Fixed Overdraft w/o baro or key damper

[laynes69]

We bought our Caddy 2 years ago and last winter was the first season. Our chimney was too big and it caused some serious condensation to form from a cool chimney and little draft from lower temperatures. Then this Summer I installed a 5.5" rigid liner. Fixed the low draft, but caused a bad overdraft. Having a 32' chimney I used a barometric damper to keep things in control. About 2 months ago I noticed that the flue temps were less than 100*f at the top of the chimney. The baro was cooling the flue too much and I was worried about condensation and installed a key damper. Well the key damper is not recommended from the maker due to the possibility of low draft speeds and possible backup of combustion products. My burn times were reduced because a key damper still wouldn't bring the speeds down to a managable level, but did help with the temps.

Well a couple of days ago I was reading about the Florida Bungalow Syndrome and decided to investigate. EPA stoves and furnaces were tested with a 15' chimney and not a 32' chimney like ours. I always had decent operation of the furnace, but at times things didn't seem right. I thought why not reduce the inlets on the furnace to make things operate normally. I started by blocking the secondary air ports by 1/2 and seen little difference. When I blocked them at 2/3 the secondaries lit up. There was too much air entering the secondaries which was having the opposite effect. Then I blocked the primary air by about 1/2 to 2/3. Sure the factory inlets are okay, but on a much shorter chimney.

Instantly the fire burned brighter, the flames became lazy and the rate of combustion slowed. I left the aluminum tape there and now for the last 3 days and nights theres no barometric damper or key damper in the flue. The unit can go from full on to full off and have no smoke. The burn times have increased as well as output. Finally after 2 years I no longer have to tinker with a key damper a barometric damper and I don't have the low temperatures in the chimney.

Being the furnace operates from a wall thermostat, its very important that if the damper remains open during a call for heat that things don't overfire. Now that the key damper isn't there or the baro there is no smoke spillage, which was a problem here and there before.

I hope in the future that manufacturers can supply something to control the incoming air based on chimney height. Either that or have different settings for different heights. Those with manual dampers don't have so much of a problem, but with an automatic damper it can make things difficult to fine tune.

https://www.hearth.com/econtent/index.php/wiki/why_stoves_overfire_from_too_much_draft

 

[tobes]

I am currently looking at putting a Caddy wood/oil combination furnace in our house for next winter. We currently heat with a 16+ year old oil furnace and are not liking the direction oil prices have headed. We figured if we are going to go with a wood furnace we may as well get rid of the old oil unit. We have access to my wife's family wood lot, so other than my time and the cost of running a chain saw the wood is free. We bought our two storey century farm house two years ago and other than the 4-5 cords of wood we have put through our wood cookstove I am a newby to wood as a fuel. I called around and got a good price on a Caddy ($3500 Wood/Oil) and was pretty much set when I decided to make a few more calls to compare prices. One dealer I was talking to said he wouldn't install a Caddy unit in my house because my 35' chimney was too high for that unit. He said that he has put 9 of them in over the past year and taken 3 of them out. He said the customers were having draft problmes, that the Caddy wasn't designed for long chimneys ? Like I said, I am a little ignorant on the matter and am not ashamed to admit it. Do you have any advice ? Should I be looking at a different unit ?

 

[laynes69]

Draft problems, Did he specify? Theres always a risk of an overdraft with a chimney that tall, unless the draft is reduced. I did have ours installed and the first year we had some issues because our flue was too large and there were draft and condensation issues. Now with an insulated liner, those problems are fixed. You should figure out your heat loss on the home to get an idea on what furnace to buy. Whats the square footage of the home, and how tight/insulated is it? I would guess you would need a barometric damper on the oil side as well as the main flue. Ours is wood only, so the oil side I'm not familiar with the oil side. I think the Caddy line of furnaces are nice units that are well built. I would do my homework and make sure though you get a unit thats sized correctly for the home. As far as not being designed for tall chimneys, thats partially true. The test chimneys used are around 15 feet. But its like saying no stove or furnace will work on a tall chimney. Thats why in the manual for the Caddy's, it specifies draft requirements at .04"min to .06" max. I have spoke with them and they say you can safely go up to .08" but no higher for you will lose efficiency. Theres a man in Michigan that installed a Caddy on a 35+ foot insulated chimney, he runs a barometric damper and doesn't have any problems with the operation of the furnace.

 

[afblue]

This is some great insight. I have an EPA rated fireplace insert that has a chimney liner in a 20ft chimney. I think its over drafting, so when I install my Paxo in the fall, in the next flue over, its going to be 30+ ft, and I am sure its going to mess with the secondary controls to get an optimal burn.

 

[tobes]

He didn't specify, I was at work and couldn't get into too many details at the time. Our house is around 1600 square feet, I added insulation in the attic last summer but there is still lathe and plaster on 75% of the exterior walls, and mainly old slider windows throughout. So it's probably not the most efficient house right now. We were weighing our options whether or not to replace the windows and re-insulate the walls, but with a limited budget we figured switching furnaces right now would be the most economical choice, that is with easy and free access to wood through her family lot.
Now that you mention it i think our existing oil furnace does have a damper on it. I would have to increase the chimney from a 5" to a 7" for the wood/oil combo.

 

[laynes69]

I would have an energy audit done, or at very least a blower door test done to check for air leakage. When you insulated the attic, did you seal all penetrations from the living space to the attic? Does your walls currently have insulation? If your old furnace is terribly inefficient then it would be a good move for a new furnace. I'm not a fan of combo units, but if thats whats needed then its understandable. I like having 2 seperate units, I feel you get the most efficiency from both fuels.

 

[tobes]

I sealed off the attic the best I could. Our upstairs bedrooms have partially sloped ceilings so I couldn't seal the attic right off without cutting off the air flow to the trusses that extend past the walls. There was a combination of vermiculite and fibreglass insulation already in the attic and I added another layer of fibreglass on top of that (I had the vermiculite tested and it doesn't contain asbestos). As for the walls, they are a double brick wall with 2" strapping and lathe and plaster. There is insulation, but only 2" of it. I will eventually tackle tearing out the lathe and plaster and either go with styrofoam/drywall or build out 2x4 walls and insulate with fibreglass. Not sure which one I'll go with yet, I can afford to lose a little space if I decide to go with the 2x4 walls, but that is all down the road.
I had the oil furnace serviced this past fall and the tech said that it was running well, I guess I just figured that a new combo would run more efficiently than an older furnace. I was looking down the road as well, I though a newer oil furnace would look a little better to a buyer than a 16+, or by than 20+, year old furnace. I looked at the caddy add-on and the price is obviously alot less. I obviously don't know alot about furnaces, but is the s-duck that connects it to the existing furnace efficient ? I mean it just seems odd that when your original furnace is called for heat it has to force air down and then through the caddy. Forcing warm air down seems counter productive. Have you noticed if it has made your original furnace any less efficient ?

 

[laynes69]

Our furnace is installed in parallel, which is illegal as far as I know in Canada. With a series install with the add-on, as long as you are heating with wood, the firebox will be warm to hot. If the oil furnace would fire there would be little heat loss. On the other hand if wood was rarely used, you would use a few btus to heat up the wood furnace so there would be a slight loss of efficiency from the oil furnace. Basically the same with a combo unit. As far as forcing air downward, I don't think that will effect efficiency. When installing an add-on, you need to match the original flow from the old furnace after installing the wood furnace. I guess there are pros and cons to each setup.

 

[tobes]

Thanks alot for the input. It's very helpful to hear from some people who have already installed them and worked out the kinks.

 

[Willman]

I will eventually tackle tearing out the lathe and plaster and either go with styrofoam/drywall or build out 2x4 walls and insulate with fibreglass. Not sure which one I’ll go with yet,

You will want to check out buildingscience.com for the correct way to insulate brick walls. Fiberglass is not very kind to the brick as well as foam board. I am doing a rehab on a double wythe brick home and framed it with 2x's and had it spray foamed.

Will

 

[pybyr]

A restrictive damper (like a key damper) in the flue of a combustion appliance that has a blower would make me really uncomfortable- changed conditions could lead to significantly pressurized combustion gases in the flue-- which will then find a path of least resistance into the living space...

Not familiar with the details of the Caddy, but if it has a blower, seems as if it would be better to either try to reduce blower RPMs (if that can be done properly, depending upon type of motor) or reduce air intake via some sort of partial shutter

 

[laynes69]

The Caddy has natural draft, not forced draft. There's just a single servo that opens and closes a damper.

 

[tobes]

I will eventually tackle tearing out the lathe and plaster and either go with styrofoam/drywall or build out 2x4 walls and insulate with fibreglass. Not sure which one I’ll go with yet,

You will want to check out buildingscience.com for the correct way to insulate brick walls. Fiberglass is not very kind to the brick as well as foam board. I am doing a rehab on a double wythe brick home and framed it with 2x's and had it spray foamed.

Will

Thanks, I'm always open to suggestions. I will take a look at that site.

 

[TCaldwell]

exhausto makes some controllers that will send a control sig to a damper or fan for just that purpose, maybe a little over the top but will give ideas.