heat loss up flue with boiler off

Dave_61

I have a Lochinvar EBN 300 (Efficiency +) boiler. The fan is actually inside the boiler jacket and pulls fresh air from outside the house and pushes it into the burner chamber.
With the boiler off, the stainless steel vent is somewhat warm but not hot (and some of that could be just normal cooldown from when the burner had run).
In this situation, if we're not losing much heat out via the vent, I would probably not repipe my system.

What do you think?

Dave_61

heat loss up flue with boiler off

We have a direct vent copper tube boiler to which we added an indirect as a buffer tank as the boiler was constantly shortcycling and overheating. Currently, the buffer tank is in series with the boiler so that when any individual zone calls for heat, the boiler pump also must run and water is carried from the buffer tank through the boiler heat exchanger with the burner off until the temp gets to 150 and then the burner fires. We have had no issues with inlet temps below 140.
We are probably going to do some repiping in the spring so that the boiler pump does not have to run when any zone calls.
But I was wondering how much heat is lost up the flue when the hot water is passed through the heat exchanger at 25 gpm? It seems like the burner does not come on for 15-20 minutes at a time, and the stainless steel flue is warm, but not hot to the touch. Is there really a significant amount of heat/efficiency lost by running the water (175 degrees) through the heat exchanger with the burner off? The system works great otherwise.
Thanks.
Dave

hr

A bit like a cooling tower

circulating hot water through an unfired boiler. Especially one without a draft damper. Heat transfer is a two way street. Hot will always go to cold.

Generally cold combustion air falls to the floor and gets vented through the flue pipe to the roof. As it passes through that boiler HX it takes a portion of your fuel dollars along with it

That's one reason PS piping is stressed for efficient buffer tank connection.

hot rod

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

Brad White_34

Hot Rod is right

as usual. An unfired boiler with continuous circulation is a reverse heat exchanger. If I were a bird with a cold butt I would be sitting on top of your flue outlet right now...

John Mills_5

Direct vent

I wouldn't expect much heat lost out the flue of a direct vent boiler. It would be far from the old natural draft boilers with no damper that pour heat up the chimney in the off cycle.

Dave_61

John,

It is direct vent but there is no damper in the flue. It just has an induction fan that pulls fresh air in.

John Mills_5

Model?

Usually they use a draft inducer, the fan is in the flue and little heat is lost through it. There will still be some heat lost but minimal compared with the chimney vented boilers where the chimney sucked air 24 hours a day through the boiler.

Dave_61

Hot Rod,

do you agree that there isn't nearly as much heat loss through a direct vent application with burner off?
I'm trying to decide whether to repipe or not.
Thanks.
Dave

Dave_61

Hot Rod,

do you agree with some of the statements in the thread below that there isn't nearly as much heat loss through a direct vent application with burner off? I'm trying to decide whether to repipe or not. Thanks. Dave

Weezbo

Dave ,*~/:)

here is something that i have done and it totally convinced me that indeed there is a tremendous amount of parasitic heat loss in many instances through the HX and out the stack.

I stuck a basketball in the outside flu, lashed up an electric water heating "tool" and collected numbers for a month...

Stack sensors and intake and exhaust dampers pre and post purge could go a long way to reduce these losses. Insulating the stacks to a higher degree than code to a min standard of your own might also increase the longevity of these stacks.

hope that helps

Dave_61

Weezbo,

I'm confused. You say you put something (basketball) in flue...how did you run the boiler with flue blocked?
Would I be able to put damper in flue without causing a problem with my boiler? I could just put it to open when intake fan comes on. What do you think?

hr

That boiler can be vented

in a number of ways. if it is a vertical vent (conventional negative draft system .02-.05 inches negative) probably more loss than the sidewall configuration.

hot rod

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

Weezbo

Oh Good !

Now er.... running a boiler with a basket ball in the flue might be a bit of a CO issue

Howsomever an electric boiler might be an eye opener.

Hot Rod has spun a number of these smaller systems with electric water heaters take a real real good look at some of the installs. a search under Electric boilers on this site will reveal many very sharp reasonings and rational behind the usefulness of a better use of those "errant" BTU's *~/:)

Howdy Hot Rod caught your post after plinking in a reply

Yup Dave. do yourself a huge favor though and ask some questions of some service out fits in your area as to the whys and where fores of venting and its effects on stacks.

Dave_61

The boiler is vented

out the sidewall currently. The flue travels out of the top of the boiler for about 5 feet and then makes a 90 degree bend and goes about 4 feet before terminating out the sidewall.
If this does not cause large decrease in efficiency with hot water travelling through HX with burners off, I may leave it vs risk having other problems with water flow if we repipe the house returns teed into the boiler supply.
I plan to have the plumber measure the head in the main loop (we have a circuitsetter with test ports in the primary/boiler loop)and this will give us an idea of the backpressure (if that's the correct term) that the zone returns would have to work against if we repipe.
What do you think?
Thanks again.
Dave