Garn Flue losses?

[Swamp Yankee]

Hello, I'm new to the forum. Well, new to posting anyway.. I have been reading the posts on here for a while. I have a Garn 1500 installed for two heating seasons now and am pretty happy with it. The other day when it was very cold I borrowed a FLIR thermal camera to check the garn shed and house for losses and was a bit surprised at the warm spot that could be seen at the garn exhaust. It makes sense that there would be some convection through the tank, but I never thought about it before. So, it got me thinking that I there could be losses through the flue I hadn't figured on.
Anyone tried to quantify the losses out the flue?

I did a back of the envelope calculation: Q=mdot*CpAir*DeltaT where Q=heat flow, mdot=mass flow rate in lbs/min, CpAir=Specific Heat Air, delta T=difference in temp. See screenshot of spreadsheet attached.

I don't know what the flow rate would be through the flue.. so I figured maybe 1 cubic foot per minute? Anyone have an idea what that might be? Eh.. I could just be obsessing over losses too much?

I also added a row for how much I'm losing per minute and per hour if I set the blower timer too long. I think you guys are on the right track getting an auto-shutoff on the blower when the burn is complete.

Any thoughts?

 

[Jim K in PA]

Welcome to the Boiler Room, Swamp Yankee. Good to see another GARN-aholic here.

Were you taking readings with the draft fan off or on? I am presuming off. I have never used a FLIR, so don't know what it's capabilities are. Can it differentiate between radiant heat and actual warm air flow? Could you have perhaps been reading the radiant standby loss from the flue pipe rather than actual convective loss?

What is the GARN CFM rating for the draft fan? Is that the 750 figure you have at the bottom of the table?

Fan off draft is probably quite low, probably well under 5 (as you calculated) based on purely unscientific (hand under the elbow) measurement on mine.

Tom Caldwell has installed a turbulator in the last run of his flue and has shown a measureable increase in heat transfer/drop in final flue temps. That is something that I want to incorporate in my 2000. But now you have me wondering if the turbulator might INCREASE standby losses through the flue when the fire is out.


Were you thinking of a damper of some sort? I am not sure what would last long in that environment without failing/sticking (closed, of course-Murphy loves wood boilers).