Watercoil in stack of OWB

[hondaracer2oo4]

Seeing as I have a Hardy OWB and the stack temperature is pretty high, wasting alot of heat warming the outdoors I was thinking about a water coil in the stack that would act much like a sidearm heat exchanger using natural thermosyphon to move the water through the coil. You would simply drill a hole at the bottom and top of the stack where the coil would come out to head to and from the boiler. The coil would run up the walls of the chimney extracting the heat that is going out. It would work much like those portable hot water pressure washers that basically are just a barrel with a coil that goes up with an oil burner at the bottom. You would run each end of the coil back to the boiler. What do you guys think? Anyone thought or seen something like this? It would be pretty easy to make up a prototype to see how well it worked. The only thing that I can see being an issue is the creosote which would get all over the coil and be difficult, if not impossible to clean off. I get very little cresote myself and any that I get is not sticky and is easy to brush off. Just thought it would be a simple yet effective means to extract more heat from the unit. Let me know what you guys think!

 

[atlarge54]

Personal opinion and worth exactly what you're paying for it--------don't waste your time. The gains won't be worth the efforts expended.

 

[1grnlwn]

Sounds like a good place for creosote to collect.

 

[leon]

water heater coil

About your hot water coil,

The problem is both heat loss and creosote build up.

You require a very large amount of bent pipe/tubing to acccomplish
some level of heat absorption. The other huge issues are metal fatigue
and simple corrosion.

The gentleman I pirchased my wood and coal boiler
from made and may still make a heat exchanger that
is a square water filled or dry box with a 4 pipe exhanger that
is added to the rear exhast of his wood and coal boilers and
pulls that much more heat from the chimney exhaust through
the flue pipe and into the water in the heat exchanger.

Assuming your water stove is a forced draft type the size of the
exchanger will not be a problem and all the heat lost now will not be
lost after you install a horizontal heat exhanger.

He made these exhangers up tro 20 feet long with a four tube system.

You would need to make a trestle brace system as you are going to have
a lot of water weight and steel weight and install an elbow to direct the
flue gas exiting the boiler through the heat exchanger and then to a
small vertical stack and rain cap.

The 4 tube heat exhanger has a cleanout plate in the end that allows
you to brush the 4 pipes clean when ever it needs to be done.

The man does quality work and I still have the last inside the house
wood and coal boiler in service that he made in 1982.


This gentlemans name is

Gary Switzer of Switzer Furnaces in Dundee, New York

 

[hardy steve]

My H2 already has one around stack.It sits right on top of fire box in water box.Doesnt yours have that?

 

[applefarmer]

Seeing as I have a Hardy OWB and the stack temperature is pretty high, wasting alot of heat warming the outdoors I was thinking about a water coil in the stack that would act much like a sidearm heat exchanger using natural thermosyphon to move the water through the coil. You would simply drill a hole at the bottom and top of the stack where the coil would come out to head to and from the boiler. The coil would run up the walls of the chimney extracting the heat that is going out. It would work much like those portable hot water pressure washers that basically are just a barrel with a coil that goes up with an oil burner at the bottom. You would run each end of the coil back to the boiler. What do you guys think? Anyone thought or seen something like this? It would be pretty easy to make up a prototype to see how well it worked. The only thing that I can see being an issue is the creosote which would get all over the coil and be difficult, if not impossible to clean off. I get very little cresote myself and any that I get is not sticky and is easy to brush off. Just thought it would be a simple yet effective means to extract more heat from the unit. Let me know what you guys think!

This idea has came up a few times now. If you have a easy way to make a prototype TRY IT! Then make sure you report back with your results. Sure would be nice to catch some of that heat going out the stack!

 

[hondaracer2oo4]

I think I could make up a protype pretty easily. You guys think I could just use a coil of copper or I would have to go with SS. A 60 ft piece of 1/2" copper is about 100 bucks. Just not sure if using thermospyphon would work fast enough to move the water to keep it from flashing to steam on the way through the coil since the stack temps can get pretty high! I was thinking about putting a 50 gallon drum on the ground and just plumbing the coil to that and seeing how it works. I just feel like I am shooting alot of heat up the stack which is still recoverable. Im just surprised no one else has attempted this yet.

 

[atlarge54]

Does your boiler have a blower for combustion air? I'm running (7 years now) a modified Global Hydronics unit and the very first time I saw it operate I disconnected the blower. All the aquastat does is open the air inlet.

My neighbor and I are lifetime woodburners and when I lit the boiler with that blower running it was truly an OMG moment. It's just crazy what that blower was sending out the stack.

 

[hondaracer2oo4]

Yes I do have a forced draft blower. I don't think that just opening the small flapper door on the fan which opens and the fan turns on when the aquastat calls for heat would be enough to pick the fire up to recover the water temp fast enough. The water jacket is only 130 gallons. I am heating a 210 year old house 2500 sf house with minimal insulation. My return water temps are 18 degrees lower than the boiler temp and my FHA in the house runs pretty steady anything below 30 degrees out. Even with the forced draft fan running it sometimes has a hard time keeping up.

 

[atlarge54]

Ever considered reducing your heat load? Exactly how many of those 2500 sq ft really NEED heated all the time? Can you close off any rooms that don't get used? Maybe time for buttoning up the place. Insulation is the gift that keeps on giving. Maybe even a setback T-stat. Ever consider installing some floor radiant in areas with good access, it's a wonderful way to heat.

Back to the original question------I have experimented with similar ideas in the past and doubt that the effort required will be justified.

 

[hondaracer2oo4]

This is our first year in the house so I have not got a chance to start insulating yet. I plan to do it room by room, it is a historical house with all of the original features so I plan to keep all of the features and do things right. Buttoning up and insulating are at the top of the priority list. I just think that it would not be alot of effort to make a coil in the stack and run it back down to the boiler and could have significant heat loss recovery from it. Not sure why everyone is passing this off as little gain since the heat going out of the stack is in the 300 degree range.

 

[applefarmer]

Goggle "aquastack" it seems the company is out of business but they have some pictures online. If your boiler has another set of inlet outlet ports, my idea was to run another pump dedicated to the "stack" tied into the fan. That way your not pumping water when there is no heat coming out.

 

[rossman97]

I know of a guy that makes em here in wi. Heres the link to his website Bauer Mechanical - Home I know he does quality work although i dont have any first hand knowledge on the waterstack

 

[hondaracer2oo4]

Aquastack | Wood Boiler, Outdoor Wood Boiler, Indoor Wood Boiler

Hmm looks like you can still buy them! $275 is actually a decent price too.

 

[applefarmer]

Good luck getting them to answer the phone! If you get a responce let me know please.

 

[Felman]

Considered downsizing the firebox by adding firebricks?

 

[hondaracer2oo4]

No real good way for me to add firebricks since the boiler is SS, I can't weld any channels easily onto the sides of the firebox to hold the brick in. Also that still wouldn't recover much of the heat that is going up the chimney, but it would keep the boiler water at temp longer and absorb some more of the heat extra heat.

 

[NC4AB]

If you want to add a coil inside the stack it needs to be Stainless, copper won't stand up very long inside the stack. Also you will have potential to plug the stack with creosote when you cool the temperature down.

I also had thought of ways to reduce the stack temperature and have had pretty good success with using a two stage RANCO controller to operate the fan and damper separately, which helps with stack temperature / wood consumption under low load conditions.

Operation is pretty straightforward, when the water reaches 170 the damper opens up and if it can't maintain the temperature the fan cuts on at 160.

If you are marginal on heating capacity on the Hardy then this option won't do you much good.

 

[hondaracer2oo4]

I do like the idea of opening up the damper first at 170 and then kicking the fan on at 160 if the damper was not enough. That would work for me on days with temps around 30 or so but anything below probably would not. It looks like the 'aqua stack' really just has two boxed up metal containers which probably have fins on them if I could guess to attempt to transfer heat into the boxes with two nipples to connect to your return line. Im thinking about trying to make one of these things. I just can't believe no one has EVER bought one!

 

[STLfirewood]

Yes I do have a forced draft blower. I don't think that just opening the small flapper door on the fan which opens and the fan turns on when the aquastat calls for heat would be enough to pick the fire up to recover the water temp fast enough. The water jacket is only 130 gallons. I am heating a 210 year old house 2500 sf house with minimal insulation. My return water temps are 18 degrees lower than the boiler temp and my FHA in the house runs pretty steady anything below 30 degrees out. Even with the forced draft fan running it sometimes has a hard time keeping up.

I'm not up on boilers like most of you guys but 18 degrees doen't seem like much of a drop. I run a boiler for my kiln and I get up to a 40 degree drop in my return water. I run my fan across the exchanger 24/7. Once the kiln heats up I get a 10 degree drop. I use a natural draft CB. The natural draft keeps up pretty good. It does allow a little to much temp drop when I'm throwing wet (rained and snowed on) wood in it. That is my fault for not having it covered. If done right I think you have a good idea. The hardy is a nice looking stove that lasts a long time. I know several people who have had them for close to 20 years. They just use a lot of wood and loose a lot of heat out of the top.

Scott