Exhaust Flue Heat Exchanger? Idea I had - what are your thoughts.

[nvr-enuf]

I have a Napolean NZ26 fireplace - I have this hooked up to my cold air return on the furnace therefore it pulls air in reverse: sucks air out of the top vent and goes into 2, 8" duct work pipes to the furnance to distribute heat throughout the house, works pretty good. Well, I have an addition going on the house (14 by 20') in which I was going to install the pex tubing in the concrete. Here is what my thought is - what if I build a 2' tall exhaust pipe that instead of the foam, I had a pipe with a 1" cavity for water? I run the duravent pipe that is 6" ID and 8" OD - the 1" in between is a foam filled pipe. The mfg stated the exhaust temp is typically between 500F and 800F. I do not need a lot of heat transfer - my desire would be around 5000 btus, maybe a little more. My thought was to use a hydronic water pump into the concrete slab and then into a radiator with a 12volt fan system on a thermostat switch therefore it can disperse the heat if the water temp would get too hot. The radiator would also act as a pressure relief system. I understand I will be cooling the exhaust temp - I do think the upper pipes will maintain temp thus I do not believe I will have draft issues. I tend to run the fireplace on medium to high heat settings.

Any thoughts or suggestions on existing parts out there especially the flue exchanger would be helpful. I am not too keen on drilling holes into my firebox that is why i am leaning towards the exhaust flue water / heat exchanger.

Thanks
Scott

 

[Hobartian]

In these situations sometimes you have to 'bite the bullet' and give it try. However, I have serious doubts that you will extract very much heat as when you dramatically cool the flue by encasing it with a water jacket it will attract and condense any creosote that was not burnt in the fire. That accumulation of creosote will impede efficient heat transfer and of more concern will be a great fire hazard.

Have you had a look at proven commercial designs to see how they work?

 

[Duetech]

Hobartian is in line with this one. Condensed creosote attracted to the cold spot will act as an insulator and impede thermal transfer. It will also "bottle neck" your units exhaust and could hamper proper draft and cause a back pressure. When starting a fire or when the blaze begins to diminish the overall exhaust temperature will not be sufficient to keep the chimney at a temp that prevents condensation so you will already have two circumstances generating the need for periodic cleaning. A cold spot in the chimney will just accelerate the need for cleaning and the eventual unsafe condition of a semi-clogged chimney. In some fireplaces there is often a draft that flows down a chimney to help supply air for the fire in the fire place when the home is fairly well sealed and tight and that cold air will also be a factor in creosote build-up. If less than ideal low moisture content wood (well seasoned) is used in the mix the potential for dangerous creosote buildup and chimney fires spikes dramtically. There was a product called "magic heat" I think that insurance companies favored the removal from use and even relegated to a no-no when issuing policies because of it's tendencies clog with creosote and it only used air for heat transfer and not water.

 

[Singed Eyebrows]

The boiler needs to be in the insert, not past, cooling down the flue gas. Can you make some sort of a tank that will go inside? When creosote forms you can just scrape it off. The tank will pick up radiant also & help with getting a decent output, Randy