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Calm_Blue_Ocean 01-01-2008 06:56 AM

Removing Extended Plenum
Hi all,

I'm new here. I'm installing a basement apartment (I'm starting from scratch) into a small semi-detached home (townhouse). I've got lots of questions....I'll start with my ductwork.

First, some relevant background. My home is two stories and only 1050 square feet (not including the basement). I share one wall with my neighbor. I have a 75,000 BTU furnace which is only 2 years old. I live in northern Alberta, where it is not uncommon for winter temperatures to hit -50C. The furnace is situated in one corner of the basement. The house is well insulated and the furnace currently does a great job of keeping things nice and cozy. The temperature is uniform throughout the house (which is an impressive feat for this climate).

Now, my problem. I am putting an apartment into the unfinished basement. There are rectangular supply and return ducts running below the joists which greatly reduce the amount of available headspace (you can see these extending to the left in my first photo). There are 6 supply pipes attached to the supply duct and an additional 7 pipes attached directly to the supply plenum on top of the furnace (opposite where the duct comes off). The supply plenum measures approximately 3x2x1.5 feet. Rather than build a very low soffit in what will become the bedroom, I’d like to run the pipes directly to the plenum on top of the furnace. Is this feasible? Essentially I want to convert an “extended plenum system” into a “radial system” (there is plenty of room on the plenum to do this). I realize that this is not ideal as my furnace is not in the centre of the house.

Also, there are two ducts bringing combustion air to the furnace; you can see these in my second photo. The black insulated flex pipe brings air from outside and dumps it on the floor between the furnace and water heater. A second insulated flex pipe (the silver pipe in the upper corner of the photo, running between the joists) dumps outside air into the return duct. Is this overkill? (I don’t mind too much if it is, I’m not a fan of carbon monoxide…however that’s a lot of cold air to heat). Can I tie the silver flex pipe directly into the vertical return air duct on the side of the furnace? If I do this and connect a second return from upstairs using 6” pipe I can do away with horizontal return duct as well. (Incidentally I removed the humidifier after the second shot was taken).


redline 01-03-2008 10:33 AM

Does the townhome have an attic?

Calm_Blue_Ocean 01-03-2008 04:11 PM

Yes, there is an attic. The apartment I'm building in the basement will have one bedroom, a small livingroom, bathroom and kitchenette. There are currently two registers in the basement, I will probably add one more. The first floor consists of a kitchen, half bathroom, and livingroom. There is an open stairwell to the top floor. The top floor has three bedrooms (two of them are very small) and a full bathroom.

redline 01-04-2008 08:44 AM

Install the furnace in the attic. Install a tankless water heater in the basement. This will give you more space in the basement and you will not need a supply air from outside going to the basement.

Calm_Blue_Ocean 01-04-2008 02:31 PM

Hi Redline. Although moving the furnace out of the basement would certainly free up a lot of space (I could add an extra bedroom) I really don't have the time or money to do that. Rerouting the ducts would be a major headache. I've also never heard of anyone doing that in this climate (the attic is pretty chilly, there would be substantially more cold air to heat).

redline 01-04-2008 02:53 PM

Does code in your area require to points of egress for a basement dwelling?

redline 01-04-2008 02:57 PM


Originally Posted by Calm_Blue_Ocean (Post 85552)
Rerouting the ducts would be a major headache. I've also never heard of anyone doing that in this climate (the attic is pretty chilly, there would be substantially more cold air to heat).

The ducts would not be removed you would just conect them from above.

How much cold air comes in from the supply air from outside? (it seems like it would be like having a window open 24/7)

redline 01-04-2008 02:59 PM

You should be able to hire a heating contractor to reposition the ducts into a more compact area and save you space.

bigMikeB 01-04-2008 04:33 PM

First off there is no way you need to have outside air come in to the return duct. If you were worried about carbon monoxide you should have capped the return near the floor on the return stack, so you don't pull return air from near the furnace.
Have you looked into the legal and code aspects of adding a basement bedroom to your place? Do you have egress windows?
You need to have a furnace room that has outside air for combustion and the duct that is currently there doesn't make it. You need to check these things out with the code enforcement offical in your town.

Calm_Blue_Ocean 01-06-2008 12:42 AM

Redline, I checked the installation manual for my furnace and an attic installation like you described, although legal in the US, is not allowed in Canada.

I bought the house less than a month ago. The building inspector thought it was odd that I had two inlets for combustion air but didn't think it was a problem. There will be one bedroom where the window is currently. The tape on the floor shows where I plan to put my walls (not to be confussed with the unpainted line - this was a wall I removed). I intend to replace the window with one that is a bit larger to meet the code for egress.

I don't see that I lack the required combustion air (I think I have too much). The black pipe provides air for both the water heater and furnace (both of which are gas). The inspector thought this was a good thing. This pipe was added when the furnace was installed less than two years ago (presumably to bring things up to code).

Contractors are extremely expensive in this town and to meet my budget I need to do everything myslef if possible. Codes are taken rather lightly here (despite the fact that Canadian codes are rather strict)... of course I want to make sure I do everything safely.

LawnGuyLandSparky 01-06-2008 07:47 AM

You're doing everything yourself because everything you're doing is illegal. You want to make sure you do everything safely, with the proviso that the income from the illegal apartment is first and formost, and trumps the obvious illegality and lack of egress from the entire uninspected, unregulated project.

bigMikeB 01-06-2008 08:37 AM

Looking at the window in the pic you will have a hard time adding an egress window, which requires an opening large enough for a fireman, @30"x30" clear. As for combustion air, it's one high and one low grill, both one square inch of free area for each 1000btu's of burner, the water heater is @45,000 btu's. Just a few thoughts toward the future health of your apartment dwellers.

Calm_Blue_Ocean 01-06-2008 05:21 PM

Thanks BigMike, I'll check the codes for combustion air my area. I'm not entirely sure what you mean by "one high and one low grill". Where should these be with respect to the furnace?

Lawn Guy:

What am I doing that is illegal?Here is the code for my area:

"Section 3B (b) Emergency Egress – For buildings of 3 storeys or less and except where a bedroom door provides access directly to the exterior or the suite is sprinklered, each bedroom shall have at least one outside window which may be opened from the inside without the use of tools or special knowledge.

(i) Windows referred to above shall provide unobstructed openings with areas not less than 0.35 m2 (3.8 ft2 ), with no dimension less than 380mm (15”).

(ii) If the windows referred above are provided with security bars, the security bars shall be installed so they may be opened from the inside without the use of any tools or special knowledge.”

I have already shopped for a window and have found one that will fit this space and comply with these guidelines. (Incidentally, the rough guideline here is that a 250 pound man needs to be able to exit through the window - a tight fit no doubt). Note that the window currently in place is much smaller than the opening in the foundation (it is shimmed in place with several 2 by 4s).As I mentioned in another post, this town is an anomaly. Because of the oil boom here the population rises 9% per year. The city is ‘forgiving’ when it comes to codes because it needs housing. Construction costs are double what they are elsewhere. I couldn’t possibly afford a ‘real contractor’ (and I would be on a waiting list for several months). Most people have these suites built by shotty freelancers who ignore the codes. I fully intend to keep things up to code and I do good work. My experience is, however, limited mainly to carpentry. I will sell the house in a couple years and I assume the buyer will hire an inspector. The rental income is indeed very important to me. I don’t work for one of the oil companies and thus I don’t get a massive monthly living allowance tacked onto my salary. I need that income so that I can continue to live here and pay the mortgage on a half-million dollar home. People rent out unfinished garages here; a sofa in someone’s living room goes for over $500/month. I am going to build a nice suite and I will be one of the few people here who does it above board. I will have “house guests” rather than ‘renters’ in the basement. There will be a leasing (house guest) contract – this is a legal arrangement here (admittedly it’s a bit of a loop-hole). (A fully legal rental suite has to be self sufficient - i.e. with a separate furnace, water heater, etc). My insurance company is also fully aware of my intentions and I will meet their requirements as well. Before you accuse me of anything, at least know what you are accusing me of!

Now back to the original issue. I will likely leave the combustion air set-up as is, as my inspector said it looked good. He also checked everthing out with a carbon monoxide tester and found nothing. I intend to install a CO detector in the basement (along with a hardwired smoke detector). I'll leave the fresh air intake (the silver pipe) as well, although perhaps I could put a regulator on it (I'll see what me first heating bill is like).

Can I get rid of that plenum, or at least take it back a few feet? Thanks.

Calm_Blue_Ocean 01-06-2008 05:45 PM

Incidentally redline, I didn't mean to suggest I would need to reduct the entire house to put in an attic furnace. But we are talking about 4 levels (including the basement and attic) and the wall layout is different on each floor. I would need to open the walls and floor/ceiling in several places to tie all the ducts together. I suspect the basement would end up being rather cold and getting an even temperature throughout the house without being cooked upstairs (i.e. forcing the hot air down three levels) would be very difficult (impossible without blowers). I also think this would waste a lot more heat and energy. The passive setup I have now works very well and I'd rather not interfer with that.

Calm_Blue_Ocean 01-07-2008 02:38 PM

BigMike, I hadn't noticed your suggestion to cap the intake at the base of the return. I intend to do that, although not for the reason you suggested. Cold air from upstairs and from the outside intake is dumped through this opening when the furnace is not running. If carbon monoxide was a problem (it's not) closing this off would stop it from being distrubuted throughout the house, however it would allow it to pool in the basement - I'm not sure that would be better.

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