Is this correct?

Kevin_54

Does a 1 degree tempature drop in a boiler represent a savings of 3 BTU's? I thought i heard this once but i can't remember where.

Brad White_9

Only if

you are dealing with three pounds of water and you are only speaking of net BTU's available after combustion.

Otherwise I am not really sure of the context.

Kevin_54

radiant

It would be a radiant system running @ a lower tempature using a condensing boiler. So running the boiler @ say 120 opposed to 180 just trying to figure out what kind of potential savings.

ALH_4

Chart

Does this chart help? It's not linear.

Steve Ebels_3

It's been said.......

The folks at a Viessman class I attended referred to the fact that for every 3 degree drop in boiler water temperature, below 140*, you pick up 1% in boiler efficiency. I think Andrew's chart bears this out. Low temp systems with condensing boilers ROCK!!! Nothing beats looking at exhaust temps that are 10-20* LOWER than your supply temp.

Mike T., Swampeast MO

This is a digest of information--theoretical, manufacturer supplied and as recorded in my "wired" system.

Using a condensing/modulating boiler

Presuming 120F supply temperature at design load and presuming some form of reset and presuming a well-sized boiler you can sincerely count on 98% gross system efficiency for the heating season as a whole.

If the boiler and all piping are in the conditioned envelope, you can consider seasonal net system efficiency very close to 98% as well.

Significant daily setback is really the only way to reduce this seasonal efficiency as it will require higher water temperatures and/or higher boiler output during the recovery phase. Granted there is a savings from reduced load from lower room temperatures during the setback phase, but my personal opinion and measurements (substantiated here by someone else with a "wired" system) are that these essentially cancel each other out and there is no net savings. My personal opinion and measurements (not yet substantiated here) is that not using setback results in a savings if you maintain space temperature lower than the high setting when using setback.

Using a conventional boiler with a fixed operating temp of 180F in the same system

If you achieve 70% gross seasonal efficiency, consider yourself to be among the finest hydronic engineers--and pray that the always problematic vent damper isn't abandoned...

More typically, you can expect 40% - 60% gross seasonal efficiency. I say with absolute sincerity that the more the customer will try to save with such a system, the lower the gross seasonal efficiency will be. The fuel savings will still outpace the efficiency hit, but not by much!

If you operated the condensing/modulating boiler at a fixed temp of 180F in the same system

Yes, it's possible even if it is rather stupid. You could expect a gross seasonal efficiency in the range of 85% - 92%. My personal opinion is that efficiency would wind up nearer the low end of this range than the higher.

-----------------------------------------------

Yes, I know these numbers are at complete odds to AFUE. Condensing/modulating boilers and conventional boilers simply cannot be compared via AFUE. Search here and you will find innumerable reports from both homeowners and contractors to verify.