How do I isolate two separate air handlers on the same line from one another?

[dogwood]

I have a forced hot air system. I am going to put a water to air heat exchanger in the plenum over the furnace to serve my Solo Innova 50. The plenum already has an air conditioning coil in it. If the current furnace fan doesn't produce enough cfms, when set on high speed, to blow sufficient air through both the a/c coil and the w/a hx, I may have to put in a separate air handler for the w/a hx.

The existing furnace and new air handler will both feed into the same extended plenum in the attic, that now only the existing furnace's plenum is connected to. I think I would need to put in some kind of automated, motorized damper in both the existing furnace's plenum and the new air handler's plenum, so only one plenum is open at a time. My question is, are there motorized dampers of a size and design that would fit into a plenum. The ones I looked at online to cover my boiler room's outside air intake were fairly expensive even for covering a small 14" x 14" opening, much less the size of a plenum. Maybe there is a simpler kind of one flap damper that might do the job. I don't know what might work or may be available.

An alternate solution to the problem I've considered, might be to put a fan in the existing plenum to provide supplemental cfm's to the furnaces current fan. Then I wouldn't have to buy an additional air handler. Has anyone ever tried to do that, or know of a fan that might fit into a plenum and provide supplemental cfm's. I am leary of trying to change out and upsize the current fan and fan motor in my Lennox Pulse gas furnace. I couldn't find one in the Graingers catalog that would fit into the limited space anyway. Any suggestions would be appreciated on either idea. Thanks.

Mike

 

[rkusek]

I thought about using a 2nd air handler to avoid any issues with my heat pump but couldn't find viable way to control a motorized damper to do that without spending a fortune. Why do you think you will not have enough CFM's? Typically cooling requires more than heating and many systems run the blower at higher speeds in cool mode vs. heat. My Lennox 5 ton heat pump has a 1/2 hp blower fan that heats a 2200 sq ft. ranch with a walkout basement. The blower has 4 speeds but it was set up to use only 1 speed from the way it was wired. I believe it was the medium (2nd lowest), I actually re-wired it for the lowest setting and it has done the job on that setting for more than 1 year now. I installed my HX in the plenum just above the air handler output using aluminum angle irons to form a "tray". This would allow me to take it out in summer if it created too much resistance. It handled the cooling just fine this summer so I only believe I will be taking it out every few years to clean. All I needed to hook it up was a 2nd thermostat, relay, and a aquastat(Johnson Control A419). After seeing how well the heat pump handled our needs the first winter (w/o boiler) compared to the high efficiency natural gas unit in our previous home I was sold on the low temp heat and the wood boiler water to air HX is very similar. The gas furnace was always hot-cold-hot-cold and scorched the air so that we had to have humidifers in every bedroom and even the living room. The heat pump runs much longer and more frequent to maintain the temp setting but it is a low temp heat and more comfortable to live in. Granted this time we had a built in Aprilaire humdifier which also might help with maintaining the right humidity. The aquastat is currently set to pump water through the HX and run the blower when the water is even below 100*. Unless your house has high heat loss, I think you will be very pleased with the results from the wood boiler and water to air HX.

 

[dogwood]

The problem I am currently having is that there is not enough air blowing through the system now to adequately heat our 3000 sq. feet on colder days. Hardly any air comes out the vents in the farthest away upstairs bedroom as it is. We only have a 1/3 hp motor on our Lennox Pulse gas furnace. The furnace adequately heated the home when it was 2000 square feet, before I added a 1000 square foot addition upstairs. When building our home twenty years ago I specified to the HVAC contractor that when sizing the furnace he allow for the upstairs addition. He obviously didn't. I figure when I add the oversized heat exchanger in the plenum over the a/c coil the problem will only worsen. So I am trying to plan alternatives if that doesn't work out.

By the way our central a/c unit is only a 3 ton I believe, though the house always seems to be adequately cooled in the summer. The house is well insulated. Appreciate your help huskers.

Mike

 

[Hunderliggur]

Why not just get a more powerful (and adjustable?) motor and replace the 1/3 HP unit? One possible problem you may have is that your air ducts are inadequately sized for the CFM (BTU) you need to push through it. You may want to consult a reliable HVAC contractor if you don;t want to just try upgrading. You need to be aware of other comfort issues like noise, vibration, etc. when you force more air through your current system.

 

[rkusek]

Since your home is well insulated I would at least try using the Lennox. Don't be afraid if the blower runs a lot to maintain temp. 1/3hp is only 250W and trust me slow steady even heat is better than the typical gas furnace output. You will also want a second tsats to control the fan and pump for wood. A Honeywell non programmable set for an electric resistance setting works well for me since it maintains the setting within 1 degree. My heat pump tst is set 2 degrees lower and only comes on when my water cools below 90 degrees. You can always add the second air handler if it doesn't keep up.

 

[dogwood]

Thanks for the advice Hunderligger and Huskers. Hunderligger, what do you mean by an "adjustable' motor". My current furnace motor has three set speeds. I spent a good part of yesterday looking at blower motors and their fans in the Graingers catalog. There are so many different types. It is really confusing. My original plan was to upgrade the motor as you suggest. I even contacted Lennox a while back, who said "don't do it". I guess their blowers are carefully matched to the combustion chamber with the Lennox Pulse technology in it. The other problem is that the bigger capacity blowers seemed to need a larger space to fit properly than the 17.5 inches height available in the Lennox's fan chamber. I then thought that maybe I could just change out the blower motor and not the fan housing it is in, but read somewhere that this may not be advisable; that bigger motors require bigger fans. What do you think? I think my extended plenum which runs about three-quarters the length of the attic has sufficient capacity to handle even a lot more cfm's and is not providing any air restiction.

My wife says a whole new furnace wouldn't cost all that much more and then everything would be sure to work okay. And then she'd have adequate heat from the furnace when I am away at work and not available to load and tend the Solo Innova, which is currently four days, three nights a week. I hate to admit it, but she's probably right. The biggest Goodman gas furnace on the Alpine site was only a couple hundred more than their air handler unit alone. I wlll give it a try, as you suggest Huskers, with the current setup, adding the hx and running the fan more. Maybe if that it is minimally okay, I can put in an in-line booster to get hot air into the farthest bedroom. And have a Happy Thanksgiving.

Mike

 

[Hunderliggur]

I think the Lennox Pulse requirements trumps the larger motor. Do what Huskers says while you work on what your wife says ;-) I have to chime in the the universal Hearth.com response - how well are you insulated and can you effectively add some more, reducing your heat load. Minimizing heat loss is the best payback. The plenum is in the attic, and I bet the attic is not conditioned space. Insulate the heck out of the plenum, and a few batts over the ceiling while you are up there, then maybe your old Lennox Pulse will keep the wife happy while you are away. You could even get it done this weekend.

 

[dogwood]

You're right about insulating the extended plenum in the unheated attic. It's the kind of duct with insulation on the inside and I've been planning on wrapping the outside for the past twenty years. When I run the black iron pipe from the new boiler room in the garage to the heat exchanger in the furnace, I'll be running it right beside the plenum, so I'll insulate both then. As it is, I've got R-26 in the walls and R-30 in the ceilings with a vapor barrier all around. I should probably roll some more out in the attic too. There's always more work to do. Good advice though. Thanks again Hunderligger.

Mike