here is -21f (-30c) but it does get into the -30's now and then (-40 is the coldest Ive seen here). At the design temp, my calc is 70k. The unit Im (most likely) getting is 92Mb (80 into the water per AHRI). So I was considering soft-capping it at 90% burn (maybe 80% someday, via trial and error). Its a 4" slab (6" in the attached garage), so it's got some thermal mass, but I have no idea how its going to do)

hot_rod

this isn't the exact webinar Im thinking of, but good info on mod con combustion.

https://www.youtube.com/watch?v=N5qpYSqkbLE

Dave Carpentier

Ya, watched that one. Very cool explain on the chemistry of combustion. I wonder if someday these things will have an oxygen sensor in the exhaust stream, steering a trim procedure for the air under various conditions, sort of like in an ICE vehicle ?

Adding - So if a boiler was set up in warm weather, it could have far too much excess air in the middle of winter. But if it is set up in the middle of winter, it might suffer from incomplete combustion in the shoulder season ? Is the trick usually to setup at the start of heating season for "just enough" excess air and then just live with the extra excess air (and loss of eff) mid-winter ?