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kjacobs4242 09-26-2010 02:28 PM

fans and duct work
I want to get a fireplace grate blower, to improve the efficiency of burning wood as a heat source. I may buy one similar to the third picture at this site, or make my own. To the moderator, I am using this link so every one can understand what a grate of this type looks like, I suspect the fans they use as being an inadequate source of forced air.

From what I have read the blowers on most don't really move much air, so I am thinking about using a stonger fan, and putting it in the basement to reduce noise, and running a flex pipe thru the floor next to the fireplace and hooking it up where the fan is attatched. My first question is what is the best type of blower fan to use in a system like this? Air is forced into what looks like 3x5 or 4x6 inch channel branching off into many (depending on size) one inch round tubes. I know just enough abot ducting to know there is a lot of friction involved, and out put at top of pipes will be diminished.
Thinking about using a duct booster fan ( 2 styles available, standard inline fan blades or squirrel cage type that attaches on the bottom of duct) in the basement, either attatched to the cold air return or attached to nothing. Would this work (attatched) and not cause problems? Or would I get more forced air out of the system with a stand alone squirrel type built in its own box. Been mulling all these things over for a year or so, and have a ton of HAND split wood I would like to get the most use out of, so I gotta get someting going. TIA,

nap 09-26-2010 02:54 PM

a few thoughts:

it makes no sense to use a fan that blows more air than the fireplace can heat

drawing heat in this manner causes the exhaust gases to be cooled which can result in creosote build up in the chimney. Be sure to keep an eye on that. I have been told the sound a chimney fire makes is pretty neat but obviously not wanted.

you need to use caution so you do not draw gases (CO and others) into the home. Depending where your intake is, how well the flue flows and other considerations, you could cause a seriously unsafe situation

kjacobs4242 09-26-2010 04:15 PM

Good points raised thank you. Do you think these fireplace grates maybe unsafe in general? The intake for airflow thru the grate would be in the basement well away from the fireplace or furnace for that matter. The fire place is an inexpensive prefab builders model, with no separate intake, the draw that feeds the flames is the open front of the fireplace, as far as I can tell.
When you mention drawing gases into the house, do you mean the tubes coming out the top might create a flow that would draw gases out of the fire? And the stronger the flow the more gases? The motor I am looking at can be used with a speed rheostat to set it to any speed. So that could be fine tuned. We burnt wood all last winter, and noticed no measurable change in propane usage, so, as is, majority of heat is going up chimney. How would one monitor too much cooling of gases? I sweep the chimney myself, so letting it get "dirty" may not be an issue.
I think the Idea of tying it in to the cold air might not be good, at first I thought it might circulate the entire house better, but the chance that it might affect the furnace operation may not be worth it, I recently read that most houses have been built with too small of a return system, thru faulty airflow physics assumptions. Plus all I gotta do is turn the furnace fan on by itself, to achieve that effect.
I have read all the reviews from owners of the various makers of grates, and all complained of too little airflow. The tubes are welded to the rectangular intake chamber at right angles, with no rounding, which sets up way more friction than home heating ducts, plus on the models at the web site, the type of fan they use looks like it might be subsceptable to a lot of flow slippage.
You can also get the latest manufactured, high quality prefabs, or inserts, with well designed chambers that use a lot of forced air. I looked at one, and you could stand back 10 feet and feel the flow of heated air striking you. Just trying to replicate that. They are very expensive. Starting around 4k, and that doesn't include install or chimney or extras. The really nice thing about them is they are airtight, and use outside air to both fuel flames, and to rlease into the home as heated air. Which causes positive air pressure in the home and helps keep cold air from sneaking in thru cracks and such. I would think heating outside air during winter would affect flue-rise more than what I might. All interesting points to consider. Thanx again


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