Duct Design For Woodshop HVAC

[Rob Will]

The machine room in my new woodworking shop is 32' wide x 48' long x 16' high. I have decided to replace the Lennox Pulse Furnace currently installed in my house and re-install it in the woodworking shop along with the A-coil and condensing unit. The system will be well protected from woodworking dust. The furnace is an 80,000 btuh and the condensing unit is a 3 ton.

I must install the furnace at one end of the shop. At the opposite end is an entry foyer and a small office. Basically I need a straight 48' long supply duct with openings toward the main woodworking room about every 8'. At the far end of the duct there will need to be two additional outlets for the 8'x8' foyer and 8'x8' office.

Here is the question:
Can somebody please help me size this duct?
Should it reduce down in size every time the duct goes by an outlet?

Thanks,

Rob

[Jim Becker]

Rob, a system like that normally is stepped. Perhaps your local HVAC supply company can help you with sizing. OR...maybe you could bribe your HVAC contractor who is doing the replacment in the house to run the design for you. (You'll need them to charge the AC portion of the old system, anyway)

[Al Willits]

Bit hard with out the charts and they're at work, but if your gonna just head down one end of the bldg to the other, start with at least the same size duct as the outlet of the furn, if you can radius the outside curve where the duct goes from vertical out of the furn to horizontal that will help a lot.

You have a long run there so I wouldn't reduce duct size to quick.

You running a return or just a filter at the furnace?
If the office is a closed room, remember what heat goes in, needs to come back as colder air.

Might want to bump up the anticipator setting on the thermostat, to get a bit more blower time, and try to move the thermostat far enough away to help prevent short cycles that don't heat/cool the far end of the shop.
All this is with out seeing what you have, and just ball park suggestions.

btw, you know some of the older pluse furnace had a problem with chambers leaking and Lennox reccomends a chamber test every year on them?
Google "lennox recalls" and you should be able to get years affected, 1994 or so I think.
Al

[Rob Will]

btw, you know some of the older pluse furnace had a problem with chambers leaking and Lennox reccomends a chamber test every year on them?
Google "lennox recalls" and you should be able to get years affected, 1994 or so I think.
Al

Thanks Al,
Mine tested ok a couple of years ago but I have not heard about the annual tests.

In the woodshop, there will be a pre-filter plenum at the furnace. I have thought about using some filter grilles on the side of the return plenum. That would flow into the existing SpaceGuard pleated filter.

Rob

[Al Willits]

Be careful with how much you filter, it's my understanding filters are to keep furnaces clean, not your house/shop and over filtering is pretty easy with todays high end filters.

Your furnace should have a temp rise rating on it, prob 40-80 degree's or something like that (still at home) when she's up and running take a inlet air temp and outlet temp, it should be with in the specified range.
Don't have/can't find the plate PM me and I'll look if I ever get to work..:)

If the temp rise is to high, it usually means not enough air flow, dirty blowers and to restrictive filters and poss a plugg evap coil are the three main problems, too small a ductwork also, come to think of it.

The 4" pleated ones seem to work nice, but a bit spendy, tightwad here just uses a 99 cent fiberglass one from one of the borgs.
Make sure ya can get to the filter easily btw.. :)

I'd also use the regsisters that have a closeable damper on them, helps direct airflow and how much.
Just remember, where ever ya send heat to, its gotta come back, think loop.

Just a thought.

Al

[Mark Hubler]

I have a .pdf copy of an article from Journal of Light Construction that will help you design your system. I can email it to you - just send me an address.

One thing you need to know first is how many CFM your furnace puts out, because that will determine the size of the ductwork you need. You will want to make sure that the duct velocity is not too high or the system will be noisy. As a guess, I think that the starting duct size would be 14" - 16" round.

I would suggest that you find the local HVAC supply center and use spiral duct. There are vents that mount directly into the spirial duct - very easy to build. After each set of vents, the duct size is decreased (my 75,000 BTU home system goes from 14" to 4" in 2" increments).

Home Depot used to carry an air flow calculator (slide rule) by Leslie Locke, that would help the process.

Hope this helps