High Velocity Furnace

[stephanemunger]

I'd like to find a high velocity furnace capable of delivering high static pressure. This is for a high-end residential application where I would like to conceal a duct which would be ideally as small as possible.

I've looked into the Unico system, which from what I can gather is not truly a furnace but a fan coil unit with a high velocity blower. It still only gives up to 2" W.G. static, which is in my opinion marginally better that the 0.8" W.G. I'm getting from a standard G61 Lennox furnace.

My specs are 80,000 Btu/hr, so approx. 1300 cfm depending on the temperature rise. I have a duct run of approx. 100', including all elbows. I was hoping to use a 6" round duct, even a 4" if at all possible, but I know this is pushing it quite a bit. So I'm looking at least 12" W.G. static. Is there any off-the-shelf high-velocity furnace capable of doing this ? Thanks in advance for your help everybody.

 

[willard3]

You will need a 3-5 hp fan for this air and I don't think that a residential furnace with a motor of that hp is made.

Think also of fan type: most residential fans are forward curved and I don't see many FC fans that will do 12" w.c.

 

[stanlsimon]

You could add a booster fan in series with the furnace. You would have to carefuly balance the system to make sure you got the right air flow through the furnace.

The high air pressure drop of these systems makes them expensive to operate. If you don't have room for conventional sized ducts, how about fishing hydronic pipes up through the joist spaces? You could go with fancoils or water source heatpumps. Then you are talking 3/4" sized pipes plus insulation.

 

[KiwiMace]

If this is a high end installation, the 6" duct is 3x the most velocity i'd put without serious acoustic treatment. Consider the noise before going down this path. Sounds like crazy talk to me.

 

[designbuild]

Keep your duct outlets in mind as well. For most grilles if you get in the 700 FPM face velocity you are pretty loud. 6000+ FPM to 700 FPM is a pretty tall order. I would say it is time to use your people skills to show that they should spend a little bit of it on their mechanical system, or at least in giving the mechanical systems enough room to operate properly.
As a side note my company did a 7 Mil house that the architect and owner decided to take out radiant floor heating and insisted that the return for a main unit had to be high up with the supply. Despite our best attempts to persuade them otherwise we were forced to put it in that way. Guess what? In the winter the house gets cold, especially in this main area. Three years after the fact the architect makes the comment that he wished he had been told that it would be such a big problem. Nothing like a folder full of documentation to remind him that we had. Luckily at this point the owner realized their mistake and was very accommodating from that point on to try and fix the problem.

 

[stephanemunger]

Thanks for your thoughts ! CinciMace you're right this is pretty much crazy talk !!! I was thinking so much but threw this idea out there anyway. I was actually splitting my 1300 cfm in two 6" ducts so it would not have been as bad as it looks velocity-wise. Duct outlets weren't too much of a concern for me as I was going with linear slots over the 30' of sliding doors.

But here's the rest of the story...

This was to heat the winter garden of this house. More or less a greenhouse attached to the house but with 30' of sliding glass doors on the south wall and a retractable roof. This room is kept closed in the winter and is open to the outdoors during the summer. Radiant floor heating was impossible as almost all the floor space is taken up by earth for the plants. Radiant heating from above would have been brutal for the plants, also impossible.

So I initially decided to go with a linear floor diffuser at the base of the glass doors, with below grade ductwork encased in concrete. Sounded fine until I thought about the room being exposed to the elements during the summer. My floor diffuser basically becomes a storm trench drain. Hmmm, not good.

I then suggested that we might be better going with ductwork at high level above the sliding doors. The architect wasn't too keen on having a 10" duct in this room with glass walls and ceiling. So I tried to figure out a way to go high velocity and get the duct size down. This is where this question comes in.

In the end, the client preferred to go with the trench linear diffuser in spite of the drainage problem. I sloped the trench away from my supply duct and connected a drain line to the house weeper system. Hopefully it'll do the trick and the plumbing inspector won't give me a hard time. Drawings are now out and it is getting built soon. I'm confident it's ok on paper but how will it perform when the trench hasn't been cleaned for a year ? I hope I'll never find out...