Add-on furnace - return air flow.

[Backhoe]

As I start to do so serious thinking and reading about installing my Englander 28-3500 into my existing oil fired hot air furnace duct work, a few questions come to mind. I can’t find any direct answers so I thought I’d try here. My house is about 10 years old and therefore fairly tight. Hooking in the feed presents no problems, it’s the return air flow that puzzles me. The 28-3500 manual really doesn’t address the situation and I can’t find it in the forums - exactly. In one discussion, stoveguy2esw (Mike) mentioned he was concerned about the static pressure putting in a return line so that kind of tells me that normally you would not install a return line, hence the lack of a mention in the manuals. If no return line is the normal case, then, in a new, tight house, wouldn’t you create a partial vacuum in the basement as you are pushing air out of the basement but how is the air getting replaced in the basement? Wouldn’t that cause chimney draft problems or no draft at all and smoke in the room? The blower is rated at 850 cfm so it is substantial air movement. If we assume the air in the basement is being replaced via normal air leaks in the house going to the basement, then I’m going to be heating the basement which seems like a terrible waste. What am I missing? It seems like I need a return line to the stove.

 

[Eric Johnson]

Since I know nothing about it and there's no mention of a return-connection strategy in the manual, I guess I'd be inclined to run it as and see what happens.

 

[SeanD]

I have the 28-3500. You are right - there is no provision for return air. My home is fairly snug and I haven't had a problem. Usually I leave the door to the basement open a bit, but even when I don't there is no problem. Never had draft issues or smoke in the basement. Also have a gas fired hot water tank in the area, vented into a separate flue. Have a CO detector about 10 feet away. No draft problems in 3 years.
The 28-3500 does heat the basement, but it is radiated heat from the stove pipe and front of the stove, not forced air stuck in the basement.

 

[JustWood]

I have a similar model called a Woodchuck and have had no problems without a old air return . Although when I installed it I raised the whole unit 10" off the floor so I wouldn't have to bend over to load it ( I'm 6'5"). I also figured raising it would keep the intake further off the floor so less dust would end up in the system.

 

[Backhoe]

I guess I had determined that I should provide some type of return air flow to the basement to prevent potential draft issues. I could cut some type of vent to the cellar but the whole idea of using cellar air with all the associated dust, etc seems like I wouldn't want to do that if I could help it. Also, it seems like I'd be taking in, say in January, 55-60 degree cellar air when I could be taking in 70 degree house air. Seems like I'm sort of heating the basement which takes more wood. I was sort of thinking of building a metal box around the return air fan on the furnace and supplying it with two 8 inch return lines. As there is one output 8 inch feed to the existing duct work maybe that would reduce the static pressure to acceptable limits. Don't know, just thinking. Thanks for the feedback.

 

[JustWood]

Like Eric said I would try it without a return 1st. Could save yourself some money on ductwork. Mine works fine without a return.

 

[Nofossil]

I had a different model in a smallish house. I added a return in the staircase risers. Reduced complaints about draftiness. The air WILL find a way to make the round trip (unless it's leaking out of the upstairs and being drawn in through leaks in the basement). It's up to you to decide what the return path will be.

If the air quantity is low enough, there may be no real problem. I remember sitting in the path of the cold air that was flowing down the stairs - very dramatic.

 

[begreen]

If the basement is uninsulated, I'd tie into the return air duct to keep the system a closed loop. Otherwise, the basement might get warmer, but the heat loss will be significant, perhaps up to 25%.

 

[smokinj]

sould be as easy as any furance to put in a cold air return,I was able to get a prefab cold air return for mine but it was just a starting point

 

[ccwhite]

I have a cold air return on mine and I will make sure my new Woodchuck has one too.

 

[glacialhills]

I just installed My Kuuma vapor fire 100 wood furnace and didnt want to draw cold dirty air from the basement either. we ran round prefab duct from the fan back to the forced air gas furnaces return air duct. was very easy and we now draw warm air from living room and hall and also it gets filtered twice before entering the furnace. The gas furnace has a 5" outside makeup air attached to the return so no problem with lack of make up air. Here is one pic showing the back and some of duct.

 

[ThosMN]

I just discovered this site and this string, and it's got me thinking (which can be dangerous). My wood furnace is 20' away from our NG furnace, in a drafty, earth-bermed tuck-under garage/basement. The wood furnace blower didn't have enough ooomph to run through the whole duct system when I tried to hook it up that way, so I just ran one dedicated duct straight up from the furnace, it blows wood heat into the Kitchen/Dining room area, as a supplement at best. My NG furnace is a 15yo high-eff. model, and my wood furnace is an older Hot Shot, which could be a local brand for all I know. A good fire can get it up over 500 F, when I run down every hour or so to feed it, but the lazy, single blower never seems to help heat the house up much.

I'm thinking about replacing the NG furnace with a newer one, and/or possibly an air source heat pump to get in on both efficiency and tax credits. And I'm intrigued by this discussion. Any suggestions for me?

Since I'm new to this--I came here looking for info on replacing my limit control fan-controlling module--is there a primer link anyone knows that can help me learn about the gas furnace/wood furnace interconnect system? Sounds like it might be worth a try. We have pretty cold winters here in N. Minn. Thanks.

 

[laynes69]

As far as a cold air return, it should be required with a wood furnace. Heat circulation is much better. I have my furnace in series, but no returns in the basement. I was worried about pressure differences. If you don't want to tie in the ductwork of the main furnace, then I would run a seperate duct in the home for return. Plus backhoe has it right. You are putting warmer air around the jacket, therefor the blowers will run longer when the furnace is running. Thos I had considered a new system with a heatpump. I burn wood to cut as much propane usage as possible. If you run a heatpump, you will not need as much wood. If you get a new furnace, and still want a wood furnace installed then get one that will install in series. That way they will work as a set. Series has many benefits over a parallel installation when it some to forced air systems.

 

[ccwhite]

You cannot run a wood furnace in series with a heat pump. If you put your A-coil after the wood burner it will jack up your pressures and screw up your coil or even melt the drain pan(most are plastic). It is illegal to run your A-coil on the cold air side in series with a wood burner because in the event that you have a leak the refrigerant will get on the fire box and when heated create a poisonous gas which could kill everyone in the house. I am installing my system presently. I have my Woodchuck up and running and I will finish up my Goodman heat pump tomorrow. I will post pictures once I'm done. I just ran my 2 units side-by-side sharing a common cold air return and air filter cabinet. I'll put a manual damper on the Woodchuck and a electric damper on the air handler. This has been a learning experience for me as I wanted to put the units in series. I heard about the pressure problems and the melting pans. So then I wanted to put it on the cold air side. 2 different contractors told me that was illegal and the reason why. So we came up with this solution. Anyway hope this helps and sorry if I got long winded.

 

[mazz1111]

Hi there.

CCWhite- I think my set up is similar to you---

I'm new to this forum but found your thread on the woodchuck through a google search...I have a woodchuck 2900 in series with a heat pump as well. with no return...was thinking of adding a return..Currently the chuck and pump air handler are in a utility room in the basement(10'x10') with a louvered door. The room is always filthy with soot and dust ...our ductwork and air quality is filthy as a matter of consequence...I was hoping you could share your thoughts and your experience ...

thanks,
miike

 

[laynes69]

I wouldn't share a cold air return, unless its in series. You can draw heat from the wood furnace into the return of the central furnace, therefore compromising your coil. And no your coil doesn't go after the wood furnace. I have talked to many hvac techs about the installation and it was safe. If yours leaked refrigerant with you sharing a cold air return the same thing can happen with the poisonous gas. The issues with a woodfurnace is if needed, both units should be able to operate safely at the same time. Whether its a heatpump and wood furnace, electric resistant and woodfurnace, or gas and a woodfurnace.

 

[ccwhite]

Hi Mazz,
If You click this https://www.hearth.com/talk/threads/31660/ You can see another post I made when I finished up my install. You'll see I used a full plenum on top of the woodchuck and then ran the heat pump right into the side of that big plenum. The dampers are the real trick. If I had it to do over again I would have used both manual dampers. I would try to find dampers that are louvers so you just have to turn a lever to open or close. The way mine is now I have a large plate that I slide into a track to completely block the plenum about 8" - 10" above the woodchuck when I want to run exclusively on the heat pump. I also have an automatic (electric) louvered damper right before the elbow on top of the heat pump pours into the plenum. This works fine but the auto dampers are very pricey and I have found that I am always running on one heat source or the other so the manual dampers would be just fine and cost much less. As far as the return air goes ... you'll see in the pictures I have one large return air drop that then goes through a Trion - Air Bear (there are plenty of other brands available. they all do the same job this is just what I ended up getting) media filter cabinet. below the filter cabinet you see a large tin box. Once the air comes through the filter it can either go over to the air box on the back of the woodchuck or into the tin air box under the air handler (the technical name for the electric furnace with the A coil in it). This works very well because 1. I only have one filter to worry about 2. All air gets filtered whether heating with wood or heat pump or cooling 3. It is a very efficient filter (20"X25"X5") MERV 8 and I could get a MERV 11 if I wanted to. In your post you said that your units are in series I'm hoping that you meant parallel. If you read my post just above yours then you read how I learned the dangers of hooking them up in series. I know I got a little long winded but I wanted to describe things properly. If I can answer any other questions or if you want any specific pictures just let me know I'd be glad to help in any way I can.

 

[mazz1111]

Hi Charlie,,

Thanks for the post...to answer your question about parallel- I have the woodchuck....BIG plenum on top that immediatley breaks left OVER the air handler where it feeds in to the plenum...it then has another bend and leaves the room and goes throughout the house.

Both units have open returns at the bottom. Are you saying you use dampers at the Plenum on the outgoing air b/t the units??

Is that to prevent air from say your woodchuck, from traveling "out" of the return on the air handler, and vice versa?

Are my units in the right order, and should I consider individual returns?? sorry for so many questions.......I'm still a little green when it comes to homeownership, but prefer to learn as much as I can and be responsible for my own maintennance...

thanks,
mike

 

[ccwhite]

Hey mike,
No problem with the questions. I had to ask plenty and research a lot to figure this out. I thought it would be such a simple thing. I made up a quick drawing of my setup to show better where the dampers are. The damper for the woodchuck is in the plenum approximately 12" above the top of the unit. The damper for the air handler is in the elbow just before the air from the air handler goes into the plenum. This can be accomplished in several different ways but the key is that with one damper closed and the other damper open you can run the unit with the open damper and not "back feed" through the other unit. for instance you can close the damper on the heat pump and burn a wood fire and the blower on the woodchuck can not blow the hot air back down through the air handler and out into your basement or into the return air duct. Without the damper you would not only be wasting heat that should be going into the house, but if you would try to use the heat pump at that time (because its cold in the house with the heat blowing around in the basement) the heat from the wood fire would then have the "A" coil very hot and could seriously damage your equipment due to excessively high pressures.
With this setup I run a single return air drop into a single air filter and then have openings out of the tin box that the air filter is on top of going to both the woodchuck blower cabinet and into the bottom of my air handler. No need for dampers on the return air side as it is already blocked by the dampers on the supply air side.
If I understand your description of your setup correctly it sounds like your system is virtually the same as mine except you have two plenums which are then somehow ducted into your main trunk. Does that sound correct? If you could, put up some pics. I hope this drawing helps. Forgive me ... it's kind of crude I just whipped it up real quick to give you a better idea of what I have going on. Let me know if I can be of any further help.