[b-boy]

I bought a house that had a 2 1/2 car heated garage. It was used as a shop by the prior owner. He was a boat mechanic. It's well insulated.

I use the garage as a shop too. I'd like to keep the temperatures around 45F during the winter, and crank it up to 65F when I'm working out there.

The existing heater was an old 80% efficient forced air gas furnace. It looks like the prior owner rigged it up himself. It vented through the roof. There was no ductwork to speak of. The plenum simply blew hot air into the garage. There was no place for a filter. It was a rust bucket that had seen better days. I've already ripped it out and scrapped it. It seemed pretty sketchy.

I'd like to replace it. I found a 96% efficient 40K BTU forced air furnace that should fit the bill. I will run PVC to vent it right through the garage wall.

My question concerns the plenum. I can't find anything pre-made, and I don't have much experience working with sheet metal. Can I use fiberglass duct board to build a plenum? I've seen kits online, but I've also seen some posts indicating that it is a fire hazard.

If I can use it, how do I attach it to the furnace? I'm assuming sheet metal screws with washers followed by metal tape.

What is the general consensus?

 

[roughneck]

Use metal, you can buy pre-made pieces from most supply houses.
It should be noted, 45* return air is too low for all the condensing furnaces I know of. Most have a minimum of somewhere around 55*-60*. Heat exchanger damage can occur if it's run too low.

 

[beenthere]

You can use fiberglass duct board.

You may have a problem with a 95% furnace though. As most are not rated to work well with return air temps below 50°(for normal cycles), some are a min of 55°. The old air causes moisture to condense in the primary heat exchanger, and rot it out.

 

[b-boy]

I can bump the temperature if it will cause problems. It sounds like 55F is about as low as I should go. As I mentioned, that garage is well insulated, so it shouldn't be a problem.

Is there a standard way to attach a duct board plenum to the furnace? I can't seem to find that information anywhere.

 

[yuri]

The problem you may have is the air at the floor is colder than 50-55F as hot air rises. Also you need to drain or pump the water away that condenses in the furnace. If the heat exchanger gets corroded and fails because of low temp they will void the warranty and can tell by the color and metullargy of it.

Where do you live (Canada or America?) in the US you can still buy 80% efficient furnaces so I would get one. In Canada it has to be over 92% and you cannot install your own furnace w/o a licensed gas fitter and permit. Fail to do so and you may have no insurance if there is a fire or explosion. Same may be true with parts of the US.

 

[jomama45]

Why not just hang a small modine or "Hot Dawg" heater from the ceiling and save valuable floor space, and skip the duct work all together?

 

[beenthere]

I can bump the temperature if it will cause problems. It sounds like 55F is about as low as I should go. As I mentioned, that garage is well insulated, so it shouldn't be a problem.

Is there a standard way to attach a duct board plenum to the furnace? I can't seem to find that information anywhere.

Foil tape.

 

[SeniorSitizen]

This is a perfect opportunity to install a heater that doesn't require AC power to function. If purchased power fails there will be a place to stay warm other than the Mother Inlaw's house.

 

[supers05]

For the cold air part, I haven't seen literature to support this, of course I haven't read everything. I even just went back to look over the last manual I installed, and only found a section on installation environment temperature when using class I venting. (32*F, freezing) Condensing appliances don't use class I Venting. (Generally using class IV Venting)

Condensation is a direct function of heat rise, which is a direct function of air flow. Too much air, and you will condense, too little, and you will overstress the heat exchanger. If the Temp rise is within specifications, you should not have any condensation in the primary, if the appliance is firing correctly.



A short plenum is all that is required, with a grill on the front. This just allows it to run within its static specifications. The lower limits will be in the manual with the furnace. Here, we only use metal for plenum, but I hear lots of people use duct board just fine. Different jurisdictions have different rules. Its not too difficult to bend sheet metal into a plenum. Lots of videos on it out there. I've done it a few times without a break, or other shop style tools. It might look the prettiest without the plasma cutter, but will come out just fine. Self-tapping pan screws is what we use to fasten it to the furnace. I then use foil back tape to stop air-leaks, which in your case won't really matter.


For the venting, some places require system636 or other listed venting. The "glue" must also be listed for the same.


The manual will specify almost everything about the installation, generic to the country. More specific rules may also apply.

Good luck.

 

[beenthere]

For the cold air part, I haven't seen literature to support this, of course I haven't read everything. I even just went back to look over the last manual I installed, and only found a section on installation environment temperature when using class I venting. (32*F, freezing) Condensing appliances don't use class I Venting. (Generally using class IV Venting)

Condensation is a direct function of heat rise, which is a direct function of air flow. Too much air, and you will condense, too little, and you will overstress the heat exchanger. If the Temp rise is within specifications, you should not have any condensation in the primary, if the appliance is firing correctly.

You have a 45°F return air temp, and a condensing furnace will have condensate in its primary heat exchanger. Even if temp rise is at its high end. Call the the rep for the brand you install and ask him.

 

[supers05]

You have a 45°F return air temp, and a condensing furnace will have condensate in its primary heat exchanger. Even if temp rise is at its high end. Call the the rep for the brand you install and ask him.

I'll call my sales rep next week, but we are partners with a few big name brands. Been to several of their specific courses, and haven't seen anything written backing this up.

Cheers!

 

[sktn77a]

Why not just use an 80% furnace - no worries about PVC venting, condensate freezing, etc. Based upon your usage description, you'll never get a payback for the 95%.

 

[yuri]

I'll call my sales rep next week, but we are partners with a few big name brands. Been to several of their specific courses, and haven't seen anything written backing this up.

Cheers!

Low return temp disclaimer.

I have seen it written in some Lennox install manuals I believe and may be in other brands. Right where they say the furnace is not supposed to be used as a temporary construction heater in new housing.

 

[user_12345a]

Have you thought about a 80% efficient unit heater?

These units don't have the condensate to get rid of and won't get plugged up with dust. They have axial fans in them, no filter needed.

Can get a larger unit and only quickly heat the garage only when needed. Get something with a stainless steel heat exchanger and it shouldn't rot out as eaily from condensation when you first turn it on and it's cold.

 

[supers05]

Low return temp disclaimer.

I have seen it written in some Lennox install manuals I believe and may be in other brands. Right where they say the furnace is not supposed to be used as a temporary construction heater in new housing.

With those search terms, I finally found one that even mentioned it. Not all my manuals say anything about this, but a newer York manual had it. Thanks Yuri! I'm still going to call the rep next week, but the literature is gospel.

Cheers!

 

[beenthere]

Why not just use an 80% furnace - no worries about PVC venting, condensate freezing, etc. Based upon your usage description, you'll never get a payback for the 95%.

An 80%er will have the same min return air temp restrictions as a 90%er does.

Also will have its own venting and combustion air problems.

 

[yuri]

With those search terms, I finally found one that even mentioned it. Not all my manuals say anything about this, but a newer York manual had it. Thanks Yuri! I'm still going to call the rep next week, but the literature is gospel.

Cheers!

Low Temp Disclaimer

It is in the Install Manual which are hard to find on the net. Usually you get the owners and service manuals.

 

[supers05]

Low Temp Disclaimer

It is in the Install Manual which are hard to find on the net. Usually you get the owners and service manuals.

I get them through davenet/upgnet/comfortsite/etc. Carrier is the only large NA brand that doesn't require a trade account to access all of the manuals. A leader IMHO. (I never have problems getting manuals from companies like Samsung and Mitsubishi) However, I did look in about a dozen install manuals that I have saved and only found the one that had it.

Cheers!

 

[yuri]

At the end of the day it is not a good idea to just stick any 80 or 92% furnace in a garage and run it w/o proper ductwork. The fan needs some resistance to work against to get up to speed properly plus like the OP said the other one turned into a rust bucket. Gas fired unit heater or electric heater is better depending on the elec rate.

In the OLD days of belt drive furnaces you could get away with no ductwork but not anymore.

 

[rjniles]

Go back and look at post #6. A heater made specifically for this application.