Thread: Wood heaters and dampers in flues

I am opening a discussion on the need still to have chimney and flue dampers in wood fires and stoves used for heating purposes.

I understand the need for full air flow and the reasoning behind dampers not being standard in flue kits, but I do not understand why they are not available as extras for those individual situations where they may be needed.

My argument is that when a fire is closed down for the night the lack of a flue damper allows all the hot air to escape up the chimney and out into the atmosphere, thus wasting a lot of the energy and loosing all the stored heat that could have gone to moderating the temperature of the house overnite.

Your opinions on this matter are of importance to me in my decision making and while i have firm views they can be changed by cogent argument with attached facts

Moondog, my woodheater has a damper in it. My problem was the they left off a quarter of the insulation tape on the door to increase burning efficiency (which it did, at the expense of keeping the fire in overnight - fairly essential where I am). What wood heaters are you looking at that do not have these? I am even considering having one fitted to the chimney that will have an open fire in it, so I can close of the flue when not in use, and thus reduce heat loss more effectively than just shutting the door to the room.

The benefit of dampers is not only reducing heat loss but also controlling the rate of burn in the heater - I can not imagine any sort of modern heating system that would not use them - tell us more ...

Modern theory says not to damp the chimney but to regulate the incoming airflow and for the area of the inflow to be greater than the area of the chimney ( as a rule of thumb ) we have an older model Coonara inbuilt and the local Coonara dealer says they are not allowed to put a damper in the flue.

Wood fires do burn cleaner when the chimney is allowed to draw but it is only when the firing phase is over that I want to shut the air flow down.
BC if you remember my thread on adding thermal mass and insulating the chimney, I want to stop the hot air from escaping the flue which I cannot do effectively when all I have is an air intake control

Ah, yes, I can see that problem. I had a closer think about how my heater works and actually the damper stops the smoke from going directly up the chimney but it does so by redirecting the smoke around the body of the heater (to heat up the oven down below) after which the smoke goes up the chimney un-obstructed. So in fact, no chimney damper as you say. However, in redirecting the smoke, the heat is absorbed by the cast iron body of the heater and radiates out on all four sides (with beneficial effects on the chimney bricks immediately behind the heater which heats my bedroom on the other side of the wall).

I am not convinced by the argument that lack of a damper enhances effective burning, however, as the airflow option seems to simply accelarate burning with the result that the fuel usage rate is higher than it should be and there is still a fair accumulation of ash (which should not be happening if it is burning 'efficiently'. Even with the modifications I have recently made the fire still burns wood at a rather higher rate, though I have at least achieved hot coals still present in the mornings, which I was not getting last year. This is in part also due to larger, denser firewood pieces (last years load was mainly branches).

Do you know a handy blacksmith? I know that a few of the locals have fabricated their own dampers to solve their problems with their newfangled woodheaters. Perhaps that is the only solution?

For my open fire I am thinking of getting one of those counter-weighted flaps for the top of the chimney which stay shut until you pull on a chain to open them when you light a fire. Presumably these could be redesigned so as to have a part-open option as well.

My plumber and I have been discussing options for hydronic heating since I missed the boat on installing a wetback on the heater (because my first plumber was a drongo). Options we have considered are installing a water container between the body of the heater and the side wall of the fire place which could at least have a constant supply of hot water for cups of tea, and creating a copper coil around the flue to draw off heat and redirect it around the other rooms. We discarded that second option on the grounds it would cool the flue too much and thus reduce draw and possibly contribute to coke build-up inside the flue - something to consider with any option that reduces the heat going up the chimney too much.

The pot boiler is still simmering away in the back of my head for a rainy day, but at present, the combination of more effectively slowing down the heater for overnight burns and the installation of insulation means that the entire house is now fairly cosy even on seriously chilly nights (minus 2 night before last and snug as a bug all night - may have to retire the electric blanket ...)

A water jacket around a flue is VERY efficient at robbing heat and does lead to a soot and tar build up' but a copper coil is less so and may well work; the cost of a few meters of 12mm copper is worth experimenting with I think.
Those chimney dampers for open fires were on sale here in Melbourne a few years ago but I haven't seen any recently
Heat exchange grates are also worth looking at if you really want to keep the open fire and not fit a closed box insert, if you have a pipe bender it would take only a couple of hours to make, assuming you can scavenge some 50mm pipe in reasonable lengths

Yes, the heat exchange grate is the option I would consider, or lining the hearth with heavy iron plates is another option. The room it is in will ultimately be the dining room but is not terribly large so it may end up being purely ornamental anyway as it would be jolly warm in there with the heater going in the loungeroom next door as well. Keeping my options (and the chimney) open until the room is set up and I can see how it will work. Behind that fireplace is another with a cast iron fire surround which I would quite like to keep viable, so I may yet split the chimney in half to enhance 'draw' for the little coal grate of the iron fire (in the sewing room and thus far the coldest room in the house) and seal off the open fireplace altogether just using it for a dry flower arrangement or something equally awful ...

Originally Posted by Black Cat

seal off the open fireplace altogether just using it for a dry flower arrangement or something equally awful ...

please not, not the pretty dried flowers, I'd be using it to put a sub-woofer in LOL. But if not being used close it off to stop the cold draught, if properly made originally the chimney should already have two flues or at least be already divided down the center

Originally Posted by Black Cat

(minus 2 night before last and snug as a bug all night - may have to retire the electric blanket ...)

Minus 2 in April?

I take it you're up on the Central Plateau or somewhere like that?

Anyway, it took me a while to realise that my wood heater (Saxon) doesn't have a damper on the flue. There's two knobs on the front, but both of them control inlet air only. Until I investigated it, I had assumed that one of them was controlling a flue damper.

I had a pot belly stove in another house. It had a damper and just as well too. Load it up with wood, leave the bottom air inlet wide open and that thing would end up literally glowing red hot and making an incredible roaring sound too. The damper did help in bringing it under control at times, though care was needed otherwise any back pressure would simply blow the top lid off the stove.

I'm thinking that there's probably a reason why wood heater manufacturers don't fit baffles. Maybe they would burn out too quickly due to being in the path of the fire?

That's a good point smurf, but i thought it had to do with clean burning, where I would want to fit the flue damper is towards the top of the flue where it exits the chimney.
In this instance I would be using it to stop stored heat from flowing up the flue and escaping.

Cast Iron Flue dampers are available on Ebay, as are hot water coils for your wood fires. My Stove has it all built in which makes life easy!

Cheers,

Stork

Are they?/
I could not find any assemblies at all, although there are some old style inserts available from the UK and USA.

I'll have to keep looking

While we are talking about wood heating; does this ebay seller represent value for money??

WOOD HEATER AIR TRANSFER KIT Open Fire 3 rooms B/NEW | eBay

I am convinced that an air transfer system is the best for our situation but Cecile less so, our Coonara is in the smallest room of the house and it can get too hot when close to the fireplace even when it is damped down

I have always had my doubts about these air transfer systems as well. Unless boosted by a fan there is no reason why hot air would suddenly start falling into other rooms from ceiling vents. I'm still a firm believer in having doors so you can direct the heat to the rooms in use, and keep it out of the rooms you want to remain cool. Even the curtains I am temporarily using while waiting to get round to repairing the doors that came with the house are very efficient in that respect.

All the transfer systems I am looking at are fan boosted, doors and curtains are great at keeping heat in a room but opening a door or curtain does not let the heat escape the room with the fire in it quickly enough, Cecile likes to have the chill taken off all the rooms but does not like the feeling of being close to a blast furnace, which is how she once described the heat from the Coonara

The air transfer kit would be one of the best things I've ever done around the house. Love it.

Wood heater is in the middle of the house in the kitchen / dining and I've ducted it through to the 3 bedrooms. It's made a huge difference, both in heating the bedrooms and through not having to turn the kitchen into a sauna so as to heat the lounge.

I've gone from one room at 30 degrees, the lounge at 22 and the bedrooms at 16 to having the whole house at 21 - 23. A vast improvement.

My system has a 250mm fan and intake duct, with 3 x 150mm ducts to each bedroom.

I've also got a 3.5kW electric heater built into the system (using the same ducts and fan) such that I can heat the bedrooms without having to light the fire. That's quite handy if arriving home late etc.

The fans and ducts are easy to get. You could do most of the work yourself, just need an electrician to wire up the fan to a switch somewhere convenient. The in-line heater was hard to get - I got it from a company in NZ.

Yeah Smurf I remember the thread, is the asking price for the system with the 3 outlets and a 200mm fan reasonable??

G'day Moondog

I am interested in this thread as I am planning a slow combustion wood fire in the lounge room of the new house. The room will be about 47 square metres with a 4.5 metre high exposed beam cathedral ceiling. The flue will be in a concrete block chimney 8 metres high. With such a long flue located in the chimney, I expect the air inside to get quite hot and so I plan to use the chimney as a duct to transfer hot air to the upstairs bedrooms. By using vents in the chimney at the apex of the cathedral ceiling, in the upstairs bedroom above (which the chimney passes) and at the bottom near the wood heater, I hope to be able to direct the heated air in any required direction. A system of vent dampers and / or fans will be used but I haven't worked out the details yet.

Any thoughts / comments would be appreciated.

I don't like the term "slow combustion" myself as it brings up images from my past of smoke every-where in winter.
But i know what you are talking about.

Couple of thingsI have worked out and researched; the need for the flue to be absolutely smoke tight, and if any of the chimney is exposed to the outside air it needs to be heavily insulated so the heat stays in the house and doesn't escape to the cold night air.

Return air needs to be away from the warm air inlet, preferable on the other side of the room, in a 2 story house a simple vent in the floor may be good enough, make the chimney solid enough with enough thermal mass so the chimney stores heat and then you can build big hot fires that burn clean and produce almost no pollution.
If the chimney is inside the building envelope then you may not need to use any additional vents, just a matt black finish on the masonry and let IR radiation do the work passively, but if you do use a vent it need not be too big, I would think just as big as one masonry unit would do the job
An 8meter flue is going to give a huge draw to the fire

Thanks for the feedback Moondog

The engineer in me would like to experiment with different ideas and refine the details with the data collected but the building process doesn't allow for much reworking.
Closing up a vent will be easier than making a new one, so I will allow for them and see how things work out in practice. I will have to give some serious thought to insulating the exterior portion of the chimney! Maybe even block off the last couple of metres above the roof line so that section is not part of the "internal" thermal mass.
How would one go about creating a thermal break in a block work chimney? Hebel layer maybe??

You might also like to consider one of those reverse ceiling fans that has a 'winter' setting that blows hot air back down to the habitable zone from up there in that glorious cathedral ceiling (which will otherwise be wasted by lingering up there).

Vents ARE easier to block than retro fit so I agree with that scenario; Hmmm? Thermal break in the masonry? Good point but why not just insulate the whole lot?? adds all that extra blockwork as thermal mass and increases the total stored energy

As far as temperatures near the ceiling are concerned, I've measured 37 degrees coming out of the vents from the duct system (with the in-line electric heater off).

I've no idea how hot it used to get at the ceiling above the wood heater before the fan and ducts were installed, but I'd guess we're talking about something over 60 degrees since it's a lot cooler up there now - but still in the high 30's.

Smurf where those ducts the same as the standard air-con ducting with a jacket of polyester about 30mm think??

I have always thought that those tubes could be more heavily insulated, i wonder how they would be if you put one inside the other for a wall thickness of about 75 / 90mm

I'm no expert on ducts, but they're flexible ducts with aluminium foil inside and plastic on the outside. Inbetween there's a layer of insulation.

One of the things on my "to do" list is to measure the difference in temperature between the inlet and outlets of the system to see how much heat loss there is. Thus far, I've only measured the outlet not inlet temp.

Originally Posted by Smurf

I'm no expert on ducts, but they're flexible ducts with aluminium foil inside and plastic on the outside. Inbetween there's a layer of insulation.

One of the things on my "to do" list is to measure the difference in temperature between the inlet and outlets of the system to see how much heat loss there is. Thus far, I've only measured the outlet not inlet temp.

Look forward to seeing that information, I was just at my parents house and noticed how the ducting was loosing heat to the roof space, the wall of the duct was much cooler than the air temp inside the house, I must get some prices on that flexible duct from a couple of local suppliers